Effects of a shelf polynya on flow and water properties in the northern Bering Sea

Abstract

We present long‐term (∼230‐day) measurements from three moorings in the vicinity of Saint Lawrence Island on the northern Bering Sea shelf. One mooring was deployed near the polynya which often exists south of the island. These data confirm the existence of strong (∼0.15 m s−1) flow toward Bering Strait northwest of the island (in Anadyr Strait) and suggest that regional circulation results in a moderate (∼ 0.03 5 m s−1) mean flow eastward along the southern coast of Saint Lawrence Island. Coherent variations in the regional circulation and variations in the geostrophic wind account for much of the low‐frequency current fluctuation south of Saint Lawrence Island. However, there were 11 events when offshore (northerly) winds coincided with the temperature following the freezing point, increasing salinity, and reversal of the current. This suggests that ice formation and the ensuing brine rejection affect flow in the vicinity of the polynya. Scaling of a simplified momentum equation indicates that the cross‐shelf density gradient was a possible mechanism for some of the reversals. Salinity data show a seasonal cycle as well as individual events of brine rejection. During an average event time of 65 hours, the salinity increased by 8×10−3 g kg−1 h−1; the heat flux can be estimated as 535 Wm−2. These fluxes correspond to an ice production of ∼5 m during winter 1981. Extrapolation of our results and comparison with the freshwater cycle suggest that brine rejection is an important component of the regional salt budget.

Tidal currents in the St. Lawrence Island region

“…In the St Lawrence Island region, the manifestation of this semiannual period ( Figure 6) results in approximately 2.5 cm/s difference between February/August (2.5 cm/s) and November/May (5 cm/s). Although 5 cm/s is not a particularly energetic flow, it is of the same order of the mean background flow (3.6 cm/s) at NC25A [Schumacher et al, 1983] and is approximately 40% of the annual mean tidal flow and 14% of the peak tidal flow. The major seasonal cycle for the diurnal currents is not related to the annual cycle of density that dominates behavior of M 2 , but the semiannual cycle caused by the linear superposition of the K 1 and P 1 constituents.…”

“…Ice cover progresses from north to south [Niebauer et al, 1999] and by the end of December the entire northern shelf is ice covered. During the winter, a polynya is generated immediately to the south of St. Lawrence Island by periodic strong northerly wind events [Schumacher et al, 1983;Johnson and Kowalik, 1986]. Opening and closing of the polynya occurs until late April, when increasing solar radiation and warmer air temperatures melt the sea ice.…”

Temporal changes in dissolved oxygen of the intermediate water in the subarctic North Pacific

“…The necessary component for deep ventilation is the salt supply. It can be added to seawater as brine during the formation of ice in the shelf areas of the Bering Sea [Schumacher et al, 1983]. Another source of salt for the surface layer of the Bering Sea is the advection of the more saline water from low latitude areas through the Aleutian Islands straits.…”

Impact of eastern Arctic shelf waters on the Nansen Basin intermediate layers

“…Significant quantities of brine-enriched bottom water are assumed to form on the Siberian shelf seas as a result of sea ice generation [Cavalieri and Martin, 1994]. Dense, brine-enriched water has also been observed in the Bering Sea [Schumacher et al, 1983] and in the Beaufort Sea [Melling and Lewis, 1982], which may contribute to maintaining the Arctic Ocean halocline. The plumes have regionally different temperature-salinity signatures, so that they mix with the ambient water at different depths.…”