RELATIONS BETWEEN ATMOSPHERIC CIRCULATION AND MASS-BALANCE AT SOUTH CASCADE GLACIER, WASHINGTON, USA

The yearly net mass balance of South Cascade Glacier, Washington, has decreased since the mid-1970s. Results show that the decrease is prima rily caused by a significant decrease in the winter mass balance. The decrease in winter mass balance is caused, in part, by changes in wint er mean atmospheric circulation that began during the mid-1970s. Appro ximately 60% of the variability in winter mass balance can be explaine d by variations in winter mean 700-mb heights over western Canada. Sin ce the mid-1970s, there has been an increase in winter mean 700-mb hei ghts over western Canada and the northern western contiguous United St ates and a decrease in winter mean 700-mb heights in the eastern North Pacific Ocean centered near the Aleutian Islands. These changes in at mospheric circulation indicate a decrease in the movement of storms an d moisture from the Pacific Ocean into the western contiguous United S tates. In addition, the increase in winter mean 700-mb heights over we stern Canada and the northern western contiguous United States indicat es an increase in subsidence, which results in a warming and drying of the air that further reduces precipitation and also increases the rat io of rain to snow during the cold season. These factors contribute to below-average winter mass balances.