Robert G. Crane, Department of Geography & Earth Science Center, Pennsylvania State University, University Park PA 16802
Sea ice in the Bering Strait forms in shore leads in Norton Sound and off St. Lawrence Island. Northerly winds then advect the ice southward, where it melts in the warmer water near the edge of the Bering Shelf. This "conveyor belt" of ice formation, advection, and melt is driven by the constant stream of low pressure systems that track across the northern North Pacific. Day-to-day variability in the ice cover reflects changes in the atmospheric circulation caused by low pressure systems moving through the region, while the interannual variability depends on the preferred location of the storm track during the season. A high frequency of storms tracking northward along the western margin of the Bering Sea produces southerly winds that push the ice margin northward. Conversely, a northward track along the eastern margin of the Sea, produces northerly winds that result in extensive ice cover.
Interannual variability in the storm tracks is related to larger scale features of the atmosphere-ocean system in the Pacific. Prior to 1978, there is a clear relationship between alternating patterns of El Nino-La Nina events, the circulation over the northern North Pacific, and the lagged response of the ice cover in the Bering Sea. In the late 1970's, there is a step change in the North Pacific climate regime. The circulation switches to an almost continuous El Nino mode, there is a shift in the location of the Aleutian Low, and reduction in Bering Sea ice extent (Niebauer, 1998). Since that time, there has been a slight increasing trend in winter and spring-time ice extent. Despite the present trend toward increasing ice cover, the mean ice extent for 1978-1996 is approximately 5% lower than for the period 1947-1977.
Niebaur, H.J., 1998. Variability in Bering Sea ice cover as affected by a regime shift in the North Pacific in the period 1947-1996. Journal of Geophysical Research, 103(C12): 27, 717-27, 737.
