Variations in passive microwave measurements of melt on the Greenland ice sheet and associated Northern Hemisphere atmospheric circulation

A simple microwave emission model was used to simulate 37 GHz brightness temperatures associated with snowpack melt conditions for locations across the Greenland ice sheet. The simulated values were utilized as threshold values and compared to daily, gridded SMMR and SSM/I passive microwave data, in order to reveal regions experiencing melt. The spatial extent of the area classified as melting was examined on a daily, monthly and seasonal (May through August) basis for 1979 to 1991. The typical seasonal cycle of melt coverage shows melt beginning in late April, a rapid increase in the melting area from mid May to mid July, a rapid decrease from late July through mid August, and cessation of melt in late September. Seasonal averages of the daily melt extents demonstrate an apparent increase in melt coverage during the 13-year period of approximately 3.8 percent annually (significant at the 95% confidence interval). This apparent increase is dominated by statistically significant positive trends in melt coverage during July and August in the west and southwest portions of the ice sheet. The melt extent values were compared to several mid-tropospheric teleconnection indices during May 1979 to June 1989. The teleconnection indices were derived by performing a rotated principal components analysis on a 100-point subset of the 700 hPa heights from the NMC octagonal grid. The component scores were used as predictor variables in a multiple regression analysis of surface melt extent. The results from the regression analysis show that the circulation accounts for 56 percent of the trend identified in melt extent between 1979 and 1989. Nine common synoptic patterns of 700 hPa heights were extracted by performing cluster analysis on the component scores. The mean melt extents for eight topographically defined regions of the ice sheet were compared among the nine different synoptic patterns, demonstrating the importance of synoptic-scale atmospheric circulation on the regional variations in melt extent