OCS Study MMS 2006-031 

Final Report 

Circulation, thermohaline structure, and cross-shelf transport in the 

Alaskan Beaufort Sea 

Principal Investigator: Thomas Weingartner 

School of Fisheries and Ocean Sciences 

University of Alaska 

Fairbanks, AK 99775 

May 2006 
ii 

Contact information 

e-mail: carter@sfos.uaf.edu 

 phone: 907.474.1811 

 fax: 907.474.1188 

 postal: Coastal Marine Institute 

 School of Fisheries and Ocean Sciences 

 P.O. Box 757220 

 University of Alaska Fairbanks 

 Fairbanks, AK 99775-7220
iii 

Table of Contents 

List of Figures..............................................................................................................................................iv 

List of Tables ..............................................................................................................................................vii 

Abstract......................................................................................................................................................viii 

I. Introduction ...............................................................................................................................................1 

II. Objectives................................................................................................................................................9 

III. Methods ...............................................................................................................................................10 

IV. Results..................................................................................................................................................15 

IV.1 Winds...............................................................................................................................................15 

IV.2 Currents...........................................................................................................................................18 

IV.3 Spatial coherence structure of the currents.....................................................................................19 

IV.4 Wind-current relationships..............................................................................................................33 

IV.5 Temperature and Salinity variability...............................................................................................42 

IV.6 Submarine Hydrography .................................................................................................................48 

IV.7 Beaufort Slope Dynamics ................................................................................................................50 

V Summary And Recommendations..........................................................................................................54 

VI. References............................................................................................................................................56 
iv 

List of Figures 

Figure 1: A representative vertical profile of temperature (red) and salinity (blue) obtained along the 

Beaufort Sea continental slope in April 1999 from XCTDs deployed from the USN Hawkbill. The 

halocline represents the region between 50 and 200 m depth over which the salinity, and hence 

density increases rapidly..........................................................................................................................2 

Figure 2: Map of the Alaskan Beaufort Sea and North Slope with place names and subdivisions. .............3 

Figure 3: Regional circulation schematic of the Beaufort Sea illustrating connections to adjacent shelves 

and the Canada Basin. .............................................................................................................................4 

Figure 4: Time-longitude plots of mean monthly a) east-west (~along-slope) and b) north-south (~crossslope) wind component along 72.5o

N. Contour interval is 0.2 m s-1 [Furey, 1998]. Positive values 

indicate winds blowing to the east or north and negative values indicate winds blowing to the west or 

south. .......................................................................................................................................................6 

Figure 5: Upper panel: Mean daily transport through Barrow Canyon (black lines) and 33-day running 

mean of the daily transports (blue line). Lower panel: Mean monthly transports in Bering Strait 

(blue; from Roach et al., 1995) and from Barrow Canyon (red; from Weingartner et al., in prep.).......7 

Figure 6: Vertical profiles (0 – 800 db) of temperature and salinity along the western Beaufort Sea 

continental slope from summer (red) and winter (blue). Summer data provide by K. Shimada 

(JAMSTEC).............................................................................................................................................8 

Figure 7: Map showing the location of the moorings (open circles) and hydrographic stations (centered 

on the numbers). Stations 13 and 19 had faulty XCTDs and are not shown. The dotted line shows 

the approximate location of the submarine’s along-slope transect from which temperature and 

salinity data were collected continuously while underway. ..................................................................10 

Figure 8: Mean monthly Barrow wind speeds for the along-slope (east-west) and cross-slope (northsouth) velocity component. Filled circles are the mean monthly values and bars indicate the monthly 

standard deviation. Positive values indicate winds blowing to the east or north and negative values 

indicate winds blowing to the west or south..........................................................................................15 

Figure 9: Mean monthly ECMWF (71o

 12.8’N, 147o 22.5’W) wind speeds for the along-slope (eastwest) and cross-slope (north-south) velocity component. Filled circles are the mean monthly values 

and bars indicate the monthly standard deviation. Positive values indicate winds blowing to the east 

or north and negative values indicate winds blowing to the west or south. ..........................................16 

Figure 10: 3-day and 15-day running means of the cross-slope (upper panel) and along-slope (lower 

panel) ECMWF winds at 71o

 12.8’N, 147o 22.5’W. .............................................................................17 

Figure 11: Along-slope wind stress estimated from ECMWF winds at 71o

 12.8’N, 147o 22.5’W after 

smoothing with a 3-day (blue) and 15-day (red) running mean. ...........................................................18 

Figure 12: Along-slope (U) velocity at mooring BFK in the western Beaufort Sea...................................22 

Figure 13: Cross-slope (V) velocity at mooring BFK in the western Beaufort Sea....................................23 

Figure 14: Along-slope (U) and cross-slope (V) velocities at mooring B1 in the central Beaufort Sea.....24 

Figure 15: Along-slope (U) velocities at mooring BFS in the central Beaufort Sea. .................................25 

Figure 16: Cross-slope (V) velocities at mooring BFS in the central Beaufort Sea. ..................................26 

Figure 17: Along-slope (U) velocities at mooring B3 in the central Beaufort Sea.....................................27 

Figure 18: Cross-slope (V) velocities at mooring B3 in the central Beaufort Sea......................................28 
v 

Figure 19: Along-slope (U) velocities at mooring B5 in the eastern Beaufort Sea. ...................................29 

Figure 20: Cross-slope (V) velocities at mooring B5 in the eastern Beaufort Sea. ....................................30 

Figure 21: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BFK (western Beaufort Sea) and B1 (central Beaufort Sea). The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

........................................34 

Figure 22: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at 101 m at BFS (central Beaufort Sea) and at 128 m at B5 (eastern Beaufort 

Sea). The horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

.......................35 

Figure 23: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at 254 m at B3 (central Beaufort Sea) and at 227 m at B5 (eastern Beaufort 

Sea). The horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

.......................36 

Figure 24: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BFK (western Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

........................................37 

Figure 25: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B1, 72m (central Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

........................................38 

Figure 26: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BF_S, 101 m (central Beaufort Sea) and along-slope winds at Barrow. 

The horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

.................................39 

Figure 27: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B5, 28 m (eastern Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

........................................40 

Figure 28: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B5, 128 m (eastern Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

........................................41 

Figure 29: Potential temperature-salinity diagram describing water mass nomenclature for waters 

observed at depths < 250 m. ..................................................................................................................42 

Figure 30: Temperature and salinity at mooring BFK in the western Beaufort Sea...................................44 

Figure 31: Temperature and salinity at mooring B1 in the central Beaufort Sea. (No salinity data at 

52 m depth.)...........................................................................................................................................45 

Figure 32: Temperature and salinity at mooring BFS in the central Beaufort Sea. ....................................46 

Figure 33: Temperature and salinity at mooring B3 in the central Beaufort Sea........................................47 

Figure 34: Temperature and salinity at mooring B5 in the eastern Beaufort Sea. ......................................47 

Figure 35: Along-slope temperature (red) and salinity (blue) at 117 m (± 1 m) from the 

temperature/conductivity recorder in the submarine sail, April, 1999. .................................................48 

Figure 36: Along-slope section of temperature (left) and salinity (right) based upon the XCTD data. The 

along-slope XCTD stations are indicated at the top of each figure and their location is shown in 

Figure 7..................................................................................................................................................49 

Figure 37: Along-slope dynamic topography referenced to a) 400 db and b) 900 db based on the alongslope XCTD stations indicated at the top of each figure. The location of these stations is shown in 
vi 

Figure 7. Blue (negative) numbers indicate southward geostrophic flow and red (positive) numbers 

indicate northward geostrophic flow. Velocity units are cm s-1. ..........................................................50 

Figure 38: Composite map of mean currents over the Chukchi shelf and Beaufort slope. Only current 

vectors from ~100 m depth have been used in constructing this figure. ...............................................51 
List of Tables 

Table 1: 1998 Beaufort Sea Mooring Deployments....................................................................................11

Table 2: Record lengths for the western Beaufort mooring array. Current meters and T/C 

recorders.................................................................................................................................................12 

Table 3: Record lengths for the central Beaufort mooring array. Current meters (R) and T/C 

recorders (S)...........................................................................................................................................13

Table 4: Record lengths for the eastern Beaufort mooring array. Current meters (R) and T/C 

recorders (S)...........................................................................................................................................14

Table 5: Current statistics for Western Beaufort Moorings. Results from the principal axis 

decomposition are presented in terms of the variance explained (%) and orientation of the 

major axis (ΘP), the integral time scale is τ , mean values for each velocity component V 

(cross-slope; north-south), U (along-slope; east-west) are in braces (“<>”), and the sample 

variance is denoted s

2

. Italicized values of the velocity components indicate that these are 

significant at the 95% confidence level (from the student’s t-test) after accounting for the 

integral time scale. .................................................................................................................................20

Table 6: Record length current statistics for Central and Eastern Beaufort moorings. ..............................21 

Table 7: EOF results for the along-slope velocity components at the eastern Beaufort mooring 

(B5). The entries in the first four rows of the table are the correlation between the original 

data and the mode. The last row lists the percent variance explained by a given mode. ......................31

Table 8: Except for the EOF results for the cross-slope velocity components at the eastern 

Beaufort mooring (B5). The entries in the first four rows of the table are the correlation 

between the original data and the mode. The last row lists the percent variance explained by 

a given mode..........................................................................................................................................31 

Table 9: EOF results for the along-slope velocity components at the central mooring (B1, BF_S, 

B4). The entries in the first four rows of the table are the correlation between the original 

data and the mode. The last row lists the percent variance explained by a given mode. ......................32 

Table 10: EOF results for the along-slope velocity components at the central mooring (B1, 

BF_S, B4). The entries in the first four rows of the table are the correlation between the 

original data and the mode. The last row lists the percent variance explained by a given 

mode.......................................................................................................................................................32 
viii 

Abstract 

This project was undertaken to investigate 

the along-slope structure and variability in the 

circulation and thermohaline fields of the 

Alaskan Beaufort Sea. This was accomplished 

from five current meter moorings deployed from 

July 1998 – October 1999 along the shelfbreak 

and slope of the western, central, and eastern 

Alaskan Beaufort Sea. The mooring data was 

supplemented by a novel hydrographic data set 

obtained from the USN submarine Hawkbill in 

April 1999. The major findings are: 

1. The mean monthly along-slope component 

of the winds was eastward in all months 

except January 1999, when winds were 

weakly westward. The monthly wind 

variance shows no seasonal variation in 

contrast to climatology, which indicates a 

doubling in variance from summer to winter. 

2. The mean subsurface flow at depths between 

80 and 200 meters is eastward along the 

western and central Beaufort slope. This 

flow is subject to frequent reversals, which 

are accompanied by upwelling, but there is 

no distinct seasonal signal in the shelfbreak 

flow. This finding contrasts with the large 

seasonal signal in transport in Bering Strait 

and over the Chukchi shelf 

3. The spatial coherence of the flow field 

suggests an along-slope decorrelation scale 

of at least 200 km between the western and 

central Beaufort shelfbreak. However, 

along-slope current variations were virtually 

incoherent between the central and eastern 

Beaufort Sea. The cross-slope correlation 

structure suggests that the slope flow field 

has a complicated structure, which was not 

adequately resolved with the sampling 

design and equipment failures. 

4. The coherence between winds and alongslope currents varies as a function of depth 

and distance seaward of the shelfbreak. The 

largest correlations occur for upper level 

currents (≤ ~100 m depth) in the western 

and central Beaufort Sea. Deeper currents 

and currents in the eastern Beaufort showed 

little or no coherence with the winds. The 

low coherence might be explicable in terms 

of seasonal variation in the sea ice 

distribution. It appears that during the 

winter of 1999, the pack ice was immobile 

for extensive periods of time in the eastern 

Beaufort Sea. This implies that little of the 

surface wind stress is transmitted into the 

water column, hence the wind-current 

correlation might be weak due to seasonal 

and shorter period variations in the ice-water 

stress. However, in conjunction with 

conclusion 3, the results suggest that the 

slope flow is largely influenced by the deep 

ocean and/or by remotely forced 

topographic waves. 

5. Our results, along with measurements made 

along the Mackenzie Beaufort slope, suggest 

that the eastward-flowing Beaufort 

undercurrent might not extend beyond the 

eastern portion of the Alaskan Beaufort 

slope. Instead, the current measurements 

and hydrography indicate that the eastern 

Alaskan Beaufort slope might be one of 

along-slope convergence, wherein the 

eastward flow in the undercurrent meets the 

westward flow along the Mackenzie 

shelfbreak. The merged flows are likely 

diverted offshore in the form of eddies. 

6. We emphasize that the eastern Beaufort 

mooring may not be representative of 

conditions at the shelfbreak because the 

continental slope broadens from west to 

east. Consequently, this mooring was 

further seaward of the shelfbreak than the 

moorings to the west. Very likely we have 

missed important shelfbreak processes in 

this region and this caveat must be borne in 

mind when considering the above 

conclusions. 

7. It seems probable that there is a substantial 

along-slope density gradient in the vicinity 

of the Canada/U.S. EEZ line and that this 

contrast extends across the shelf and has 

important implications for cross- and alongshelf flows. 
1 

I. Introduction 

This study examines the subsurface flow 

along the Alaskan Beaufort continental slope 

using moored current meters deployed between 

fall 1998 and 1999 and wintertime hydrography 

collected by the USN submarine Hawkbill in 

April 1999. The slope is the seaward boundary 

for the 80 km wide Alaskan Beaufort Sea shelf 

and slope processes that regulate exchanges 

between the shelf and basin may be dynamically 

important to both the shelf and deep basin. For 

example, Aagaard et al. [1989] suggest that 

during strong westward winds, eastward 

momentum can be upwelled from the slope onto 

the Beaufort Sea shelf, perhaps as far inshore as 

(at least) the 50 m isobath. In such cases, the 

normally wind-forced westward outer shelf flow 

reverses and/or slows. 

In addition, much of the thermohaline and 

biogeochemical structure of the Arctic Ocean’s 

interior bears the signature of waters from its 

adjoining continental shelf seas and from the 

Pacific and Atlantic oceans [Aagaard et al., 

1981; Jones and Anderson, 1986]. Central basin 

waters are therefore largely a result of lateral 

water mass advection across the shelf/slope 

margin and through Bering and Fram straits 

[Coachman and Barnes, 1962; Rudels et al., 

1994]. Over the Chukchi shelf, the mean flow is 

northward and carries Pacific waters into the 

Arctic Ocean. Dynamic principles suggest that 

much of the Chukchi outflow is first deflected 

eastward along the continental slope and 

shelfbreak before entering the central basin. The 

mechanisms and locations by which this water 

enters the basin are not well understood, 

however. 

The circulation in the interior ocean (far from 

the continental shelves and slopes) is weak on 

average and highly variable in space and time. 

Instead, the principal advective signatures 

consist of boundary currents that flow in the 

direction of Kelvin wave propagation (the 

bottom shoals to the right of the direction of 

flow in the northern hemisphere) along the 

continental slopes and mid-ocean ridges 

[Aagaard, 1989; Rudels et al., 1994]. The 

boundary currents are subsurface flows and 

move in a direction opposite to the wind-driven 

surface circulation (which extends to ~50 m 

depth). For this reason Aagaard [1984] called 

the sub-surface currents undercurrents. 

Hydrographic measurements suggest that the 

boundary current along the Alaskan Beaufort 

slope is 10 – 20 km wide [Pickart, 2004] and 

extends over several hundred meters depth. 

Along the Eurasian continental slope these 

currents transport Atlantic Water (AW) that 

enters the eastern Arctic Ocean through Fram 

Strait and across the Barents Sea shelf [Steele et 

al., 1995; McLaughlin et al., 2002; Woodgate et 

al., 2001], mixtures of Barents and Kara shelf 

waters with Atlantic Water [Schauer et al.,

1997] and shelf waters (if sufficiently dense) 

derived from the Laptev and East Siberian seas. 

(The shelf water contributions are complex 

products resulting from river discharge, freezing 

and melting, and shelfbreak upwelling [Melling 

and Moore, 1995; Melling, 1993]. All of these 

processes affect the density of the shelf water 

and therefore the depth to which shelf water 

sinks as it flows across the shelfbreak.) Along 

the Chukchi and Beaufort shelfbreak, the 

boundary current incorporates Pacific Ocean 

waters modified in transit across the Bering and 

Chukchi shelves [Aagaard, 1984; Carmack, 

1986; Weingartner et al., 1998; Pickart, 2004; 

Pickart et al., accepted; Weingartner et al., 

accepted; Woodgate et al., accepted]. Thus, the 

boundary current transports water masses from a 

variety of sources with each water mass having 

its own distinct water property signature (which 

is often distinguishable on the basis of 

temperature-salinity characteristics or chemical 

tracers, such as nutrient concentrations or ratios, 

other naturally occurring dissolved substances, 

or contaminants). The various water masses 

contributing to the boundary flows are of 

different densities, which leads to along-slope 

variations in density structure. For example, the 

Pacific outflow from the Chukchi shelf is, on 

average, less dense than the Atlantic Water 

modified on the Barents and Kara shelves, 

which, in turn, is less dense than Atlantic Water 

flowing through Fram Strait [McLaughlin et al., 

2002]. Consequently, there is a progressive 
2 

thickening of the stratification of the Arctic 

Ocean as one proceeds eastward from Fram 

Strait to the Beaufort Sea [Carmack, 1986]. 

Although the stratification varies seasonally (as 

discussed below), the most prominent (and 

perennial feature) aspect of the Arctic Ocean’s 

stratification is the halocline between 75 and 

200 m depth [Figure 1]. Stratification and 

stratification gradients can affect boundary 

current dynamics and the exchange of water and 

momentum between the shelf and slope 

[Huthnance, 1995]. The Alaskan Beaufort 

continental slope extends 600 km eastward from 

the Chukchi Sea to the Mackenzie continental 

shelf in the Canadian Exclusive Economic Zone 

(Figure 2). The bottom slope and width of the 

continental slope (as determined by the distance 

between the 100 and 1000 m isobaths) varies 

from ~0.05 and 20 km in the west to ~0.02 and 

~50 km in the east and near the Mackenzie shelf. 

Seaward of the 3000 m isobath, depths more 

gradually decrease to more than 4000 m. 

Figure 1: A representative vertical profile of temperature (red) and salinity (blue) obtained along the 

Beaufort Sea continental slope in April 1999 from XCTDs deployed from the USN Hawkbill. The 

halocline represents the region between 50 and 200 m depth over which the salinity, and hence density, 

increases rapidly.
3 

 

Figure 2: Map of the Alaskan Beaufort Sea and North Slope with place names and subdivisions.

Figure 3 is a schematic of the regional 

circulation based upon many of the references 

cited above. The near-surface (0 – 50 m depth, 

at least) circulation over the continental slope is 

westward and consists of the southern limb of 

the wind-forced Beaufort Gyre. Between 50 and 

175 m, the flow is eastward and transports 

waters derived from the Chukchi Sea shelf. 

(Hereafter, we follow Aagaard’s [1984] 

terminology and refer to this eastward 

subsurface flow as the undercurrent.) These 

waters originate in the Pacific Ocean (Bering 

Sea) and flow northward through Bering Strait 

at a long-term mean rate of 0.8 Sv [Sverdrups; 1 

Sv = 106

 m3 s-1; Roach et al. 1995]. North of the 

strait, the Pacific waters flow across the Chukchi 

shelf along three principal pathways associated 

with major topographic depressions in the 

western, eastern, and central Chukchi Sea 

[Weingartner et al., accepted; Woodgate et al.,

accepted]. The density of these outflows varies 

seasonally, but the denser fractions, formed in 

winter through cooling and brine enhancement 

during ice formation, feed the shallower portion 

of the undercurrent and upper halocline. 

Kuparuk 

River 
4 

 

Figure 3: Regional circulation schematic of the Beaufort Sea illustrating connections to adjacent shelves 

and the Canada Basin.

Of particular relevance to the present study is 

that the easternmost branch of the Chukchi Sea 

outflow joins the Beaufort slope at the mouth of 

Barrow Canyon, although some of this outflow 

might spill onto the Beaufort Sea shelf as well 

[Aagaard, 1984; Pickart et al., accepted]. 

Waters from the western and central Chukchi 

outflows also appear to flow eastward along the 

slope to eventually join with the outflow from 

Barrow Canyon. Not all of the water exiting the 

Chukchi shelf enters the boundary current, 

however, for Shimada et al. [2001] show that the 

less dense waters leaving the Chukchi shelf in 

summer enter the near-surface layers of the 

Beaufort Gyre and are carried westward and 

thence northwestward into the Canada Basin. In 

addition, the slope current is inherently unstable 

[Pickart, 2004] and therefore capable of 

generating the numerous mesoscale eddies (20 

km diameter and ~100 m vertical extent) that 

populate the interior of the Canada Basin 

[Newton et al., 1974, Manley and Hunkins, 

1985; Muench et al., 2000]. Recent NSFsponsored research conducted by Aagaard, 

Pickart, Weingartner, and Woodgate along the 

northern Chukchi slope suggests that the eddies 

(depicted as small clockwise rotating circles in 

Figure 3) are common features of the slope 
5 

between at least the central Chukchi and western 

Beaufort slope. Whether or not eddy formation 

occurs along the central and eastern portions of 

the Alaskan Beaufort Sea remains unknown. 

Beneath the halocline (Figure 1), salinities 

slowly increase with depth, but temperatures 

increase to a subsurface maximum of ~0.7o

C at 

300 m, and then decrease more slowly to 0.0o

C 

at about 800 m depth. This warm layer 

originates in the Atlantic Ocean and flows 

around the continental slopes and along the 

major topographic ridges of the Arctic Ocean. It 

is modified en route through cooling with 

overlying ice (in the Nansen Basin) and through 

contributions from the Eurasian shelf seas 

[McLaughlin et al., 2002]. Most likely the 

majority of the Atlantic-influenced waters flow 

southward along the Northwind Ridge north of 

the Chukchi Sea and then turn eastward along 

the Beaufort slope (Figure 3). There may also 

be an additional eastward flow of Atlanticmodified water along the Chukchi continental 

slope. The thermohaline characteristics of 

Atlantic waters do not vary seasonally, although 

there have been substantial changes in the 

temperature of these waters in the last decade 

[McLaughlin et al., 2002, 2004]. 

The eastern sector of the Alaskan Beaufort 

continental slope connects to the Mackenzie 

shelf/slope in the vicinity of Mackenzie Canyon 

near 140o

W (Figure 2). Here the along-slope 

isobaths diverge so that the continental slope 

broadens and becomes not as steep. Moreover, 

we expect that the thermohaline properties over 

the slope will also change in this region due to 

the outflow of water from the Mackenzie shelf. 

Mackenzie shelf water is substantially 

influenced by the year-round discharge from the 

Mackenzie River [Carmack et al., 1989; 

Macdonald et al., 1989; Macdonald and 

Carmack, 1991] although its density varies 

seasonally and interannually depending upon the 

winds and the amount of ice produced on the 

Mackenzie shelf [Melling, 1993]. In summer, 

brackish Mackenzie water can spread far across 

the shelf and into the Canada Basin as a shallow 

surface layer (~25 – 50 m thick). The horizontal 

extent of this dispersal probably varies from 

year to year depending upon the strength, 

frequency, and duration of northeasterly 

(upwelling-favorable) winds (which prevail on 

average) over this shelf. Mackenzie shelf waters 

have been detected along the continental slope 

of the Chukchi and western Beaufort Sea as far 

west as 160o

W [Guay and Falkner, 1997; 

MacDonald et al., 1999a]. In years of frequent 

or strong downwelling winds, however, much of 

the Mackenzie River’s summer discharge is 

probably advected northeastward along Banks 

Island or into the Canadian Archipelago 

[Melling, 1993]. In winter, the Mackenzie shelf 

can be a source of moderately saline shelf water 

formed through enhanced ice production (and 

brine expulsion) in the flaw lead that develops 

offshore of the landfast ice edge [Macdonald 

and Carmack, 1991]. This dense water can 

ventilate the upper halocline [Melling, 1993; and 

Melling and Moore, 1995] and, by so doing, 

alter the along-slope density gradient. 

The slope flow is expected to vary seasonally 

given the substantial annual variation in the 

regional winds, the Chukchi shelf transport that 

feeds the undercurrent, and the density of the 

outflowing shelf waters from both
6 

Figure 4: Time-longitude plots of mean monthly a) east-west (~along-slope) and b) north-south (~crossslope) wind component along 72.5o

N. Contour interval is 0.2 m s-1 [Furey, 1998]. Positive values 

indicate winds blowing to the east or north and negative values indicate winds blowing to the west or 

south. 

the Chukchi and Mackenzie shelves. Furey’s

[1996] analysis of the 1981–1993 surface winds 

(derived from the U.S. Navy’s forecast wind 

fields) shows a distinct annual cycle in the 

along-slope winds in both the east-west and 

north-south components (Figure 4) for the 

region between 152.5o

E (East Siberian Sea) to 

125o

W (Mackenzie shelf). The winds are 

primarily zonal year-round and over the Alaskan 

Beaufort Sea they are westward in all seasons 

except in summer when they are weakly 

eastward. Maximum wind speeds occur in fall 

(Nov.–Dec.) and early spring (Apr.–May) and 

are centered over the western Beaufort slope. 

The north-south wind component is weak 

throughout the year and is weakly southward 

from fall through spring and weakly northward 

in summer. While the winds vary over a range 

of time scales shorter than the seasonal, the 

along-slope coherence scale of these variations 

is generally ~1000 km [Furey, 1998]. 

The western Beaufort slope incorporates the 

outflow from Barrow Canyon, with that 

transport varying both seasonally and 

synoptically [Figure 5; Weingartner et al., in 

prep.] in response to the along-slope winds 
7 

[Weingartner et al., 1998; accepted]. The longterm mean transport through the canyon is about 

0.3 Sv (into the Arctic Ocean). It has a 

maximum of ~0.6 Sv in August and a minimum 

(0 Sv) in December. The average transport in 

the canyon is less than half the mean transport 

through Bering Strait, but it varies seasonally 

and in-phase with the Bering Strait transport 

[Figure 5; bottom panel]. Mean daily canyon 

transports can vary rapidly and range from -2 Sv 

(negative values indicate upcanyon transport or 

transport onto the Chukchi shelf) to 1.5 Sv 

downcanyon. 

The density of the Chukchi shelf outflow 

varies seasonally with the saltiest (densest) 

fraction exiting the shelf from winter through 

late summer and the freshest flowing offshelf in 

late summer and fall [Weingartner et al., 1998; 

accepted; Woodgate et al., accepted]. The 

different densities ventilate the continental slope 

at different depths with the fresher components 

mixing with the surface waters over the slope 

while the most saline fraction can ventilate the 

halocline between 100 and 175 m [Weingartner, 

accepted; Woodgate et al., accepted]. In 

summary the transport, temperature, and salinity 

of Chukchi shelf waters (nearly all of which are 

derived from Pacific Ocean waters flowing 

through Bering Strait) vary seasonally and affect 

Figure 5: Upper panel: Mean daily transport through Barrow Canyon (black lines) and 33-day running 

mean of the daily transports (blue line). Lower panel: Mean monthly transports in Bering Strait (blue; 

from Roach et al., 1995) and from Barrow Canyon (red; from Weingartner et al., in prep.). 
8 

conditions along the Alaskan Beaufort slope and 

shelfbreak. 

The differences in the vertical profiles of 

temperature and salinity for winter and summer 

are shown in Figure 6, (both profiles collected 

along the continental slope of the western 

Beaufort Sea in April [winter] and September 

[summer]). The largest seasonal differences are 

in the upper 150 m. In summer, the temperature 

profile shows a complex structure associated 

with interleaving lenses of relatively warm 

water, from 1 – 3o

C, with cold (~-1.0oC). Much 

of the warm water is the inflow of Bering 

Summer Water from the Chukchi shelf (perhaps 

mixed with dilute but warm ice melt waters). 

Salinities range from 29 – 31 in the upper 100 m 

and are fresh relative to winter due to terrestrial 

runoff and ice-melt. Winter temperatures in the 

upper 100 m are within a few tenths of a degree 

of the freezing point. Salinities in the upper 50 

m are vertically uniform at ~31 and rapidly

Figure 6: Vertical profiles (0 – 800 db) of temperature and salinity along the western Beaufort Sea 

continental slope from summer (red) and winter (blue). Summer data provide by K. Shimada 

(JAMSTEC).
9 

increase with depth to about 34.5 at 200 m. The 

lower halocline, as defined by salinities between 

33 – 34, is approximately 35 m shallower in 

winter than in summer. 

To date there has been no systematic 

investigation of undercurrent dynamics in the 

Arctic Ocean, although several hypotheses have 

been advanced to explain its existence. For 

example, Holloway [1992] and Haidvogel and 

Brink [1986] show that eddy-topographic 

interactions can generate rectified (mean) 

currents that flow in the direction of Kelvin 

wave propagation. Of particular relevance to the 

Alaskan Beaufort setting is Gawarkiewicz’s 

[2000] finding that the undercurrent might arise 

through rectification of an eddy flow field 

generated when dense shelf water flows across 

the shelfbreak. As noted above, dense outflows 

of halocline source waters occur nearly yearround from the Chukchi shelf. The 

undercurrents might also arise as a geostrophic 

adjustment of the inflows from the adjacent 

shelves and/or oceans as found for 

undercurrents elsewhere in the global ocean 

[Ponte, 1995; Kundu and McCreary, 1986]. 

Other dynamics might also be involved, 

however. For example, the Beaufort 

undercurrent could be dynamically analogous to 

the poleward flowing countercurrents found 

along the eastern boundaries of mid-latitude 

oceans [McCreary, 1981; Philander and Yoon, 

1982] or due to the existence of an along-slope 

density (or baroclinic pressure) gradient over a 

continental slope, e.g., the “JEBAR” (Joint 

Effect of Baroclinicity and Relief) mechanism 

[Huthnance, 1984, 1995] or “pycnobathic” 

currents [Csanady, 1985]. While it is probably 

that more than one process is operating along the 

Chukchi-Beaufort slope, we show below that 

there is a substantial along-shelf density gradient 

that suggests that JEBAR dynamics could be an 

important component in the dynamics of the 

flow field along the Alaskan Beaufort slope. 

Our results also raise important questions 

regarding the continuity of the undercurrent 

along the eastern Beaufort slope.

II. Objectives 

The purpose of this program was to 

provide a kinematical and, where possible, 

dynamical description of the circulation, 

thermohaline structure, and cross-slope 

transport along the Beaufort Sea shelfbreak 

and slope. The field and data analysis 

portions of the program were designed to 

address the following issues: 

1. Determine the mean transport over 

the outer shelf and slope and the 

cross-slope and vertical scales of the 

mean flow field. 

2. Determine the magnitudes of 

transport variability and the dominant 

temporal and spatial scales associated 

with this variability. 

3. Determine the cross-slope fluxes of 

heat, salt, and momentum. Determine 

if these are related to instabilities 

(eddy generation mechanisms) of the 

along-slope flow. 

4. Determine the relationship between 

observed flow and density variations 

and the surface wind field. 

5. Compare the results obtained from 

the proposed field program with 

those collected in 1987/88 by 

Aagaard et al., [1989], to determine 

if the present set of measurements are 

comparable to those obtained earlier. 

We were only modestly successful in 

achieving these goals due, in part, to 

equipment failure and loss. In particular 

neither estimates of along-slope transport 

nor cross-slope fluxes could be reliably 

made. 
10 

III. Methods 

The field program consisted of two 

distinct elements: an array of six moorings 

sponsored by MMS (and conducted in 

collaboration with the Japan Marine Science 

and Technology Center; JAMSTEC) and a 

winter hydrography program led by 

Weingartner and supported by the Office of 

Naval Research. Figure 7 shows the 

locations of the moorings and the 

hydrographic stations occupied during April 

1999 by the U.S. Navy submarine Hawkbill. 

Six moorings (two, BF-K-98 and BF-S-98, 

were sponsored and prepared by JAMSTEC, 

a matching partner in this program) were 

deployed in September 1998 from the Sir 

Wilfred Laurier, a Canadian Coast Guard 

icebreaker. (Ship logistics constituted 

additional matched support from the 

Canadian Coast Guard.) The original plan 

called for 1 mooring (BF-K-98) to be 

deployed in the western Beaufort Sea and 

for five moorings (B1, BF-S-98, B3, B4 and 

B5) to be deployed across the central 

Beaufort Sea slope. After the first four 

moorings were deployed along the central 

slope, several days of severe weather 

prevented additional work. Although the 

vessel stood by for 2 days awaiting the 

weather to abate, it did not, and scheduling 

requirements necessitated that it continue 

eastward. We therefore elected to deploy 

mooring B5 along about the 1000 m isobath 

over the 

Figure 7: Map showing the location of the moorings (open circles) and hydrographic stations (centered 

on the numbers). Stations 13 and 19 had faulty XCTDs and are not shown. The dotted line shows the 

approximate location of the submarine’s along-slope transect from which temperature and salinity data 

were collected continuously while underway. 
11 

eastern Beaufort slope. All moorings, 

except B4 were recovered in October 1999 

by the Sir Wilfred Laurier. B4 did not 

respond to acoustic release commands and 

poor weather and the ship schedule 

constraints required that we abandon 

mooring search and recovery operations. In 

fall 2000, the Sir Wilfred Laurier once again 

transited eastward along the Alaskan 

Beaufort Sea and spent an additional day 

conducting an unsuccessful search for 

mooring B4. The mooring was likely 

dragged from its position by drifting sea ice 

and either destroyed or moved far outside 

the search grid. It is also possible that the 

batteries failed in the acoustic releases 

(although these had sufficient power for a 

three-year deployment). All the moorings 

were taut-line, subsurface moorings having 

a variety of instruments, including 

temperature-conductivity recorders (Seabird 

SeaCats or MicroCats), acoustic Doppler 

current profilers (ADCP), electromagnetic 

(Aanderaa RCM8 or S4), or Aanderaa 

current meters equipped with rotors (RCM 

4, 5, or 7). The mooring positions, 

deployment depths and the institution 

responsible for the mooring (JAMSTEC, 

UAF, or UW) are listed in Table 1.

Instrument types, record lengths, and data 

quality comments for each instrument are listed 

in Tables 2 – 4. Numerous instrument problems 

plagued the data set, especially with the 

Aanderaa meters equipped with rotors. The two 

most common problems were rotor stalls and/or 

premature battery failure. The former were due 

either to clogging by suspended material or 

because current speeds were below the rotor

threshold speed (~3 cm s-1). There were also 

several failures with the temperature/conductivity recorders resulting in either reduced data 

record length or outright failure of the 

instrument due to leakage upon deployment. 

The number of failures was unusually large 

based on prior experience and suggests an 

unlucky combination of events. However, the 

large number of short Aanderaa record lengths 

might also be due to a batch of bad batteries (all 

batteries were new prior to deployment). Many 

of the Aanderaa RCM 4, 5, and 7s were also old 

(> 15 years old), which might have contributed 

to some of the failures in this instrument pool. 

Finally, we have had only one failure due to 

flooding of a Seacat T/C recorder in over 10 

years of deploying these instruments. 

Winter hydrographic measurements were 

made with expendable conductivitytemperature-depth (XCTD) probes and continuous underway temperature and salinity 
12 

(UTS) measurements from a temperatureconductivity recorder mounted in the 

submarine’s sail. The XCTDs are launched 

through the submarine’s torpedo tubes at depth 

(~110 m). After launch, the probe rises to a 

depth of about 12 m and then begins its descent 

to a nominal terminal depth of 750 m or 

shallower if the bottom is encountered. CTD 

data are transmitted through a wire connected to 

the submarine during the descent. At the end of 

the profile the XCTD wire breaks and the cast is 

completed. The UTS measurements consist of 6 

minute-averaged samples of temperature and 

salinity at a depth of 117 m (±1 m) collected 

along about the 500 m isobath while the 

submarine was in transit. 

Pre- and post-calibrations suggest that the 

salinity measurements are better than 0.03 for 

salinity (using the non-dimensional Practical 

Salinity Scale) and better than 0.01o

C for 

temperature for the Aanderaa instruments and 

better than 0.01 for salinity and <0.01o

C for 

temperature from the SeaCats or MicroCats, 

including the SeaCat mounted on the 

submarine’s sail.
13 
14 

For the western Beaufort mooring (BFK), 

we use the ADCP record for velocities, 

since these are similar to the Aanderaa 

current 

meters, but yielded longer record lengths. 

However, we use the temperature and 

salinity data recorded by the Aanderaas at 

this mooring. 
15 

IV. Results

IV.1 Winds 

The mean monthly east-west (~alongslope) and north-south (~cross-slope) winds 

over the Beaufort Sea slope using the 

observed winds at Barrow and the European 

Center for Medium-Range Weather 

Forecasting (ECMWF) winds from July 

1998 – October 1999 are shown for 

comparison in Figures 8 and 9. The 

monthly statistics (means and standard 

deviations) for the two sites are similar, 

although the ECMWF cross-slope wind 

speeds are generally greater than the 

observed winds at Barrow. Those 

differences may be real or an artifact of the 

boundary layer correction applied to the 

ECMWF geostrophic winds. In agreement 

with climatology, the mean monthly winds 

were primarily westward and maximum 

wind speeds occur in October–November 

and April–May. However, the mean winds 

during summer (July and August) were 

westward, whereas the climatology indicates 

mean eastward winds during these months. 

In fact, only in January 1999 were the 

monthly-averaged winds eastward. 

Moreover, the observed monthly variances 

are more or less uniform throughout the year 

in contrast to the climatology, which 

indicates that wind variance doubles 

between summer and winter [Furey, 1996]. 

Figure 8: Mean monthly Barrow wind speeds for the along-slope (east-west) and cross-slope (northsouth) velocity component. Filled circles are the mean monthly values and bars indicate the monthly 

standard deviation. Positive values indicate winds blowing to the east or north and negative values 

indicate winds blowing to the west or south.
16 

Figure 9: Mean monthly ECMWF (71o 12.8’N, 147o 22.5’W) wind speeds for the along-slope (east-west) 

and cross-slope (north-south) velocity component. Filled circles are the mean monthly values and bars 

indicate the monthly standard deviation. Positive values indicate winds blowing to the east or north and 

negative values indicate winds blowing to the west or south.

The large monthly standard deviations are a 

consequence of shorter-period wind variations 

shown for the along-slope wind velocity and 

wind stress (Figures 10 and 11, respectively). 

Westward winds promote westward surface flow 

and upwelling and eastward winds force 

eastward surface flow and downwelling. The 

strongest westward or upwelling-favorable wind 

stress occurred from October – November 1998, 

April – June 1999, and August 1999. Eastward 

or downwelling favorable wind stress 

magnitudes are smaller and downwelling events 

rarely last for more than 10 days, although, as 

noted above, downwelling favorable winds 

prevailed in January 1999.
17 

Figure 10: 3-day and 15-day running means of the cross-slope (upper panel) and along-slope (lower 

panel) ECMWF winds at 71o

 12.8’N, 147o 22.5’W. 
18 

Figure 11: Along-slope wind stress estimated from ECMWF winds at 71o 12.8’N, 147o 22.5’W after 

smoothing with a 3-day (blue) and 15-day (red) running mean. 

IV.2 Currents 

Record length current statistics are 

summarized in Table 5 (mooring BFK) and 

Table 6 (central moorings, B1, BFS, B3 and 

eastern mooring B5). At the western mooring 

(BFK) the mean flow components are both 

statistically significant with the flow toward the 

east-southeast at a mean speed of ~5 cm s-1. The 

mean southward velocity of ~1 cm s-1 (not 

statistically significant) is nevertheless 

consistent with the dynamic topography 

obtained from the submarine hydrography 

(shown later), which indicates a mean southward 

geostrophic flow of the same magnitude over the 

upper 100 m of the water column. The flow 

varies primarily in the east-west direction as 

indicated by the principal axis projection, with > 

90% of the flow variance aligned in the alongslope direction and parallel to the isobaths. 

Mean flows at the central mooring site are also 

eastward on average, with the majority of the 

flow variance (> 75%) oriented in the alongslope direction. 

For all moorings the principal axis of 

variance is roughly aligned parallel to the 

isobaths, with approximately 80% or more of the 

variance explained by this projection. The 

integral time scales at moorings BFK, B1, and 

BFS are all < 6 days, indicating relatively short 

period current fluctuations. However, at 

mooring B3, the integral time scales are longer, 

suggesting that longer period phenomena 

dominate the motion moving seaward and 

deeper in the water column. 

In summary, both the western and central 

moorings show a mean eastward flow, albeit 

with substantial current variability. Both the 

mean and varying components of the flow 

parallel the isobaths suggesting that the currents 

are largely steered by the topography. 

In contrast to the western and central 

moorings, the east-west velocity components at 

the eastern mooring are not statistically 

significant, except at 476 m, where an eastward 

flow of about 1 cm s-1 is indicated. The northsouth (~cross-slope) flow components are small 

(~1 cm s-1), although significant, and indicate a 

mean northward flow at 28 m depth but a mean 
19 

southward flow at the deeper depths. As with 

the central mooring there is a dichotomy in the 

integral time scales between the shallowest 

meter and the deeper instruments suggesting 

longer period motions at depth and shorter 

period motions at the 28 m. At the three 

shallowest depths only about 65% of the 

variance is aligned along the principle axis of 

variance and the orientation of these axes is in 

the east-northeast sector. Only at 476 m is the 

axis of principle variance oriented toward the 

southeast (as are the isobaths) and here ~87% of 

the variance is accounted for by this projection. 

These features suggest that the topographic 

control on the flow at the eastern site is weaker 

than at the central or western moorings except at 

476 m. This is most likely due to the decreasing 

bathymetric gradient in the continental slope of 

the eastern Beaufort Sea. The lack of the mean 

eastward flow at 128 m and 227 m indicates that 

the undercurrent has disappeared or that is has 

migrated further inshore and closer to the 

shelfbreak. 

In general, the variance decreases with depth 

at all moorings, although the maximum variance 

was observed at ~100 m depth at BFS_R2 along 

the central array. The reason for this is not 

known, although the instrument is located near 

the heart of undercurrent (Aagaard, 1984; 

Pickart, 2004). 

Low-pass filtered velocity component time 

series for each good quality current record are 

shown in Figures 12 – 20. (Note that the 

velocity scales change among plots.) The time 

series illustrate the rich temporal and spatial 

structure in the variability of the along- and 

cross-slope currents. While inspection of the 

various records suggests some coherency in the 

current fluctuations, the overall appearance is 

that correlation structure is complicated and 

variable both spatially and temporally. 

For example, both the along- and cross-slope 

velocity components at mooring BFK in the 

western Beaufort Sea are highly coherent, with 

most of the current variance between 82 – 122 m 

in-phase and coherent. There is, however, no 

strong seasonal signal in these data in contrast to 

the large seasonality identified in the Barrow 

Canyon transport climatology (Figure 5). We 

explore the wind-current relationships below 

after first describing in more detail the spatial 

and temporal structure of the current field. 

IV.3 Spatial coherence structure of the 

currents 

Our description of the spatio-temporal 

structure of the currents relies on empirical 

orthogonal functions (EOFs) and the spectral 

distribution of coherence (γ

2

) and phase (φ). The 

EOFs partition variance of the current time 

series into coherent and independent 

(orthogonal) modes. They are, however, 

incapable of detecting propagating phenomena. 

Consequently, these are not appropriate 

techniques for detecting the along-slope 

structure in current variance if, for example, 

shelf waves and/or topographic Rossby waves 

are present in the data set (which we believe is 

true). Thus we restrict the EOF analyses to 

looking at the vertical structure of the currents 

and/or the cross-slope and vertical structure at 

the central and western moorings only. The 

EOFs computed on the correlation matrix, to 

ensure equal weighting of time series. 

The coherence squared function, γ

2

, 

quantifies the fraction of the variance in one 

time series that is correlated with the second 

series as a function of frequency (or period) 

while the phase function, φ, describes the phase 

relationship as a function of frequency for 

coherent signals. This spectral approach is 

appropriate for detecting propagating 

phenomena.
20 
21 
22 

Figure 12: Along-slope (U) velocity at mooring BFK in the western Beaufort Sea. 
23 

Figure 13: Cross-slope (V) velocity at mooring BFK in the western Beaufort Sea. 
24 

Figure 14: Along-slope (U) and cross-slope (V) velocities at mooring B1 in the central Beaufort Sea.
25 

Figure 15: Along-slope (U) velocities at mooring BFS in the central Beaufort Sea.
26 

Figure 16: Cross-slope (V) velocities at mooring BFS in the central Beaufort Sea. 
27 

Figure 17: Along-slope (U) velocities at mooring B3 in the central Beaufort Sea. 
28 

Figure 18: Cross-slope (V) velocities at mooring B3 in the central Beaufort Sea. 
29 

Figure 19: Along-slope (U) velocities at mooring B5 in the eastern Beaufort Sea. 
30 

Figure 20: Cross-slope (V) velocities at mooring B5 in the eastern Beaufort Sea. 
31 

The EOF results for each velocity component 

for the eastern mooring (B5) are summarized in 

Tables 7 and 8. For the along-slope component 

only the first two modes are statistically 

significant by North’s criterion and together 

these explain 90% of the variance. Mode 1 

(69% of the variance) captures in-phase motions 

throughout the water column, but is most 

strongly correlated with the currents in the upper 

227 m. Mode 2 (22% of the variance) is 

primarily correlated with currents at 28 m and 

128 m depth. For the cross-slope component, 

Mode 1 captures 70% of the variance, with this 

mode once again projecting predominately on 

the three uppermost depths. Mode 2 describes 

out-of-phase motions between the surface deeper 

meters, and correlates best with the cross-slope 

flow at 28 m. However, it is not significant by 

North’s criterion. The EOF results for each 

velocity component for the central moorings 

(B1, BF_S, and B3) are listed in Tables 9 and 

10. For the along-slope component only the first 

mode is statistically significant. It indicates that 

~55% of the along- slope current variance is inphase in the vertical and across the slope. The 

correlations between
32 

this mode and the original data are largest at 

depths less than 250 m indicating that this mode 

primarily projects onto the shallower currents. 

Mode 2 (20% of the variance) largely captures 

variance in the shallow meters closest to the 

shelfbreak. The other modes (not shown) are 

not significant, and generally correlate with only 

one or two depths. For the cross-slope 

component, Mode 1 captures 70% of the 

variance, with this mode once again projecting 

predominately on the three uppermost meters. 

Mode 2 describes out-of-phase motions between 

the surface deeper meters, and correlates best 

with the cross-slope flow at 28 m. However, it 

is not significant by North’s criterion. The 

cross-slope velocity component has a 

complicated structure with the first mode (37% 

of the variance) primarily correlated with 

mooring B3 and the second mode projecting 

onto the shallow instruments at moorings B1 and 

BF_S. 

Overall, the EOF analysis indicates a 

complicated vertical and cross-slope velocity 

structure over the central Beaufort slope as the 

variance is broadly distributed over a number of 

modes that are not well resolved. This further 

suggests that the sampling design (in terms of 

numbers of instruments and cross-slope 

distribution and/or record lengths) was 

insufficient to adequately describe the horizontal 

and vertical resolution of our instruments. On 

the other hand the result for the eastern mooring 
33 

(B5) suggests a relatively simple current pattern 

in the vertical, with most of the current variance 

accounted for by the 1st mode (e.g., the currents 

over the uppermost 500 m coherent) and the 2nd

mode capturing the near surface variability. 

We next consider the results from the 

coherence analysis. Figure 21 reveals that the 

along-slope currents at the shelfbreak between 

the western (BFK) and central Beaufort Sea (B1) 

are generally coherent at periods between 72 and 

240 hours (3 – 10 days) and at periods > 18 

days. At shorter periods the phase is negative 

(when γ

2

 exceeds the 95% significance level) 

indicating eastward phase propagation. For 

example, at periods of ~6 days, the phase lag is 

about 1 day, suggesting that current events at 

this period are propagating eastward at a phase 

speed of ~2.3 m s-1. For periods > 18 days 

currents are in-phase. In contrast, the alongslope currents between B1 and B5 are incoherent 

(not shown) and coherence is small between the 

deeper currents at either BFS or B3 and B5 

(Figures 22 and 23). 

IV. 4 Wind-current relationships 

Figures 24 – 28 summarize the correlation 

(in terms of γ

2

 and φ) between along-slope winds 

(using Barrow winds) and along-slope currents 

for several of the moorings. (Similar results 

were obtained using the ECMWF winds.) 

Winds and currents are coherent for the 

shelfbreak moorings in the western (BFK; 

Figure 24) and central (B1) Beaufort Sea 

(Figure 25) and for the currents at 101 m at 

mooring BFS (Figure 26). However, the windcurrent coherence is stronger at shorter periods 

(< 0 days) over the western slope than over the 

central slope. Winds and along-slope currents 

are generally incoherent for depths > 100 m for 

the central moorings (BFS or B3; not shown) or 

in the western Beaufort Sea (B5 at 28 m; Figure 

27 and at 128 m; Figure 28). 

The statistical analyses suggests that the 

along-slope currents at the shelfbreak (BFK and 

B1) have an along-slope correlation scale of at 

least 200 km, but that seaward of the shelfbreak 

and for depths ±100 m, the along-slope 

correlation scale is < 200 km. Caution should be 

exercised in assuming that these scales hold 

along the entire Beaufort slope, however. For 

example the bathymetric slope in the eastern 

Beaufort Sea is much gentler than in the central 

and western Beaufort and mooring B5 is further 

seaward of the shelfbreak than the moorings to 

the west. Indeed, mooring B5 might not have 

been deployed in the undercurrent and so current 

fluctuations here might not be representative of 

undercurrent variability closer to the shelfbreak. 

In addition, we show in the next section that the 

along-slope density gradient changes between 

the eastern and western Beaufort slope. This 

will alter the dynamics governing the 

undercurrent and degrade the correlation 

between the eastern and central Beaufort slope. 

The wind-current relationship also weakens, 

moving eastward along the slope and deeper into 

the water column. The latter result may suggest 

that deep ocean motion forces remotely 

dominate the subsurface current variations. The 

lack of a coherent response at shallower depths 

at the central and eastern mooring sites is harder 

to understand, however. While these sites may 

also be responding to deeper ocean dynamics 

that are unrelated to local wind stress, the 

reduction in coherence might be associated with 

variations in the regional ice conditions. 

Analysis of 10-day repeat Synthetic Aperture 

Radar (SAR) imagery by A. Mahoney and H. 

Eicken (pers. communication) indicates that the 

pack ice was immobile over the eastern and 

central Beaufort slope for at least 30 days (and 

possibly longer) in April 1999, whereas highly 

mobile pack ice covered the western Beaufort 

slope. The differences in ice mobility should 

result in along-slope variations in the surface 

stress, which would degrade the coherence. 

Mahoney and Eicken show that similar ice 

conditions occurred in the winter of 2000, while 

in the five other years they examined freely 

drifting pack ice extended along the entire 

Beaufort slope. This issue deserves theoretical 

attention because it implies that the wind-current 

response and slope flows could vary 

substantially between years. 
34 

Figure 21: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BFK (western Beaufort Sea) and B1 (central Beaufort Sea). The horizontal 

red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
35 

Figure 22: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at 101 m at BFS (central Beaufort Sea) and at 128 m at B5 (eastern Beaufort 

Sea). The horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
36 

Figure 23: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at 254 m at B3 (central Beaufort Sea) and at 227 m at B5 (eastern Beaufort Sea). 

The horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
37 

Figure 24: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BFK (western Beaufort Sea) and along-slope winds at Barrow. The horizontal 

red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
38 

Figure 25: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B1, 72 m (central Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
39 

Figure 26: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at BF_S, 101 m (central Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
40 

Figure 27: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B5, 28 m (eastern Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
41 

Figure 28: Coherence squared (γ

2

; upper panel) and phase (φ; lower panel) as a function of frequency for 

the along-slope currents at B5, 128 m (eastern Beaufort Sea) and along-slope winds at Barrow. The 

horizontal red line on the γ

2

 plot indicates the 95% significance level for γ

2

. 
42 

IV. 5 Temperature and Salinity variability 

There are four broad water mass classes 

observed at the moorings, of which three are 

shown in Figure 29. Bering Sea Summer 

Water, which occurs above the halocline has 

temperatures > 0o

C and salinities ±32.2. 

Winter–Modified Shelf Water, occupies the 

upper halocline and has temperatures at or near 

the freezing point and a salinity range of 32 – 

33.1. Atlantic-Modified Water occupies the 

lower halocline with a temperature range of -

1.5o

C to 0.5oC and salinities > 33.1. The fourth 

water mass, observed only at mooring B5 at 28 

m depth (Figure 33), is Arctic Ocean mixed 

layer water (salinities < 31.2 and temperatures 

generally < 0o

C). With this classification and the 

vertical profiles of temperature and salinity 

shown in Figure 6 as guides, the time series 

plots of temperature and salinity from each 

instrument (Figures 30 – 33) are more

Figure 29: Potential temperature-salinity diagram describing water mass nomenclature for waters 

observed at depths ≤250m.
43 

easily understood. These show considerable 

complexity and variability among sites, although 

several general conclusions can be drawn. First, 

the warmer fractions of Bering Sea Summer 

Water are only observed at the western (BFK) 

and central (B1) shelfbreak moorings at depths < 

73 m. Although present at BFK in August 1998 

and August – September 1999, it occurred at B1 

only in September 1999. It appeared at B1 

approximately 30 days after its first appearance 

at BFK in August 1999, with this time difference 

consistent with the mean flow along the 

shelfbreak being 6 – 7 cm s-1. There are also 

several prominent upwelling events in which 

Modified Atlantic Water was upwelled. These 

events appear as rapid increases in both 

temperature and salinity at BFK (104 and 116 

m; Figure 30) and at mooring BFS (101 and 133 

m; Figure 31) in October and late November – 

December 1998 and April and May 1999 and are 

most likely related to the strong upwellingfavorable wind pulses during these months. 

While the fall 1998 upwelling signals are 

evident in the shelfbreak moorings (BFK and 

B1), they are not reflected at 73 m depth at 

mooring BFS (Figure 32), which is seaward of 

the shelfbreak. This suggests that upwelling into 

the near-surface layer is stronger at the 

shelfbreak than further offshore. If we assume 

that the temperature change at 133 m at mooring 

BFS in October is solely due to upwelling (no 

horizontal advection and/or mixing) then the 

vertical velocity was ~4.5 m/day at this location. 

These are relatively large vertical velocities, 

being comparable to the vertical velocities 

observed during moderately strong upwelling 

events on mid-latitude shelves. 

The upwelling episodes discussed above are 

associated with very low-frequency 

displacements of the lower halocline (water 

depths > 190 m) evident at moorings BFS and 

B3. For example, at mooring BFS the lower 

halocline waters upwelled from September – 

November, 1999, March – May 1999 and 

August – September 1999, when westward wind 

stress was greatest. The downwelling periods of 

December 1998 – January 1999 and June – July 

1999 were associated with westward winds 

(December – January) or relaxation in westward 

wind stress (June – July). We note, however 

that we do not find any consistent relationship 

between upwelling of the undercurrent and 

along-shelf currents at the shelfbreak. During 

some upwelling events the shelfbreak currents 

accelerate eastward (suggesting vertical 

advection of eastward momentum) while other 

events are associated with westward 

acceleration. 

Temperature and salinity variations over the 

eastern Beaufort slope (mooring B5; Figure 34) 

differ substantially from those to the east. For 

example, both the high and low-frequency 

vertical displacements of the halocline are 

largely suppressed throughout the record. The 

reason for this is not at all clear, for even in the 

absence of a local wind stress we might have 

expected substantial halocline variability in the 

eastern Beaufort Sea due to eastward 

propagating topographic waves. The SeaCat 

record at 28 m shows no evidence of upwelling 

or Bering Sea Summer Water. The principle 

seasonal transitions at 28 m are the rapid cooling 

(from 0o

C to -1.5oC) and freshening (from 32.3 

to 28.5) that began in late November 1998 and 

continued through January 1999 followed by a 

rapid salinity increase in February. These 

transitions appear to be associated with 

horizontal advection. The cooling and 

freshening event occurred during a period of 

strong westward flow, whereas the salinity 

increase occurred in conjunction with 

southeastward flow. 
44 

Figure 30: Temperature and salinity at mooring BFK in the western Beaufort Sea. 
45 

Figure 31: Temperature and salinity at mooring B1 in the central Beaufort Sea. (No salinity data at 52 m 

depth.) 
46 

Figure 32: Temperature and salinity at mooring BFS in the central Beaufort Sea. 
47 

Figure 33: Temperature and salinity at mooring B3 in the central Beaufort Sea. 

Figure 34: Temperature and salinity at mooring B5 in the eastern Beaufort Sea. 
48 

IV.6 Submarine Hydrography 

Submarine hydrographic data are used to 

illustrate the along-slope thermohaline (and 

density) structure, which is an important 

component in the dynamics of the along-slope 

flow. Figure 35 shows the along-slope 

temperature and salinity structure collected by 

the temperature-conductivity recorder in the 

submarine sail during transit along the slope. 

The measurements were made in the upper 

halocline at 117 m (± 1 m) depth between 

longitudes 159o

W (eastern end of the Chukchi 

Sea slope) and 142o

W (eastern end of the 

Beaufort Sea slope). Two prominent spatial 

scales of variability are evident. The short-scale 

variations are primarily concentrated at ~30 km 

(as determined from spectral analyses, not 

shown). Interestingly, the along-slope bottom 

topography also exhibits a spectral peak at ~30 

km suggesting that the short-wavelength 

fluctuations might be internal waves generated 

by flow over the topography, although inertial 

waves and/or eddies might also contribute to the 

small scale fluctuations. The vertical excursions 

of the isopycnals associated with these smaller 

scale fluctuations are about 10 – 15 m as 

estimated from neighboring XCTD profiles 

similar to those shown in Figure 1. The larger 

scale mode of variability is associated with the 

eastward increase in salinity from 32.8 to 33.2 

between 159o

W to 147oW and suggests that the 

halocline shoals over this portion of the slope 

and then deepens to the east. 

This large-scale change is also present at 

greater depths as evident in the along-slope 

profiles of temperature and salinity (Figure 36). 

Temperatures are between -1.4o

C and -1.7oC in 

the upper 100 m and then increase to ~0o

C at 

225 m depth near the base of the halocline. The 

0o

C and -1.0oC isotherms both bow upward by 

~25 m over the central portion of the slope and 

then plunge downwards at the eastern and 

western ends of the transect. The deeper 

isotherms are flatter, although there is east-towest cooling in the warm (~0.5 C) core of the 

Modified Atlantic water centered at ~400 m. 

Figure 35: Along-slope temperature (red) and salinity (blue) at 117 m (± 1 m) from the 

temperature/conductivity recorder in the submarine sail, April, 1999. 
49 

The salinity section also shows that the 32 

isohaline shoals between the western and 

central portions of the section (in agreement 

with the submarine sail data) and the 33 and 

34.5 isohalines bow upwards in the central

Beaufort, similar to the isotherms at ~200 m 

deep. Thus, the along-slope structure of the 

halocline is that of an inverted bowl with the 

halocline depth at the western and eastern 

edges of the bowl ~25 m deeper than the 

inverted base of the bowl over the central 

slope. Later we argue that this structure of 

the halocline has important implications for 

the dynamics of the slope current field. 

Note also that this large-scale 25 m doming 

of the halocline exceeds the range in vertical 

scales of the halocline fluctuations 

associated with the small along-slope 

wavelength eddies and internal waves 

inferred from the submarine sail data. On 

this basis we suggest that the coarser scale 

XCTD sampling is not aliased by these 

smaller scale features. Changes in the depth 

of the halocline imply along-slope density 

and pressure gradients as manifested in the 

dynamic topography (vertically integrated 

density anomaly) referenced to either 400 or 

900 db (Figure 37).

Figure 36: Along-slope section of temperature (left) and salinity (right) based upon the XCTD data. The 

along-slope XCTD stations are indicated at the top of each figure and their location is shown in Figure 7. 
50 

The dynamic topography slopes downward 

from the western to the central Beaufort and 

then upward to the east at depths above 300 m. 

At greater depths the dynamic topography is 

nearly flat. This indicates, then, that the 

pressure gradient is largely associated with 

changes in the depth of the halocline and in 

changes in the density of the polar mixed layer, 

but not with deeper along-slope density 

variations. If, to lowest order, the cross-slope 

flow is in geostrophic balance then there is a 

southward cross-slope geostrophic flow in the 

western Beaufort Sea and a northward crossslope geostrophic flow in the eastern Beaufort 

Sea. These cross-slope flows are weak, 

however, having a magnitude of ~1 cm s-1 above 

250 m depth, which is consistent with the weak 

cross-slope velocities measured by the current 

meters.

Figure 37: Along-slope dynamic topography referenced to a) 400 db and b) 900 db based on the alongslope XCTD stations indicated at the top of each figure. The location of these stations is shown in Figure 

7. Blue (negative) numbers indicate southward geostrophic flow and red (positive) numbers indicate 

northward geostrophic flow. Velocity units are cm s-1. 

IV.7 Beaufort Slope Dynamics 

Figure 38 is a map of record-length mean 

currents ~50 m depth (for the Chukchi Sea) and 

at ~100m depth (along the Beaufort slope). We 

have chosen the 100 m depth as being most 

closely representative of the flow at the 

shelfbreak. The Chukchi measurements are 

from Weingartner et al. [accepted] and 

Woodgate et al. [accepted] and span the period 

1990–1995. The Beaufort slope measurements 

from this report are combined with the current 

measurements made in 1986 – 87 by Aagaard et 

al. [1986]. Mean currents along the Mackenzie 

continental slope are from Kulikov et al. [1998].
51 

Figure 38: Composite map of mean currents over the Chukchi shelf and Beaufort slope. Only current 

vectors from ~100 m depth have been used in constructing this figure.

Although made in different years, in 

aggregate, the results suggest that waters 

flowing from the Chukchi shelf contribute to the 

eastward (subsurface) flow along the Alaskan 

Beaufort slope at least as far east as 144o

W. 

Some caution is required in interpreting the 

flows in the eastern Beaufort (B5) and from 

Kulikov et al., however. For example, B5 might 

not be a good representation of the shelfbreak 

flow since it is further seaward of the shelfbreak 

(owing to the broadening of the continental 

slope) than the measurements to the west. 

Moreover, Kulikov et al.’s measurements near 

the Mackenzie Canyon might suffer from local 

topographic effects associated with the mouth of 

this canyon. Finally, their measurements along 

the Mackenzie slope (~135o

W) are only from 

summer, although they find that the flow here is 

consistently steady and westward. While there 

is some question on how representative some of 

the measurements are of the eastern (and 

Mackenzie) Beaufort slope, the observed 

currents, when combined with the hydrography, 

raise critical questions pertaining to the alongslope continuity of the undercurrent, and in 

particular, the extent to which it continues 

eastward into the Mackenzie Beaufort shelfbreak 

and slope. 

In spite of the considerable spatial and 

temporal variability in the along-slope density 

field our results suggest that the along-slope 

variation in the vertical displacement of the 

halocline along the Chukchi-Beaufort slope is 

that of an inverted bowl, with the halocline 

shallow along the central Beaufort slope and 

deeper to the east and west. Csanady [1985] 

examined theoretically the steady shelfbreak 

flow fields associated with just such an alongslope pycnocline structure and uniform alongslope bathymetry. His solution for the steady 

flow field (with wind stress neglected) is similar 

to that described based on the submarine 

hydrography and consists of: 1) onshore 

geostrophic flow along the eastern 

Chukchi/western Beaufort slope, 2) offshore 

geostrophic flow in the eastern Beaufort, and 3) 

along-slope eastward flow at the shelfbreak that 

connects the two regions. For this case, the 
52 

result suggests that undercurrent waters are 

carried offshore where the halocline deepens in 

the eastern Beaufort Sea. Moreover, Csanady’s

solution indicates a weak westward flow at the 

shelfbreak in the eastern Beaufort Sea. Hill

[1995] examined the response of a steady alongslope barotropic current over a continental slope 

which broadens in the direction of the flow and 

finds that the current also broadens as it flows 

downstream. Both theoretical results are 

consistent with the suggestion that the 

undercurrent weakens as it enters the eastern 

Beaufort Sea. 

Adding an opposing along-slope surface 

wind stress to the along-slope density gradient 

will alter the vertical structure of the flow, but 

not necessarily change the basic dynamics of the 

undercurrent. For a westward wind stress the 

near surface flow will be westward, with the 

westward flow diminishing with depth and 

reversing where the along-slope pressure 

gradient exceeds the stress. The eastward flow 

is eventually balanced by bottom stress so that at 

steady state a vertically sheared flow develops 

that is westward at the surface and eastward at 

depth. We consider this to be the lowest order 

along-slope momentum balance over the 

Beaufort slope. Quite possibly along-slope 

advection of eastward momentum is important in 

the momentum balance in the western Beaufort 

Sea, for it appears from preliminary calculations 

that this term is about the same magnitude as the 

vertically integrated wind stress. In contrast, 

vertical advection of eastward momentum 

appears to be an order of magnitude smaller on 

average. 

Since the along-slope density field appears 

crucial to the eastward flow of the undercurrent 

along the Beaufort slope, it must be maintained 

against the eroding influence of both advection 

and diffusion. For the western Beaufort Sea we 

suggest that the density gradient is maintained 

by the (more or less steady) outflow of 

moderately saline Pacific waters from the 

Chukchi shelf. These primarily affect the upper 

150 m of the slope. This pool of moderately 

saline water is continually replenished by the 

outflow from the Chukchi Sea shelf. There is no 

comparable source of moderately saline water 

that feeds the upper halocline along the Alaskan 

Beaufort shelf. Moreover, it is conceivable that 

the upwelling response to westward winds is 

greater over the central Beaufort, where shelf 

outflows are absent, than over the western 

Beaufort and Chukchi continental slopes. Such 

a response would further enhance the alongslope density gradient between the western and 

central Beaufort slope. 

The reversal in sign of the along-slope 

density gradient might be related to outflows 

from the Mackenzie shelf. The density of this 

outflow varies seasonally and is large and fresh 

in summer due to the massive Mackenzie River 

discharge at this time and by modest salinization 

of the discharge in the flaw lead system that 

forms in winter over this shelf [Melling, 1993; 

MacDonald and Carmack, 1991]. If these 

outflows pool along the Mackenzie continental 

slope, they could effect a reversal in the sign of 

the along-slope density gradient and force a 

westward flow. This hypothesis is consistent 

with the current observations of [Kulikov et al.,

1998] shown in Figure 38. They found a mean 

westward flow at 133 m of ~10 cm s-1 in a water 

depth of 200 m along the bathymetrically 

smooth Mackenzie shelf break (134.5o

W) during 

spring and summer 1987. They also find a mean 

westward flow of ~4 cm s-1 at the mouth of 

Mackenzie Canyon (~138.5o

W) in a water depth 

of 200 m for the period March 1987 – March 

1988. We emphasize, however, that these 

conclusions are tentative for the reasons cited 

earlier. Moreover, there is little long-term 

information from this region so that the 

measurements described herein might be an 

indication of interannual variability and not an 

indicator of mean conditions. If, in fact, the 

slope flows are convergent in the vicinity of the 

Canada-U.S. EEZ boundary, then both 

shelfbreak currents turn offshore, most probably 

decomposing into eddies that propagate into the 

interior. 

On a broader scale our results raise questions 

regarding the pathway that Pacific waters follow 

to Fram Strait, where they are returned to the 

North Atlantic Ocean. Pacific waters appear to 

leave the Arctic Ocean via an eastward flow 

along the north Greenland continental slope 

[Newton and Sotirin, 1997]. It was presumed 
53 

that this flow represented an extension of the 

Beaufort undercurrent, which continued around 

the margin of the Canada Basin. Our findings 

call into question this premise. 

If a large along-slope density gradient is a 

persistent feature of the Beaufort continental 

slope, it is unlikely to influence the nearshore 

circulation because, as Csanady’s [1985] results 

demonstrate, the along-slope pressure gradient is 

attenuated inshore of the shelfbreak, with the 

shelfbreak effectively insulating the nearshore 

from the slope dynamics. This is not 

inconsistent with the results from Weingartner et 

al. [2005] who found that there was no mean 

along-slope pressure gradient on the inner 

Beaufort Sea shelf. Nevertheless, the question 

remains how far inshore the slope pressure field 

might extend. This cannot be addressed with 

our data, although our hypothesis that the alongslope dynamic topography gradient is 

established by outflows of relatively low density 

water from the Chukchi and Mackenzie shelves 

does have implications for the Alaskan Beaufort 

continental shelf. It would appear likely that 

these outflows would result in along-shelf 

density gradients over the Beaufort Sea shelf. 

Most likely this gradient will be largest in the 

western Beaufort Sea due to the persistent 

discharge from the Mackenzie River compared 

to the small (in summer) or negligible (in 

winter) river discharge onto the Alaskan 

Beaufort Sea. While we suspect that most of the 

canyon outflow is constrained by the canyon’s 

topography to continue to the shelfbreak before 

turning eastward, some should also flow 

eastward over the shelf. Very likely the density 

difference between Beaufort and Chukchi shelf 

water is not that large (especially in winter) so 

that the along-shelf density gradient could be 

small. Nevertheless, if these gradients exist they 

could be an important element in the dynamics 

of the Beaufort Sea shelf, for they would tend to 

drive a mean cross-slope flow whose magnitude 

is proportional to the strength of the density 

gradient. For example, in the eastern Beaufort 

Sea the low density water associated with 

Mackenzie shelf water would tend to force an 

offshore flow over the shelf. 
54 

V. Summary and Recommendations 

1. The mean monthly along-slope component 

of the winds was eastward in all months except 

January 1999, when winds were weakly 

westward. The monthly wind variance shows no 

seasonal variation in contrast to climatology, 

which indicates a doubling in variance from 

summer to winter. 

2. The mean subsurface flow, at depths 

between 80 and 200 meters, is eastward along 

the western and central Beaufort slope. This 

flow frequently reverses, with reversals 

accompanied by upwelling, but there is no 

distinct seasonal signal in the shelfbreak flow. 

This finding contrasts with the large seasonal 

signal in transport in Bering Strait and over the 

Chukchi shelf 

3. The spatial coherence of the flow field 

suggests an along-slope decorrelation scale of at 

least 200 km between the western and central 

Beaufort shelfbreak. However, along-slope 

current variations were virtually incoherent 

between the central and western Beaufort Sea. 

The cross-slope correlation structure suggests 

that the slope flow field has a complicated 

structure, which was not adequately resolved by 

the sampling design and equipment failures. 

The decorrelation scale between the central and 

eastern Beaufort Sea is smaller, but could not be 

determined from this study. 

4. The coherence between winds and alongslope currents varies as a function of depth and 

distance seaward of the shelfbreak. The largest 

correlations occur for upper level currents 

(≤~100 m depth) in the western and central 

Beaufort Sea. Deeper currents and currents in 

the eastern Beaufort showed little or no 

coherence with the winds. The low coherence 

might be explicable in terms of seasonal 

variation in the sea ice distribution. It appears 

that during the winter of 1999, the pack ice was 

immobile for extensive periods of time in the 

eastern Beaufort Sea. This implies that little of 

the surface wind stress is transmitted into the 

water column, hence the wind-current 

correlation might be weak due to seasonal and 

shorter period variations in the ice-water stress. 

However, in conjunction with conclusion 3, the 

results suggest that the slope flow is largely 

influenced by the deep ocean and/or by remotely 

forced topographic waves. 

5. Our results, along with measurements made 

along the Mackenzie Beaufort slope, suggest 

that the eastward-flowing Beaufort undercurrent 

might not extend beyond the eastern portion of 

the Alaskan Beaufort slope. Instead, the current 

measurements and hydrography indicate that the 

eastern Alaskan Beaufort slope might be one of 

along-slope convergence, wherein the eastward 

flow in the undercurrent meets the westward 

flow along the Mackenzie shelfbreak. The 

merged flows are likely diverted offshore in the 

form of eddies. 

6. We emphasize that the eastern Beaufort 

mooring may not be representative of conditions 

at the shelfbreak because the continental slope 

broadens from west to east. Consequently, this 

mooring was further seaward of the shelfbreak 

than the moorings to the west. Very likely we 

have missed important shelfbreak processes in 

this region and this caveat must be borne in 

mind when considering the above conclusions. 

7. We hypothesize that there is a substantial 

along-slope density gradient in the vicinity of 

the Canada/U.S. EEZ line and that this gradient 

extends across the shelf. The existence of such a 

gradient has important implications for both the 

cross- and along-shelf flow structure in this 

region. 

Future measurements in this region should 

address the following issues: 

1. There are no data upon which to test the 

hypothesis advanced in point #7. An effort 

should be made to assess the alongshelf/slope density gradients from synoptic 

scale measurements made at high horizontal 

and vertical resolution. This could involve 

CTD casts and/or continuously recording, 

towed instrument packages. 

2. The measurements obtained in this 

program were unable to assess the onshelf 

extent to which shelfbreak and slope 

processes extend. This has been a problem 

because of the risk to mooring integrity by 
55 

deep ice keels in relatively shallow water. 

However, the use of near-bottom ADCPs, 

such as those employed on the inner shelf 

by Weingartner et al. [2005], considerably 

reduce the risks entailed in undertaking 

such measurements. 

3. Nor were we able to provide estimates 

of the transport along the continental slope 

and how this transport changes between the 

western and eastern Beaufort Sea. This is a 

key set of measurements that will help 

diagnose the regional flow dynamics and 

serve as an important comparison for 

regional circulation models that may be 

used in establishing oil spill risk 

assessments. 

4. Future shelfbreak and slope current 

measurements need to incorporate ADCP 

technology and moored profiling CTDs 

because these instruments can resolve the 

complex vertical structure of the flow field, 

including the uppermost 20 m. Such 

moorings have recently been used with 

great success in the western Beaufort Sea 

by R. Pickart (pers. comm.) as part of the 

NSF and ONR sponsored Shelf-Basin 

Interaction in the Western Arctic Program.
56 

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