Sea Ice Response to Atmospheric and Oceanic Forcing in the Bering Sea

A coupled sea ice–ocean model is developed to quantify the sea ice response to changes in atmospheric and oceanic forcing in the Bering Sea over the period 1970–2008. The model captures much of the observed spatiotemporal variability of sea ice and sea surface temperature (SST) and the basic features of the upper-ocean circulation in the Bering Sea. Model results suggest that tides affect the spatial redistribution of ice mass by up to 0.1 m or 15% in the central-eastern Bering Sea by modifying ice motion and deformation and ocean flows. The considerable interannual variability in the pattern and strength of winter northeasterly winds leads to southwestward ice mass advection during January–May, ranging from 0.9 × 1012 m3 in 1996 to 1.8 × 1012 m3 in 1976 and averaging 1.4 × 1012 m3, which is almost twice the January–May mean total ice volume in the Bering Sea. The large-scale southward ice mass advection is constrained by warm surface waters in the south that melt 1.5 × 1012 m3 of ice in mainly the ice-edge areas during January–May, with substantial interannual variability ranging from 0.94 × 1012 m3 in 1996 to 2.0 × 1012 m3 in 1976. Ice mass advection processes also enhance thermodynamic ice growth in the northern Bering Sea by increasing areas of open water and thin ice. Ice growth during January–May is 0.90 × 1012 m3 in 1996 and 2.1 × 1012 m3 in 1976, averaging 1.3 × 1012 m3 over 1970–2008. Thus, the substantial interannual variability of the Bering Sea ice cover is dominated by changes in the wind-driven ice mass advection and the ocean thermal front at the ice edge. The observed ecological regime shifts in the Bering Sea occurred with significant changes in sea ice, surface air temperature, and SST, which in turn are correlated with the Pacific decadal oscillation over 1970–2008 but not with other climate indices: Arctic Oscillation, North Pacific index, and El Niño–Southern Oscillation. This indicates that the PDO index may most effectively explain the regime shifts in the Bering Sea.