OCS Study MMS 2001-068 

Final Report 

Beaufort Sea Nearshore Under-Ice Currents: 

Science, Analysis and Logistics 

by 

Thomas J. Weingartner

Stephen R. Okkonen 

Institute of Marine Science 

University of Alaska Fairbanks 

Fairbanks, AK 99775-7220 

weingart@ims.uaf.edu 

okkonen@alaska.edu 

December 2001 
Table of Contents 

List of Tables ......................................................................................................................... iii 

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

Abstract ................................................................................................................................... 1 

Background and Relevance to Framework Issues .................................................................. 1 

Goal and Objectives ................................................................................................................ 3 

Results ..................................................................................................................................... 6 

Currents and winds ............................................................................................................. 6 

Wind and current correlations .......................................................................................... 13 

Temperature, salinity, and transmissivity ......................................................................... 14 

Conclusions ........................................................................................................................... 20 

Acknowledgements ............................................................................................................... 20 

Study Products ...................................................................................................................... 21 

References ............................................................................................................................. 21 

ii
List of Tables 

Table 1. Mooring specifics ............................................................................................................. 5 

Table 2. Mooring instrumentation ................................................................................................. 5 

Table 3. Currents and winds summary statistics ............................................................................ 6 

Table 4. Cross-correlation between winds and currents .............................................................. 13 

List of Figures 

Figure 1. Map of Prudhoe Bay and Stefansson Sound, Alaska with the location 

of the three mooring sites—Argo, Dinkum, and McClure—indicated ............................. 4 

Figure 2. Winds and currents projected along their principal axis of variance ............................... 8 

Figure 3. Time series of (unfiltered) currents projected along their principal axis 

of variance during the landfast ice period ........................................................................ 9 

Figure 4. Time series of a) ice thickness (smoothed with a 5-day running mean) 

at Dinkum, b) bottom-track speed, c) vertical shear of the current speed, 

and d) unfiltered currents along the principal axis ......................................................... 11 

Figure 5. Histograms of current speed during the landfast ice (left-hand plot) 

and open water periods (right-hand plot) ....................................................................... 12 

Figure 6. Scatterplot of daily temperature and salinity values at Argo and Dinkum .................... 14 

Figure 7. Time series of a) ice thickness, b) temperature, c) salinity, and 

d) transmissivity ............................................................................................................. 16 

Figure 8. Mean daily discharge from the Sagavanirktok River for May–September 

2000 based on USGS discharge records collected near Pump Station 3 

(60.015°N, 148.75°W) .................................................................................................. 18 

Figure 9. Time series for May–June 2000 of a) mean daily discharge from the 

Sagavanirktok River, b) ice thickness (solid line) and transmissivity (dashed 

line), c) velocity difference (shear) for the along-shore and cross-shore 

velocity components, and d) the along- and cross-shore velocity ................................. 19 

iii
iv
Abstract 

We measured velocity, temperature, salinity, and transmissivity (a proxy for suspended sediment 

load) hourly for one year within the landfast ice zone of the Beaufort Sea near Prudhoe Bay, Alaska. 

The purpose was to measure under-ice and open water currents for evaluating oil-spill trajectories and 

sedimentation risk in this nearshore environment. The measurements were made from an array of 

three instrumented moorings spanning an along-shore distance of ~28 km within the western portion 

of Stefansson Sound. The circulation dynamics vary seasonally and in response to the formation and 

disappearance of the landfast ice. The landfast ice formed in mid-October 1999 and remained intact 

until the end of June 2000. Current speeds were typically about 3 cm s–1 and most of the current 

variability was at subtidal periods. Current speeds greater than 10 cm s–1 were observed less than 5% 

of the time at two locations and about 10% of the time at one location. During the open water season, 

current speeds typically exceeded 10 cm s–1 and the maximum currents were greater than 10 cm s–1. 

Winds and currents were strongly correlated during the open water season but uncorrelated once the 

landfast ice was established. The highest transmissivity signals were observed in early October 1999 

during a strong gale and in early June, when the landfast ice was still intact and winds were weak. We 

believe that the June turbidity signal was associated with the summer freshet from the Sagavanirktok 

River, which flows beneath the landfast ice and (at least) 10 km offshore of the river mouth. 

These data represent the first year-round (including the freeze-up and ice melt seasons) current 

measurements for this area. This information is critical for designing oil spill response protocols for 

offshore drilling operations specifically at the nearby Northstar and Liberty fields and generally for 

similar landfast ice regions of the Alaskan Beaufort Sea. The data should also be useful in evaluating 

regional numerical circulation models that would be used for oil spill trajectory predictions. While 

these were the principal reasons for this study, the data will have broader scientific applications 

relevant to other arctic shelves. For example, the low current energy beneath the landfast ice zone 

implies little vertical mixing, which has important implications for the fate of river waters entering the 

shelf in early summer. Consequently, this data will lead to a better understanding of mixing processes 

and circulation on the innermost portion of arctic shelves. 

Background and Relevance to Framework Issues 

Within the vicinity of Prudhoe Bay, the Northstar oil field is being developed and the Liberty 

prospect is under construction for further development. Both prospects lie offshore and there is some 

concern that development might impact nearshore biota. The Minerals Management Service (MMS) 

Alaska OCS Region expects that these concerns will require integrated studies of key issues including 

long term impacts on benthic/kelp communities and other sensitive resources. 

Understanding the under-ice currents is a necessary precursor to estimating potential effects of oil 

spills on sensitive resources in the landfast ice zone, and in particular at the Liberty project. The 

landfast ice zone typically lies shoreward of the 20-m isobath [Reimnitz 1978]. Such spill trajectory 

assessments cannot be modeled with the standard MMS oil spill trajectory analysis based on regional 

circulation. Detailed under-ice current measurements are necessary. Another important question is 

whether the under-ice currents could transport suspended sediments from the development area to the 

nearby Boulder Patch, and endanger kelp during the critical under-ice growth period. 

1
Under-ice current speed and direction are important because currents of 10–20 cm s–1 will move 

spilled oil along the underside of the ice. The one study of under-ice currents supported by MMS 

[reported by Matthews 1981] was made within Stefansson Sound. Matthews did not measure the 

under-ice currents directly but rather inferred them from near-bottom measurements with assumptions 

that the flow field was unstratified and that two-dimensional mass balance held. He inferred that 

under-ice currents averaged 6 cm s–1 and had a maximum velocity of 35 cm s–1 directed onshore. The 

latter speed is more than sufficient to move oil beneath the landfast ice. Indeed, if these currents are

sustained they imply onshore displacements of about 3 and 20 nmi d–1. Matthews found that average 

currents under landfast ice appeared to be related to brine drainage from growing sea ice and that the 

highest currents were associated with wind-forced, negative surges. Along-shore currents appeared to 

have similar magnitudes. Our expectation is that wind-forced currents should dominate in this region, 

as tidal currents are about 4 cm s–1 [Kowalik and Proshutinsky 1994]. 

Matthews’ [1981] measurements, made from mid-November 1978 through February 1979, were 

obtained within Stefansson Sound, a shallow lagoon bounded offshore by barrier islands. We 

anticipate that the lagoon circulation is less energetic than the flow offshore of the barrier islands. In 

this regard, his measurements are probably more applicable to the Liberty development than to 

Northstar. Moreover, his measurements did not include the early fall period of vigorous ice growth 

and the developing landfast ice cover. We expect under-ice currents to be swifter during this period as 

the ice is still mobile and therefore effective in transmitting momentum from the winds to the 

underlying ocean. We expect that until the landfast ice cover is established, the drag of the ice over 

the ocean will impart momentum into the ocean via a frictional boundary layer (e.g., very large 

vertical gradients in ocean velocity) near the surface. Once formed, the landfast ice is an immobile lid 

that inhibits the flux of wind energy into the water column. A priori we expect that the portion of the 

inner shelf covered by landfast ice will be relatively quiescent. Finally, Matthews’ measurements did 

not include the period of river runoff, which increases rapidly in early summer when the landfast ice 

is still present. This region receives a large and sudden influx of river water (primarily from the 

Sagavanirktok River), with much of the discharge probably flowing under the landfast ice [Carmack 

et al. 1989; Dean et al. 1994]. Whether this fresh water mixes with ambient shelf water and, if it does, 

how this mixing evolves, is crucial in the seasonal evolution of the nearshore and shelf circulation 

[Chapman and Lentz 1994; Yankofsky and Chapman 1997; Weingartner et al. 1999]. The river plume 

will spread under the ice, possibly forming strong vertical velocity gradients with maximum speeds 

near the bottom of the ice. 

2
Goal and Objectives 

The goal and objectives of this project were to provide direct measurement of winter under-ice and 

open water currents in the vicinity of the Liberty and Northstar projects to help MMS and the Alaska 

Department of Environmental Conservation evaluate oil-spill and sedimentation risk to the nearshore 

Beaufort Sea, including the Boulder Patch and other local resources. The specific objectives were to: 

1. Deploy three instrumented moorings in the vicinity of the Liberty and Northstar 

projects for a period of one year (Figure 1). The moorings collected velocity, 

temperature, salinity, and one collected transmissivity data. Transmissivity is a 

proxy for the suspended sediment load in the water column. Our moorings were 

deployed in the western end of Stefansson Sound—west, east and offshore of the 

Sagavanirktok River delta. 

2. Analyze the current meter and local wind records to quantify the magnitude of 

current and wind variability and their relationship to one another. 

3. Determine the vertical structure of the currents throughout the water column and 

how this structure changes with the development of the landfast ice through winter 

and in summer when the ice melts and rivers flood the inner shelf. 

Tables 1 and 2 summarize details on the mooring nomenclature, locations, measurement period, and 

bottom depths and instrumentation used. Mooring McClure was within the region of the extensive 

kelp beds known as the Boulder Patch. Currents at each location were measured with a 1200 kHz 

acoustic Doppler current profiler (ADCP) which collects backscattered sound integrated over 0.5-m 

depth bins throughout the water column and to within about 1.0 m beneath the ice. These instruments 

transmit sound at 1200 kHz and then measure the Doppler shift of the backscattered signal. The 

ADCPs depend upon water column scatterers (density discontinuities, plankton, and suspended 

matter). Internal software reconstructs the current profile throughout the water column at a high 

vertical and temporal resolution. The ADCPs were mounted on gimbals in a non-ferrous plastic 

frame, which ensured that the instrument was vertically oriented throughout the measurement period. 

The transducer head for each ADCP was located about 0.85 m above the bottom. Valid velocity 

measurements were acquired above the moorings at 0.5-m intervals beginning at 1.0 m (1.5 m at 

Argo) above the ADCP transducer head. Due to interference from transducer beam sidelobes, the 

shallowest valid velocity measurements were acquired at about 1 m below the open-water surface or 

ice-bottom surface. In the following discussion, the shallowest measurements are referred to as 

shallow-bin measurements and the deepest as deep-bin measurements (the deep-bin measurements 

were acquired at ~2.35 m above bottom at Argo, and ~1.85 m above the bottom at Dinkum and 

McClure). Temperature, salinity, and water pressure were measured with a MicroCAT (manufactured 

by Sea-Bird Electronics, Inc.) at station Argo. Temperature, salinity, and transmissivity were obtained 

from SEACATs (manufactured by Sea-Bird Electronics, Inc.) at stations McClure and Dinkum. All of 

these variables were sampled at half-hour intervals from sensors located about 0.5 m above the 

bottom. However, due to an error in setting the sampling rate for the McClure SEACAT, only four 

days of data were acquired at this mooring. These data are not presented in this report. The mooring 

deployment period was 382 days in length. 

Wind speed and direction measurements at the Deadhorse airport were available for the period from 

19 September 1999 to 13 September 2000. The sampling rate was variable, but typically once per 

3
hour. The wind record was interpolated to hourly intervals for comparison with the current meter 

records. 

Argo

Dinkum

McClure

Prudhoe

Bay

Sagavanirktok

River Delta

–149 –148 30′ –148 –147 30′ –147

70 12′

70 24′

70 36′

–149 –148 30′ –148 –147 30′ –147

70 12′

70 24′

70 36′

Liberty

S t e f a n s s o n S o u n d

Deadhorse

Airport

ALASKA

Northstar

Figure 1. Map of Prudhoe Bay and Stefansson Sound, Alaska with the location of the three mooring 

sites—Argo, Dinkum, and McClure—indicated. 

Table 1. Mooring specifics. 

Mooring Latitude 

(N) 

Longitude 

(W) 

Time Deployed 

(1999, GMT) 

Time Recovered 

(2000, GMT) 

Bottom 

Depth 

4
Argo 70° 27.177′ 148° 12.722′ 13 Aug, 1821 1 Sep, 0556 8.4 m 

Dinkum 70° 24.352′ 147° 53.656′ 14 Aug, 1837 1 Sep, 0318 6.8 m 

McClure 70° 20.204′ 147° 32.701′ 14 Aug, 2007 1 Sep, 0054 6.7 m 

Deadhorse 

Airport 70° 11.7′ 148° 27.7′ 19 Sep 13 Sep 

Table 2. Mooring instrumentation. Each of the ADCPs was situated about 1 m above the 

seabed and the SEACATs and MicroCAT were situated about 0.5 m above the seabed. 

Mooring Data Analysis Period Instrument Variable Measured 

Argo 19 Sep 99 – 1 Sep 00 1200 kHz ADCP current velocity 

Argo  ̋  ̋ MicroCAT temperature, salinity and water pressure 

Dinkum  ̋  ̋ 1200 kHz ADCP current velocity 

Dinkum  ̋  ̋ SEACAT temperature, salinity and transmissivity 

McClure  ̋  ̋ 1200 kHz ADCP current velocity 

McClure  ̋  ̋ SEACAT temperature, salinity and transmissivity 

Deadhorse  ̋  ̋ Anemometer wind speed and direction 

5
Results 

Currents and winds 

Mean statistics for the currents and moorings are summarized in Table 3. All statistics are based on 

the common time period of 0000 on 19 September 1999 to 0000 on 1 September 2000 (GMT). The 

principal axes calculation shows that most of the flow variability (>~90%) at all mooring sites is 

oriented along the length of Stefansson Sound, e.g., the flow is predominantly along the northwest–

southeast axis. Thus flow variations in the cross-sound direction are relatively weak. The winds are 

also strongly rectilinear, with 90% of the wind variability oriented along the east–west axis. While the 

mean current velocities are small the variability about the mean is large. The variance changes with 

the ice cover. Standard deviations during the open water period are 3–6 times greater than during the 

ice covered period. The integral time scales, τ, of the current and winds are similar, with each being 

about three days. This is the decay time scale for flow (and wind) events and it is the time required to 

gain a new “degree of freedom” from a record of N data points. We use it to estimate the effective 

number of degrees of freedom [Davis 1976] in our data set, which is defined as Neff = NΔt/τ, where Δt

is the sampling interval. 

Table 3. Currents and winds summary statistics. The current statistics are given for a depth about 1.5 

m above each instrument. 

Mooring Net Velocity Principal Axis of 

Variance 

Speed 

(cm s–1) 

Direction 

(°T) 

Variance 

Explained

% 

Direction

(°T) 

Max 

Speed 

(cm s–1) 

Integral 

Time Scale

(days) 

Mean±SD 

North–South 

Velocity 

(cm s–1) 

Mean±SD

East–West 

Velocity 

(cm s–1) 

Full Record 

Argo 4 278 97 97 99 3 0 ± 3 –3 ± 14 

Dinkum 2 305 96 119 114 3 1 ± 8 –2 ± 4 

McClure 2 2 90 151 82 3 2 ± 11 0 ± 7 

Deadhorse 

winds 

1.3 

(m s–1) 250 90 77 25 (m s–1) 3 –1 ± 5 (m s–1)

–2 ± 12 

(m s–1)

Landfast Ice Period 

Argo 2 283 88 99 24 3 0 ± 2 –2 ± 6 

Dinkum 0 – 83 145 18 2 0 ± 4 0 ± 3 

McClure 1 12 82 173 15 3 1 ± 4 0 ± 2 

Open Water Period 

Argo 3 270 97 97 99 4 0 ± 14 –3 ± 18 

Dinkum 2 322 95 119 114 4 2 ± 11 –1 ± 18 

McClure 2 360 95 150 82 4 2 ± 13 0 ± 9 

6
Figure 2 shows time series of the current from the bin about 1.5 m above the ADCP with wind 

components projected onto their principal axes. Each of the current meter time series in this figure 

was low-pass filtered to suppress tidal motions. The wind time series was smoothed with a 25-hour 

running mean. The time series show that the current variations are seasonally modulated, with most of 

the variance and the highest current speeds occurring from July through October. The current variance 

between mid-October and the end of June is a factor of 5–10 times smaller than the variance prior to 

and after these dates. As shown below, this is the time period when we believe that landfast ice was 

established in the sound. Flow events exceeding 25 cm s–1 are common prior to mid-October 1999 

and after 1 July 2000, but between these dates flow speeds seldom exceeded 10 cm s–1. The midOctober through June period is highlighted in Figure 3, which shows the unfiltered current time series 

at the three mooring sites. Although the currents are comparatively weak, there are several events in 

winter, each of several days duration, when the flow is quite persistent. For example, beginning in late 

December, there is a 5-day event when current speeds averaged about 10 cm s–1 at all locations. Such 

events can transport suspended material over a distance of more than 40 km. 

Inspection of the current time series in Figures 2 and 3 suggests that these are all well correlated with 

one another throughout the record. Quantitatively this is supported by the results of an empirical 

orthogonal function (EOF) analysis (based on the correlation matrix) on the current components 

aligned along their principal axes. The first EOF mode accounts for 92% of the variance while the 

second and third modes are not significant, following North et al.’s [1982] procedure. The dominance 

of the first EOF mode implies that flow variations along Stefansson Sound are coherent over spatial 

scales of at least 28 km. The results also suggest that a single mooring would suffice in capturing 

along-shore flow variations in this region. 

7
Figure 2. Winds and currents projected along their principal axis of variance. The direction of the positive and negative 

values at each location is indicated along the ordinate. The currents were low-pass filtered to suppress diurnal and 

semi-diurnal tidal fluctuations. The winds were smoothed with a 25-hour running mean.

Deadhorse Winds

McClureDinkumArgo

1510

50

–5

–10–15–20

755025

0

–25–50–75

755025

0

–25–50–75

–100

755025

0

–25–50–75

–100

(77 T)

(257 T)(150 T)(330 T)(119 T)(299 T)

(98 T)

(278 T)

1999 2000

cm s –1 cm s –1 cm s –1 m s –1

24 Sep14 Oct3 Nov23 Nov13 Dec2 Jan22 Jan11 Feb2 Mar 22 Mar 11 Apr1 May21 May10 Jun30 Jun20 Jul 9 Aug

8
Figure 3. Time series of (unfiltered) currents projected along their principal axis of variance during the 

landfast ice period. 

10

50

–5

–10

2015

10

–

5

5

(119 T)

(299 T)

–1 cm s 0

–10–15

(150 T)

(330 T)

–1 cm s

McClureDinkumArgo

10

–5

–10

50

–20–15

(98 T)

(278 T)

–1 cm s

15

15 Oct4 Nov24 Nov14 Dec3 Jan23 Jan12 Feb3 Mar 23 Mar 12 Apr2 May22 May11 Jun1 Jul

1999 2000

9
We next consider how the velocity and current shear vary with the seasonal formation of the landfast 

ice. Figure 4a shows 5-day running mean estimates of the sea ice thickness as determined from the 

ADCP. The thickness B was estimated as the difference between the bottom depth Z (Table 1), and 

the sum of the height of the transducer above the bottom H and the range from the transducer at which 

the intensity of the reflected acoustic signal was maximum R (e.g., B = Z – [H + R]). The uncertainty 

in the thickness measurement is ±0.25 m because the ADCP integrates over a 0.5 m layer. Sea ice 

appears in early October and steadily increases to a thickness of ~1.5–2.0 m by mid-March. Ice 

thickness remains constant through mid-June but then rapidly ablates and is completely gone by midJuly. (Floating sea ice is probably present prior to mid-October and after breakup in July, but the 

exact dates at which sea ice first appears and then completely disappears cannot be unequivocally 

determined from the ADCP data.) Figure 4b shows time series of the bottom-tracking mode. For 

ADCPs deployed in an upward-looking orientation, the bottom-tracking mode provides measurements 

of sea ice drift speed and direction when sea ice is present. In the absence of ice, surface current 

speeds will not be accurate because of sidelobe contamination. The bottom-tracking mode shows that 

the near surface, including the ice, is in motion until mid-October, at which point the bottom-tracking 

speeds fall to zero and remain there until the end of June (when the ice thickness is ~0.5 m). In 

aggregate, these figures suggest that the period of landfast ice lasts for about eight and one-half 

months. Figure 4c shows the difference in speed (or current shear d|V|/dz, where |V| is the speed and z

is the depth coordinate) between the shallowest depth bin beneath the ice or sea surface and the 

deepest depth bin. At Dinkum the depth range over which these differences are computed is ~4 m 

during open water periods and ~2 m during maximum ice thickness. Current speeds typically differ 

by about 5 cm s–1 over the measured depth range when landfast ice is present. However, shears can be 

quite large when the ice is absent, with differences of up to 80 cm s–1 observed. But more typically the 

differences are between 20 and 30 cm s–1. Finally, Figure 4d is the unfiltered velocity time series, 

which shows that the largest current speeds and greatest current variance occur before the landfast ice 

is established and after it breaks up. The frequency distribution of the variance also changes 

seasonally. For example, approximately 80% of the variance is subtidal (periods longer than 30 hours) 

during the landfast ice period, while more than 90% of the variance is in the subtidal periods during 

the open water period. 

We conclude this section with the seasonal speed histograms shown in Figure 5. The left-hand plot 

shows histograms of current speed during the landfast portion of the year and the right-hand panel 

shows histograms for the open water ice period. For the latter period, the majority of current speeds 

are less than 10 cm s–1 at all locations, while during the remainder of the year the majority of the 

current speeds exceed 10 cm s–1. For example, at Argo, 10.5% of the current speeds exceeded 10 cm 

s

–1 during the landfast ice period. At Dinkum and McClure the comparable values were 2.4% and 

1.3%, respectively. Matthews [1981] inferred from continuity arguments that under-ice current speeds 

could be as large as 35 cm s–1. In contrast, our direct current measurements suggest that under-ice 

current speeds exceed 10 cm s–1 less than 10% of the time. 

10
Figure 4. Time series of a) ice thickness (smoothed with a 5-day running mean) at Dinkum, b) bottom-track speed, c) vertical shear of 

the current speed, and d) unfiltered currents along the principal axis. All measurements are from Dinkum except the bottom 

track speed, which is from Argo.

DinkumArgoDinkumDinkum

2

0.5

0

100

50

0

–0.05

75

5025

0

–25–50–75

–100

(119 T)

(278 T)

1999

200150

0.200.150.100.050.00

100

1

1.5

abcd

2000

19 Sep4 Oct 3 Nov 18 Nov3 Dec 2 Jan 17 Jan1 Feb 2 Mar 17 Mar1 Apr 1 May31 May 15 Jun 30 Jun 30 Jul29 Aug

–1 cm s

) –1 (s dz

Bottom-track speed Ice thickness d V

(m)

19 Oct 18 Dec 16 Feb 16 Apr16 May 15 Jul14 Aug

) –1 (cm s

11
0 4 8 2 12 16 0 24

McClure

2500

2000

1500

1000

500

0

Number of Observations

Speed Class (cm s–1)

0 20 6040 80 100 120

McClure

500

400

300

200

100

0

Number of Observations

Speed Class (cm s–1)

2500

2000

1500

1000

500

0

500

400

300

200

100

0

Number of Observations

Number of Observations

0 4 8 2 12 16 0 24

Speed Class (cm s–1)

0 20 6040 80 100 120

Speed Class (cm s–1)

Dinkum Dinkum

2500

2000

1500

1000

500

0

500

400

300

200

100

0

Number of Observations

Number of Observations

0 4 8 2 12 16 0 24

Speed Class (cm s–1)

0 20 6040 80 100 120

Speed Class (cm s–1)

Argo Argo

Landfast Ice Period Open Water Period

12
Figure 5. Histograms of current speed during the landfast ice (left-hand plot) and open water 

periods (right-hand plot). 

Wind and current correlations 

The landfast ice is a rigid, immobile cover that should effectively decouple the ocean from the direct 

effects of wind stress. Consequently the correlation between the winds and the currents should be 

different between the open water and ice-covered periods. This is in fact the case, as shown by crosscorrelation analysis (Table 4) between the wind and current components projected onto their principal 

axes. Cross-correlation analyses of Deadhorse winds and currents at the three mooring sites were 

computed for: 1) the open water/drifting ice period from 19 September 1999 through 14 October 

1999, 2) the landfast ice period from 15 October 1999 through 30 June 2000, and 3) the open 

water/drifting ice period from 1 July 2000 through 31 August 2000 (Table 4). The results show that 

currents are significantly correlated with the prevailing winds at lags of one or two hours during the 

periods of open water/drifting ice at all mooring locations. The higher correlations associated with the 

earlier open water period are likely attributable to the nearly two-week period of strong east–

northeasterly winds in early October. Correlations associated with the landfast period are not 

statistically significant at all locations. Thus, the results are consistent with the frictional decoupling 

of the wind from the water by the landfast ice. The behavior of the flow under the landfast ice differs 

markedly from that observed under freely-drifting ice, as shown for the Beaufort shelf by Aagaard 

[1984] and the Chukchi Sea by Weingartner et al. [1998]. At these locations the winter, under-ice 

flow can be strong and is strongly correlated with the surface winds. 

Table 4. Cross-correlation between winds and currents. Values denoted by an asterisk are 

significant at P = 0.01. Neff is the effective number of degrees based upon an integral time 

scale of 3 days. 

Mooring 19 Sep 99 – 14 Oct 99 

Neff = 9 

15 Oct 99 – 30 Jun 00 

Neff ~ 86 

1 Jul 00 – 31 Aug 00 

Neff ~ 21 

Argo 0.94* (1 hr) 0.09 0.70* (5 hr) 

Dinkum 0.94* (2 hr) –0.02 0.73* (5 hr) 

McClure 0.94* (1 hr) –0.13 0.74* (4 hr) 

13
Temperature, salinity, and transmissivity 

Water property parameters also vary considerably throughout the year. The annual ranges in 

temperature (T) and salinity (S) are shown in Figure 6, which is a scatterplot or TS-plot of daily 

temperature and salinity values at Argo and Dinkum. Salinity ranges from 20–35 psu and 

temperatures range between 4 to approximately –1.7° C. In general, low salinities are associated with 

high temperatures and high salinities with low temperatures. The solid line in the TS-plot is the 

freezing point curve as a function of salinity. Many of the observed values fall on this line or slightly 

above it as might be expected in these shallow areas. We also note that because the MicroCATs and 

SEACATs were re-deployed upon recovery we could not post-calibrate these instruments. The cluster 

of points on the freezing point provides one indication that the sensors maintained their calibrations at 

least through most of the year. 

22 24 26 28 30 32 34

Salinity (psu)

0

–2

–1

1

2

3

4

20

Temperature ( C)

Argo

Dinkum

Figure 6. Scatterplot of daily temperature and salinity values at Argo and 

Dinkum. 

14
The seasonal variation in water properties is seen in Figure 7, which shows time series of a) ice 

thickness, b) temperature, c) salinity, and d) transmissivity. Temperature decreases rapidly from late 

September through mid-October. It remains at the freezing point (approximately –1.7° C) until late 

June, whereupon it slowly increases to about 0° C by late July. At that time there is a rapid increase to 

4° C before decreasing to about 0° C by the end of the record. Salinity varies between 28 and 32 psu 

prior to the establishment of the landfast ice in mid-October and then increases to 34–35 psu by 

January. This increase is related to the formation of sea ice, which expels salt as it forms. Thereafter, 

salinities vary between 33 and 35 psu throughout the remainder of winter and then slowly start to 

decrease in June. The temperature increase and the associated drop in salinity beginning in late June 

and continuing through July coincide with the ablation and breakup of sea ice and probably also 

reflect the addition of fresh water to the system due to river runoff. In August, salinity decreases and 

temperature increases very rapidly. We suspect that this sudden freshening (a salinity decrease from 

about 28 to 20 psu) and warming is associated with the mixing of river water throughout the water 

column, as these coincide with the strong current shears observed in early August (Figure 4c). These 

shears were associated with the strong westerly winds of early August (Figure 2), which should 

enhance vertical mixing of a stratified water column. Assume that the ~7-m-deep water column in 

Stefansson Sound can be approximated as a two-layer system with the river plume at the surface, 

having an unknown thickness but with a presumed salinity of 5 psu. The lower layer has a salinity of 

28 psu (as observed before the rapid salinity decrease in early August). Mass balance considerations 

suggest that the plume thickness is about 2.5 m. (For plume salinities of 10 and 0 psu, the plume 

thicknesses are 3 and 2 m, respectively.) We suspect that the water column begins to stratify in June 

when North Slope rivers breakup. River plumes have been observed under the landfast ice at this time 

of the year where they establish strongly stratified conditions [Carmack et al. 1989; MacDonald et al. 

1989; Dean et al. 1994]. Our experimental design precluded obtaining the vertical gradients of salinity 

and temperature needed to unequivocally establish that river water is present beneath the ice and 

above our mooring. Searcy et al. [1996] show that under-ice river plumes can strongly affect the 

summer breakup of landfast ice. 

15
2

0.5

0310

80

0

60

100

–1

343230

142

1.5

–2

2826242220

4020

1999 2000

abcd

Dinkum

Argo

30 Sep 15 Oct14 Nov 29 Nov 14 Dec13 Jan 28 Jan 12 Feb13 Mar 28 Mar 12 Apr 27 May 11 Jun 26 Jun 26 Jul25 Aug 30 Oct 29 Dec 27 Feb 27 Apr 12 May 11 Jul10 Aug

Salinity

(psu)

Temperature

C)

(

Ice thickness

(m)

Transmissivity

(%)

16 Aug 31 Aug 15 Sep

Figure 7. Time series of a) ice thickness, b) temperature, c) salinity, and d) transmissivity. 

16
Transmissivity is a relative value that is proportional to the amount of suspended material in the water 

column. (Additional measurements would be needed to relate transmissivity to suspended load.) It, 

too, shows a strong seasonal cycle. Transmissivity is relatively high (low suspended material) in 

winter and low in summer and fall when the lagoons are ice free and/or covered with drifting ice. 

Interestingly, transmissivity dropped to very low levels in mid-June when the landfast ice was intact 

but melting rapidly. Current speeds and shear were also low at this time. There are several possible 

sources of suspended material that contributed to the low transmissivity: The first is sediment that was 

incorporated into the ice during the previous fall freeze-up and which drops out when the ice melts. 

The second is ice algae flushed from the ice during melt. The third is sediment carried by river plumes 

flowing under the ice but above the depth of mooring. Conceivably this suspended material drops out 

as the river plume flows offshore under the ice. The fourth is sediment scoured from the bottom from 

a nearby “glory hole”. While all of these sources could be contributing simultaneously, we believe 

that the latter two are most likely since the low transmissivity signal occurs during the period of 

maximum increase in river discharge from the Sagavanirktok River at Pump Station 3. The mean 

daily Sagavanirktok River discharge for 1 May 2000 to 30 September 2000 is shown in Figure 8. 

Discharge is negligible through May, increases rapidly over the first week of June and attains a peak 

of ~700 m3

 s–1 on 9 June. Discharge then falls off rapidly by late June and remains relatively constant 

at ~150 m3

 s–1 through August. Note that the peak discharge on 9 June precedes by two days the rapid 

decrease in transmissivity (increase in suspended sediment load) observed at mooring Dinkum on 10 

and 11 June (Figure 7). In fact, the vertical velocity structure also changes as the discharge peaks. 

This is evident in Figure 9, which shows time series of the discharge, ice thickness, transmissivity, 

vertical velocity differences, and velocities beneath during the May–June 2000 period at the Dinkum 

mooring. Both components of the velocity shear increase as the discharge peaks. The increase in shear 

suggests a two-layer structure in the water column. There is also an increase in the cross-shore 

velocity consistent with the offshore spreading of the plume. The acoustic backscatter signal (not 

shown) suggests an increase in backscatter intensity at about 1.5 m beneath the ice. This is consistent 

with the estimates made above based on salinity changes for the plume thickness. The increase in 

backscatter intensity at this depth is presumably related to the density difference between the river 

plume and seawater and/or the (presumed) increase in suspended material within the plume. All of 

these changes coincide with the rapid increase in discharge and its presumably high sediment load. 

Note also that following the peak discharge the ice began to thin rapidly. Presumably this decrease is 

related to the relatively warm water associated with the river plume. River plume temperatures should 

be at least 1.5° C to 2.0° C above the melting point of sea ice. Searcy et al. [1996] estimate that rivers 

might provide up to one-third of the heat required to melt landfast ice in the vicinity of river mouths. 

17
0

400

350

300

250

50

100

Discharge (m3 s–1)

28 Sep

1 May

21 May

10 Jun

30 Jun

20 Jul

9 Aug

11 May

31 May

20 Jun

10 Jul

30 Jul

19 Aug

29 Aug

8 Sep

18 Sep

150

200

450

500

Sagavanirktok River

(Mean daily discharge May–September 2000)

550

600

650

700

Figure 8. Mean daily discharge from the Sagavanirktok River for May–September 

2000 based on USGS discharge records collected near Pump Station 3 

(60.015°N, 148.75°W). The maximum daily discharge occurred on 

9 June 2000. 

18
Figure 9. Time series for May–June 2000 of a) mean daily discharge from the Sagavanirktok River, b) ice thickness (solid 

line) and transmissivity (dashed line), c) velocity difference (shear) for the along-shore and cross-shore velocity components, and d) the along- and cross-shore velocity. The oceanographic data were from mooring Dinkum. Solid lines de

pict the alon

g-shore com

ponents and dashed lines de

pict the cross-shore com

ponents. 

750500250

0

2.01.51.0

0

0.5

840

–41510

0

–5

5

–10

1510

0

–5

5

–10

75

100

5025

0

Enhanced Shear

Ice thicknessTransmissivityAlong-shoreCross-shore

z = 2.5 m

Sagavanirktok RiverMay–June 2000

2000

1 May

Velocity difference ) –1 Along-shore (cm s ) –1 (cm s Discharge (m Ice thickness (m) 3 ) –1 s

Cross-shore (cm s–1 Transmissivity (%) )

11 May

6 May

16 May

21 May

26 May

31 May

5 Jun

10 Jun

15 Jun

30 Jun

25 Jun

20 Jun

a

b

c

d

19
Conclusions 

The year-long records of oceanographic conditions at three near-shore sites in Stefansson Sound near 

Prudhoe Bay describe two different regimes. During the summer months when there is open water 

and drifting sea ice the currents are wind driven, generally swift (>10 cm s–1), highly variable, and 

strongly sheared. After landfast ice is established, the currents are decoupled from direct forcing by 

the wind and uncorrelated with the local wind field. There is also a substantial reduction in current 

magnitude (with speeds generally <10 cm s–1), variability, and shear relative to the wind-driven 

regime. However, the current fluctuations are spatially coherent year round over at least 28 km. The 

low correlation between local winds and currents in winter suggests that other mechanisms are 

responsible for the low-frequency currents observed at this time of the year. These could be 

fluctuations in atmospheric pressure and/or by offshore oceanic motions. Both are of broad enough 

scale to account for the spatial coherence observed in the currents. For example, Aagaard [1984] and 

Aagaard and Roach [1990] have suggested that remotely forced oceanic motions could account for 

fluctuations in the flow over the Beaufort Shelf. 

Our measurements suggest the possibility that in June, prior to the breakup of landfast ice, portions of 

Stefansson Sound might be influenced by under-ice river plumes that carry a large burden of 

suspended material that subsequently sinks to the bottom. 

Finally, we note that our measurements have been made in Stefansson Sound, which is somewhat 

protected by a barrier island complex. There are wide and, in some places, relatively deep channels 

between these islands so that communication between the sound and offshore waters is not as 

restricted as elsewhere along the Beaufort coast. For example, Simpson Lagoon lies to the west of 

Prudhoe Bay and is bounded by a relatively extensive chain of barrier islands having fewer and 

shallower passes than those associated with the barrier islands offshore of Stefansson Sound. 

Moreover, Simpson Lagoon is shallower and narrower in width. We believe that flow conditions in 

Simpson Lagoon will be comparable to those measured in Stefansson Sound and that our 

measurements are probably representative of lagoon flows elsewhere along the North Slope. 

However, it is not clear if our measurements within Stefansson Sound reflect the magnitudes of 

currents beneath the landfast ice at locations seaward of the barrier islands. 

Acknowledgements 

Dave Leech was responsible for mooring design, fabrication, deployment, and recovery. This research 

was supported by the MMS Coastal Marine Institute in cooperation with the Alaska Department of 

Environmental Conservation. British Petroleum, Inc. provided logistic support for the field crews. 

Study Products 

Portions of data collected and analyzed in this project were presented as follows: 

20
Weingartner T.J. 2000. Beaufort Sea nearshore under-ice currents: Science, analysis and logistics, p. 

75. In University of Alaska Coastal Marine Institute Annual Report No. 6. OCS Study MMS 

2000-046, University of Alaska Fairbanks and USDOI, MMS, Alaska OCS Region. 

Weingartner T.J. 2000. Beaufort Sea nearshore under-ice currents: Science, analysis and logistics, p. 

37–41. In University of Alaska Coastal Marine Institute Annual Report No. 7. OCS Study MMS 

2000-070, University of Alaska Fairbanks and USDOI, MMS, Alaska OCS Region. 

Weingartner T.J. 2001. Beaufort Sea nearshore under-ice currents: Science, analysis and logistics. 

University of Alaska Coastal Marine Institute Annual Research Review, February 2001, 

Fairbanks. 

Weingartner, T.J. 2001. Beaufort Sea nearshore under-ice currents: Science, analysis and logistics. 

MMS Information Transfer Meeting, April 2001, Anchorage. 

Weingartner, T.J. 2001. Going to extremes: Buoyancy forcing on arctic continental shelves. Gordon 

Conference on Coastal Ocean Circulation, June 2001, New London, NH. 

Dr. Weingartner was an invited speaker at this conference and presented some of the results from 

this project at the meeting. 

Weingartner T.J., and S.R. Okkonen. 2001. Beaufort Sea nearshore under-ice currents: Science, 

analysis and logistics. Final Report. OCS Study MMS 2001-068, University of Alaska Coastal 

Marine Institute, University of Alaska Fairbanks and USDOI, MMS, Alaska OCS Region, 

22 p. 

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