Variations in snow and ice distributions of the Western Arctic are partly attributed to changing patterns of atmospheric circulation, especially during spring. This is demonstrated by comparing subsets of "early" and "late" years of melt at BRW in conjunction with analyses of PMW results and corresponding synoptic-scale circulation patterns. For example, the west-east coupling of Low and High pressure centers indicated in Figure 1b favors an early rather than a late snow melt (Figure 1a) because the flow of warm, moist Pacific air into the region is enhanced. Early onset tends to lengthen the melt season, possibly resulting in an overall loss of ice volume. Concern arises as to whether or not the observed trends are manifestations of natural, low-frequency oscillations or are anthropogenically forced.
Figure 1. Average sea ice concentration and maximum Normalized Difference Vegetation Index (NDVI) during 21-31 May for (a) late years (1985, 1986, 1987, 1988) of snowmelt at BRW compared with (b) early years (1990, 1996, 1998) of melt. The geopotential height contours show, schematically, the mean synoptic patterns and associated prevailing winds at 850 hPa averaged for March, April, and May of the respective years. White over land areas is snow cover with deeper green shading indicating advanced growth. Gray over ocean areas represents open water, with progressively lighter shading indicating higher sea ice concentrations. By the end of May, during years of early melt at BRW, snowmelt is also well advanced over eastern Siberia, and low ice concentrations are observed over much of the western Arctic Ocean. The west-east coupling of Low (L) and High (H) pressure centers favors an early onset of melt due to enhanced flow of warm Pacific air into the western Arctic region. An overall decline of sea ice volume may result if such a pattern persists in a succession of years.
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