Date of this Version
Six years of current meter and water property data were collected year-round (1999 – 2007) from the landfast ice zone of the nearshore Alaskan Beaufort Sea (ABS). The data show large seasonal differences in the circulation that is defined by the set-up and breakup of the landfast ice. During the open water season (July – mid-October) mid-depth currents often exceed 20 cm-s-1, whereas during the landfast ice season (mid-October – June) these currents are generally <10 cm-s-1. Tidal currents are feeble (<3 cm-s-1) year-round and probably do not play a dynamically significant role on the inner shelf.
Most (>90%) of the current variability is in the along-shore direction year-round. In general the mean currents are not statistically different from zero over the whole record or in individual seasons. Open water currents are significantly correlated with the local winds, but currents beneath the landfast ice are not. Calculations conducted over both seasons suggest along-shore sea-level gradients are about 10-6, with the magnitude of these gradients being only slightly larger during the open water season than during the landfast ice season. These gradients are presumably set-up by the winds during the open water season, but their origin during the landfast ice season is unknown. However, preliminary model studies indicate that spatial variations in the underice friction coefficient are capable of establishing along-shore pressure gradients of this magnitude. During the open water season upwelling-favorable winds force westward flows that are strongly sheared in the vertical and with maximum currents at the surface. In contrast, downwelling favorable winds are weakly sheared in the vertical. The asymmetric current structure is presumed due to differences in stratification; strongly stratified during upwelling (westward) winds and weakly stratified during downwelling (eastward) winds.
Cross-shore flows are generally small (~3 cm s-1) compared to along-shore currents. However, cross-shore flows of ~10 cm-s-1 were observed during the landfast ice season when the spring freshet resulted in an offshore spreading of a buoyant plume beneath the landfast ice. Although measured cross-shore flows are generally small, satellite imagery suggests that frontal instabilities associated with low-salinity nearshore plumes can transport inner shelf waters offshore to the Beaufort shelfbreak during the open water season. Observations from elsewhere in the Arctic suggest that cross-shore current speeds associated with instabilities can be as large as 30 cm s-1.
Our results suggest that oil spilled beneath the landfast ice will stay within the vicinity of the oil spill source as current speeds will rarely exceed the threshold velocity required to transport an oil slick once it has attained its equilibrium thickness. We find that an underice oil spill has a 90% probability of remaining within 20 km of its origin over a 12-day period. Because of the broad spatial coherence in the flow field (~100 km in along-shore extent), underice currents could be monitored at one point and transmitted real-time to cleanup crews in the event of an underice spill. This information would verify the current speeds and whether oil would stay in the vicinity of the spill. Oil spilled during the open water season could be rapidly dispersed over great distances (~200 km in 12 days) in both the along- and cross-shore directions, however.
Water properties also vary seasonally in response to ice formation and melting, the spring freshet, and wind-mixing. Salinities increase and temperatures decrease throughout the winter due to freezing and brine expulsion from sea-ice. During the spring freshet, the inner shelf is strongly stratified and remains so until the ice retreats and downwelling winds mix the water column. The annual suspended sediment cycle, based on transmissivity measurements, suggests rapid deposition of river borne sediments beneath the landfast ice during the spring freshet, with re-suspension and transport occurring throughout the open water season depending upon storm frequency. Re-suspension and transport is also vigorous during the formation of landfast ice and we conclude that much sediment is incorporated into the ice matrix at this time of the year. Ice-incorporated sediments are either transported with the ice or returned to the water column during melting the following summer.
There are several important issues that we believe need to be addressed in the future. Modeling of the landfast ice zone requires an understanding of the role that ice-water friction plays in this region. Measurements of the spatially and temporally varying underice topography are critical to understanding the dynamics of this shelf. Second, the source and magnitude of the along-shore pressure gradients responsible for the underice currents needs to be determined. Third, it is not clear if the findings based on current measurements made in water depths ≤17 m apply to deeper portions of the landfast ice zone. Hence the cross-shore coherence in the underice circulation field needs to be determined. Fourth, the introduction of freshwater creates stratification that can lead to an asymmetric current response to wind-forcing during the open water season. Observations on the thermohaline structure of the Beaufort shelf are needed in order to understand and model the circulation field during the open water season. Cross-shore salinity fronts, established by river runoff, can become unstable and cause energetic cross-shelf flows capable of carrying pollutants far offshore. The dynamics and kinematics of these features need study. Fifth, sediments can adsorb pollutants and be incorporated into the ice along with oil; hence we recommend that consideration be given to the potential role that ice plays in the transport of sediments and pollutants on this shelf.