Satellite-tracked Drifter Measurements in 

the Chukchi and Beaufort Seas 

Principal Investigator 

Thomas Weingartner1

Collaborators 

Cayman Irvine1, Elizabeth Dobbins1, Seth Danielson1

Leandra DeSousa2*, Billy Adams2*, Robert Suydam2*, Willard Neatok3*

1University of Alaska Fairbanks, Institute of Marine Science 

2North Slope Borough Department of Wildlife Management 

3Village of Pt. Lay

*Under support from the North Slope Borough-Shell Baseline Studies Program

FINAL REPORT 

April 2015 

OCS Study BOEM 2015-022 
Contact Information: 

email: CMI@alaska.edu 

phone: 907.474.6782 

fax: 907.474.7204 

Coastal Marine Institute 

School of Fisheries and Ocean Sciences 

University of Alaska Fairbanks

P. O. Box 757220 

Fairbanks, AK 99775-7220 

This study was funded in part by the U.S. Department of the Interior, Bureau of Ocean Energy Management (BOEM) through 

Cooperative Agreement M11AC00001 between BOEM, Alaska Outer Continental Shelf Region, and the University of Alaska 

Fairbanks. This report, OCS Study BOEM 2015-022, is available through the Coastal Marine Institute, the Department of Interior 

library, and electronically from http://www.boem.gov/BOEM-Newsroom/Library/Publications/Alaska-Scientific-and-TechnicalPublications.aspx. 

The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the 

opinions or policies of the U.S. Government. Mention of trade names or commercial products does not constitute their 

endorsement by the U.S. Government. .
Table of Contents

 Page

List of Figures ................................................................................................................................iii

List of Tables..................................................................................................................................ix

Abstract ........................................................................................................................................... x

I. Introduction and Background ............................................................................................... 1

Ia. Mean Circulation and Water Masses............................................................................... 2

Ib. Wind-Forced Variability ................................................................................................. 3

II. Project Objectives.................................................................................................................. 4

III. Methods................................................................................................................................. 6

IIIa. Drifters and Winds........................................................................................................ 6

IIIb. Hydrographic Data...................................................................................................... 13

IIIc. Drifter Clusters: Relative Dispersion and Kinematic Characteristics......................... 14

IV. Results and Discussion........................................................................................................ 17

IVa. Winds.......................................................................................................................... 17

IVb. Sea Ice Distribution .................................................................................................... 18

IVc. Hydrography ............................................................................................................... 19

IVd. Overview of Selected Trajectories ............................................................................. 38

2012 Drifter Deployments and Data Distributions.................................................... 39

13 August 2012 Deployments: Central Chukchi Shelf.............................................. 41

10 August Microstar Deployments: Inner Shelf of the Chukchi Sea......................... 55

10-11 August 2012 iSphere Deployments: Inner Self of the Chukchi Sea ............... 64

2 September 2012 MS Drifter Deployments in Burger ............................................. 71

17 August 2013 Deployments: Pt. Lay Inshore and Offshore (NSBW-Shell).......... 77

24 August 2013 Deployments: Wainwright Inshore and Offshore (NSBW-Shell)... 78

8 September 2013 Deployments: Northeast Hanna Shoal (BOEM).......................... 97

9 September 2013 Deployments: Northwest Hanna Shoal (BOEM) ...................... 103

IVe. Relative Dispersion and Kinematics......................................................................... 109

IVf. Wind-Drifter Correlations........................................................................................ 132

V. Conclusions....................................................................................................................... 158

VI. Future Recommendations with Respect to Drifter Studies .............................................. 166

VII. Acknowledgements........................................................................................................... 167

VIII. Project Presentations ......................................................................................................... 167

IX. References......................................................................................................................... 168

ii
List of Figures

Figure 1. Mean depth-integrated streamlines in the Chukchi Sea, after Spall (2007). ................. 1

Figure 2. Schematics of the CODE-type 1-m drogued Microstar drifter and the SVP-type 10-m 

drifter ................................................................................................................................. 8

Figure 3. Underwater photo of a Microstar drifter in operation. ................................................. 8

Figure 4. Aerial photo of two Microstar drifters offshore of Wainwright in 2013....................... 9

Figure 5. iSphere surface drifters.................................................................................................. 9

Figure 6. Time of occurrence of high quality position data for the drifters deployed in 2013... 10

Figure 7. Time of occurrence of all quality position data for the drifters deployed in 2013 ...... 10

Figure 8. Locations of meteorological buoys in the Chukchi and Beaufort Seas since 2008..... 11

Figure 9. Time series of winds based on the NARR model and observed at the Burger 

meteorological buoy in 2012. .......................................................................................... 11

Figure 10. Time series of winds based on the WaveWatch model and observed at the Pt. Lay 

meteorological buoy in 2012. .......................................................................................... 12

Figure 11. Bathymetric map of the Chukchi Sea showing the approximate location of the 

Statoil, Burger, and Klondike prospects.......................................................................... 13

Figure 12. Example schematic of a cluster of drifters dispersing through time. ........................ 14

Figure 13. Vector plots of winds from the central Chukchi Sea shelf from August through 

November for 2011, 2013 and 2013. ............................................................................... 17

Figure 14. Ice-edge location as a function of year and date between August and September.... 21

Figure 15. Regional ice concentration maps by year and date for August 15 and 30, and 

September 15 and 30........................................................................................................ 22

Figure 16. Regional ice concentration maps by year and date for October 15 and 30, and 

November 15 and 30........................................................................................................ 23

Figure 17. Temperature-salinity diagrams from the CSESP stations in the central Chukchi Sea 

for 2011 and 2013. ........................................................................................................... 24

Figure 18. Plan views of surface and bottom temperatures and salinities averaged over the 

upper and lower 10 m of the water column in August and September 2011................... 28

Figure 19. Plan views of surface and bottom temperatures and salinities averaged over the 

upper and lower 10 m of the water column in August and September 2012................... 29

Figure 20. Plan views of surface and bottom temperatures and salinities averaged over the 

upper and lower 10 m of the water column in August and September 2013................... 30

Figure 21. Vertical sections of temperature and salinity from 2011 and 2012 along the 

southwest to northeast transect shown in Figure 14. ...................................................... 31

Figure 22. Top: location of the west-to-east transect of glider 191 from 10 – 14 August, 2013. 

Bottom: Vertical sections of temperature, salinity, and density along the transect......... 33

Figure 23. The Leg H vertical section derived from the towed-CTD (Acrobat) on 11 - 12 

September 2013. .............................................................................................................. 34

iii
Figure 24. As in Figure 23, but for Leg D, from northwest of Hanna Shoal to the coast ...........34

Figure 25. As in Figure 23, but for Leg G, a short transect offshore of Wainwright..................35

Figure 26. As in Figure 23, but for Leg Q, a south to north transect across Hanna Shoal..........35

Figure 27. As in Figure 23, but for Leg B, a short west to east transect along the northeastern 

flank of Hanna Shoal........................................................................................................36

Figure 28. As in Figure 23, but for Leg O, a short north to south transect along the northeastern 

flank of Hanna Shoal........................................................................................................36

Figure 29. As in Figure 23, but for Leg R, a short north-south transect across Hanna Shoal.....37

Figure 30. As in Figure 23, but for Leg C, a short transect along the northwestern side of Hanna 

Shoal ...............................................................................................................................37

Figure 31. Trajectories of all drifters between mid-August and 22 November 2011, released in 

the central Chukchi Sea....................................................................................................38

Figure 32. Trajectories of 10-m drogued drifters between mid-August and 22 November 2011, 

released in the central Chukchi Sea..................................................................................39

Figure 33. Deployment times, locations, and drifter types for the UAF and NSBW drifters used 

in 2012..............................................................................................................................40

Figure 34. Data distribution in 15’x15’ grid cells for all drifters, SVP drifters, Microstar 

drifters, and iSphere drifters for the UAF and NSBW deployments in 2012. .................41

Figure 35. The trajectory of MS-1 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record.....................................................................................44

Figure 36. The trajectories of MS-4 and 5. .................................................................................45

Figure 37. The trajectories of MS-7 and 9. .................................................................................46

Figure 38. The trajectory of SVP-2 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record.....................................................................................47

Figure 39. The trajectory of SVP-3. ............................................................................................48

Figure 40. The 2012 trajectory of SVP-5 and the time series of its zonal (U), meridional (V), 

and sea surface temperature (SST) record........................................................................49

Figure 41. The 2013 trajectory of SVP-5 and the time series of its zonal (U), meridional (V), 

and sea surface temperature (SST) record........................................................................50

Figure 42. The 2012 trajectory of SVP-6 and the time series of its zonal (U), meridional (V), 

and sea surface temperature (SST) record........................................................................51

Figure 43. The 2013 trajectory of SVP-6 and the time series of its zonal (U), meridional (V), 

and sea surface temperature (SST) record........................................................................52

Figure 44. The trajectory of SVP-8 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record.....................................................................................53

Figure 45. The trajectory of SVP-10 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. .............................................................................54

Figure 46. The trajectory of NSBW-1 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. .............................................................................56

iv
Figure 47. The trajectory of NSBW-2 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. ............................................................................. 57

Figure 48. The trajectory of NSBW-3 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. ............................................................................. 58

Figure 49. The trajectory of NSBW-4 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. ............................................................................. 59

Figure 50. The trajectories of NSBW-5 and 6. ........................................................................... 60

Figure 51. The trajectories of NSBW-7 and 8. ........................................................................... 61

Figure 52. The trajectories of NSBW-9 and 10. ......................................................................... 62

Figure 53. The trajectories of NSBW-11 and 12. ....................................................................... 63

Figure 54. The trajectories of NSBW-13.................................................................................... 64

Figure 55. The trajectory of iSphere-63180 and the time series of its zonal (U), meridional (V), 

and sea surface temperature (SST) record. ...................................................................... 66

Figure 56. The trajectories of iSphere-60180 and 60190............................................................ 67

Figure 57. The trajectories of iSphere-61180 and 62300............................................................ 68

Figure 58. The trajectories of iSphere-64180 and 65180............................................................ 69

Figure 59. The trajectories of iSphere-65190 and 66180............................................................ 70

Figure 60. The trajectory of MS-13-2 and the time series of its zonal (U), meridional (V), and 

sea surface temperature (SST) record. ............................................................................. 72

Figure 61. The trajectories of MS-7-2 and 8-2. .......................................................................... 73

Figure 62. The trajectories of MS-9-2 and 10-2. ........................................................................ 74

Figure 63. The trajectories of MS-11-2 and 12-2. ...................................................................... 75

Figure 64. The trajectories of MS-14-2 and 15-2. ...................................................................... 76

Figure 65. The trajectory of MS-16-2......................................................................................... 77

Figure 66. The Pt. Lay Inshore trajectory of MS-54 and the time series of its zonal (U), 

meridional (V), and sea surface temperature (SST) record. ............................................ 79

Figure 67. The Pt. Lay Inshore trajectories of MS-57 and 58 .................................................... 80

Figure 68. The Pt. Lay Inshore trajectories of MS-59 and 62 .................................................... 81

Figure 69. The Pt. Lay Inshore trajectories of MS-63 and 67 .................................................... 82

Figure 70. The Pt. Lay Inshore trajectories of MS-68 and 70 .................................................... 83

Figure 71. The Pt. Lay Inshore trajectories of MS-71 and 75 .................................................... 84

Figure 72. The Pt. Lay Inshore trajectories of MS-76 and 77 .................................................... 85

Figure 73. The Pt. Lay Inshore trajectory of MS-78................................................................... 86

Figure 74. The Pt. Lay Offshore trajectory of MS-53 and the time series of its zonal (U), 

meridional (V), and sea surface temperature (SST) record. ............................................ 87

Figure 75. The Pt. Lay Offshore trajectories of MS-55 and 56 .................................................. 88

v
Figure 76. The Pt. Lay Offshore trajectories of MS-60 and 61...................................................89

Figure 77. The Pt. Lay Offshore trajectories of MS-64 and 65...................................................90

Figure 78. The Pt. Lay Offshore trajectories of MS-66 and 69...................................................91

Figure 79. The trajectory of MS-29 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record.....................................................................................92

Figure 80. The trajectories of MS-27 and 32. .............................................................................93

Figure 81. The trajectories of MS-36 and 38. .............................................................................94

Figure 82. The trajectories of MS-40 and 41. .............................................................................95

Figure 83. The trajectories of MS-43 and 44. .............................................................................96

Figure 84. The trajectory of MS-35 and the time series of its zonal (U), meridional (V) and sea 

surface temperature (SST) record.....................................................................................98

Figure 85. The trajectories of MS-28 and 30. .............................................................................99

Figure 86. The trajectories of MS-34 and 35. ...........................................................................100

Figure 87. The trajectories of MS-36 and 38. ...........................................................................101

Figure 88. The trajectories of MS-43 and 44. ...........................................................................102

Figure 89. The trajectory of MS-45...........................................................................................103

Figure 90. The trajectory of MS-39 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record...................................................................................104

Figure 91. The trajectories of MS-29 and 31 ............................................................................105

Figure 92. The trajectories of MS-32 and 33 ............................................................................106

Figure 93. The trajectories of MS-37 and 40 ............................................................................107

Figure 94. The trajectories of MS-41 and 42 ............................................................................108

Figure 95. Drifter tracks and cluster centroid for the cluster of drifters deployed in Burger in 

early September 2012.....................................................................................................109

Figure 96. Zonal and meridional Burger drifter cluster centroid velocities, and zonal and 

meridional WaveWatch wind velocities evaluated at the location of the centroid through 

time ...............................................................................................................................110

Figure 97. Time series of relative zonal and meridional dispersion, the cross-correlation in 

relative dispersion, and the relative dispersion for the Burger drifter cluster. ...............111

Figure 98. Time series of the horizontal divergence, relative vorticity, stretching deformation, 

and shearing deformation for the 2012 Burger drifter cluster........................................112

Figure 99. Time series of the terms comprising the absolute vorticity balance for the Burger 

drifter cluster. .................................................................................................................113

Figure 100. Mean cluster trajectory and individual drifter tracks for the Pt. Lay Inshore drifter 

cluster. ............................................................................................................................114

Figure 101. Mean cluster trajectory and individual drifter tracks for the Pt. Lay Offshore drifter 

cluster. ............................................................................................................................114

vi
Figure 102. Mean zonal and meridional Pt. Lay Inshore drifter cluster centroid velocities, and 

zonal and meridional WaveWatch wind velocities evaluated at the location of the 

centroid through time..................................................................................................... 115

Figure 103. Mean zonal and meridional Pt. Lay Offshore drifter cluster centroid velocities, and 

zonal and meridional WaveWatch wind velocities evaluated at the location of the 

centroid through time..................................................................................................... 116

Figure 104. Time series of relative zonal and meridional dispersion, the cross-correlation in 

relative dispersion, and the relative dispersion for the Pt. Lay Inshore drifter cluster. . 117

Figure 105. Time series of relative zonal and meridional dispersion, the cross-correlation in 

relative dispersion, and the relative dispersion for the Pt. Lay Offshore drifter cluster 118

Figure 106. Time series of the horizontal divergence, relative vorticity, stretching deformation, 

and shearing deformation for the Pt. Lay Inshore drifter cluster................................... 119

Figure 107. Time series of the horizontal divergence, relative vorticity, stretching deformation, 

and shearing deformation for the Pt. Lay Offshore drifter cluster................................. 119

Figure 108. Time series of the terms comprising the absolute vorticity balance for the Pt. Lay 

Inshore drifter cluster..................................................................................................... 120

Figure 109. Time series of the terms comprising the absolute vorticity balance for the Pt. Lay 

Offshore drifter cluster................................................................................................... 120

Figure 110. Mean cluster trajectory and individual drifter tracks for the Northeast Hanna Shoal 

cluster............................................................................................................................. 121

Figure 111. Mean zonal and meridional NE Hanna Shoal cluster centroid velocities, and zonal 

and meridional wind velocities evaluated at the location of the centroid through time.122

Figure 112. Time series of relative zonal and meridional dispersion, the cross-correlation in 

relative dispersion, and the relative dispersion for the NE Hanna drifter cluster.......... 123

Figure 113. Time series of the horizontal divergence, relative vorticity, stretching deformation, 

and shearing deformation for the NE Hanna drifter cluster........................................... 124

Figure 114. Time series of the terms comprising the absolute vorticity balance for the NE 

Hanna Shoal drifter cluster. ........................................................................................... 124

Figure 115. Mean cluster trajectory and individual drifter tracks for the Northwest Hanna Shoal 

drifter cluster.................................................................................................................. 125

Figure 116. Mean zonal and meridional NW Hanna Shoal drifter cluster centroid velocities, and 

zonal and meridional NARR wind velocities evaluated at the location of the centroid 

through time................................................................................................................... 127

Figure 117. Time series of relative zonal and meridional dispersion, the cross-correlation in 

relative dispersion, and the relative dispersion for the NE Hanna drifter cluster.......... 128

Figure 118. Time series of the horizontal divergence, relative vorticity, stretching deformation, 

and shearing deformation for the NW Hanna drifter cluster. ........................................ 129

Figure 119. Time series of the terms comprising the absolute vorticity balance for the NW 

Hanna Shoal drifter cluster. ........................................................................................... 129

Figure 120. Log-log plot of the total relative dispersion based on the average of the five clusters 

deployed in 2012 and 2013............................................................................................ 130

vii
Figure 121. Log-log plots of the relative zonal and meridional dispersion based on the average 

of the five clusters deployed in 2012 and 2013..............................................................131

Figure 122. Definition map of the domain locations for the wind-drifter regressions..............132

Figure 123. Scatterplots and regression statistics for the iSphere drifters in the Coastal 

domain............................................................................................................................139

Figure 124. Scatterplots and regression statistics for the MS drifters in the Coastal domain...140

Figure 125. Scatterplots and regression statistics for the SVP drifters in the Coastal domain. 141

Figure 126. Scatterplots and regression statistics for the iSphere drifters in the Barrow Canyon 

domain............................................................................................................................142

Figure 127. Scatterplots and regression statistics for the MS drifters in the Barrow Canyon 

domain............................................................................................................................143

Figure 128. Scatterplots and regression statistics for the SVP drifters in the Barrow Canyon 

domain............................................................................................................................144

Figure 129. Scatterplots and regression statistics for the iSphere drifters in the Chukchi Shelf 

domain. ..........................................................................................................................145

Figure 130. Scatterplots and regression statistics for the MS drifters in the Chukchi Shelf 

domain............................................................................................................................146

Figure 131. Scatterplots and regression statistics for the SVP drifters in the Chukchi Shelf 

domain............................................................................................................................147

Figure 132. Wind-current comparisons between MS-9 (released in Statoil in mid-August) and 

MS8-2 (released in Burger in early September).............................................................148

Figure 133. Scatterplots and regression statistics for the iSphere drifters in the Beaufort Shelf 

domain............................................................................................................................149

Figure 134. Scatterplots and regression statistics for the MS drifters in the Beaufort Shelf 

domain............................................................................................................................150

Figure 135. Scatterplots and regression statistics for the SVP drifters in the Beaufort Shelf 

domain............................................................................................................................151

Figure 136. Scatterplots and regression statistics for the iSphere drifters in the Continental 

Slope domain..................................................................................................................152

Figure 137. Scatterplots and regression statistics for the MS drifters in the Continental Slope 

domain............................................................................................................................153

Figure 138. Scatterplots and regression statistics for the SVP drifters in the Continental Slope 

domain............................................................................................................................154

Figure 139. Scatterplots and regression statistics for the MS drifters in the Canada Basin 

domain............................................................................................................................155

Figure 140. Scatterplots and regression statistics for the SVP drifters in the Canada Basin 

domain............................................................................................................................156

Figure 141. Scatterplots of zonal (U) and meridional (V) velocities between drifters and 

mooring BC2 in Barrow Canyon....................................................................................157

viii
Figure 142. Scatterplots of zonal (U) and meridional (V) velocities between drifters and the 

high-frequency radar for the Burger drifter cluster of September and October 2012. .. 157

Figure 143. Plan views of temperature and velocity vectors for the M1 and M2 on Day 25 at the 

depths indicated. ............................................................................................................ 162

Figure 144. Cross-channel sections of temperature and salinity, overlain with contours, at 

along-channel distance km 84 from M1 (a and b) and M2 (c and d) on Day 25........... 163

List of Tables

Table 1. Regression summary for the Alaska Coastal domain ............................................... 136

Table 2. Regression summary for the Barrow Canyon domain ............................................... 136

Table 3. Regression summary for the Chukchi Shelf domain................................................... 136

Table 4. Regression summary for the Beaufort Shelf domain .................................................. 137

Table 5. Regression summary for the Continental Slope domain............................................. 137

Table 6. Regression summary for the Canada Basin domain ................................................... 137

Table 7. Regression summary for the Pt. Lay Inshore and Offshore, and NE and NW Hanna 

Shoal drifter clusters...................................................................................................... 138

σθ 

ix
Abstract

In fall of 2011, 2012, and 2013, numerous satellite-tracked drifters were deployed in the 

northeast Chukchi Sea in conjunction with collaborators from the North Slope Borough. The 

drifters included non-drogued surface iSpheres, 1-m drogued Microstar (MS) drifters, and 10-m 

drogued Surface Velocity Profiler (SVP) drifters. Drifter deployments included sets of 12 – 20 

drifters of mixed type, which were subsequently tracked until the drifters were beached, ceased 

operating, or were trapped in sea ice. There was considerable interannual variability in the 

resulting trajectories. In 2012, nearshore (within 10 miles of the Chukchi coast) drifters moved 

northeastward through Barrow Canyon. After exiting Barrow Canyon, these drifters moved 

eastward onto the Beaufort Sea shelf or the Beaufort slope, or northwestward along the Chukchi 

Sea slope. Drifters deployed further offshore in the northeast Chukchi Sea in 2011 and 2012 

generally moved eastward toward the coast and eventually entered Barrow Canyon. After 

exiting Barrow Canyon they also moved eastward along the Beaufort shelfbreak (or shelf) or 

northwestward along the Chukchi shelfbreak. Many of the drifters that moved westward on the 

Chukchi shelfbreak turned southward and back onto the northeast Chukchi Sea shelf. All of the 

drogued drifters that returned to the northeastern Chukchi Sea shelf stopped transmitting in this 

region. However, many of the iSphere drifters continued southward over the shelf in September 

and October and eventually came ashore in Ledyard Bay, along the coast of the Lisburne 

Peninsula. One iSphere was caught in the ice south of Pt. Hope and was eventually crushed near 

the Siberian coast. 

The SVP drifters deployed in 2012 had the greatest endurance and several drifted, trapped in sea 

ice, for several months before being crushed or losing battery life. The ice carried these drifters 

as far afield as the western Chukchi Sea (near Wrangel Island) and the Mackenzie Shelf in the 

eastern Beaufort Sea. Several SVPs continued transmitting through winter, while locked in the 

landfast ice zone of the Beaufort Sea, and then made broad zonal excursions over this shelf in 

early summer 2013 before they stopped transmitting.

Linear regressions were used to examine the statistical relationship between drifter velocities 

and the local surface winds over six regions, defined on an ad hoc basis, as the Chukchi shelf, 

the inner Chukchi Sea shelf (Alaskan Coastal Domain; water depths <30m and exclusive of 

Barrow Canyon), within Barrow Canyon, the Chukchi-Beaufort continental slope, the Beaufort 

Sea shelf, and the Canada Basin. The iSphere drifters consistently exhibited a strong statistical 

relationship with winds, often explaining more than 60% of the drifter velocity variance. For the 

MS drifters, winds explained more than 60% of the velocity variance only within the Alaskan 

Coastal Domain and the Beaufort Sea Shelf. Influence of winds explained less than 30% of the 

velocity variance of the SVP drifters in all regions. The spatial differences in response likely 

reflect dynamical differences over the region. These differences can be ascribed to stratification, 

mesoscale motions associated with unstable meltwater fronts, instabilities in shelfbreak 

currents, and/or time-varying geostrophic currents within the regions. Drifters that entered the 

Canada Basin were entrained in eddies most likely spawned by shelfbreak current instabilities. 

There were also seasonal differences in the wind-driven response within the same general area 

x
of the shelf and we tentatively ascribe these differences to the absence/presence of fronts. Windcurrent correlations among drifters far from fronts were substantially better than for those likely 

encountering fronts.

The 2013 trajectories were markedly different from those of 2012 as few escaped the 

northeastern Chukchi Sea. Instead, most drifters moved westward in response to strong 

northeasterly winds of August and September 2013. Approximately 60% of the zonal velocity 

variance for drifters deployed south of Hanna Shoal was due to the winds, while wind-current 

regressions north of Hanna Shoal were substantially poorer. Relative dispersion characteristics, 

based on clustered drifter deployments in 2012 and 2013, indicate anisotropic dispersion, with 

the zonal relative diffusivity being about twice that of the meridional diffusivity. Relative 

dispersion increases according to a time2.5 power law after ~2 days, which is the Lagrangian 

decorrelation time scale.

xi
I. Introduction and Background

There is strong industry interest in exploring the hydrocarbon potential of the Northeast Chukchi 

Sea Shelf. These activities require physical oceanographic data pertinent to the environmental 

and engineering concerns of both industry and the Bureau of Ocean Energy Management 

(BOEM). The measurements described herein provide information on upper-ocean and surface 

currents during the open water season (August through November) of 2011, 2012, and 2013.

The Chukchi and Beaufort Seas are the northernmost shelf seas bordering Alaska. Although 

properly a part of the western Arctic Ocean, both shelves are linked, atmospherically and 

oceanographically, to the Pacific Ocean. These connections profoundly influence the wind 

regime, the seasonal distribution of sea ice, the regional hydrologic cycle, and the water masses 

and circulation characteristics of the Chukchi Shelf. The atmospheric connection is primarily via 

the Aleutian Low, whose time-varying position and strength, and interactions with the 

anticyclonic Beaufort High (Zhang et al., 2013) affect regional meteorological conditions. The 

oceanographic link is via the mean northward flow through Bering Strait, which draws water 

from the Bering Sea Shelf and Basin, and is sustained by a large-scale pressure gradient between 

the Pacific and Atlantic oceans (Coachman et al., 1975; Aagaard et al., 2006; Figure 1).

Figure 1. Mean depth-integrated streamlines (white lines) in the Chukchi Sea, after Spall (2007). The bathymetry is 

colored and major bathymetric features are labeled. The lettered boxes denote the approximate locations of the 

Klondike (K), Burger (B), and Statoil (S) exploration areas.

1
The northward flux of mass, heat, nutrients, carbon, and organisms through the strait bequeaths 

the Chukchi Shelf with physical and ecological characteristics that are unique among arctic 

shelves. For example, spring retreat occurs earlier, and fall onset of sea ice later, than most other 

arctic shelves because of the northward heat flux through the Strait. Woodgate et al. (2006) 

estimate that summer Pacific waters provide a heat source capable of melting nearly the entire 

(~640,000 km2) 2-m thick ice cover of the Chukchi Sea and Shimada et al. (2006) contend that 

this flux may be an important source of interannual variability in the ice cover of the western 

Arctic Ocean. Similarly, the enormous biological productivity of this shelf (Walsh et al., 1989; 

Springer and McRoy, 1993), including its ability to support large and diverse marine mammal 

populations, is due to the carbon and nutrient loads carried through Bering Strait.

The water properties of the Strait throughflow reflect the time-varying output of physical

processes occurring over the Bering Shelf and northern North Pacific. These fluxes are a result of 

the net effects of upwelling from the deep Bering Sea basin and regionally-integrated heat and 

freshwater fluxes (Aagaard et al., 2006), including the freezing and melting of sea ice 

(Danielson et al., 2006), river runoff, atmospheric moisture and heat fluxes, and heat and 

freshwater contributions from the Gulf of Alaska (Weingartner et al., 2005a). These factors all

ultimately affect the heat and salt budgets of the Chukchi Sea Shelf (Coachman et al., 1975; 

Woodgate et al., 2005a).

Ia. Mean Circulation and Water Masses

The shallow (~50m) Chukchi Sea Shelf extends ~800 km northward from Bering Strait to the 

shelfbreak at about the 200 m isobath. The mean flow over much of the shelf is northward due to 

the Pacific-Arctic pressure gradient and opposes the prevailing northeasterly winds. This 

pressure gradient propels the Bering Strait throughflow northward along three principal 

pathways that are associated with distinct bathymetric features (Figure 1); Herald Canyon, the 

Central Channel, and Barrow Canyon. Herald Shoal separates Herald Canyon from the Central 

Channel, and Hanna Shoal is between Barrow Canyon and the Central Channel. Regions of 

industrial interest with respect to hydrocarbon exploration are to the south of Hanna Shoal and 

east of the Central Channel and denoted by boxes in Figure 1 as Klondike (K), Burger (B), and 

Statoil (S).

The mean circulation, as sketched in Figure 1, includes a western branch that flows 

northwestward from the Strait and exits the shelf through Herald Canyon. While most of this 

outflow probably descends through Herald Canyon, some of it may spread eastward across the 

central shelf. A second branch flows northward through the Central Channel and then splits; 

some water continues eastward toward the Alaskan coast along the south flank of Hanna Shoal 

(Weingartner et al., 2005b; Weingartner et al., 2013a) while the remainder flows northeastward 

toward the continental slope. The third branch flows northeastward along the Alaskan coast 

towards Barrow Canyon at the junction of the Chukchi and Beaufort shelves. In summer this 

flow includes the northward extension of the Alaskan Coastal Current (ACC) that originates 

2
south of Bering Strait. At the head of Barrow Canyon the ACC is joined by waters flowing 

eastward from the central shelf, with the merged flow then continuing down-canyon as a narrow, 

but strong, coastal jet (Weingartner et al., 2013b).

From summer through early fall, the Strait transport is northward on average and includes three 

major water masses, which following the nomenclature of Coachman et al. (1975) and Walsh et 

al. (1989) are: cold, salty, nutrient-rich Anadyr Water; warm, fresh, nutrient-poor Alaskan 

Coastal Water (ACW); and Bering Shelf Water. The latter has properties intermediate between, 

but nonetheless distinct from, the Anadyr and Alaskan Coastal water masses. Coachman et al. 

(1975) maintain that the Anadyr and Bering Shelf water masses mix to form Bering Sea Water 

(BSW) north of the Strait, whereas ACW maintains its properties on the Chukchi Shelf. In 

summer and fall, Chukchi bottom waters often include near-freezing, saline (dense) waters that 

formed in winter by freezing over both the Bering and Chukchi seas. In addition, shallow plumes 

of cool, dilute, surface waters, formed by ice melt, may also be present.

Bering Sea Water is transported to the northwest Chukchi, over the central shelf, and northward 

through the Central Channel (Weingartner et al., 2005b; Woodgate et al., 2005b). Weingartner et 

al. (2005b) and Weingartner et al. (2013a) suggest that, south of Hanna Shoal, some centralshelf waters flow eastward toward the coast, in agreement with the circulation models of Winsor 

and Chapman (2004) and Spall (2007). North of the Central Channel, where there are no longterm current measurements, both models suggest the average flow follows the bathymetry 

around the western and northern flanks of Hanna Shoal before turning southward along the 

eastern side of the Shoal and eventually entering Barrow Canyon. However, the models also 

predict that some of the water along the eastern flank of Hanna Shoal penetrates southwestward 

along the southern flank of the Shoal before turning eastward towards the coast. Hydrography 

suggests that this indeed does occur, at least episodically (Weingartner et al., 2013a). The ACW 

flows northeastward within the Alaskan Coastal Current toward the head of Barrow Canyon. 

Here it merges with waters flowing eastward from the central shelf to form the canyon outflow. 

Hence in summer and fall, the canyon outflow contains a horizontally- and vertically-structured 

complex of water masses (Pickart et al., 2005; Shroyer and Plueddemann, 2011) that include 

ACW, meltwater, dense winter waters, BSW, and mixtures of each.

Ib. Wind-Forced Variability

The mean circulation is due to the large scale pressure field between the Pacific and Arctic 

oceans and opposes the mean winds, which are from the northeast at ~4 m s-1 on average. The 

winds are the principal cause of flow variations, which can be substantial. Wind forcing varies 

seasonally with the largest variations being in fall and early winter and the smallest being in 

summer. Weingartner et al. (2005b) found that current fluctuations were coherent with wind 

velocity variations over the northeast shelf over spatial scales of at least 300 km. These 

adjustments reflect wind-induced modifications via coastal convergences and divergences to the 

shelf pressure field. Although the adjustment envelops a broad area, the magnitude of the current 

3
response varies over the shelf. In particular, both wind-forced and mean currents are more 

vigorous in regions of steep topography (Central Channel and Barrow Canyon) than in areas of 

gentler bottom relief. On occasion, and most frequently in fall and winter, strong northeasterly 

storm winds can reverse the shelf flow field or even redistribute the flow from one of the main 

flow pathways to another.

The time-varying shelf pressure field may result in forces that oppose the surface wind stress, a 

dynamic with important implications. In particular, it suggests that the flow in a “thin” surface 

layer, which absorbs the bulk of the momentum imparted by the wind to the water column, may 

differ from deeper currents. The thickness of this wind-shear layer will vary due to wind 

velocity, bathymetry, and the stratification. Since moored current meters have difficulty 

measuring the uppermost ~5 m of the surface, understanding the surface circulation is critical for 

assessing potential pathways of drifting organisms and contaminants in the upper ocean. The 

purpose of this project was to assess the near-surface circulation in the northeast Chukchi Sea by 

means of satellite-tracked drifters.

Satellite-tracked drifters are quasi-Lagrangian, free-drifting instruments often used to study large 

oceanic regions. Their individual trajectories are largely unpredictable and often complex (and 

surprising). Consequently, the analytical methods applied are different than those for Eulerian 

measurements. The Lagrangian analytical approach is largely statistical in nature and meaningful 

analyses depend upon large numbers of drifters and/or trajectories. A substantial literature exists 

that discusses these techniques (e.g., Davis, 1991; LaCasce, 2008; and the references therein).

II. Project Objectives

The overall goal of this program was to evaluate the near-surface currents and their response to 

winds in the northeast Chukchi Sea where hydrocarbon exploration is underway and future 

development may take place. In particular, we sought to determine:

1. If the surface (upper 1 m) and near-surface (10 – 15 m) circulation field diverges and 

differs from the sub-surface circulation, as captured by current meter measurements, and

2. If the differences in the surface and sub-surface flows are related to seasonally-varying 

winds, bathymetry and hydrographic conditions.

These were the original goals of the Coastal Marine Institute (CMI) funded University of Alaska 

Fairbanks (UAF) project. As originally conceived, the one-year (2011) study would deploy 48 

drifters, half drogued at 10 m and half at 1 m. The deployments were planned as clusters of 4 – 8 

drifters (half at each depth) so differences in surface and subsurface circulation could be 

examined. Deployment was planned for areas with differing hydrographic conditions and within 

the regions of industry interest (e.g., Klondike, Burger, and Statoil). The North Slope Borough, 

Department of Wildlife Management (NSBW) undertook a companion program to deploy 24 1-

m drogued drifters within 10 km of the coast in the Chukchi Sea. Unfortunately, construction 

4
flaws by the manufacturer (Technocean, Inc. of Coral Gables, FL) resulted in a massive failure 

rate (>80%) of the deployed CMI project drifters, leaving only nine functioning drifters in 2011. 

Flaws in the Technocean drifters included the lack of functioning thermistors and, in our opinion, 

improper seals on the pressure canister that housed the electronics. As a consequence, many of 

the failed drifters presumably leaked within hours or days after deployment. We were also 

unable to decode the data string according to the manufacturer’s instructions and technical 

advice. However, Dr. Danielson (UAF) eventually developed a successful decoding strategy. 

The NSBW experienced similar failure rates with their Technocean drifters and halted 

deployments shortly after beginning their program. The bulk of the drifter data discussed in this 

report thus derives from additional drifters deployed in BOEM and NSBW projects in 2012 and 

2013. Only a few of these additional drifters were drogued at 10 m depth. Rather, most had 1-m 

drogues and some of the NSBW drifters deployed in 2012 were simply surface floats 

(undrogued). Due to the equipment failures and lack of 10-m drogued drifters, there were 

insufficient data sets to address all of our objectives in a statistically meaningful way. However, 

we applied statistical analyses where we felt that the data density allowed. 

Quasi-Lagrangian measurements provide a unique description of the ocean circulation by 

yielding information on the pathways of water parcels and the various circulation structures in 

which they are embedded. To our knowledge, the drifter data assembled in this report represents, 

by far, the most comprehensive deployment activity in the open water season for any Arctic 

continental shelf. Consequently, we feel that the trajectory descriptions are an important aspect 

of the study, and that a sense of the complexity and diversity of the circulation field can only be 

attained through such an overview.

Nevertheless, the descriptive approach is limited by the enormous temporal and spatial 

variability of the ocean. As will become evident, two parcels or drifters deployed simultaneously 

at nearly identical positions often evolve along quite different pathways. Indeed, the drifters 

discussed herein executed a variety of complex and unique trajectories that crossed through 

dynamically different regimes (Chukchi Shelf, Chukchi-Beaufort shelfbreak, coastal currents, 

and the Canada Basin). Where the data density permitted, this variability is described 

statistically. Several of the deployments were made as “cluster” deployments and these allow us 

to examine relative dispersion (how particle pairs, initially deployed close to one another in time 

and space, move with respect to one another) and several kinematic properties of the flow field. 

We also explore wind-current relationships. In these analyses, we, a priori, regarded drifters in 

different dynamical regimes as providing statistics from a distinct “population”, so that it would 

be inappropriate to group all the data into a single population. Consequently, we have examined 

the wind-current relationship separately for each area. Similarly, we have limited our analyses of 

relative dispersion and kinematic properties to the Chukchi Shelf because this region provided 

the largest number of data points for these estimates. We regard the results from the Chukchi Sea 

Shelf reported herein to be preliminary. In 2014, projects operated by the NSBW and the U.S. 

Department of Interior’s Bureau of Safety and Environmental Enforcement (BSEE) deployed 

5
about 100 more drifters on this shelf. Results of those exercises will be combined using the same 

analyses as described below. The intention is to produce a climatological description of relative 

dispersion on this shelf. The analyses are climatological in the sense that they cover deployments 

made over a number of years and at a number of locations on the shelf. This approach will lead 

to firmer confidence in the derived statistics.

III. Methods

IIIa. Drifters and Winds

The sampling approach used two different types of satellite-tracked drifters; the CODE-type and 

the SVP-type (Figure 2). Additional photographs showing deployed CODE drifters are shown in 

Figures 3 and 4. The NSBW used a combination of the CODE-type and iSphere drifters (Figure 

5; manufactured by Metocean). CODE drifter performance characteristics (Davis, 1985) are that 

the drifter slippage is ~1 cm s-1 and thus small compared to the expected magnitudes of the 5 –

50 cm s-1 current velocities typical of the Chukchi Sea. CODE drifters (referred to herein as

Microstar or MS) measure the upper 1 m of the water column and a SVP drifter drogue is at 10 

m depth. These drogue depths were chosen to address our two goals and because these depths are 

the standard manufacturer designs. Each drifter also included a surface thermistor. Drifter 

positions were determined by satellite GPS fixes and recorded approximately hourly along with 

sea surface temperatures. The data were stored aboard the drifter and then transmitted via 

Service Argos (2011) twice per day or by Iridium link (2012 and 2013) hourly. All data were 

then interpolated to the top of the hour and checked for outliers in position data. Outliers were 

infrequent in 2012 and replaced with linearly interpolated positions. In contrast, the 2013 drifter 

deployments included periods of time when there is either no or only lower quality position data 

available. We believe that these periods coincide with rough seas when the surface float was 

periodically submerged and incapable of establishing sustained communications with the GPS 

satellites. As a consequence, drifter positions are determined by three different methods, derived 

from the quality flags in the raw data sets. The highest quality GPS fixes are calculated using 

signals from three GPS satellites, the lower quality positions are obtained from only two GPS 

satellites, and the poorest quality fixes are obtained from the Iridium satellite. Initially, we used 

only the highest quality data, as we had done in 2012. However, in 2013, this left hundreds of 

small gaps in the drifter positions; approximately 66 of the gaps were longer than one day, with 

the longest being four days. These longer gaps were distributed across all MS drifter 

deployments, but were concentrated during specific time periods (Figure 6).

To fill these gaps, a second data set was created that includes lower quality data. However, these 

data exhibit a variety of errors. The poorer quality data included positions that are duplicates of 

previous positions but at later times, repeated duplicate positions when the drifter was obviously 

not beached, sections offset from the track-line, and generally noisy data. Some of the offset 

sections were corrected using time adjustments provided by the drifter manufacturer, Pacific 

6
Gyre. Most of the gaps were filled on this pass of editing. The remaining erroneous positions 

were replaced with error codes (NaNs) manually. Velocities calculated using the poor quality 

data are somewhat noisier than those using higher quality data. Some gaps remained (Figure 7) 

especially in the BOEM drifters deployed northeast or northwest of Hanna Shoal; the only gap 

greater than one day in the NSBW drifter positions occurred at the end of the NSBW-I-30

deployment. All remaining gaps were filled with a linear interpolation of positions. Velocities 

based on interpolated positions are highlighted in the time series for each drifter. The final data 

file has the position data flagged as follows: 0 for good quality, 1 for questionable positions that 

passed quality control, 2 for positions that failed quality control, 3 for when there was no original 

data at all, and 4 for interpolated values across gaps of three or more hours.

In all years the drifters were deployed from research vessels operating south, east, or north of 

Hanna Shoal in approximately 40 m water depth. In 2011, drifter deployments were made by 

research vessels under charter to Shell, Conoco-Phillips, and Statoil as part of their Chukchi Sea 

Environmental Studies Program (CSESP). In 2012, the drifter deployments were made from the 

USCG Healy and from the Norseman II. The NSBW drifters were deployed within 10 miles of

the coast in water depths of <20 m offshore of Icy Cape, Wainwright, and Barrow. Unlike either 

the SVP or CODE drifter, the iSphere is analogous to a partially submerged basketball sitting

half in and half out of the water, making them subject to considerable wind drag.

Regional wind data used in the analyses were obtained from the National Oceanic and 

Atmospheric Administration’s WaveWatch forecast model and/or from the National Center for 

Environmental Prediction, North American Regional Re-analysis (NCEP-NARR; Mesinger et 

al., 2006). Both wind data sets are available at three-hourly intervals on a 32 km grid. 

Comparisons were made using either wind product and the winds measured at meteorological 

buoys in the northeast Chukchi Sea, specifically those at Pt. Lay, Klondike, Burger, and Hanna 

SE for the period August – October 2012 (Figure 8). Our analyses indicate that the skill exceeds 

0.9 and that the root mean square error differences are ~2.5 m s-1 for both wind components in all 

cases. Figure 9 shows a comparison of the NARR and Burger observed winds for the same 

period and Figure 10 shows a comparison between the WaveWatch model wind components and 

those recorded at the Pt. Lay buoy. 

7
Figure 2. Schematics of the CODE-type 1-m drogued Microstar drifter (top) and the SVP-type 10-m drifter 

(bottom). Both drifter types shown are manufactured by Pacific Gyre, Inc.

Figure 3. Underwater photo of a Microstar drifter in operation. Note the structure of the drogue and the white, semisubmerged surface float.

8
Figure 4. Aerial photo of two Microstar drifters offshore of Wainwright in 2013 (Photograph is courtesy of Vikki 

Beaver and Janet Clarke).

Figure 5. iSphere surface drifters (manufactured by Metocean).

9
Figure 6. Time of occurrence (solid colors) of high quality position data for the drifters deployed in 2013. High 

quality position data is derived from three GPS satellites.

Figure 7. Time of occurrence (solid colors) of all quality position data for the drifters deployed in 2013. The 

remaining gaps were linearly interpolated through time.

10
Figure 8. Locations of meteorological buoys in the Chukchi and Beaufort Seas since 2008. Colored ring on each 

buoy indicates the year for which data is available.

Figure 9. Time series of winds based on the NARR model and observed at the Burger meteorological buoy in 2012. 

Upper panel shows the east-west wind component and lower panel shows the north-south wind component. The skill 

in both cases is >0.95.

11
Figure 10. Time series of winds based on the WaveWatch model and observed at the Pt. Lay meteorological buoy 

in 2012. Upper panel shows the east-west wind component and lower panel shows the north-south wind component. 

The skill in both cases is >0.95.

A description of the interannual variability in the winds over the Chukchi Sea are based on the 

NARR winds at grid point 71° 3.75’N, 165°3.75’W, located within Klondike. The wind 

estimates are generated at three-hourly intervals and the NARR ice-edge positions are mapped 

based on the analyses of the National Ice Center.

The drifter deployments in 2011 and 2012 were made in the areas shown in Figure 11. These

deployments were conducted in regions providing supplementary data that included shipboard 

and/or towed conductivity-temperature-depth (CTD), subsurface moorings, and surface velocities. 

The latter were obtained from shore-based high-frequency radars (HFR) located in the 

communities of Barrow, Wainwright, and Pt. Lay. 

12
Figure 11. Bathymetric map of the Chukchi Sea showing the approximate location of the Statoil (dark blue box), 

Burger (red box), and Klondike (green box) prospects. The orange arcs show the approximate radar masks for HFRs 

at Pt. Lay, Wainwright, and Barrow. The solid black line extending northwest from Wainwright encompasses an 

array of six subsurface moorings present during the 2011 period. Approximate locations of UAF drifter releases in 

2011 (circles) and 2012 (squares) are shown. The approximate locations of NSBW drifter deployments in 2012 are 

shown by triangles.

Drifter trajectories were updated daily and posted to a website available to the public 

at http://dm.sfos.uaf.edu/chukchi-beaufort/data/drifters/. The website also contains the data files 

for each drifter. Individual trajectories and their time series of derived velocities and 

temperatures, can be viewed at http://dm.sfos.uaf.edu/chukchi-beaufort/data/drifters/2012_png/

for 2012 deployments, http://dm.sfos.uaf.edu/chukchi-beaufort/data/drifters/2013_png/ for 

drifters deployed in 2013, and http://dm.sfos.uaf.edu/chukchi-beaufort/data/drifters/2014_png/

for drifter deployments carried out in 2014 by BSEE and the North Slope Borough-Shell 

Baseline studies programs. An animation of the 2012 drifter trajectories can be viewed 

at http://mather.sfos.uaf.edu/drifters/Chukchi2012/Plots/DrifterMovieC.html.

IIIb. Hydrographic Data

Hydrographic data was shared with this program from other programs, including the CSESP

program and the BOEM-funded “Characterization of the Circulation on the Continental Shelf

Areas of the Northeast Chukchi and Western Beaufort Seas”. The CSESP CTD data were 

Icy Cape

13
collected using a Seabird, Inc. SBE-19+V2 CTD sampling at 4 Hz and lowered through the 

water column at ~10 m min-1. The CTD data were processed following the manufacturer's 

procedures and were then averaged into 1 dbar bins (Weingartner et al., 2013a). Comparison of 

pre- and post-season calibrations (performed by Seabird) of the temperature and conductivity 

sensors indicate that the data are accurate to better than 0.005°C for temperature and 0.02 for 

salinity. The BOEM CTD data were obtained from a towed, CTD vehicle (Acrobat). The 

Acrobat vehicle yo-yos between the surface and bottom and samples with a high horizontal 

resolution. Its data were averaged into 1-dbar bins to produce water column profiles with a 

nominal 250 m horizontal spacing.

IIIc. Drifter Clusters: Relative Dispersion and Kinematic Characteristics

Relative dispersion is the mean square distance between pairs of drifters and indicates how a 

group of particles spreads through time about the center of mass of a cluster of drifters (LaCasce 

and Ohlmann, 2003; LaCasce, 2008). The calculation depends upon the initial starting point and 

the initial drifter pair separation distance. The relevant quantities are computed as follows:

( ) ( ( ) ( ))

( ) ( ( ) ( ))

( ) ( ) ( )

( ) ( ( ) ( ))( ( ) ( ))

2 2

2 2

2 22

1

1

1

x i j

i j

y i j

i j

x y

xy i j i j

i j

D t xt x t

M

D t yt y t M

Dt Dt Dt

D t xt x t yt y t M

≠

≠

≠

= −

= −

= +

=−−

∑

∑

∑

Here, x(t) [y(t)] is the zonal [meridional] position of drifters i and j, relative to the center of mass 

of the cluster at time t. The terms ( ) 2 D t x and ( ) 2 D t

y are the mean square dispersion in the zonal 

and meridional directions, respectively. The last two equations represent the relative mean square 

dispersion ( ( ) 2 D t ) and the cross-correlation ( D t

xy ( ) ). The variable M is the number of drifter 

pairs used in the estimate. Schematically, the calculation is performed as indicated in Figure 12, 

beginning with the initial drifter separation distances at time = t0 and proceeding through time = 

tn.

Figure 12. Example schematic of a cluster of drifters dispersing through time. The dispersion calculations are made 

amongst all possible drifter pairs.

14
We calculated the dispersion quantities in the Chukchi Shelf domain only based on several sets 

of MS drifters from 2012 and 2013. There are three reasons for this approach. First, these 

clustered deployments were all within several meters of one another at the time of deployment. 

Second, given the observed differences in the wind-drifter regressions we do not include the 

SVP-drifter results in these calculations. There were too few SVP trajectories available for a 

meaningful estimate of relative dispersion at 10 m depth. Third, we do not include drifters that 

entered Barrow Canyon in this calculation, since the dynamical regime (and presumably the 

dispersion characteristics) within the canyon is different from that of the shelf. Five clusters 

satisfied these criteria; one cluster from 2012 that was deployed in Burger on 2 September and 

four clusters deployed in 2013. The 2013 clusters used include two deployed offshore of Pt. Lay, 

another cluster deployed northeast of Hanna Shoal, and a fourth deployed northwest of Hanna 

Shoal. Each cluster involved from 9 – 13 drifters and the dispersion calculations are based on 

time series ranging from about 25 – 40 days depending upon the varying attrition rate of the 

drifters in the clusters. We excluded drifters that entered ice in the dispersion calculations.

Okubo and Ebbesmeyer (1976) presented a convenient way to summarize many of the kinematic 

properties of drifter trajectories based on clusters (groups of drifters deployed close to one 

another). Based on observed positions and observed velocities of each drifter (i) in the cluster, 

  ut vt i i ( ), ( )  is, at each time step, expanded in a Taylor series about the centroid position located 

at xt yt ( ), ( )     accordingly:



( ) ( ) ( )

 ( ) ( ) ( )



( ) ( ) ( )

( ) 

( )  ( ) ( ) ( ) ( ) 

( ) ( ) ( )

ii i

observed tbd known known tbd tbd

tbd tbd

known known observed tbd

ii i

u t u t

u t ut x t xt y t yt u t

x y

v t v t

v t vt y t yt y t yt v t

x y

e

e

∂ ∂

=+ − + − +       ∂ ∂

∂ ∂

=+ − + − +       ∂ ∂

))))( ))))(

))+)) ))+))

The terms denoted indicated by “tbd” are determined from a least squares fit based on all the 

drifters in the cluster. The higher order terms u t ,v t ε ( ) ( ) are considered turbulent motions no

discussed herein. The results allow us to estimate the centroid velocity components ut vt ( ), ( )    

and the velocity gradients. The following kinematic quantities were then estimated:

( )

( )

( )

( )

Horizontal divergence: = 

Relative vorticity: = 

Stretching deformation rate: 

Shearing deformation rate: 

t ux vy

t vx uy

t ux vy

ht v x u y

g ∂ ∂ +∂ ∂

z ∂ ∂ −∂ ∂

a =∂ ∂ −∂ ∂

=∂ ∂ +∂ ∂

15
By considering a “box” to denote a water parcel captured by a drifter, these terms can then be 

described schematically as follows:

1. Horizontal divergence describes increases (divergence) or decreases (convergence) in 

parcel’s area:

2. Relative vorticity describes the rotation of the parcel: 

3. Stretching deformation describes dilatation of the parcel along a particular axis without 

changing the area of the parcel:

4. Shearing Deformation describes distortion of the parcel due to the velocity gradients:

Note that a parcel may undergo one, some, or all of these changes simultaneously. The time rate 

of change of the fluid’s absolute vorticity (z+f) is given by:

( ) ( ) ( )

 rate of change stretching vorticity of absolute vorticity

 sources and sinks for d u v f f f dt x yζζζ   

+ ++ + =   +

))(   ∂ ∂

))))))(

The first time on the right describes the rate of change of the absolute vorticity and the second 

term is vortex tube stretching due to convergence or divergence within the fluid. Vorticity 

sources or sinks arise from torques associated with the surface wind or bottom stresses, or 

baroclinic torques arising from the density field. Our data allow us to evaluate the terms on the 

left hand side only. If there are no vorticity sources or sinks then the left hand side should sum to 

zero.

16
IV. Results and Discussion

IVa. Winds

On long-term average, the winds over the northeast Chukchi Sea Shelf are from the eastnortheast (Brower et al,. 1988) and roughly oppose the mean bathymetrically-steered flow 

established by the pressure gradient between the Pacific and Arctic oceans. Based on moored 

measurements, Weingartner et al. (1999) found that the Alaskan Coastal Current reversed and 

flowed southwestward in Barrow Canyon when northeasterly wind speeds were >5 m s-1

. 

Northeasterly winds also promote coastal upwelling along the Chukchi coast, which should 

include a southwestward alongshore flow and an offshore flow at the surface. Weingartner et al.

(2013a) found that, in summer, the currents in Klondike reversed from being eastward to being 

westward when winds had a westward component that exceeded ~6 m s-1. Both sets of 

observations are consistent with the high-frequency radar measurements of the regional surface 

currents south of 71.5°N reported by Weingartner et al. (2013b). The radar data sets also indicate 

that the winds explain ~50% of the surface current variance in ice-free regions. With these 

results in mind, we briefly review the October – November wind fields for 2011 – 2013 in the 

form of three-hourly vector plots based on the NARR grid point in Klondike (Figure 13).

Figure 13. Vector plots of winds from the central Chukchi Sea Shelf from August through November for 2011 

(top), 2012 (middle) and 2013 (bottom). North is oriented toward the top of the page and the vectors point in the 

direction toward which the wind is blowing.

17
As seen in the figure, winds were primarily from the northeast in each of these years, but there 

was considerable interannual variability both seasonally and synoptically. For example, 

northeasterly winds were dominant from August through October in 2011 and 2013 and, 

compared to 2012, there was relatively little synoptic variability. In November of 2011 and 2013, 

the wind directions were much more variable with nearly equal frequencies of southerly and 

northerly winds. In contrast, the winds in 2012 were from southerly quadrants in August and 

through much of October. There was considerable directional variability in the winds in 

November of all years, however, wind speeds in November 2012 were generally <5 m s-1, much 

weaker than in either November of 2011 or 2013. While the NARR winds provide an overview 

of the regional winds, we have used winds prepared by NOAA’s WaveWatch wave prediction 

model for most of the statistical analyses between winds and currents derived from the drifters. 

As shown previously, both products agree well with measured winds, and with one another.

IVb. Sea Ice Distribution

From the perspective of this drifter study, sea ice plays two roles. As will be seen, it is a major 

source of cold, dilute seawater that influences the hydrography, especially the stratification and 

frontal structures. Sea ice will respond differently to the winds than the ocean so that drifters 

caught in ice are likely to not reflect the ocean currents. Maps of the ice-edge position over the 

northeastern Chukchi Shelf for selected dates (1 and 15 August; 1 and 15 September) for 2008 –

2013 are shown in Figure 14. The seasonal pattern of ice retreat consists primarily of a 

northward regression of the ice-edge such that by mid-September the region south of Hanna 

Shoal is completely ice free (although this was not the case in 2012). Note that the ice retreat 

does not proceed uniformly across the region. Ice lingers longer over Herald Shoal, centered at 

70.5°N and 170°W, than to the east of the shoal where by mid-August the ice-edge is typically at 

71°N. Similarly, ice frequently persists over Hanna Shoal even though the shelf to the west (e.g., 

within the Central Channel) and to the east of the Shoal may be ice free.

The interannual variations in ice-edge position were substantial. The heaviest ice years occurred 

in 2008, 2012, and 2013. In each of these years the ice-edge was nearly zonally-oriented along 

~71°N south of Hanna Shoal and varied little in position throughout August. In 2008 and 2013 

the ice retreated to northwest of Hanna Shoal and over the outer shelf by mid-September. The 

same northwestward retreat occurred in 2012, but ice remained over Hanna Shoal through midSeptember. It appears that in both 2012 and 2013 heavily-deformed ice grounded atop Hanna 

Shoal in late winter and that much of this ice remained grounded well into summer (Dr. A.

Mahoney, UAF, personal communication, January 2015). The lightest ice years were in 2009 

and 2011. In 2009, the ice retreated very rapidly between 1 and 15 August such that the 

northeastern shelf was completely ice free by mid-August. In 2011, the northeastern shelf was 

virtually ice-free in early August. The ice retreat in 2010 was modest in comparison to the heavy 

and light ice years. By early August 2010, ice had retreated northward in the Central Channel, 

but still encompassed Herald Shoal and extended eastward along 71.5°N between 165°W and the 

18
coast. By mid-August the ice had retreated to the north of 72°N and by 1 September the shelf

was largely free of ice.

The causes of this interannual variability are not at all clear and may depend upon a number of 

factors discussed later. We note, however, that these differences in ice extent are not simply 

related to the local winds. For example, the winds during August and September of 2008 and 

2013 (heavy ice) and 2011 (light ice) were persistently from the northeast. In contrast, winds 

were from the south and southwest in 2009 (light ice) and in August of 2012 (heavy ice) while 

winds were variable in both direction and strength in 2010.

Figures 15 and 16 show the ice concentrations based on AMSRE and/or AMSR2 satellite 

imagery for the Pacific Arctic sector including the Chukchi and Beaufort Seas. Note that this 

imagery is based on passive microwave satellites and is acquired at a coarser horizontal 

resolution than the NIC ice edges of the previous figure. In all years, the main concentrations of 

ice in the Chukchi sector were north of 73°N through September. In 2011 and 2012, ice remained

north of this latitude through the end of October. In 2013, the ice was further south, between 

72°N and 73°N by mid-October, and advancing to between 70°N and 72°N by the end of 

October. Ice advanced rapidly southward through early November, with this advance being 

largest in 2012 and least in 2011. Much of the Chukchi Shelf was covered by ice by the end of 

October in all years. Over the Beaufort Shelf, the main concentrations of ice remained north of 

72°N through the middle of October in all years and through the end of October in 2012. In 2011 

and 2013, the ice expanded westward along the Beaufort Shelf to between 145° and 150°W and 

to the south of 72°N through the end of October. By mid-November, ice had encompassed the 

entire Beaufort Shelf, except for a small fraction of the shelfbreak region in 2013.

IVc. Hydrography

We begin the discussion of hydrographic variability using the CSESP data from 2011 – 2013. 

These data have been published in the associated annual reports for that program 

(https://www.chukchiscience.com/Downloads). Here, we present the data in terms of plan views 

of temperature and salinity averaged over the surface and bottom 10 m of the water column for 

each cruise in each of these years, with two vertical sections from 2011 and 2012 for 

comparison. Additional hydrography from 2013 is presented in the form of vertical sections 

taken from either gliders or the Acrobat vehicle. As a prelude to this section we include a 

temperature-salinity diagram (Figure 17) that illustrates the water masses present. The diagram 

is based on the 2011 and 2013 CSESP data (2012 T/S diagrams are similar to 2013 so not 

shown). Three basic water masses and mixtures are generally present. These include Winter 

Water (WW), which is very cold (near freezing) and saline, warm and moderately saline Bering 

Sea Water (BSW), and Meltwater (MW), which is fresh and cool. All of these water masses were 

present in 2012 and 2013, but MW was absent in 2011. There was an additional water mass 

present in 2013 on some of the Acrobat sections discussed later. This is Alaskan Coastal Water 

19
(ACW), which is as warm or warmer than BSW, but fresher, with salinities generally <30. With 

this nomenclature established, we next consider the hydrographic properties of the shelf in these 

years.

In August 2011 (Figure 18) the surface waters over the central shelf were largely occupied by 

BSW with the warmest and most saline surface waters found in Klondike. Recall that the iceedge in August 2011 was far to the north by 1 August and so the survey area had been flushed of 

MW by the time of the sampling. Bottom water properties in Burger and Statoil consisted 

primarily of WW, while those in Klondike were warmer and fresher, suggesting that some 

mixing had occurred between the BSW and WW. In September, BSW occupied the entire 

surface layer so that horizontal temperature and salinity gradients were weak. In contrast, there 

was considerable spatial variability in the bottom waters. Those in Klondike consisted of BSW, 

while bottom waters in Burger and those to the north and east of Hanna Shoal consisted entirely 

of WW. Bottom waters in Statoil had warmed and freshened compared to the prior month 

indicating infiltration of BSW into this area.

20
Figure 14. Ice-edge location as a function of year and date between August and September. The dashed line in the 

2011 map shows the location of the vertical sections for 2011 and 2012 shown in Figure 21.

21
Figure 15. Regional ice concentration maps by year (columns) and date (rows) for August 15 and 30, and 

September 15 and 30.

22
Figure 16. Regional ice concentration maps by year (columns) and date (rows) for October 15 and 30, and 

November 15 and 30.

23
Figure 17. Temperature-salinity diagrams from the CSESP stations in the central Chukchi Sea for 2011 (upper 

panels) and 2013 (lower panels).

The conditions in 2012 (Figure 19) were quite different from those in 2011. Although BSW 

occupied Klondike in August, surface temperatures and salinities decreased toward the northeast 

indicating the presence of MW in both Statoil and Burger. Bottom water properties in Klondike 

were cooler and saltier than the surface layer, but nonetheless these suggest that BSW had 

displaced much of the WW. Within Statoil and Burger, the bottom water properties consisted of 

WW. By September, MW occupied the surface waters in the entire region north of 71.5°N, 

except over the westernmost portion of the survey area. Here a tongue of water with BSW 

properties extended northward within the Central Channel and to the west of Statoil. With the 

exception of the coastal stations near Wainwright, September bottom waters consisted of cold 

BSW in the southwestern portion of the study area with these being colder and saltier in 

September than in August. Hence, in 2012, the typical seasonal progression in Klondike bottom 

water properties was reversed. Elsewhere WW prevailed, with the coldest and saltiest waters 

found encircling the southern flank of Hanna Shoal.

24
In many ways 2013 conditions were similar to 2012. In August of 2013 (Figure 20), a prominent 

surface front extended along 71.25°N and separated BSW in the south from MW to the north. 

Although bottom waters were cold throughout the area, the salinities increased from the south to 

the north suggesting the intrusion of cold BSW into Klondike. By September, surface 

temperatures had cooled and the frontal structure evident in the salinity field had weakened,

however, there was little change in the distribution of bottom water properties.

We next examine vertical sections of temperature and salinity from September 2011 and 2012 

(Figure 21). The sections consist of the same stations along a transect that extends from the 

southeast corner of Klondike, skirts the eastern side of Hanna Shoal (indicated by “HS” in the 

figure) and continues to the northeast side of Hanna Shoal (the location of the section is shown 

on the 2011 ice-edge map in Figure 14). In 2011, the upper 25 m of the entire section contained 

BSW with salinities between 31 and 32. There was very little horizontal variability in the upper 

layer, except for the weak thermal gradient at ~km 100. The pycnocline was centered at ~25 m 

depth across the entire section and varied in strength along the section. Southwest of km 100 it 

coincided with a weak halocline (0.05 m-1

) and a modest (~0.2°Cm-1) thermocline. Northeast of 

this location the pycnocline strengthened due to a much stronger halocline (0.2 m-1

) and 

thermocline (~0.8°Cm-1). The strengthening of the pycnocline was a consequence of the change 

in bottom water properties; cold (<0oC), salty (>32) winter waters occupied the bottom 15 m of 

the water column on either side of Hanna Shoal. In contrast to the upper layer, the bottom waters 

included a strong thermohaline front at ~km 100, which separated the BSW to the southwest 

from the WW to the northeast. The corresponding section in 2012 was substantially different in 

two important respects. First, the upper layer was substantially more heterogeneous in consisting 

of both BSW and MW. The latter was largely centered over Hanna Shoal and separated from the 

BSW to the southwest by a strong thermohaline front at ~km 125. Second, a prominent 

pycnocline extended across the entire section, although it was stronger to the northeast than to 

the southwest. Northeast of the front the pycnocline was shallow, centered at ~15m depth, and 

largely due to the vertical salinity gradient (~0.5 m-1

). In contrast, the pycnocline to the 

southwest was deeper (~22m) and associated with vertical gradients in both temperature and 

salinity. Third, there is the suggestion of an anticyclonic eddy embedded in the pycnocline in 

2012. This feature is indicated by the patch of 2.5o

C water centered at ~km 210 on the transect, 

and it is accompanied by vertical spreading of the isohalines. As outlined later, this feature was 

likely an eddy shed baroclinic instability of the MW/BSW front.

In spite of the large hydrographic differences between 2011 and 2012, the vertical sections 

shared a common feature in terms of the large pool of WW that was distributed to the south and 

north of Hanna Shoal. Indeed, this pool of WW and its location appears to be a conspicuous and 

recurrent feature of the shelf hydrography as evidenced by the September bottom water property 

distributions in Figures 18–20 and in previous years as described by Weingartner et al. (2013a). 

While there are variations between years in the temperature and salinity of the WW (in 2011 it 

was somewhat warmer and fresher than in 2012), these differences are much smaller than the 

25
large interannual variations in temperature and salinity observed at the surface and along the 

bottom elsewhere throughout this region.

As will be seen, most of the 2011 and 2012 drifters deployed on the Chukchi Sea Shelf moved 

into Barrow Canyon and out onto the Beaufort and Chukchi shelfbreaks. However, the drifters 

deployed in 2013 mainly remained on the Chukchi Sea Shelf throughout their lifetime. A

considerable amount of hydrography collected by Dr. Winsor (UAF) under separate BOEM 

funding in September 2013 from the northeast Chukchi Sea Shelf is relevant to the 2013 drifters. 

This hydrographic data set provides a glimpse of the hydrographic structure of the shelf that

many of the drifters passed across within their first month. We first review a subset of these 

vertical sections. The Acrobat measured temperature, salinity, chlorophyll, particle 

concentrations, and color-dissolved organic matter (CDOM) as a function of pressure, depth and 

density were computed from temperature, salinity, and pressure.

From 10 – 14 August, and just prior to the release of the Pt. Lay drifters, glider 191 (deployed by 

Dr. Winsor) transited eastward from the Central Channel toward the Alaskan coast along 69.5o

N 

completing a nearly 120 km long section. Figure 22 shows the glider transect its related vertical 

section. (Note that the glider did not extend inshore of ~30 isobath and only came within ~40 km 

of the coast.) Warm (8 – 9o

C), fresh (<29.5) ACW occupied the uppermost 20 m of the section 

between km 100 – 120. Beneath this layer, waters were cooler (~1oC) and saltier (31). This 

region was separated from offshore waters by a front located at ~km 100. Seaward of the front, 

waters were cooler (7 – 8o

C) and saltier (>~30) and more weakly stratified. Below 20 m depth, 

temperatures were 5 – 6oC and salinities were ~31. The winds at the time of this transect were 

upwelling-favorable and very likely the offshore extent of the ACW reflects surface near-surface 

displacement by Ekman transport. We do not, however, see evidence of coastal upwelling in the 

form of upward sloping isopycnals toward the coast on this section. The most likely reason is 

that the glider did not come sufficiently close to the coast to capture upwelling of subsurface 

waters into the surface layer. 

In the following paragraphs we describe the hydrography collected by the Acrobat-CTD system. 

Leg H (Figure 23), occupied on September 11 – 12, extended ~260 km westward from offshore 

of Wainwright and across the Central Channel. Nearshore surface temperatures were cool (2 –

3o

C) and moderately fresh (<31) while offshore temperatures were warmer (>3oC) and slightly 

saltier (~31.5). Maximum temperatures were ~6 o

C along the eastern side of the Central Channel

and are associated with the northward flow of BSW within the Channel. Cold (< 0oC), salty 

(32.5) waters occupied the lower half of the water column everywhere but along the eastern flank 

of the Central Channel. Density stratification was moderate over the entire section, except in the 

eastern portion of the Central Channel, where the water column was weakly stratified. Thus 

surface temperatures along the western end of this section were cooler than those observed by 

the glider in mid-August.

26
Leg D, occupied on 10 September, extended from the west side of Hanna Shoal to the head of 

Barrow Canyon offshore of Wainwright (Figure 24). The lower half of the water column 

consisted of cold (< 0oC), salty (~32.5) WW. Surface waters between km 0 and 80 were cool 

(~1oC) and fresh (~27) indicating MW. Between km 100 – 190, there was a plume of warmer (~3 

– 4oC) and saltier (29 – 31.5) surface waters, most likely ACW that was displaced offshore by 

the upwelling-favorable winds. Colder and saltier waters were found within 20 km of the coast. 

Here the isopycnals bowed upward toward the surface nearing the coast, indicative of windinduced coastal upwelling. Across the whole transect, with the exception of this coastal 

upwelling band, the water column was heavily stratified. Leg G (Figure 25), from 8 September, 

is a shorter transect extending 60 km offshore of Wainwright to the coast where it ended ~50 km 

southwest of Leg D. This section also reflects coastal upwelling, with the upwelling signature 

present inshore to at least the 10 m isobath. Seaward of the 25 m isobath (km 15), the surface 

waters were the warmest (~3o

C) observed on this section and had a salinity of ~32. Given the 

prevalence of upwelling-favorable winds in August and September, these hydrographic 

conditions were likely similar to those prevailing at the time of the Wainwright drifter 

deployments on 24 August.

27
Figure 18. Plan views of surface and bottom temperatures and salinities averaged over the upper and lower 10 m of 

the water column in August and September 2011. Note that the temperature and salinity scales differ between the 

surface and bottom maps.

28
Figure 19. Plan views of surface and bottom temperatures and salinities averaged over the upper and lower 10 m of 

the water column in August and September 2012. Note that the temperature and salinity scales differ between the 

surface and bottom maps.

29
Figure 20. Plan views of surface and bottom temperatures and salinities averaged over the upper and lower 10 m of 

the water column in August and September 2013. Note that the salinity scales differ between the surface and bottom.

30
Figure 21. Vertical sections of temperature (left) and salinity (right) from 2011 (top row) and 2012 (bottom row) 

along the southwest (SW) to northeast (NE) transect shown in Figure 14. The location of Hanna Shoal is indicated 

by “HS”. 

Leg Q (16 – 17 September; Figure 26) extends ~220 km northward from offshore of Wainwright 

to the north side of Hanna Shoal. Proceeding northward along the transect, the hydrographic 

properties over the first 80 km consists of warm (3 – 6o

C) and moderately salty (~31) BSW in 

the upper 20 m and much colder (<0o

C) and saltier (> 32.3) waters within the lower half of the 

water column. The stratification over this part of the transect is modest, changing by 2 kg m-3

over 2 m depth, and centered at ~25 m depth. On approaching the southern flank of Hanna Shoal, 

between km 80 – 220, the water column stratification nearly doubles, with the surface layer 

consisting of meltwaters and the bottom layer being winter water. A 20-km wide prominent 

front, confined to the upper 20 m, occurs at km 80 and separates warmer saltier waters to the 

south from cooler, fresher waters to the north. Leg H, which extends westward offshore of 

Wainwright and across the Central Channel (Figure 27) reflects properties in the upper 20 m of 

the water column similar to those south of the front found on Leg Q. Note, however, that within 

the Channel, the surface waters are warmer and saltier on the east side of the channel than those 

on the west side. 

Legs B, O, and R (Figures 28 – 29) characterize the water properties on the shelf between the 

east side of Hanna Shoal and Barrow Canyon and atop Hanna Shoal. In aggregate, these sections 

indicate a strongly stratified water column, a consequence of the fresh, cold meltwaters in the 

31
upper 20 m and the saline, colder winter waters below 25 m depth. This strong stratification 

persists even over the shallow (~25 m deep) Hanna Shoal (Leg R; Figure 29). None of these 

sections show evidence of the warmer and moderately salty water that was observed on the legs 

farther south. This suggests that either these waters have yet to arrive in this region, or that 

dynamical processes are inhibiting their infiltration into this part of the shelf. 

Leg C, occupied on 9 September, extended ~70 km northwestward from the western side of 

Hanna Shoal (Figure 30). This section’s properties were similar to those found on the eastern 

side of Hanna Shoal in being primarily a composition of surface meltwaters and subsurface 

winter waters. However, there are lenses of warm, moderately salty water embedded in the 

pycnocline along this section, most noticeably between km 5 – 30 and 40 – 60. These lenses are 

signatures of warm (~3 – 4o

C) moderately saline (~31.5) waters derived from the south (most 

likely from the northward flow of BSW in the Central Channel; cf., Leg H in Figure 23).

In summary, the hydrographic structure of the northeast Chukchi Sea Shelf was similar in many 

regards in 2012 and 2013. Features in common include surface fronts that separated weakly 

and/or modestly stratified BSW and ACW in the south from strongly stratified water columns 

consisting of MW at the surface and WW at depth. These fronts tended to be oriented from west 

to east along about 71.5°N (and south of Hanna Shoal) in both years. Not surprisingly, these 

fronts were also approximately aligned with the August ice-edge in both years. 

32
Figure 22. Top: location (black line) of the west-to-east transect of glider 191 from 10 – 14 August, 2013. The green 

dot indicates the start of the transect. Bottom: Vertical sections of temperature (top), salinity (middle), and density 

(expressed as sigma-t; bottom) along the transect. The westernmost end of the transect is on the left of the figure.

33
Figure 23. The Leg H vertical section derived from the towed-CTD (Acrobat) on 11 – 12 September 2013. The 

section extends from the coast to the Central Channel as indicated by red line on the inset map. The black and green

circle indicates the beginning of the transect. From top to bottom: temperature, salinity, and density. The 

westernmost end of the transect is on the right.

Figure 24. As in Figure 23, but for Leg D, from northwest of Hanna Shoal to the coast.

34
Figure 25. As in Figure 23, but for Leg G, a short transect offshore of Wainwright.

Figure 26. As in Figure 23, but for Leg Q, a south to north transect across Hanna Shoal. The black dotted line on the 

map insert shows the approximate location of the front at km = 80.

Hanna Shoal

35
Figure 27. As in Figure 23, but for Leg B, a short west to east transect along the northeastern flank of Hanna Shoal. 

Figure 28. As in Figure 23, but for Leg O, a short north to south transect along the northeastern flank of Hanna 

Shoal.

36
Figure 29. As in Figure 23, but for Leg R, a short north-south transect across Hanna Shoal.

Figure 30. As in Figure 23, but for Leg C, a short transect along the northwestern side of Hanna Shoal.

Hanna Shoal

37
IVd. Overview of Selected Trajectories

In this section we provide a descriptive overview of the diversity and complexity of a subset of 

the drifter trajectories. Those drifter labels prefixed with MS denote the Microstar, or 1-m 

drogued drifters and those with the SVP prefix denote the 10-m drogued drifters.

Given the problems with the 2011 drifters, the data distribution was insufficient to address many 

of our goals and hence our discussion of this data set is limited. Figure 31 shows the trajectories 

of drifters deployed in mid-August 2011 (and whose lifetimes exceeded four days). All of the 

functioning 10-m drogued drifters moved eastward from south of Hanna Shoal toward 

Wainwright (Figure 32) in August under easterly winds that averaged 4 ms-1 (the background 

geostrophic flow more than compensated for the westward surface wind stress). All but one of 

these drifters then moved northeastward down Barrow Canyon to the shelfbreak, and then turned 

westward along the Chukchi shelfbreak (under the influence of strong westward winds in fall). In 

contrast, immediately after deployment three of the 1-m drogued drifters moved southwestward 

(also under strong northeasterly winds) from Hanna Shoal and into the Central Channel in 

August. From there, they moved northward toward the Chukchi shelfbreak in accordance with 

both historical (Weingartner et al., 2005b) and recent (Weingartner et al., 2013a) current meter 

data from the Central Channel.

Figure 31. Trajectories of all drifters between mid-August and 22 November 2011, released in the central Chukchi 

Sea. Green lines are SVP drifters and purple lines are the 1-m drogued drifters.

38
Figure 32. Trajectories of 10-m drogued drifters between mid-August and 22 November, 2011, released in the 

central Chukchi Sea.

2012 Drifter Deployments and Data Distribution

Deployments in 2012 were staggered in time and space. Deployments planned for northwest and 

northeast of Hanna Shoal in early August from the USCG Healy were prevented due to heavy ice 

in these regions. Instead, deployments were made along the western side of the Statoil and on the 

southeast side of Hanna Shoal. Unfortunately, several of these drifters were destroyed in ice 

(<30% concentration) shortly after deployment. Figure 33 shows the deployment distribution 

times and locations for the UAF and NSBW drifters. Figure 34 shows the eventual data 

distribution over the Chukchi Sea in terms of number of observations in each 15’ x 15’ grid cell. 

39
The various panels show the data distribution for all drifters as well as for each drifter type. It is 

evident (Figure 34) that the iSphere drifters had a distribution distinctly different than the 

Microstars and SVPs. For example, almost all the drifter observations south of 70oN were 

iSpheres. Many of these initially drifted northeastward into Barrow Canyon, then turned 

westward over the Chukchi shelfbreak before being carried back onto the shelf and southward.

Figure 33. Deployment times, locations, and drifter types for the UAF and NSBW drifters used in 2012.

All of the drifters released in 2012 were eventually crushed in ice by late fall. However, five 

continued to transmit as late as February 2013. One of these traveled ~200 km north of Wrangel 

Island and a second was eventually trapped in the landfast ice zone of the Mackenzie Shelf. A 

third was trapped in the landfast ice in Peard Bay, and two drifted slowly eastward in the pack 

ice over the Alaskan Beaufort Shelf. Each of these drifters were of the SVP-type. It is 

emphasized that once entrapped by ice these drifters move with the ice and no longer 

characterize the water motion.

40
Figure 34. Data distribution in 15’x15’ grid cells for all drifters (upper left), SVP drifters (10 m drogue; upper 

right), Microstar (1-m drogue; lower left) drifters, and iSphere drifters (lower right) for the UAF and NSBW

deployments in 2012.

13 August 2012 Deployments: Central Chukchi Shelf

The Statoil deployment consisted of five MS drifters and six SVP drifters. All were deployed in 

the northeast corner of Statoil on August 13. The drifters were deployed sequentially and as 

rapidly as possible from the stern of the USCG Healy as it moved westward at ~2 kts. Separation 

distances between individual drifters were about 50 – 100 m. A few of the trajectory maps 

include time series of the zonal (U), meridional (V) and sea surface temperature (SST). These

time series include both the hourly data (light lines) and the 35-hour low-pass filtered data (thick 

red line). For this deployment, the MS drifters are shown in Figures 35 – 38 and the SVP drifters 

are in Figures 39 – 45.

From 13 – 30 August, MS-4, 7, and 9 first moved northward and then southeastward across the 

northern side of Statoil and Burger. MS-1 and 5 initially moved northward also, then proceeded 

southward until 21 August when these drifters moved southeastward across the southern portions 

of Statoil and Burger. At this time, winds were generally from the south so that these drifter 

41
motions were not simply related to the wind field. On 29 August, winds began blowing from the 

northeast and all of these drifters moved to the southwest. Winds were generally from the 

northeast (or north) through first half of September, with the drifters making variously sized 

loops between 70o and 71oN and between 162o and 165o W. Note that as the drifters moved 

southward, the SST increased rapidly from <3oC to ~5oC, indicating that they crossed a frontal 

system with cool meltwaters present north of the front and warmer Bering Sea summer waters 

south of the front. By mid-September most of the drifters resumed a northeastward trajectory and 

entered Barrow Canyon in the last few days of September.

After entering the canyon, the drifters moved northeastward (down-canyon) for ~8 days and then 

either beached (MS-1 and 6) near Barrow or turned eastward onto the Beaufort Shelf by October 

8. Drifter MS-7 moved onto the shelfbreak at the mouth of Barrow Canyon, and then meandered 

back and forth across the canyon before eventually beaching at Barrow. In summary the MS 

drifters deployed within Statoil drifted over the northeast shelf for 1 – 1.5 months before entering 

Barrow Canyon. While on the Chukchi Shelf, MS-1 and 7 conducted a number of mesoscale 

meanders and loops, with these meanders accompanied by rapid changes in SST. The remaining 

drifters generally moved eastward (more or less) across the shelf and toward Barrow Canyon. All 

of the MS drifters then spent approximately 8 – 10 days moving rapidly to the northeast through 

Barrow Canyon. Drifter velocities on the shelf were variable with speeds typically ~10 cm s-1. 

Within the canyon, velocities were northeast at an average speed of ~25 cm s-1, however,

maximum speeds in the canyon were between 50 and 100 cm s-1

. 

The SVP drifters generally had a longer life-time as they were less prone to beaching or losing

their drogue to wave action. Hence their trajectories encompassed a much broader area than the 

MS drifters. All of the SVP drifters moved eastward across Burger. Recall from the discussion of 

the hydrography that there was a thermohaline BSW/WW front oriented zonally across the 

center of Burger in 2012. This front is reflected in the drifter data as those to the north of the 

front measured SSTs of 2 – 3o

C, while drifters to the south of the center of Burger recorded SSTs 

of 4 – 5o

C. All of the SVP drifters took from 1 and 2 months to move into Barrow Canyon and 

many of them underwent a number of complex meanders and eddying motions that are not 

evidently related to the winds as they moved toward the canyon. Most likely these mesoscale 

motions are with frontal instabilities as described later. The fact that not all the drifters were 

caught in such motions reflects the fact that these instabilities are spatially and temporally 

variable in their occurrence along the front.

After passing through the canyon, most of the SVP drifters moved to the mouth of the canyon 

and then proceeded eastward along the shelfbreak. (Recall that the MS drifters proceeded 

eastward along the inner Beaufort Shelf). SVP-2 and 3 were trapped in ice on about 8 November 

and stopped transmitting shortly thereafter. Upon exiting Barrow Canyon on 8 October, SVP-5 

moved rapidly eastward over the next 30 days before getting trapped in the landfast ice of the 

Mackenzie Shelf at ~135o

W where it was immobile through the winter and spring of 2013. Upon 

42
landfast ice breakup in late June 2013, SVP-5 slowly looped around the Mackenzie Shelf and 

then drifted westward through mid-July until it reached the head of the Mackenzie Canyon, north 

of Herschel Island. From here, it moved northwestward along the western flank of the canyon,

then westward along the Alaskan Beaufort Sea shelfbreak before it stopped transmitting in late 

July. During July, while on the Mackenzie Shelf, SVP-5 recorded large (amplitude ~10o

C) 

~diurnal oscillations in SST. These oscillations may be associated with diurnal variations in solar 

heating. Drifter SVP-6 also moved eastward along the shelfbreak after exiting Barrow Canyon. 

Its drift was interrupted by 2 – 3 cyclonic excursions that carried SVP-6 ~80 km seaward of the 

break and slowly westward. It then resumed its eastward movement along the shelfbreak until it 

reached ~140o

W where it was caught in ice and drifted westward, episodically, with the ice. The 

episodic nature of the drift suggests that ice motion occasionally stalled, possibly because it was 

temporarily attached to, or incorporated into, the landfast ice zone along the 30 m isobath. SVP-6 

continued to transmit through the winter but registered little movement until late April. At that 

point it meandered to the east and to the west several times before beginning a long westward 

excursion through June and into early July whereupon it moved eastward to offshore of 

Mikkelsen Bay. In late July it drifted westward again and stopped transmitting near Oliktok Pt.

in the first week of August. SVP-8 entered Barrow Canyon twice. Initially this occurred in late 

August passing by Pt. Barrow in early September. Afterwards, it drifted to the southwest, 

meandered over the shelf east of Hanna Shoal for the next month and then re-entered the canyon 

and sped rapidly to the shelfbreak by 25 October. From there it moved eastward to about 152o

W, 

then regressed westward, before resuming eastward motion on 7 November. Over the next 10 

days it moved eastward rapidly (~50 cm s-1

) along the shelfbreak until it was caught in ice in late 

November and ceased transmitting. SVP-10 entered Barrow Canyon around 18 September and 

then made several up and down-canyon transits before exiting onto the Beaufort Shelf. It then 

drifted eastward before being entrapped in ice on 11 November. It continued eastward with the 

ice and stopped moving on 17 November, suggesting that this ice was incorporated into the 

landfast ice zone. It continued to transmit for the next few days but ceased working on 25 

November.

43
Figure 35. The trajectory of MS-1 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

44 
Figure 36. The trajectories of MS-4 (top) and 5 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

45 
Figure 37. The trajectories of MS-7 (top) and 9 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

46 
Figure 38. The trajectory of SVP-2 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

47 
Figure 39. The trajectory of SVP-3. The colors along the drifter trajectory are color-coded according to SST. The 

green triangle and red star show start and end of the trajectory, respectively. 

48 
Figure 40. The 2012 trajectory of SVP-5 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

49 
Figure 41. The 2013 trajectory of SVP-5 (top) the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data. This drifter overwintered in the landfast ice (cf. 

Figure 30) and continued operating through early August 2013.

Herschel

Is.

50 
Figure 42. The 2012 trajectory of SVP-6 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

51 
Figure 43. The 2013 trajectory of SVP-5 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data. This drifter overwintered in the landfast ice (cf. 

Figure 42) and continued operating through early August 2013.

Mikkelsen

Bay

Oliktok

Pt.

52 
Figure 44. The trajectory of SVP-8 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

53 
Figure 45. The trajectory of SVP-10 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

54 
10 August 2012 Microstar Deployments: Inner Shelf of the Chukchi Sea 

On 10 August, the NSBW deployed MS drifters offshore of Icy Cape in water depths of ~10 m. 

This section discusses the trajectories of the 13 MS drifters deployed at this time. Figures 46 – 

54 show the trajectories of some of the NSBW drifters. There was remarkable coherence in each 

of these trajectories at least over the first few days. For example, each took ~3 days to move 

from Icy Cape to Pt. Barrow and another day to reach the mouth of Barrow Canyon. They thus 

drifted over 175 km in ~4 days (with an average speed of ~50 cm s-1). All turned eastward at the 

shelfbreak and most turned northward into the basin before reaching 150o

W. Note that each of 

these drifters were associated with warm (>4oC) water into the basin by mid-August when heavy 

ice and cold meltwaters still covered the northern Chukchi Shelf. Drifters NSBW-2, 4, 9, 10, 11, 

12, and 13 then continued to the west-northwest toward the Chukchi Sea shelfbreak. Drifters 

NSBW-1, 5, 6, 7, and 8 made one or more anti-cyclonic and/or cyclonic loops before moving to 

the west-northwest. In contrast, NSBW-3 made several anticyclonic loops in this same region 

and then drifted southeastward toward the Beaufort shelfbreak before ceasing transmissions in 

mid-October.

After reaching the Chukchi shelfbreak, NSBW-1 and 4 turned southward onto the Chukchi Shelf. 

NSBW-1 drifted eastward over the outer shelf and then re-entered Barrow Canyon where it 

moved toward the shelfbreak. From there it undertook two large looping excursions across the 

shelfbreak and slope followed by a southwestward excursion onto the Beaufort Shelf in late 

October. It then moved eastward until it ceased transmitting on 16 November. After reaching the 

Chukchi shelfbreak, NSBW-4 was carried southward over Hanna Shoal in early October and 

continued southward to the southeast side of Burger on 18 October. Over the next seven days it 

moved rapidly eastward to Pt. Barrow and then out the canyon to the shelfbreak. From there it 

moved southeastward and crossed the Beaufort Shelf nearly to the 10 m isobath. It continued 

drifting eastward until early November when it was caught in ice near 150oW. It drifted with the 

ice as far as ~145o

W where it failed on 9 November. Among the other drifters that reached the 

Chukchi shelfbreak, drifters NSBW-5, 6, 7, 8, 11 and 12 all died in late September/early 

October. NSBW-2, 9, and 13 moved southward across Hanna Shoal and either died atop the 

Shoal or south of it in mid-October. After crossing the Chukchi shelfbreak, NSBW-10 drifted 

southwest towards the Central Channel and ceased transmissions on 4 October. 

55 
Figure 46. The trajectory of NSBW-1 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

56 
Figure 47. The trajectory of NSBW-2 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

57 
Figure 48. The trajectory of NSBW-3 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

58 
Figure 49. The trajectory of NSBW-4 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

59 
Figure 50. The trajectories of NSBW-5 (top) and 6 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

60 
Figure 51. The trajectories of NSBW-7 (top) and 8 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

61 
Figure 52. The trajectories of NSBW-9 (top) and 10 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

62 
Figure 53. The trajectories of NSBW-11 (top) and 12 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

63 
Figure 54. The trajectory of NSBW-13. The colors along the drifter trajectory are color-coded according to SST.

The green triangle and red star show start and end of the trajectory, respectively. 

10-11 August 2012 iSphere Deployments: Inner Shelf of the Chukchi Sea 

NSBW made two separate deployments of iSphere drifters with the first of these coincident with 

the MS deployments on 10 August on the inner shelf of the Chukchi Sea near Icy Cape. A subset 

of iSphere trajectories is shown in Figures 55 – 59. Note that we include only one velocity 

record (from 63180) as the velocity records of the other iSphere drifters were quite similar. 

Initially, the iSphere trajectories were similar to those of the MS drifters deployed at the same 

time, e.g., they moved northeastward along the coast and through Barrow Canyon to the 

shelfbreak within 4 – 5 days. Upon reaching the shelfbreak all iSpheres veered northwestward 

along the Chukchi shelfbreak presumably in response to the intensification of the easterly and 

southeasterly winds on 19 August. None of the iSpheres turned eastward at the mouth of the 

canyon, which contrasts sharply with the MS drifters in this deployment. Recall that the MS 

drifters first moved eastward along the Beaufort shelfbreak, with many moving to the east of 

152o

W, before turning offshore and then to the northwest. All of the iSpheres moved 

northwestward to ~161oW along the Chukchi shelfbreak and then they drifted southward on 29 

August (in response to northeasterly winds) and crossed Hanna Shoal. Of particular interest is 

that the SSTs along the shelfbreak were warm (~5oC) compared to the SSTs (~0oC) over Hanna 

Shoal. Because the iSpheres were initially deployed in waters with an SST of ~9oC, the 

trajectories suggest that at least some of the warm water moving through Barrow Canyon 

64 
contributed to ice melt along the Chukchi shelfbreak north of Hanna Shoal. Drifters 65180 and 

65190 moved to the east of Burger on crossing the Shoal and subsequently beached near 

Wainwright in late September. The remaining drifters moved across Statoil and Klondike and 

into the Central Channel between 165o and 170oW. On proceeding southward, SSTs for all these 

drifters increased from ~0oC at ~72oN to 4 – 6oC at 71oN, suggesting that the drifters crossed the 

meltwater front south of Hanna Shoal and into warmer Bering Sea waters. Many of these drifters 

reached the vicinity of Pt. Hope by ~20 October. Drifter 60180 passed Pt. Hope and proceeded to 

67.5o

N on 20 October. It then turned northward, passed Pt. Hope, and eventually ran aground 

near Pt. Lay on 30 October. Drifters 61180, 63180, 64180, and 66180 performed similarly and 

all died in Ledyard Bay between Pt. Lay and the Lisburne Peninsula. Drifter 60190 made two 

large north-south loops in late October and early November, first south of Herald Shoal and then 

offshore of Pt. Lay, before passing Pt. Hope. In late November it was caught in ice and drifted 

south toward the Siberian coast before ending transmissions in mid-December. Drifter 62300 

beached near Pt. Hope on about 18 October. 

65 
Figure 55. The trajectory of iSphere-63180 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

66 
Figure 56. The trajectories of iSphere-60180 (top) and 60190 (bottom).). The colors along the drifter trajectories are 

color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

67 
Figure 57. The trajectories of iSphere-61180 (top) and 62300 (bottom). The colors along the drifter trajectories are 

color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

68 
Figure 58. The trajectories of iSphere-64180 (top) and 65180 (bottom). The colors along the drifter trajectories are 

color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

69 
Figure 59. The trajectories of iSphere-65190 (top) and 66180 (bottom). The colors along the drifter trajectories are 

color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

70 
2 September 2012 MS Drifter Deployments in Burger 

The final set of deployments that we include from 2012 consists of ten MS drifters that were 

deployed in the southeast quadrant of Burger on 2 September. The trajectories, shown in Figures 

60 – 65, were remarkably similar to one another over much of the time. Given the similarities in 

the velocity records, we show only one time series of velocity. Through ~24 September, each 

drifter moved southward and made three to four east-to-west excursions until they reached 

~70o

N, offshore of Pt. Lay and approximately 150 km south of the deployment position. As they 

moved southward they crossed a meltwater front along the southern boundary of Burger where 

the temperature rose from ~2oC to 5oC. After reaching 70oN, each drifter then moved ~100 km to 

the northeast and then eastward toward the coast over the next several days. This was followed 

by a southwestward excursion along the coast until about 1 October. Thereafter, the drifters 

reversed course and, again, proceeded to the northeast. All of the drifters, except MS 11-2 and 

12-2, went ashore between Pt. Lay and Pt. Franklin from 9 – 10 October, presumably due to the 

onset of winds from the northwest. Drifter MS 11-2 was caught in the main current in Barrow 

Canyon and proceeded to Barrow where it grounded on 9 October. Drifter MS 13-2 rounded Pt. 

Barrow on 9 October. Winds continued to be from the northwest or north at this time so the 

drifter moved eastward over the inner portion of the Beaufort Sea Shelf until it grounded in 

Harrison Bay on 18 October. 

71 
Figure 60. The trajectory of MS-13-2 and the time series of its zonal (U), meridional (V), and sea surface 

temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The green 

triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw data 

and the heavy red lines are the 35-hour low-pass filtered data.

72 
Figure 61. The trajectories of MS-7-2 (top) and 8-2 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

73 
-035

 

Figure 62. The trajectories of MS-9-2 (top) and 10-2 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

74 
 

Figure 63. The trajectories of MS-11-2 (top) and 12-2 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

75 
Figure 64. The trajectories of MS-14-2 (top) and 15-2 (bottom). The colors along the drifter trajectories are colorcoded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

76 
Figure 65. The trajectory of MS-16-2. The colors along the drifter trajectory are color-coded according to SST. The 

green triangle and red star show start and end of the trajectory, respectively. 

17 August 2013 Deployments: Pt. Lay Inshore and Offshore (NSBW-Shell) 

The NSBW deployed drifters MS-53 to MS-78 in two separate clusters (13 drifters each) within 

a few kilometers of one another seaward of Pt. Lay. We refer to these as the Pt. Lay Inshore 

cluster and the Pt. Lay Offshore cluster. The clustered deployments were designed to assess 

cross-shore gradients in the along-shore flow near the coast. In particular, we hypothesized that 

the Alaskan Coastal Current included nearshore horizontal shears that would be identified by this 

deployment scheme. As it turns out the winds during this deployment were initially upwellingfavorable and both clusters behaved similarly. The trajectories of the Pt. Lay Inshore cluster are 

shown in Figures 66 – 73 and the Offshore cluster trajectories are in Figures 74 – 78. 

From 17 August to 7 September, all of these drifters moved ~200 km westward toward Herald 

Shoal and the Central Channel at a speed of ~15 cm s-1. Initially, the inshore cluster moved 

southwestward, while the offshore cluster had a more westerly set. The drifters were moving 

~40o to the right of the wind, consistent with Ekman dynamics, with the drift speeds being ~2% 

of the wind speed. 

Near the coast, SSTs were ~6oC, but temperatures increased to 8 – 9oC offshore before 

decreasing again west of 165o W. These cross-shore temperature differences are consistent with 

the hydrography obtained from the glider and Leg H (Figures 22 and 23) in suggesting that 

77 
coastal upwelling was occurring (e.g., cooler water was inshore and warmer waters were 

offshore. 

On about 7 September, many of the drifters approached Herald Shoal and then moved northward 

along the eastern side of the Central Channel. This northward movement occurred even though 

the winds often blew strongly from the northeast and is consistent with prior observations, which 

indicate the flow here is often northward even when northerly winds prevail (Weingartner et al., 

2005a; Weingartner et al., 2013a). By early October, the drifters had migrated to the western 

side of the Channel and then started moving southward again as northerly wind speeds increased. 

Many of the drifters moved across Herald Shoal and then southward toward the Chukotkan 

coast. Those that did approach the coast often moved along-shore and toward the southeast, 

suggesting that these might have been entrained into the Siberian Coastal Current (Weingartner 

et al., 1999). 

24 August 2013 Deployments: Wainwright Inshore and Offshore (NSBW-Shell) 

Drifters MS-27 to MS-52 were deployed in two separate clusters (13 drifters each) within a few 

kilometers of one another seaward of Wainwright. As with the Pt. Lay drifters, the two clusters 

were designed to assess cross-shore gradients in the along-shore flow. However, both clusters 

behaved similarly and are, therefore, not discussed separately. Their trajectories are shown in 

Figures 79 – 83. Thirteen of these drifters ran aground shortly after deployment so their 

trajectories are not shown. 

During the first seven days after deployment these drifters moved offshore in response to 

upwelling-favorable winds and then, with the brief wind relaxation in late August, they drifted to 

the northeast. During this brief northeastward excursion, many of the drifters beached just north 

of Wainwright. From early September to mid-October, these drifters made several (to many) 

offshore-to-onshore excursions apparently in response to the waxing and waning of upwellingfavorable winds. Consistent with the hydrography that indicated coastal upwelling, the drifters 

moved into warmer water as they were carried offshore and into colder water as they moved 

onshore. By mid-October the clusters had begun dispersing with some heading northeast and out 

through Barrow Canyon (MS-27, 32, 44) and others drifting southwest or west over the Chukchi 

Sea Shelf (MS-36, 38, 41, 43). All of these drifters died by early to mid-November. Note that the 

trajectories of the 2013 Pt. Lay and Wainwright drifters contrast sharply with the trajectories of 

the nearshore drifters deployed in 2012. In that year, virtually all of the nearshore drifters moved 

rapidly through Barrow Canyon and onto the outer shelf and slope of the Chukchi and Beaufort 

Seas. These differences are largely associated with the wind conditions at the time of 

deployment. In 2012, the winds were from the southwest in early August. In 2013, wind speeds 

exceeded 6 m s-1 from the northeast at the time of deployment.

78 
Figure 66. The Pt. Lay Inshore trajectory of MS-54 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The 

green triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw 

data and the heavy red lines are the 35-hour low-pass filtered data.

Herald 

Shoal

79 
Figure 67. The Pt. Lay Inshore trajectories of MS-57 (top) and 58 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

Herald Sh

80 
Figure 68. The Pt. Lay Inshore trajectories of MS-59 (top) and 62 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

Herald Sh

81 
Figure 69. The Pt. Lay Inshore trajectories of MS-63 (top) and 67 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

82 
Figure 70. The Pt. Lay Inshore trajectories of MS-68 (top) and 70 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

Herald Sh

83 
Figure 71. The Pt. Lay Inshore trajectories of MS-71 (top) and 75 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

Herald Sh

84 
Figure 72. The Pt. Lay Inshore trajectories of MS-76 (top) and 77 (bottom). The colors along the drifter trajectories

are color-coded according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Herald Sh

Herald Sh

85 
Figure 73. The Pt. Lay Inshore trajectory of MS-78. The colors along the drifter trajectory are color-coded 

according to SST. The green triangle and red star show start and end of the trajectory, respectively. 

Herald Sh

86 
Figure 74. The Pt. Lay Offshore trajectory of MS-53 and the time series of its zonal (U), meridional (V), and sea 

surface temperature (SST) record. The colors along the drifter trajectory are color-coded according to SST. The 

green triangle and red star show start and end of the trajectory, respectively. The light lines in the time series are raw 

data and the heavy red lines are the 35-hour low-pass filtered data.

Herald Sh

87 
Figure 75. The Pt. Lay Offshore trajectories of MS-55 (top) and 56 (bottom). The colors along the drifter 

trajectories are color-coded according to SST. The green triangle and red star show start and end of the trajectories, 

respectively. 

Herald Sh

Herald Sh

88 
Figure 76. The Pt. Lay Offshore trajectories of MS-60 (top) and 61 (bottom). The colors along the drifter 

trajectories are color-coded according to SST. The green triangle and red star show start and end of the trajectories, 

respectively. 

Herald Sh

Herald Sh

89 
Figure 77. The Pt. Lay Offshore trajectories of MS-64 (top) and 65 (bottom). The colors along the drifter 

trajectories are color-coded according to SST. The green triangle and red star show start and end of the trajectories, 

respectively. 

Herald Sh

Herald Sh

90 
Figure 78. The Pt. Lay Offshore trajectories of MS-66 (top) and 69 (bottom). The colors along the drifter 

trajectories are color-coded according to SST. The green triangle and red star show start and end of the trajectories, 

respectively. 

Herald Sh

Herald Sh

91 
Figure 79. The trajectory of MS-29 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

Hanna Shoal

92 
Figure 80. The trajectories of MS-27 (top) and 32 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

93 
Figure 81. The trajectories of MS-36 (top) and 38 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

94 
Figure 82. The trajectories of MS-40 (top) and 41 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

95 
Figure 83. The trajectories of MS-43 (top) and 44 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

96 
8 September 2013 Deployments: Northeast Hanna Shoal (BOEM) 

Ten MS drifters were deployed in a single cluster within a few meters of one another northeast of 

Hanna Shoal. Their trajectories are shown in Figures 84 – 89. Initially all of these drifters moved 

slowly westward and over Hanna Shoal while remaining close to one another. They crossed the 

Shoal in early October and then began dispersing. Drifters MS-35, 38, 43 moved southwest 

toward the Central Channel, and then northward in the Channel to about 72.75oN. Note that this 

northward excursion within the Channel occurred when winds were from the northeast during 

the latter half of October. They then turned to the southeast where they either died or were 

caught in ice. Of interest is that these drifters moved into the middle of the Central Channel

before beginning their northward loops. Several others (MS-30, 36, 44, 45) also continued to the 

southwest after crossing the Shoal, but appear to have adhered to the eastern flank of the Central 

Channel before drifting to the east. The results suggest that along-channel flow in the Central 

Channel was sheared; drifters that skirted the eastern side of the Channel separated quickly from 

those that entered closer to the center of the Channel.

Drifters MS-28, 30, and 35 survived long enough to get trapped in and drift with sea ice. Both 

MS-28 and 30 drifted with the ice northward through Barrow Canyon and exited to the west of 

the Canyon along the Chukchi slope. MS-28 continued westward along the slope and outer 

Chukchi Sea Shelf, then southward in the Central Channel reaching ~72o

N by the third week of 

January. It then drifted westward over the shelf north of Herald Shoal and died in early March 

2014. MS-30 underwent a similar excursion except that it returned to its deployment position 

where it died in the latter half of December. 

Drifter MS-35 was among those carried northward in the Central Channel before drifting 

southeastward across the west side of Hanna Shoal. It entered the ice around 1 December and 

drifted southwestward at speeds that often exceeded 30 cm s

-1. It crossed Herald Shoal around 1 

January and then turned southward and died at around 69.25o

N in late January.

97 
Figure 84. The trajectory of MS-35 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

Hanna Sh

98 
Figure 85. The trajectories of MS-28 and 30. The colors along the drifter trajectories are color-coded according to 

SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

99 
Figure 86. The trajectories of MS-34 and 35. The colors along the drifter trajectories are color-coded according to 

SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

100 
Figure 87. The trajectories of MS-36 and 38. The colors along the drifter trajectories are color-coded according to 

SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

101 
Figure 88. The trajectories of MS-43 and 44. The colors along the drifter trajectories are color-coded according to 

SST. The green triangle and red star show start and end of the trajectories, respectively. 

Hanna Sh

Hanna Sh

102 
Figure 89. The trajectory of MS-45. The colors along the drifter trajectory are color-coded according to SST. The 

green triangle and red star show start and end of the trajectory, respectively. 

9 September 2013 Deployments: Northwest Hanna Shoal (BOEM) 

Eleven drifters were deployed in a single cluster within a few meters of one another northwest of 

Hanna Shoal. A subset of their trajectories is shown in Figures 90 – 94. Initially, all proceeded 

west-northwestward at speeds of 10 – 30 cm s-1. Most crossed into the Russian EEZ on 4 

October and continued westward until about 10 October when they started drifting 

southwestward into Herald Canyon. All but one (MS-37) entered or crossed to the west side of 

the canyon and then milled about the middle of the canyon before dying in late October/early 

November, one drifter (MS-39) moved southward along the west side of the canyon and died to 

the east of Wrangel Island on 25 October. 

Throughout September, SSTs were 0 – 2oC, but decreased to <0oC in early October with 

minimum SSTs at latitude 73oN; the northernmost portion of the drifts. Based on the sea ice 

concentrations (Figure 16), the drifters were closest to the sea ice during this portion of their 

trajectories. Afterward, temperatures increased slightly as the drifters moved to the southwest 

along the western side of Herald Channel.

Hanna Sh

103 
Figure 90. The trajectory of MS-39 and the time series of its zonal (U), meridional (V), and sea surface temperature 

(SST) record. The colors along the drifter trajectory are color-coded according to SST. The green triangle and red 

star show start and end of the trajectory, respectively. The light lines in the time series are raw data and the heavy 

red lines are the 35-hour low-pass filtered data.

104 
Figure 91. The trajectories of MS-29 (top) and 31 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

105 
Figure 92. The trajectories of MS-32 (top) and 33 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

106 
Figure 93. The trajectories of MS-37 (top) and 40 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

107 
Figure 94. The trajectories of MS-41 (top) and 42 (bottom). The colors along the drifter trajectories are color-coded 

according to SST. The green triangle and red star show start and end of the trajectories, respectively. 

108 
IVe. Relative Dispersion and Kinematics

In this section we examine five drifter clusters in terms of their relative dispersion and 

kinematical characteristics. These clusters include one from September 2012 that was deployed 

in Burger (the Burger cluster) and four clusters from 2013. The 2013 clusters are the Pt. Lay 

Inshore and offshore clusters, and the two clusters from the northeast and northwest sides of 

Hanna Shoal.

Figure 95 shows the trajectories of all drifters deployed in the Burger cluster on 2 September 

2012. The dispersion statistics were computed on this cluster for all drifters that remained west 

of 161oW, which we adopted as the cutoff longitude for drifters that entered Barrow Canyon. 

Figure 96 shows a comparison of the time series between centroid velocities estimated by leastsquares (“matrix method”) with those computed directly by central-differencing the centroid 

positions at each time step. This figure also includes time series of the winds throughout the 

period considered. The least squares approach is in good agreement with the directly computed 

velocities and supports the validity of the least square estimates. Note the large temporal 

variability in the velocities, which are largely coherent with the wind variations. Through midSeptember, drifter velocities were generally ~0.2 m s-1

, but later in the month they increased to 

between 0.4 m s-1 and 0.8 m s-1. The drifters tended to move coherently throughout the region as 

they gradually spread apart over time. 

Figure 95. Drifter tracks (color-coded) and cluster centroid (black) for the cluster of drifters deployed in Burger in 

early September 2012. The color-coded date legend corresponds to the color of the various symbols (triangles for 

individual drifters and squares for the centroid) along the tracks.

109
Figure 96. Mean zonal (top) and meridional (2nd from top) Burger drifter cluster centroid velocities and zonal (3rd

from top) and meridional (bottom) WaveWatch wind velocities evaluated at the location of the centroid through 

time. For the drifter velocities, the blue line is based on the least squares solution and the green line is computed by 

centered differences of the centroid positions divided by the 6-hour time step.

110
Figure 97 summarizes the results of the relative dispersion components. The time series of Dx

(upper left panel) indicates that zonal dispersion increased more rapidly than the meridional 

dispersion (Dy; upper right panel), but then remained fairly constant at ~700 km2 from 12 

September to 3 October. Dy remained fairly small until 24 September when it rapidly increased 

to ~1000 km2 during a strong, but short-lived, burst of northward winds. Thereafter Dxy settled 

down to about 700 km2 and remained at that level through the end of the record. The crosscorrelation term (lower left panel), although variable through time, does have several large 

values. This is consistent the large cyclonic motions on 22 – 24 and 28 – 30 September, and in 

early October (Figure 95). The relative dispersion (D2; lower right panel) indicates that it 

remains nearly constant at ~1200 km2 over most of the record.

Figure 97. Time series of relative zonal (top left) and meridional (top right) dispersion, the cross-correlation in 

relative dispersion (bottom left), and the relative dispersion (bottom right) for the Burger drifter cluster.

2

2

2

2

111
Time series of the terms in the deformation tensor are shown in Figure 101. On average these 

are small (< 10-5 s

-1

), but on occasion can be quite large. For example, both positive (counterclockwise or cyclonic) and negative (clockwise or anticyclonic) relative vorticities ~ 2 x 10-5 s

-1

(~20% the value of the Coriolis or planetary vorticity) occur early in the time series. These 

relative vorticity values imply a Rossby number of ~0.2, suggesting that non-linear motions may 

be important at these spatial scales. Of particular note is that all of these terms are much smaller 

after about 8 September. Recall (Figures 60 – 65) that shortly after deployment, these drifters 

detected a strong SST gradient associated with a MW/BSW zonally-oriented front present in 

Burger (Figure 17). The horizontal divergence term increases abruptly on 13 September 

coincident with an increase in zonal divergence, and the pulse-like changes in the meridional 

dispersion between 22 and 29 September are accompanied by relatively large changes in the 

associated vorticity, shearing, and stretching terms.

The absolute vorticity balance (Figure 99) indicates that the rate of change of absolute vorticity 

tends to compensate the stretching vorticity term, but not completely, which implies the presence 

of vorticity sources/sinks. The mechanism of these sinks or sources is not clear, but it is unlikely 

that they are due to small horizontal scale variations in surface wind stress, since wind-stress 

variations occur on much larger scales, typically hundreds of kilometers. Baroclinic instabilities 

(discussed in relation to Figure 27) are possible sinks/sources of vorticity.

Figure 98. Time series of the horizontal divergence (top), relative vorticity (2nd from top), stretching deformation 

(3rd from top), and shearing deformation (bottom) for the 2012 Burger drifter cluster. Units are 10-5

s

-1

.

112
Figure 99. Time series of the terms comprising the absolute vorticity balance for the Burger drifter cluster. Units are 

s

-2

.

We next consider the Pt. Lay Inshore and Offshore clusters (Figures 100 and 101, respectively)

that were deployed in August 2013. We discuss both clusters together because they have similar 

features. The drifters remained tightly clustered as they drifted eastward from 19 August until 

approximately 11 September even though strong northeasterly winds dominated the first 20 days 

of the deployment. The winds then varied from southwesterly to northeasterly and the drifters 

began to disperse rapidly. Nevertheless, throughout the cluster lifetime, the drifter trajectories 

were coherent with one another. Figures 102 and 103 show the centroid velocities for both 

clusters and the time series of the winds throughout the cluster’s records. The temporal variation 

in centroid velocities were large and, to some degree, coherent with variations in the wind 

velocities. Note, however, that some of the large meridional wind speeds early in the record are 

not reflected in the meridional currents. In particular, the drifters moved northward between 

about 27 August and 13 September when they entered the Central Channel, even though winds at 

this time were mainly northeasterly at 5 – 10 m s-1

. The brief southward drift on ~20 September 

was associated with strong northerly winds. These winds caused the flow in the Central Channel

to reverse southward, but the currents swiftly became northward again when the winds relaxed.

The dispersion results for the Pt. Lay clusters (Figures 104 and 105) indicate that little 

dispersion occurred as they drifted westward over the first 10 days or so of the deployment. 

Zonal dispersion was small until 3 September and then grew to ~3 – 4000 km2 by 11 September. 

It increased slowly to ~4 – 5000 km2 by early October and to ~8000 km2 by mid-October. 

Meridional dispersion began around 11 September and remained small through late September as 

the drifters began their initial northward movement into the Central Channel. By late September 

the meridional (and total) dispersion increased rapidly to ~10000 km2

.

113
Figure 100. Mean cluster trajectory and individual drifter tracks for the Pt. Lay Inshore drifter cluster. The inverted 

triangles are color-coded by date and apply to individual drifter trajectories (in color) and the black line is the mean 

trajectory with dates indicated by color-coded squares.

Figure 101. Mean cluster trajectory and individual drifter tracks for the Pt. Lay Offshore drifter cluster. The 

inverted triangles are color-coded by date and apply to individual drifter trajectories (in color) and the black line is 

the mean trajectory with dates indicated by color-coded squares.

114
Figure 102. Mean zonal (top) and meridional (2nd from top) Pt. Lay Inshore drifter cluster centroid velocities, and 

zonal (3rd from top) and meridional (bottom) WaveWatch wind velocities evaluated at the location of the centroid 

through time. For the drifter velocities the blue line is based on the least squares solution and the green line is 

computed by centered differences of the centroid positions divided by the 6-hour time step.

115
Figure 103. Mean zonal (top) and meridional (2nd from top) Pt. Lay Offshore drifter cluster centroid velocities, and 

zonal (3rd from top) and meridional (bottom) WaveWatch wind velocities evaluated at the location of the centroid 

through time. For the drifter velocities the blue line is based on the least squares solution and the green line is 

computed by centered differences of the centroid positions divided by the 6-hour time step.

116
Figure 104. Time series of relative zonal (top left) and meridional (top right) dispersion, the cross-correlation in 

relative dispersion (bottom left), and the relative dispersion (bottom right) for the Pt. Lay Inshore drifter cluster. 

Time series of the terms comprising the deformation tensor for both clusters are shown in 

Figures 106 and 107, respectively. These terms are always small (<10-5 s

-1

) for the offshore 

cluster and for the inshore cluster over most of the time series. The deformation terms did reach a 

large magnitude (~10-4s

-1

) on around 24 – 25 August in conjunction with the large clockwise 

looping at 69.3oN, 166oW (cf. Figure 100) and the rapid westward acceleration of from 0 to -50 

cm s-1 (Figure 102, upper panel) at this time. 

117
Figure 105. Time series of relative zonal (top left) and meridional (top right) dispersion, the cross-correlation in 

relative dispersion (bottom left), and the relative dispersion (bottom right) for the Pt. Lay Offshore drifter cluster.

The vorticity balance (Figure 108) for the inshore cluster indicates that the terms in the absolute 

vorticity equation vary little through time. The exception is from 24 – 25 August when the 

stretching term is not compensated for by the time rate of change in absolute vorticity. This 

imbalance occurred in conjunction with the looping motion followed by the rapid westward 

acceleration mentioned above. The implied vorticity source is unclear, though it may be due to 

baroclinic production of vorticity (at small scales) associated with the front seen in Figure 18. 

However, the time rate of change and stretching vorticity did not vary at all for the offshore 

cluster (Figure 109). The reasons for these different vorticity balances between the two clusters 

are not clear. If, however, baroclinic conversion in vorticity is indeed involved, then this process 

may be spatially and temporally episodic and thus not captured by the offshore cluster.

118
Figure 106. Time series of the horizontal divergence (top), relative vorticity (2nd from top), stretching deformation 

(3rd from top), and shearing deformation (bottom) for the Pt. Lay Inshore drifter cluster. Units are 10-5

s

-1

.

Figure 107. Time series of the horizontal divergence (top), relative vorticity (2nd from top), stretching deformation 

(3rd from top), and shearing deformation (bottom) for the Pt. Lay Offshore drifter cluster. Units are 10-5

s

-1

.

119
Figure 108. Time series of the terms comprising the absolute vorticity balance for the Pt. Lay Inshore drifter cluster. 

Units are s-2

.

Figure 109. Time series of the terms comprising the absolute vorticity balance for the Pt. Lay Offshore drifter 

cluster. Units are s-2

.

120
The NE Hanna cluster centroid and individual tracks are shown in Figure 110 for the period of 

10 September through 27 October, and the accompanying current and time series are shown in 

Figure 111. This cluster’s drift was primarily westward throughout the period of record. Initially 

the cluster drifted westward across Hanna Shoal under predominantly northeasterly winds. This 

drift stopped and reversed to the northeast over Hanna Shoal under southerly winds until about 5 

October. Winds then became northeasterly (for the most part) again and the cluster drifted to the 

southwest across Statoil and north of Klondike into the Central Channel. By late October the 

cluster tracking was ended due to too many drifters being caught by ice. The NE Hanna cluster 

had much more modest speeds (e.g., +0.25 m s-1) compared to the Burger and Pt. Lay clusters 

(speeds >0.5 ms-1).

The dispersion characteristics for the NE Hanna cluster (Figure 112) show that zonal dispersion 

grew rapidly to 10000 km2 through 10 October and remained nearly constant thereafter. 

Meridional dispersion developed much more slowly; it reached a maximum of 4000 km2 by midOctober, and then diminished toward the latter end of the record. Overall, the total relative 

dispersion was primarily associated with zonal spreading of the drifters.

Time series of the terms comprising the deformation tensor for the NE Hanna Shoal cluster are 

shown in Figure 113. These terms are always small (< 10-5 s

-1

) immediately after deployment, 

suggesting unsheared, non-divergent flows with little relative vorticity. Similarly, the vorticity 

balance (Figure 114) indicates that the absolute vorticity is conserved throughout the record.

Figure 110. Mean cluster trajectory and individual drifter tracks for the Northeast Hanna Shoal drifter cluster. The 

inverted triangles are color-coded by date and apply to individual drifter trajectories (in color) and the black line is 

the mean trajectory with dates indicated by color-coded squares.

121
Figure 111. Mean zonal (top) and meridional (2nd from top) NE Hanna Shoal cluster centroid velocities and zonal 

(3rd from top) and meridional (bottom) wind velocities evaluated at the location of the centroid through time. For the 

drifter velocities the blue line is based on the least squares solution and the green line is computed by centered 

differences of the centroid positions divided by the 6-hour time step. For the wind components the green line is

winds from the WaveWatch model and the blue line is the NARR winds.

122
Figure 112. Time series of relative zonal (top left) and meridional (top right) dispersion, the cross-correlation in 

relative dispersion (bottom left), and the relative dispersion (bottom right) for the NE Hanna drifter cluster.

123
Figure 113. Time series of the horizontal divergence (top), relative vorticity (2nd from top), stretching deformation 

(3rd from top), and shearing deformation (bottom) for the NE Hanna drifter cluster. Units are 10-5 s

-1

.

Figure 114. Time series of the terms comprising the absolute vorticity balance for the NE Hanna Shoal drifter 

cluster. Units are s-2

.

124
The NW Hanna cluster centroid and individual tracks for 11 September through 17 October are 

shown in Figure 115 and their accompanying current and time series are shown in Figure 116. 

This cluster drifted west northwestward between 73°N and 72° 20’N from 11 September to 10 

October. Until 22 September, this drift was westward at between 0.1 and 0.3 ms-1 and when 

winds were from the northeast at speeds of ~7ms-1. The westward drift continued, but slowed, 

between 22 September and 6 October when the winds were from the southwest at 6 ms-1

. Winds 

were primarily from the north from 9 – 17 October and the cluster moved to the southwest 

through this period.

The dispersion characteristics for the NW Hanna cluster (Figure 117) indicate little dispersion in 

either direction before 28 September. Thereafter, zonal dispersion grew rapidly and reached 

15000 km2 by mid-October before decreasing to ~10000 km2 by the end of the record. The 

meridional dispersion was markedly smaller; it increased to ~2000 km2 in early October and then 

decreased to ~300 km2 in mid-October before increasing again to ~2000 km2 by the end of the 

record. The total dispersion was, therefore, largely related to zonal spreading of the drifters.

Time series of the terms comprising the deformation tensor for the NW Hanna Shoal cluster are 

shown in Figure 118. These terms are all small (<10-5 s

-1

) after 17 September suggesting 

unsheared, non-divergent flows with little relative vorticity. Early on, all terms are large, 

especially the relative vorticity, which has a maximum value of 6 x 10-5 s

-1 on 13 September. 

These large values are all associated with the counter-clockwise turning of all the drifters on this 

date near 166o

W (Figure 115). The vorticity balance for this cluster (Figure 119) indicates that 

the rate of change in absolute vorticity tends to, but does not completely, compensate the

Figure 115. Mean cluster trajectory and individual drifter tracks for the Northwest Hanna Shoal drifter cluster. The 

inverted triangles are color-coded by date and apply to individual drifter trajectories (in color) and the black line is 

the mean trajectory with dates indicated by color-coded squares.

125
stretching vorticity term, indicating once again that there are vorticity sources and sinks present. 

The imbalance in the vorticity equations largely occurs before 17 September and we speculate 

that there may be meanders and/or eddies associated with meltwater fronts along the northwest 

side of Hanna Shoal.

A climatological picture of dispersion on the Chukchi Sea Shelf is established by averaging the 

dispersion properties of all drifters in the five clusters in a log-log plot of the total relative 

dispersion versus time (Figure 120). Dispersion increases by an order of magnitude from ~4 x 

10-3 to 2 x 10-2 km2 during the first 12 hours after deployment and then by another order of 

magnitude to ~4 x 10-1 km2 over the next 12 hours. At time scales greater than two days, the 

approximate Lagrangian time scale (TL) based on the autocorrelation functions for these drifters, 

the dispersion increases as a function of time to the 2.5 power.

Figure 121 shows log-log plots of the relative zonal and meridional dispersion calculations 

based on the average of the 2012 and four 2013 clusters. The data suggest anisotropic dispersion 

insofar as the zonal dispersion attains an average value ~6000 km2 in 40 days, whereas the 

meridional dispersion is ~3000 km2 over this duration. Relative diffusivities can be estimated 

from the dispersion curves to yield:

2

2 1

2

2 1

1 900 zonal diffusivity 2

1 450 meridional diffusivity 2

~

~

x

xx

y

yy

dD K m s

dt

dD

K m s

dt

−

−

=

=

where the calculation is based on values between day 2 and day 40.

,

126
Figure 116. Mean zonal (top) and meridional (2nd from top) NW Hanna Shoal drifter cluster centroid velocities and 

zonal (3rd from top) and meridional (bottom) NARR wind velocities evaluated at the location of the centroid through 

time. For the drifter velocities the blue line is based on the least squares solution and the green line is computed by 

centered differences of the centroid positions divided by the 6-hour time step.

127
Figure 117. Time series of relative zonal (top left) and meridional (top right) dispersion, the cross-correlation in 

relative dispersion (bottom left), and the relative dispersion (bottom right) for the NW Hanna drifter cluster. 

128
Figure 118. Time series of the horizontal divergence (top), relative vorticity (2nd from top), stretching deformation 

(3rd from top), and shearing deformation (bottom) for the NW Hanna drifter cluster. Units are 10-5 s

-1

.

Figure 119. Time series of the terms comprising the absolute vorticity balance for the NW Hanna Shoal drifter 

cluster. Units are s-2

.

129
Figure 120. Log-log plot of the total relative dispersion based on the average of the five clusters deployed in 2012 

and 2013. The dashed black lines denote the 95% confidence interval around the mean value (blue curve). The black 

straight line is a subjective fit to the data between 2 and 40 days. The Lagrangian time scale (TL) is between 1 and 2 

days for these clusters.

130
Figure 121. Log-log plots of the relative zonal (top) and meridional (bottom) dispersion based on the average of the 

five clusters deployed in 2012 and 2013. The dashed black lines denote the 95% confidence interval around the 

mean value (blue curve). The black straight line is an eyeball fit to the data between 2 and 40 days. The Lagrangian 

time scale (TL) is between 1 and 2 days for these clusters.

131
IVf. Wind-Drifter Correlations

We examined the relationship between the winds and the drifters based on simple linear 

regressions for the 2012 and 2013 deployments. We anticipated that the regressions might vary 

spatially and with drifter type. Consequently, for 2012 we fit the regression to each drifter type in 

six different spatial domains (Figure 122):

a) Alaska Coastal domain that encompasses the region within 40 km of the northwest coast 

of Alaska and south of Barrow Canyon,

b) Barrow Canyon including the mouth of the canyon and all points within 40 km of the 

coast of Alaska,

c) Continental Slope of both the Beaufort and Chukchi Seas, including all positions between 

the 80 and 2200 m isobaths,

d) Canada Basin, which includes all locations seaward of the 2200 m isobath,

e) Chukchi Shelf including all points seaward of the Alaska Coastal and Barrow Canyon 

domains and south of the 80 m isobath, and

f) Beaufort Shelf, between the Beaufort Sea coast and the 80 m isobath.

Figure 122. Definition map of the domain locations for the wind-drifter regressions.

The regressions did not include any velocity values from drifters that were incorporated in sea 

ice. The fits are of the form:

 and d ww d ww u aU bV c v dU eV f = ++ = ++

132
The variables ud and vd are the zonal and meridional velocity components of the drifters in cm s-1

and Uw and Vw are the corresponding wind components in m s-1

. The coefficients a, b, e, and d

have units of cm m-1 and the intercepts, c and f, have units of cm s-1. Similar fits were made 

using the wind stress components, with the wind stresses computed based on Large and Pond

(1981). We only present the results using the wind velocity components since these generally 

resulted in (slightly) better fits and may be more desirable to use in an operational setting. The 

results for 2012 are summarized in Tables 1 – 6 and those for 2013 are in Table 7. There are 

several caveats associated with the results. First, different types of drifters were in the different 

domains at various times. Hence, the regressions were not all based on the same ambient 

conditions in terms of winds and stratification. Second, there are different sample sizes involved 

in constructing the regressions (e.g., there were insufficient iSphere samples from the Canada 

Basin domain upon which to construct a regression). Third, some domains (e.g., the Chukchi 

Shelf, Barrow Canyon) include spatially varying stratification and/or fronts associated with ice 

meltwaters. The drifter responses to wind are expected to vary between these regions. We have 

made no attempt at this stage to address this issue, but emphasize that these differences will 

degrade the regression results, since the (implicit) assumption of a constant parameter system is 

almost certainly violated. Nevertheless, most of the regressions are significant at the 95% 

confidence level, although the strength of the regression varies widely across domains and drifter 

types. Figures 114 – 131 are scatterplots of the drifter velocity components versus the wind 

velocity components for 2012. For brevity, the scatterplots for 2013 are not included.

Several general features emerge from these calculations. First, the best wind-current regressions 

are for the iSpheres with most of the R2 values exceeding 0.5, and in many cases, ~0.6 or larger. 

However, the winds explained only ~20% of the variance in the iSphere velocities for the 

continental slope domain. The MS drifters have lower R2 values compared to the iSpheres. The 

most useful regressions are for the Coastal and Beaufort Shelf domains, where the R2 values 

exceed 0.5. In the Chukchi Shelf domain, the R2 values are 0.3 to 0.4. However, the variance 

explained by the wind-current regressions nearly doubles if the calculations are confined to the 

September – October period (3rd row, Table 3). This result may reflect bias in drifter locations 

because the fall tended to be south of the frontal systems for the most part. In contrast, the 

drifters deployed in August were likely within the frontal systems. Figure 132 shows a 

comparison between wind and drifter velocity components for MS-9 (released in mid-August in 

the Statoil study area) and MS-8-2 (released in early September in the Burger study area). These 

time series are similar to others in the same deployment. Visual inspection clearly shows the 

difference in wind response between the two drifters. Note that the range and variability in wind 

speeds is similar in both cases so the winds cannot be the cause of the differential response 

observed. The SVP drifters have the smallest R2 values overall. In general, less than 25% of the 

velocity signals in the SVP drifters are explained by the wind. The sole exception is the zonal 

velocity component on the Beaufort Shelf, for which the winds explain 42% of the variance. The 

133
comparatively poor regression results in the shelfbreak domain for the MS and SVP drifters 

must, in part, be to eddies and meanders in the current field here.

As seen from the regression coefficients and the figures, the zonal (meridional) current 

component is more closely related to its corresponding wind component than to the orthogonal 

wind component. The regression intercepts are also of interest. In the Alaska Coastal domain 

these suggest that the flow in the absence of any wind forcing is nominally eastward at between 

14 cm s-1 (iSpheres), 13 cm s-1 (MS) and 11 cm s-1 (SVP). Similarly, these intercepts for the 

Chukchi Shelf domain range between 6 and 8 cm s-1

. These values are consistent with forcing in 

the absence of wind by the mean pressure gradient between the Bering Sea and Arctic over most 

of the central shelf in which these drifters sampled.

The regression results allow us to determine the variation with drifter type in speed and the angle 

of rotation between the wind and the drifter. For the Chukchi Shelf domain, the iSphere drifter 

speeds are ~4% of the wind speed and they deviate to the right from the wind direction by 4o. 

The MS drifters move at 2% of the wind speed and are deflected ~30o to the right of the wind. 

The SVPs move at 1% of the wind speed and are deflected to the right by 57o

. The latter two 

values are not inconsistent with Ekman’s theory when applied to a stratified ocean. We note that 

in 2013 many of the SVP drifters were deployed in stratified waters in which a strong pycnocline 

was at ~10 – 15 m depth. In such regions we expect that the Ekman layer is effectively limited to 

the pycnocline depth, so that there should be a large vertical gradient in wind shear between the 

surface and 10 m depth. Our results bear this out in the aggregate for the Chukchi Shelf domain. 

A general conclusion from these results is that there is no single wind-current regression that can 

be used with confidence for operational purposes. Our results suggest that these regressions are 

likely variable in time and space. This suggestion is supported by the results from 2013 for 

which we computed the regressions for the Pt. Lay and Hanna Shoal clusters separately and 

results are summarized for each cluster in Table 7. WaveWatch winds were used for all clusters 

except the NW Hanna Shoal cluster. For that cluster regressions were performed using the 

NARR winds, since the WaveWatch product does not extend beyond the Russian-US convention 

line. Again, all winds products were interpolated through time to produce an estimate at each of 

the drifter positions along their trajectory.

Several results emerge from these analyses. For the Pt. Lay clusters the winds account for nearly 

twice as much of the variance in the zonal velocity component as they do for the meridional 

velocity. This difference is most likely tied to the northward excursion of these drifters in the 

Central Channel where the ambient currents are often northward regardless of the wind direction 

(Weingartner et al., 2005; Weingartner et al., 2013a). The zero-wind intercepts for the Pt. Lay 

drifters indicate that in the absence of wind the drifters would move to the north-northeast on 

average; a result consistent with notions of the mean flow over the eastern Chukchi Shelf in the 

absence of wind-forcing. The slopes for the wind regressions for both Pt. Lay drifters are not 

134
statistically different from one another nor are these slopes statistically different from all of the 

MS drifters within the Chukchi Shelf domain. The Pt. Lay slopes are, however, significantly 

different from the slopes for the September – October 2012 MS drifters (compare row 3, Table 3 

with rows 1 and 2 of Table 7).

The regression results for the NE Hanna Shoal drifters are similar to those for the Chukchi Shelf 

domain (Table 7) with respect to the fraction of variance explained by the winds. However, the 

regression coefficients are statistically different from one another. These differences may be a 

consequence of differences in stratification between the two years or in the deployment 

locations. The intercepts differ significantly between the NE Hanna Shoal drifters and those from 

the Chukchi Shelf domain in 2012. For the former, these intercepts yield a mean drift to the 

southwest, whereas for the latter the mean drift is toward the east-northeast. Finally, we note the 

extremely poor regression results for the NW Hanna Shoal cluster. These are markedly different 

from any of the other drifters released in either 2012 or 2013. The reasons for the poor 

relationship with the winds are unclear, but we suggest the following. First, many of the drifter 

trajectories stagnated in Herald Canyon. Here the mean flow is often northbound on the east side 

of this canyon, but there is a large cross-canyon velocity shear associated with a front that 

extends along the canyon (Pickart et al., 2010). If this background velocity structure existed 

when the drifters were in Herald Canyon, the response to the winds would differ in accordance 

with their location within the background velocity field. To investigate this possibility, we reran 

the regressions after splitting the analyses into two parts; before and after 5 October, with the 

trajectories after this date largely associated with the Herald Valley region. The regressions are 

statistically dissimilar to one another. There is, however, no substantial improvement in terms of 

the total amount of variance explained between the two regressions. We note also that during the 

first half of the record, the drifters moved westward along the northern Chukchi shelfbreak along 

about the 100 m isobath. Here they may be entrapped in the southern limb of the Beaufort Gyre 

and/or interacting with meltwater fronts associated with the ice-edge so that simple wind-current 

relationships are masked by other dynamics.

135
Table 1. Regression summary for the Alaska Coastal domain. The numbers in parentheses represent + the 95% confidence limits on the 

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

A b c d e f

iSphere 3.5

(3.0, 4.0)

3.3

(2.9, 3.6)

13.5

(11.4, 15.4)

0.52 0.5

(0.2, 0.8)

4.8

(4.6, 5.0)

0.4

(-0.9, 1.7)

MS (1-m) 3.4

(3.0, 3.8)

3.0

(2.7, 3.3)

12.3

(10.4, 14.4)

0.56 0.5

(0.3, 0.8)

3.8

(3.6, 4.0)

0.7

(-0.6, 1.9)

SVP (10-m) 1.4

(0.9, 1.9)

2.2

(1.5, 3.0)

10.9

(7.5, 14.4)

0.26 0.4

(0.2, 0.6)

1.5

(1.1,1.8)

1.2

(-0.4, 2.8)

Table 2. Regression summary for the Barrow Canyon domain. The numbers in parentheses represent + the 95% confidence limits on th

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

A b c d e f

iSphere 7.5

(6.8, 8.1)

6.0

(5.4, 6.6)

-2.6

(-6.0, 0.8)

0.63 1.4

(1.0, 1.8)

4.3

(3.9, 4.6)

0.1

(-1.9, 2.1)

MS (1-m) 2.4

(2.1, 2.8)

2.0

(1.6, 2.3)

13.5

(11.8,15.1)

0.23 -0.7

(-0.9, -0.6)

1.6

(1.4, 1.8)

6.2

(5.3, 7.0)

SVP (10-m) 0.1

(-0.2, 0.5)

1.9

(1.5, 2.2)

10.7

(9,12.4)

0.08 -0.4

(-0.6, -0.3)

0.8

(0.6,1.0)

4.5

(3.7, 5.3)

Table 3. Regression summary for the Chukchi Shelf domain. The numbers in parentheses represent + the 95% confidence limits on the 

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

A b c d e f

iSphere 3.8

(3.7, 3.9)

0.8

(0.7, 0.9)

7.8

(3.8, 11.5)

0.67 -0.6

(-0.8, -0.5)

4.1

(4.0, 4.2)

0.0

(-3.1, 3.2)

MS (1-m)

All months

1.8

(1.7, 1.9)

1.4

(1.3, 1.5)

6.1

(5.6, 6.7)

0.38 -0.5

(-0.6, -0.4)

1.7

(1.6, 1.8)

0.3

(-0.2, 0.7)

MS (1-m)

Sept.-Oct.

3.5

(3.3,3.8)

1.5

(1.3,1.7)

13.8

(12.6,14.9)

0.56 -0.02

(-0.18,0.14)

3.7

(3.5,3.8)

5.2

(4.3,6.0)

SVP (10-m) 0.9

(0.8, 1.0)

1.1

(1.0, 1.2)

6.0

(5.6, 6.6)

0.26 -0.3

(-0.3, -0.2)

0.2

(0.2, 0.3)

-2.9

(-3.2, -2.6)

136
Table 4. Regression summary for the Beaufort Shelf domain. The numbers in parentheses represent + the 95% confidence limits on the 

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

a b c d e f

iSphere 3.3

(2.7, 4.0)

1.8

(1.3, 2.3)

7.7

(3.8, 11.5)

0.77 -1.1

(-1.6, -0.6)

3.1

(2.7, 3.5)

0.0

(-3.1, 3.2)

MS (1-m) 2.9

(2.5, 3.4)

1.5

(1.1, 1.9)

7.6

(5.4, 9.9)

0.55 -1.1

(-1.4, -0.9)

2.2

(1.9, 2.4)

-1.1

(-2.5, 0.4)

SVP (10-m) 2.5

(2.1, 2.9)

0.4

(0.0, 0.8)

-2.8

(-5.3, -0.3)

0.42 -0.9

(-1.1, -0.6)

0.1

(-0.1, 0.4)

1.0

(-0.5, 2.5)

Table 5. Regression summary for the Continental Slope domain. The numbers in parentheses represent the + 95% confidence 

coefficients.

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

a b c d e f

iSphere 3.4

(3.0, 3.8)

0.8

(0.2, 1.3)

-11.8

(-14.0, -9.6)

0.44 -0.8

(-1.0, -0.6)

3.4

(3.1, 3.7)

4.5

(3.3, 5.7)

MS (1-m) 0.4

(0.2, 0.6)

1.8

(1.6, 2.2)

-3.6

(-4.8, -2.3)

0.11 -1.2

(-1.4, -1.0)

0.8

(0.6, 1.1)

3.4

(2.3, 4.5)

SVP (10-m) 2.4

(2.0, 2.9)

-0.9

(-1.4, -0.3)

0.2

(-2.4, 2.5)

0.23 -1.6

(-1.9, -1.3)

1.1

(0.6, 1.6)

4.8

(2.8, 6.9)

Table 6. Regression summary for the Canada Basin domain. The numbers in parentheses represent the + 95% confidence limits on t

There were insufficient iSphere values to perform this regression.

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

a b c d e f

iSphere - - - - - - - 

MS (1-m) 1.0

(0.7, 1.2)

1.7

(1.3, 2.2)

-4.0

(-5.8, -2.3)

0.19 -1.4

(-1.7, -1.2)

1.6

(1.7, 2.0)

5.3

(3.8, 6.9)

SVP (10-m) 1.8

(1.2, 2.3)

0.1

(-0.7, 0.9)

0.2

(-2.4, 2.5)

0.22 -0.9

(-1.4, -0.4)

0.9

(0.3, 1.6)

9.2

(6.0, 12.3)

137
Table 7. Regression summary for the Pt. Lay Inshore and Offshore, and NE and NW Hanna Shoal clusters. The numbers in parent

confidence limits on the regression coefficients.

Drifter

d ww u aU bV c = ++ R2d ww v dU eV f = ++ 

a b c d e f

Pt. Lay Inshore 1.8

(1.7, 1.9)

1.3

(1.2, 1.4)

2.4

(1.9, 2.9)

0.61 -0.5

(-0.6, -.5)

1.96

(1.9, 2.0)

6.0

(5.5, 6

Pt. Lay Offshore 1.7

(1.6, 1.8)

1.5

(1.4, 1.6)

4.1

(3.7, 4.6)

0.65 -0.3

(-0.4,-0.2)

1.45

(1.3, 1.5)

6.8

(6.4, 7

NE Hanna Shoal 1.0

(0.9, 1.1)

2.2

(1.5, 3.0)

-0.8

(-1.3,-0.2)

0.41 -0.8

(-0.9,-0.7)

0.97

(0.9,1.1)

-1.9

(-2.5,-1

NE Hanna Shoal 1st 0.7

(0.6, 0.8)

1.0

(0.9, 1.1)

-1.5 

(-2.1, -0.9)

0.44 -0.6

(-0.7, -0.5)

0.6

(0.5, 0.7)

0.3

(-0.3, 0

NE Hanna Shoal 2nd 1.4

(1.2, 1.5)

1.0

(0.9, 1.2)

1.4 

(0.2, 2.7)

0.35 -1.0

(-1.2, -0.9)

0.9

(0.7, 1.0)

-5

(-6.4,-3

NW Hanna Shoal 0.4

(0.3, 0.6)

0.7

(0.6, 0.9)

-7.8

(-8.2,-6.8)

0.09 -1.3

(-2.3,-1.1)

0.9

(-1.4, -1.2)

0.9

(0.8, 1

NW Hanna Shoal 1st 0.09

(-0.1, 0.2)

1.0

(0.9, 1.2)

-7.5 

(-8.3, -6.7)

0.16 -0.9 

(-1.0,-0.8)

0.6 

(0.4, 0.7)

0.8

(0.1, 1

NW Hanna Shoal 2nd 1

(0.8, 1.3)

0.2

(-0.1, 0.5)

-10.1

(-11.6, -8.6)

0.07 -1.5

(-1.8, -4.3)

0.7

(0.4, 1.0)

-5.7

(-7.2, -

138
Figure 123. Scatterplots and regression statistics for the iSphere drifters in the Coastal domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components.

139
Figure 124. Scatterplots and regression statistics for the MS drifters in the Coastal domain. Top row: zonal drifter 

velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional drifter 

velocity component versus zonal (left) and meridional (right) wind components. 

140
Figure 125. Scatterplots and regression statistics for the SVP drifters in the Coastal domain. Top row: zonal drifter 

velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional drifter 

velocity component versus zonal (left) and meridional (right) wind components. 

141
Figure 126. Scatterplots and regression statistics for the iSphere drifters in the Barrow Canyon domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components. 

142
Figure 127. Scatterplots and regression statistics for the MS drifters in the Barrow Canyon domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components. 

143
Figure 128. Scatterplots and regression statistics for the SVP drifters in the Barrow Canyon domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components.

144
Figure 129. Scatterplots and regression statistics for the iSphere drifters in the Chukchi Shelf domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components. 

145
Figure 130. Scatterplots and regression statistics for the MS drifters in the Chukchi Shelf Domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components. 

146
Figure 131. Scatterplots and regression statistics for the SVP drifters in the Chukchi Shelf domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components. 

147
Figure 132. Wind-current comparisons between MS-9 (released in Statoil in mid-August; top two panels) and 

MS-8-2 (released in Burger in early September; bottom two panels). 

Statoil release

Statoil release

Burger release

Burger release

148
Figure 133. Scatterplots and regression statistics for the iSphere drifters in the Beaufort Shelf domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components. 

149
Figure 134. Scatterplots and regression statistics for the MS drifters in the Beaufort Shelf domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components. 

150
Figure 135. Scatterplots and regression statistics for the SVP drifters in the Beaufort Shelf domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components. 

151
Figure 136. Scatterplots and regression statistics for the iSphere drifters in the Continental Slope domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components.

152
Figure 137. Scatterplots and regression statistics for the MS drifters in the Continental Slope domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components.

153
Figure 138. Scatterplots and regression statistics for the SVP drifters in the Continental Slope domain. Top row: 

zonal drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: 

meridional drifter velocity component versus zonal (left) and meridional (right) wind components.

154
Figure 139. Scatterplots and regression statistics for the MS drifters in the Canada Basin domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components.

155
Figure 140. Scatterplots and regression statistics for the SVP drifters in the Canada Basin domain. Top row: zonal 

drifter velocity component versus zonal (left) and meridional (right) wind components. Bottom row: meridional 

drifter velocity component versus zonal (left) and meridional (right) wind components.

We conclude this section with a comparison between drifter velocities and those obtained from 

moorings and/or the HF radar. Figure 141 consists of the comparison between drifters passing 

within 15 km of the acoustic doppler current profiler (ADCP) on mooring BC2 (in Barrow 

Canyon) in September and October 2012. The zonal velocities are in good agreement with the 

slope not significantly different from 1 and the ADCP explaining about 66% of the variance in 

the drifter velocities. The intercept indicates that there is no significant bias, but the confidence 

limits on this statistic are broad. The fit is poorer between the meridional velocity components 

where the ADCP explains only 38% of the variance in the drifter meridional velocity. These 

differences may be due to the rather large horizontal shears typically present in the canyon 

(Weingartner et al., 2013a). Figure 142 compares the “hourly” HFR U and V components for 

the period of the Burger cluster in September – October 2012. 

156
Figure 141. Scatterplots of zonal (U; left) and meridional (V; left) velocities between drifters and mooring BC2 in 

Barrow Canyon.

Figure 142. Scatterplots of zonal (U; left) and meridional (V; left) velocities between drifters and the highfrequency radar for the Burger drifter cluster of September and October 2012. The ellipse around the points in the 

left hand panel describes points in poor agreement on and about 24 September.

157
The results are good overall, with the U-regressions explaining ~70% of the variance and those 

for V explaining ~42% of the variance. There are several reasons for the differences between the 

drifters and the HFR. First, the drifters sample hourly and the velocity is computed at a point. 

The HFR “hourly” values are actually smoothed estimates made over three-hour intervals. The 

regressions improve if we filter both the drifter velocities and the HFR data sets with a 35-hour

cutoff filter (the r2 for U and V improve to 80 and 50%, respectively). Each HFR velocity is also 

interpolated to the position of the drifter at the location of the drifter using at least three 

surrounding HFR velocities. We have included all points in the HFR radar mask that met our 

acceptance criteria, including those at the edge of the mask that may be uncertain. In particular,

the V velocities disagree noticeably on 24 September (indicated by the ellipse in the figure). At 

that time, the drifters were all on the southwest edge of the HFR domain and velocity estimates 

there are relatively poor.

V. Conclusions

The drifter trajectories deployed in this program resulted in an unprecedented Lagrangian 

perspective of the near surface circulation of the Chukchi and Beaufort Seas and their adjoining 

shelfbreaks. This achievement was accomplished by drifter deployments made in a variety of 

different areas of the Chukchi Sea Shelf during the open water periods in 2011 – 2013. The 

results reflect differences in winds, sea ice, hydrography, bathymetry of the shelf and shelfbreak, 

and the spatially different dynamical regimes that the drifters moved across. These include the 

Chukchi Sea Shelf, the inner shelf (water depths <20 m) of the Chukchi Sea, Barrow Canyon, the 

Chukchi-Beaufort shelfbreak and continental slopes, the Beaufort Shelf, and the Canada Basin. 

Our results were also subject to interannual variations in winds, sea-ice distribution, and 

hydrographic conditions.

Winds between August and November in each deployment year were primarily from the 

northeast, which favor coastal upwelling along the Chukchi Sea coast. There was, however, 

considerable interannual variability on both seasonal and synoptic timescales. In particular,

northeasterly winds were dominant from August through October in 2011 and 2013 when, 

compared to 2012, there was relatively little synoptic variability. In November of 2011 and 2013,

the wind regime included nearly equal frequencies of southerly and northerly winds and was 

much more variable than in previous months. In contrast, the winds in 2012 were from southerly 

quadrants in August and through much of October, but northeasterly in September. While there 

was considerable directional variability in the winds in November of 2012, the wind speeds were 

generally <5 m s-1 and much weaker than the same period in 2011 and 2013.

The ice cover also differed amongst these years. In 2011, the shelf was virtually ice free by midAugust and remained so until early November, when ice started forming along the northwest 

coast of Alaska. In 2012 and 2013, sea ice extended along 71°N between the Central Channel

and the coast until mid-August and then retreated and remained over Hanna Shoal into mid158
September of 2012 and early September of 2013. Sea ice advanced more rapidly compared to 

2011 in both of these years. In 2012, the shelf north of 68.5°N was covered with ice by midNovember, whereas in 2013 ice advanced as far south as 71°N by this date.

These differences in ice extent were reflected in the hydrography. In 2011, there was virtually no 

MW detected on the northeastern shelf and WW was confined to the region around the 

southeastern and eastern sides of Hanna Shoal. Throughout the rest of the area warmer, and 

moderately saline BSW occurred. In particular, west of Hanna Shoal weakly stratified BSW was 

present in the Central Channel. In 2013, the Central Channel waters were the warmest observed 

over the northeastern shelf. These warmer waters reflect the northward flow typical within the 

Channel, which serves as an important conduit for waters flowing northward from Bering Strait. 

In both 2012 and 2013, MW was prevalent north of about 71.5°N throughout August and

September and WW extended across much of the northeastern shelf. These differences in water 

mass distribution profoundly affected the shelf stratification and frontal formation. North of 

~71.5°N, the shelf was heavily stratified and a prominent upper ocean front straddled this 

latitude in both 2012 and 2013. Not surprisingly, these fronts were also approximately aligned 

with the August ice-edge in both years. In 2011, no surface fronts were evident and the 

stratification was much weaker overall. With respect to the shallow-drogued drifters used in this 

project, the stratification affects the efficacy by which the wind’s momentum is mixed into the 

water column. Under the strongly stratified conditions associated with regions where MW 

overlies WW, the pycnocline is confined to the upper 10 – 15 m and strong vertical shears are 

expected. Under the more weakly stratified conditions found elsewhere (or at other times) on the 

shelf, the vertical shear should be weaker. Consequently, we expected to find greater divergence 

between drifters drogued at different depths where the stratification is strong.

The hydrographic data from 2013 indicated that much of the nearshore hydrographic structure 

was consistent with the upwelling-favorable winds that prevailed over the northeast Chukchi Sea 

in August and September of that year. These winds resulted in the offshore displacement of 

relatively warm and fresh ACW above colder, denser WW. Some of this dense water appeared to 

have been upwelling within at least 20 km of the coast.

Drifters deployed near the coast under upwelling-favorable winds moved quite differently from 

those deployed under non-upwelling conditions. In this regard, the drifters deployed in 2013 in 

the Northeast Chukchi Sea were substantially different from those deployed in August of 2012 

under southwesterly winds. In that year, nearly all of the drifters deployed on the shelf and 

nearshore exited the shelf through Barrow Canyon. In 2013, the Wainwright and Pt. Lay drifters 

were all deployed under upwelling-favorable winds that exceeded 6 m s-1 from the northeast. All 

of these drifters moved westward and offshore.

It is of some surprise that the year-to-year differences in water properties and ice cover do not 

appear to depend heavily on the regional winds. For example, we might have expected that the 

159
persistent northeasterly winds in August and September of 2011 would have delayed the arrival 

of BSW into the area and perhaps delayed the retreat of ice. Similarly, these winds might have 

been expected to promote the southward advection of both MW and WW. Clearly this did not 

occur. One might expect also that the southerly winds of August 2012 would have promoted 

northward advection of BSW into the area and yet the northward extent of this water mass was 

far less than in August 2011. One possible reason for these interannual differences in shelf

properties may have been due to changes in the mass and heat transports in Bering Strait. For 

example, in 2011, the northward mass and heat transports through the Strait were anomalously 

high (Woodgate et al., 2012; Danielson et al., 2014) and equivalent to that of 2007, which was 

also a year of very low ice concentrations on the Chukchi Sea Shelf. In contrast the transport in 

2012 was almost half that of 2011. One key difference between these years was that the sea 

surface height difference between the Aleutian Basin in the Bering Sea and the Chukchi Sea 

Shelf was much greater in 2011 than in the latter years (Danielson et al., 2014). This implies that 

the pressure gradient that forces flow northward through the Strait and over the Chukchi Shelf

was stronger in 2011 than in either 2012 or 2013. These transports subsequently decreased 

markedly in 2012 and 2013. Consequently, the pressure field over the Chukchi Sea Shelf was 

different among these years. Interannual changes in this pressure field will also affect the 

magnitude of the wind-forced flow field.

The amount of ice remaining over Hanna Shoal in the summer and fall may be partially a 

function of advection of sea ice into this region from December through April time frame prior to 

the open water season. If heavily deformed ice is advected onto and grounded over the Shoal, it 

may provide the basis for additional ice aggregation and deformation throughout winter. Indeed, 

there was grounded ice atop Hanna Shoal in both the winters of 2012 and 2013, with this ice 

persisting through summer (Dr. A. Mahoney, UAF, personal communication, January 2015). An 

extensive volume of grounded and melting ice over the Shoal can serve as a reservoir for MW 

that is then distributed over the shelf by winds and/or instabilities generated along the MW/BSW 

front. As a point of emphasis, the summers of 2011 and 2012 stand out as two of the three lowest 

years in terms of Arctic-wide sea ice extent (the other summer being 2007) since the dawn of the 

satellite sea ice record in 1979. Hence, the ice conditions over the northeastern Chukchi Shelf in 

summer appear to be largely a local phenomenon. The hydrographic conditions in this region are 

thus a function of local processes (fronts, winds, and the bathymetry with respect to trapping ice 

and MW) and remote (in both time and space) processes that affect heat, salt, and ice transports.

The prominent frontal structures present in 2012 and 2013 are prone to baroclinic instabilities 

that give rise to mesoscale meanders and eddies as mentioned previously. We have run a simple 

model (Lu et al., submitted) that supports our contention that these mesoscale features were 

operating in 2012 and 2013. This is indicated by Figure 143, which shows the model results 

stemming from two simple manifestations of the frontal and stratification structure of the region. 

These examples are based on the Rutgers Ocean Modeling System (ROMS) configured on an fplane for a 150 km x 150 km channel, with a constant depth of 50 m. The meridional (y) walls 

160
are impermeable, the zonal (x) boundaries are periodic, the horizontal and vertical resolutions are 

500 m and 1 m, respectively, and the integration time step is 1 min. Bottom friction is quadratic 

with a drag coefficient of CD = 5 x 10-4 m s-1. The vertical diffusivity is 5 x 10-6 m2 s

-1 and the 

horizontal diffusivity was set to minimal values to suppress numerical noise. We conducted two 

experiments that differed only with respect to their initial density (ρ) distribution (Figure 143f). 

Model 1 (M1) initially has a strongly stratified 2-layer structure confined to the northern half (y > 

75 km) of the domain. This includes a front separating MW in the upper 20 m and WW in the 

lower 30 m from a 50 m deep layer of unstratified BSW in the southern half of the channel (km 0 

– 75). The MW properties are θ = 0 °C, S = 28, and ρ = 1022.5 kg m–3. BSW properties are θ = 

5°C and S = 30 and, ρ = 1024 kg m–3, while the properties of the WW are θ = 0 °C, S = 33 and ρ

= 1026.5 kg m–3. The water properties in Model 2 (M2) are identical to those in M1, but the 

spatial structure differs in having a 2-layer structure that consists of WW extending across the 

entire width of the channel. MW occupies the upper 20 m over the northern half of the channel 

and BSW occupies the upper 20 m over the southern half.

Both models were run for 30 days. Unstable frontal waves, meanders, and dipole eddies develop 

after a few days, but for brevity we show only plan views of the temperature and velocity fields 

at the surface and 20 m for M1 (Figures 143a, b) and M2 (Figures 143d, e). These fields are 

shown also at 38 m for M1 (Figure 143c) but not for M2 where motions are extremely feeble 

and temperatures remain constant. The plots are from Day 25, before eddies reach the channel 

walls. Both models produce cyclonic and anticyclonic eddies with horizontal length-scales of 5–

20 km and azimuthal velocities of 0.05–0.25 m s–1

. For M1, the eddy signatures extend 

throughout the water column, and the horizontal velocities are strongly sheared throughout the 

water column. In contrast, the eddy motions for M2 are confined to depths above the WW. The 

M1 eddies generally have higher velocities than those of M2 and there is considerable velocity 

shear over the water column. In both models, cyclonic eddies carry MW southward into the 

region initially occupied by BSW, while cyclonic and anti-cyclonic eddies transport BSW 

northward beneath the front and in the pycnocline. In M1, the eddies transport WW southward 

across the front into the region initially occupied by BSW. The model also indicates that there 

are large horizontal velocity gradients especially around the frontal regions. These large 

gradients are reflected in the relatively large magnitudes of the deformation tensor noted in the 

Burger (Figure 98) and the Pt. Lay Inshore (Figure 106) clusters.

Vertical cross-channel sections of temperature and salinity for M1 (Figures 144a, b) and M2 

(Figures 144c, d) were made on Day 25 along km 84 (indicated by the dashed line in Figure 3f). 

The sections show a series of warm intrusions (associated with the eddies and filaments of 

Figure 144) penetrating laterally along the pycnocline between MW and WW. In approximate 

agreement with the observations, the intrusions vary in thickness from 5 – 25 m and in width 

from ~5 to ~20 km.

161
Figure 143. Plan views of temperature and velocity vectors for the M1 (143 a-c) and M2 (143d-e) on Day 25 at the 

depths indicated. Figure 143f shows the initial stratification conditions for M1 and M2. The initial location of the 

front is at y = 75 km (red line) and the location of the model cross-sections depicted in Figures 144a–d is at x = 84 

km (dashed red line).

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Figure 144. Cross-channel sections of temperature and salinity, overlain with contours, at along-channel 

distance km 84 from M1 (a and b) and M2 (c and d) on Day 25. 

Recall that the wind-drifter correlations of Section IVf showed that the various drifter types used 

in the study behaved differently from one another. There were also regional differences in the 

correlations amongst the same drifter types. For example, the iSpheres were largely controlled by 

the surface winds. On the Chukchi and Beaufort Shelves, the local winds accounted for ~70% of 

the iSphere velocity variance. Within Barrow Canyon and over the continental slope, the winds 

accounted for from 50 – 60% of the iSphere velocity variance. In contrast, for the MS (1-m 

drogued) drifters the winds accounted for 35% to 60% of the velocity variance on the Chukchi 

and Beaufort shelves, whereas for the SVP drifters the winds accounted for ~30% of the velocity 

variance on the Beaufort Shelf and within the shallow (<30 m depth) Alaska Coastal domain of 

the Chukchi Sea. Over deeper portions of the Chukchi Shelf, the winds account for 35% and 

13% of the velocity variances for the MS and SVP drifters, respectively. These differences imply 

that other forces are operating on the flow fields that propel the drifters.

A number of processes affect these in addition to the stratification and mesoscale motions 

associated with fronts discussed previously. Our review of the hydrography clearly indicated that 

there are large spatial gradients in stratification over the shelf and that the stratification can vary 

seasonally as well as from year-to-year. There are, in addition, time-varying background 

geostrophic flows established by wind-driven convergences and divergences. Weingartner et al.

(2013a) found that the sub-inertial flow is largely geostrophic and that the instantaneous winds 

were not significant contributors to the local, vertically-averaged momentum balance. Instead,

the geostrophic flow varies in time due to sea level slopes over the Chukchi Sea Shelf that are 

σθ 

163
generated by wind-driven convergences and divergences as well as the mean background flow 

described with respect to Figure 1. The magnitude and the direction of the sea-level slopes (and 

associated geostrophic flows) vary with wind speed, direction and the bathymetry. The sea-level 

slopes across and along Barrow Canyon and the Central Channel differ from, and, in general, are 

greater than those over more gently sloping regions of the shelf. The differences in the windcurrent correlations in Barrow Canyon compared to other regions of the shelf are clearly a 

manifestation of bathymetric influences on the background circulation. In addition, we noted that 

the Pt. Lay drifters and some of the NE Hanna Shoal drifters often drifted northward in the 

Central Channel even when winds were blowing to the south. The northward drift in the Channel 

is consistent with prior observations, which indicate the flow here is often northward even when 

northerly winds prevail (Weingartner et al., 2005b; Weingartner et al., 2013a). In addition to the 

shelf-wide winds, Weingartner et al. (2013b) and Danielson et al. (2014) found that the subinertial flow (and sea level slopes) within Barrow Canyon and the Alaskan Coastal domains (and 

to a lesser extent the Chukchi Shelf domain) is affected by remotely forced barotropic 

continental shelf waves propagating northward from the southern Chukchi and/or Bering seas 

along the west coast of Alaska. Although these are trapped waves, with an e-folding cross-shore 

decay scale proportion to ~140 km (the barotropic Rossby radius of deformation) some of the 

wave energy may be scattered by coastline protrusions such as at Pt. Hope and/or Cape Lisburne) 

onto the central shelf. (An assessment of these hypothesized processes would require more 

complete numerical model experiments.) These waves are almost certainly operating over the 

Beaufort Sea Shelf as well. Over continental slope, the drifters also respond to sub-inertial, 

quasi-geostrophic motions associated with barotropic and baroclinic shelf waves that propagate 

along the shelfbreak and barotropic fluctuations in the Beaufort gyre.

The bathymetry also appears to influence drifter trajectories elsewhere. For example, many of 

the drogued drifters expelled from Barrow Canyon proceeded either to the west or to the east 

along the shelfbreak. These trajectories reflect the steeply sloping shelfbreak that influence the 

width and speed of shelfbreak jets (Pickart, 2004; Nikolopoulos et al., 2009). These jets can 

become unstable and shed both cyclonic and anticyclonic eddies (Spall et al., 2008; Watanabe, 

2011; Von Appen and Pickart, 2012). Indeed several of the drifter trajectories clearly revealed 

the presence of these eddies, which carried the drifters across the continental slope and into the 

basin. Recall that the wind-current correlations for drifters in the Canada Basin were generally 

among the weakest found in our data. Although to some extent, these poor results are due to the 

small sample size, the eddy motions arise due to dynamics unrelated directly to the winds and so 

contribute to the weak correlation.

Another bathymetric influence on the drifter trajectories appears to occur at the mouth of Barrow 

Canyon. Recall that for the most part, the drogued drifters moved rapidly through the canyon. 

Once at the mouth, the trajectories tended to bifurcate with some dispersing to the east and others 

to the west (cf. Figures 46 – 52). The mouth of the canyon may, in fact, be an effective dispersal 

agent. In general the along-canyon flow is horizontally sheared (Pickart et al., 2005; 

164
Weingartner et al., 2013a) which may lead to the formation of eddies and/or meanders that 

propagate out of the canyon (D’Asaro, 1988). The along-shelf bathymetric changes associated 

with the canyon indentation, and the abrupt change in shelf-width between the Chukchi and 

Beaufort Seas’ mouth, may lend this area to shelf-wave scattering (Yankovsky and Chapman, 

1995). An additional mechanism that could enhance dispersal here may be related to the collision 

of the down-canyon currents with the nearly zonal flows along the shelfbreak.

One of the project goals was to determine if the surface (upper 1 m) and near-surface (10 –15 m) 

circulation field diverges and differs from the sub-surface circulation as captured by current 

meter measurements by Weingartner et al. (2005b) and Weingartner et al. (2013a). We cannot 

provide an unequivocal conclusion on this point. Clearly the eastward drift of the MS and SVP 

drifters in August from the central Chukchi Shelf toward the coast and Barrow Canyon is similar 

to what has been inferred from sub-surface current meter measurements for this portion of the 

shelf. Similarly, the 2011 drifters that moved northward in the Central Channel were also 

consistent with sub-surface current measurements for this region. On the other hand, the MS and 

SVP drifters deployed to the southeast of Hanna Shoal executed trajectories in areas where there 

were no prior sub-surface current measurements so such comparisons cannot be made at this 

time. Moreover, these deployments were made in heavily stratified waters in which we cannot 

assume that the upper ocean and sub-surface waters are moving uniformly. Indeed, Weingartner 

et al., (2013b) suggested that this area may have substantial velocity shear throughout the water 

column.

The results obtained from deployments of clusters of drifters are useful in understanding and 

quantifying particle dispersion (be this dissolved or suspended materials, pollutants, or 

planktonic organisms). The results presented here, albeit limited by the relatively small number 

of clusters (five clusters of ~10 drifters each, deployed in 2012 and 2013) used in the estimates, 

serve as a beginning climatology on the relative dispersion characteristics of the Chukchi Shelf. 

Moreover, the drifters released in the northeast Chukchi Sea generally remain on this shelf for 

~30 days before either moving out onto the slope via Barrow Canyon (as in 2012) or westward 

and southward (as in 2013). This reflects the rapid advective time scales of the shelf circulation 

in late summer and fall. The results imply that we may be able to attain relative dispersion 

estimates only over a brief time period (<40 days), which is relatively short compared to other 

studies (Swenson and Niiler, 1996; LaCasce and Ohlmann, 2003). Although tentative, the results 

suggest a Lagrangian decorrelation time scale of ~2 days, after which the total relative dispersion 

grows as a power law according to (time)2.5. Our results indicate that relative dispersion is 

anisotropic, with the zonal relative diffusivity being twice as large as the meridional diffusivity. 

These findings will be strengthened statistically based on the results from the 2014 drifter 

deployments that involved ~70 more drifters on the Chukchi Sea Shelf. Once obtained, these 

results can be compared with dispersion estimates from oil spill trajectory models and serve as 

an important point for evaluating the accuracy of these models. The dispersion results can inform 

improved design of oil spill mitigation procedures by offering estimates of how fast a patch may 

165
spread from a point source. We emphasize that the dispersion characteristics we are developing 

likely only apply to the ice-free season because the scales of the processes that give rise to 

dispersion during the ice-free season may be quite different when sea ice is present. Clustered 

deployments of drifters on the ice surface may be helpful in understanding the characteristics of 

the near surface dispersion in the presence of ice.

Our results are based on a relatively few drifter observations in any one dynamical domain. For 

example, the Chukchi Shelf domain has the largest number of observations, amounting to about 

2000 drifter days. However, the drifters released on the northeast Chukchi Sea Shelf thus far 

generally remain in this area for less than 30 days before moving out onto the slope via Barrow 

Canyon (as in 2012) or westward and southward (as in 2013). This reflects the rapid advective 

time scales of the shelf circulation in late summer and fall. We expect greater statistical 

confidence in the results presented here as data become available from subsequent drifter 

deployments. In this regard, BOEM deployed ~30 MS drifters in summer 2014 in the Burger 

area and the North Slope Borough-Shell Baseline Studies program deployed ~104 drifters in 

2014 in the Coastal domain of the Chukchi Sea and on the Beaufort Sea Shelf. Each of the 2014 

deployments included clusters of 10 – 26 drifters and should inform better estimates of the 

dispersion characteristics for these regions.

When viewed in aggregate, the drifter trajectories indicate the multiplicity of potential pathways 

by which waters from the Northeast Chukchi Sea may spread. Drifters ranged as far to the east as 

the Mackenzie Shelf and as far to the west as to the shelf north of Wrangel Island and Herald 

Valley. Several also drifted northward over the Northwind Ridge between 75 and 76°N, while 

others drifted southward to the Chukotkan Peninsula. Most of these excursions took place within 

3 – 4 months of deployment, which is a testament to the advectively vigorous nature of this shelf

and its adjacent areas. The results imply that the residence time of surface waters on this shelf is 

no more than a few months at least.

VI. Future Recommendations with Respect to Drifter Studies

1. As stated previously, the surface circulation and dispersion characteristics discussed in 

this study are unlikely to apply to the drift of ice and so a fundamental question arises as 

to how the ice drift and dispersion differs from the open water measurements. At present 

the Coastal Marine Institute is conducting a study of using inexpensive prototype ice 

drifters that can easily by deployed by snow machine and/or helicopter. Assuming that 

these work as designed, we suggest that clustered deployments using these drifters be 

undertaken. Deployments should be made on both the landfast ice and in the pack ice. 

Given the large interannual variability in this region, consideration should be given to 

making multi-year deployments as these will contribute to the understanding of 

variability on this time scale and toward building a dispersion climatology.

166
2. We were unable to satisfactorily determine if there is divergence between the surface (1-

m depth) and subsurface (10-m) depth motion, although our contention is that this occurs 

in the heavily stratified MW/WW areas and in the frontal regions separating BSW from 

MW. Drifter deployments in such regions are recommended, but these should be 

conducted in conjunction with process studies that include high resolution CTD sampling 

(such as afforded by the Acrobat). A process study devoted to understanding frontal 

dynamics and subduction would be well-served by having a drifter component. Given 

that this exercise well involve shipboard sampling, it may be possible to design the study 

so that the drifters can be recovered and re-used during the study.

3. Ledyard Bay appears to be critical habitat for a number of organisms including those 

important to subsistence hunters. The circulation characteristics of this region have not 

been addressed to date. A limited drifter study would provide useful information in this 

area. Such a study would be inexpensive insofar as the residents of Pt. Lay frequently 

hunt and fish in this region throughout the summer. This program has demonstrated the 

numerous advantages to be had by involving local communities in drifter deployments. A 

Ledyard Bay study would conceivably consist of multiple clustered deployments 

throughout summer and, if possible, ice drifters in winter.

4. There may be interest on the part of the Northwest Arctic Borough in conducting drifter 

studies, similar to those undertaken here, which would expand deployments into the 

Bering Strait, southern Chukchi Sea, and Kotzebue area. Data from such studies would 

provide useful information on transit times from the southern to the northern Chukchi Sea 

and enhance the development of a dispersion climatology for the Chukchi Sea Shelf.

VII. Acknowledgements

This study was supported under cooperative agreement M11AC00001 through the Coastal 

Marine Institute of the University of Alaska Fairbanks in partnership with BOEM. Additional 

support was provided by the North Slope Borough-Shell Baseline Studies Program. Support for 

the drifters and deployment logistics was provided through BOEM cooperative agreement

M12AC00008. We thank the crews of the Norseman II and USCG icebreaker Healy for 

assistance with the deployments and we are grateful to the North Slope Borough Baseline 

Studies Program for their support of the analyses, which added greatly to the value of the results 

presented herein.

VIII. Project Presentations

Weingartner, T., S. Danielson, P. Winsor, and E. Dobbins, Satellite-tracked drifter measurements 

in the Northeast Chukchi Sea. CMI Annual Review, November 2012, Fairbanks, AK.

167
Irvine, C., K. Martini, L. DeSousa, P. Winsor, E. L. Dobbins, S. Danielson, and T. Weingartner, 

Satellite-tracked drifter measurements in the Chukchi and Beaufort Seas. Alaska Marine Science 

Symposium, January 2014, Anchorage, AK.

Winsor, P., T. Weingartner, T., H. Statscewich, R. Potter, E. Dobbins, K. Martini, and S. 

Danielson, High-Resolution Observations of Hydrography and Circulation of the Chukchi Sea. 

Alaska Marine Science Symposium, January 2013, Anchorage, AK.

DeSousa, L., T. Weingartner, P. Winsor, S. Danielson, E. Dobbins, and C. Irvine, Interannual 

Variability in Surface Circulation in the Chukchi and Beaufort Seas: Satellite-tracked Drifter 

Measurements. Alaska Marine Science Symposium, January 2015, Anchorage, AK.

In addition, informal presentations that included some of these data were made to the oil industry 

and to residents of the communities of Barrow, Pt. Lay, Wainwright, Nuiqsuit, and Kaktovik by 

Dr. Weingartner as part of his membership on the NSBW-Shell Baseline Studies Program.

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171
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