Dynamics of upwelling in the Alaskan Beaufort Sea and associated shelf–basin fluxes

Abstract

Data from a high-resolution mooring array deployed across the Alaskan Beaufort shelfbreak and slope, together with an idealized numerical model, are used to investigate the dynamics of wind-driven upwelling and the magnitude of the resulting shelf–basin exchange. The analysis focuses on a single storm event in November 2002 when the sea-ice concentration was 50–70%. The normally eastward-flowing shelfbreak jet was reversed to the west, and the secondary circulation near the shelfbreak was characterized by offshore flow in the upper layer and a nearly equal amount of onshore flow at depth. Ekman theory accurately predicts the strength of the secondary circulation when one takes into account the ice–ocean stress. The depth-integrated alongstream momentum balance reveals that, near the shelf edge, the reversed jet is driven by a combination of the surface stress and divergence of cross-stream momentum flux. The reversed jet is primarily spun-down – before the winds subside – by the alongstream pressure gradient that likely results from the variation in sea surface height. The shelf–basin fluxes of heat, freshwater, and nitrate resulting from the storm are substantial. Much of the yearly supply of heat to the Beaufort shelf from the inflowing Pacific water through Bering Strait was fluxed offshore, and the amount of freshwater transported into the basin represents a substantial fraction of the year-to-year variation in the freshwater inventory of the Beaufort Gyre. The on-shelf flux of nitrate from 4 to 5 such storms could account for most of the net annual primary production that occurs on the Beaufort shelf.