ACCESS NUMBER: 15163

STUDY TITLE: Circulation, Thermohaline Structure, and Cross-shelf Transport in the Alaskan Beaufort

Sea

REPORT TITLE: Circulation, Thermohaline Structure, and Cross-shelf Transport in the Alaskan Beaufort

Sea

CONTRACT NUMBER: 1435-01-98-CA 30909 TO 15163

SPONSORING OCS REGION: Alaska

APPLICABLE PLANNING AREA: Beaufort Sea

FISCAL YEARS OF PROJECT FUNDING: 1998, 2000

COMPLETION DATE OF REPORT: July 2005

COST: FY 1998: $622,565; FY 2000: $20,000

CUMULATIVE PROJECT COST: $642,565

PROJECT MANAGER: Thomas J. Weingartner

AFFILIATION: University of Alaska Fairbanks

ADDRESS: Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775-7220

PRINCIPAL INVESTIGATOR*: Thomas J. Weingartner

KEY WORDS: Circulation, Shelf, Thermohaline, Beaufort Sea, Transport

BACKGROUND:

The development of offshore oil exploration and production facilities on the Beaufort Sea shelf might be

impacted by circulation processes over the adjacent shelfbreak and continental slope because this region

is an important offshore boundary between the shelf and basin. Moreover, shelfbreak processes can

influence circulation and water mass modification processes on the shelf, although the extent to which

this occurs on the Alaskan Beaufort Sea remains unknown. This project was undertaken to determinethe

circulation characteristics of the shelfbreak, with an emphasis on the central portion of the Alaskan

Beaufort Sea (offshore of Prudhoe Bay). Particular interest focused on obtaining year-round direct

current, temperature, and salinity measurements at sites along the western, central, and eastern Beaufort

slope. The results can be used to assist in the development and evaluation of circulation models that

may be applied to oil spill response design criteria.

OBJECTIVES:

1. Our goals were to: 1) Determine the mean transport over the outer shelf and slope and crossshore 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 relation between variations in temperature and salinity and variations in the flow

field at time scales between the synoptic to the seasonal, 4) Determine the cross-shore fluxes of

heat, salt, and momentum, 5) Determine the relationship between observed flow and density

variations and the surface wind field, 6) Compare the results obtained with those collected by

Aagaard et al. in 1987/88 in regard to changing conditions in the Beaufort portion of the Arctic

Ocean, 7) Combine this data set with other recently acquired data to provide an updated

synthesis that relates the Beaufort Sea to the large-scale circulation of the Arctic Ocean.
ACCESS NUMBER: 15163

DESCRIPTION:

The field program consisted of two distinct elements: an array of six moorings sponsored by MMS and a

winter hydrography program led by Weingartner and supported by the Office of Naval Research. The

moorings were deployed at depths ranging from 80 m to 1500m, with instruments primarily between 70

and 400 m. Hourly measurements of velocity, temperature, and salinity were recorded. Unfortunately,

one mooring was lost (presumably to drifting ice) and several instruments malfunctioned during the

course of the program, which prevented us from fully achieving our goals. Wind-current analyses used

measured winds from Barrow Alaska. The hydrographic component of the program involved deploying

expendable conductivity-temperature-depth (XCTD) probes and continuous underway temperature and

salinity (UTS) measurements from a temperature-conductivity recorder mounted in the sail of the U.S.

Navy submarine Hawkbill. The XCTDs are launched through the submarine's torpedo tubes at depth

(-110 m), with the UTS collecting measurements at about this depth throughout the cruise track. The

submarine occupied a series of transects between the NE Chukchi Sea and the U.S./Canada convention

line in the eastern Beaufort Sea.

SIGNIFICANT CONCLUSIONS:

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.

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 that is in-phase with the

annual wind cycle.

The spatial coherence of the flow field suggests an along-slope de-correlation 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, and while the de-correlation

scale was not resolved for this section of the slope in this study, it appears to be less than 200 km.

The cross-slope flow field is complex and was not adequately resolved by the sampling design and

because of the equipment failures.

The coherence between winds and along-slope 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 wind-current coherence might, in part, be due to spatial

and seasonal variations in the sea ice distribution. For example, it appears that during the winter of

1999, the pack ice was immobile for extensive periods of time in the eastern Beaufort Sea. Hence,

the transfer of momentum between the atmosphere and ocean was inhibited by the immobile pack

thereby leading to a weak wind-current correlation overall. We also found several events consistent

with the eastward propagation of remotely forced topographic waves and/or deep-ocean forcing.

Our results suggest that the eastward-flowing Beaufort undercurrent might not extend beyond the

eastern portion of the Alaskan Beaufort slope (where earlier measurements suggested a mean

eastward flow of 3 cm s1 along the shelfbreak in the eastern Beaufort Sea). Recent current meter

measurements from the Mackenzie Beaufort slope find a persistent westward flow near the

shelfbreak, which, in conjunction with our measurements suggests that the flow along the Alaskan

Beaufort slope might be convergent, wherein the eastward flow in the undercurrent meets the

westward flow along the Mackenzie shelfbreak. If indeed the along-slope flow is convergent in this

region, then we expect that slope waters are diverted offshore, possibly entraining shelf waters in

eddies that migrate into the basin interior. This conclusion is hypothetical, however, since our

measurements in the eastern Beaufort along the 1000 m isobath may not represent the shelfbreak

flow field because the continental slope broadens considerably from west to east. Since the eastern

Beaufort mooring was further offshore of the shelfbreak than the moorings in the central and western

Beaufort Sea, it is conceivable that the eastern mooring missed important shelfbreak processes in

this region.

The submarine hydrography suggested that there is a substantial along-slope density gradient in the

vicinity of the Canada/U.S EEZ line and it seems likely that this gradient extends across the shelf

proper. However, we do not know if this is a permanent hydrographic feature established by the
ACCESS NUMBER: 15163

regional wind field and inflows from the Chukchi and Mackenzie shelves or a transitory feature

captured during the synoptic sampling conducted by the submarine. Nevertheless, the existence of

such a gradient has important implications for both the cross- and along-shelf flow structure in this

region and requires additional investigation.

STUDY RESULTS:

There are no data upon which to test the hypothesis advanced in point #6 above. We recommend

that future efforts in this region include an assessment of the along-shelf/slope density gradients by

obtaining synoptic scale measurements at high horizontal and vertical resolution. This could involve

hydrographic casts over the shelf and slope and/or measurements made from a continuously

recording, towed CTD package. It would also be prudent to conduct a more comprehensive set of

current measurements in this region, with concomitant measurements made on the shelf and slope.

2. The measurements obtained in this program were unable to assess the onshelf extent to which

shelfbreak and slope processes extend because of instrument failures, inadequate spatial resolution,

and the technological limitations. The latter can easily be overcome through use of near-bottom

ADCPs that can obtain velocity profiles throughout the water column, even though the instrument

package is deep enough to minimize the risk from drifting ice. There are in addition, moored profiling

instruments now available that can make near-surface measurements, while minimizing their

exposure to ice keels. 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

parameter that will lead to a better understanding of the regional flow field and its dynamics and

serve as an important comparison point for circulation models that may be used in assessing oil spill

risks. The complex cross-slope circulation field and thermohaline structure requires that future

measurements in this region be made at a higher cross-slope spacing than was achieved in this

study.

STUDY PRODUCTS:

Weingartner T.J., 2006. Circulation, Thermohaline Structure, and Cross-shelf Transport in the Alaskan

Beaufort Sea. Final Report. OCS Study MMS 2006-031. Coastal Marine Institute, University of

Alaska Fairbanks and USD01, MMS, Alaska OCS Region. Fairbanks, AK

Weingartner, T. J. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the Alaskan

Beaufort Sea [Abstract]. In: University of Alaska Coastal Marine Institute. Annual Report No. 5.

1998. Coastal Manne Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS

Region. Fairbanks, AK

Weingartner, T. J. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the Alaskan

Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report No. 6. 2000. Coastal

Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region. Fairbanks,

AK

Weingartner, T. J. and Aagaard, K. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the

Alaskan Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report No. 7. 2000.

Coastal Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region.

Fairbanks, AK

Weingartner, T. J. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the Alaskan

Beaufort Sea. 2001. In Proceedings Alaska OCS Region Eighth Information Transfer Meeting, p.

12. Prepared by MBC Applied Environmental Sciences for MMS Alaska OCS Region. Anchorage,

AK.

Weingartner, T. J. and Aagaard, K. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the

Alaskan Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report No. 8. 2002.

Coastal Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region.

Fairbanks, AK

Weingartner, T. J. and Aagaard, K. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the

Alaskan Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report No. 9. 2003.

Coastal Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region.

Fairbanks, AK
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ACCESS NUMBER: 15163

Weingartner, T. J., Aagaard, K., Takazawa, T., and Carmack, E. C. Circulation, Thermohaline Structure,

and Cross-Shelf Transport in the Alaskan Beaufort Sea. In: University of Alaska Coastal Marine

Institute. Annual Report No. 10. 2004. Coastal Marine Institute, University of Alaska Fairbanks and

USD01, MMS, Alaska OCS Region. Fairbanks, AK

Weingartner, T. J. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the Alaskan

Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report No. 11. 2005. Coastal

Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region. Fairbanks,

AK

Weingartner, T. J. Circulation, Thermohaline Structure, and Cross-Shelf Transport in the Alaskan

Beaufort Sea. In: University of Alaska Coastal Marine Institute. Annual Report. 2005. Coastal

Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region. Fairbanks,

AK

*pls affiliation may be different than that listed for Project Manager(s).

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Focus area for "Circulation, Ttiermohaline Structure, and Cross-shelf Transport in the Alaskan Beaufort

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