DYNAMIC CHARACTERISTIC OF SNOW COVER IN WESTERN CHINA
Li Peiji (Lanzhou Institute of Glaciology and Geocryology, Chinese Academy of Sciences)
The dynamic characteristics of spatial distribution, annual cycle and secular variation ofseasonal snow cover in western China were investigated by using the best information concerning microwave derived snow depth charts produced by NASA data from SMMR (1978-1987), operational NOAA digitized weekly snow cover extent charts (1973-1987) ,short wave DMSP imagery (1978-1987), in conjuction with daily snow depth records in 175 ground climate stations covering the period from 1957 through 1987. Of all seasons, winter (dec. ,Jan. and Feb. )has the greatest snow volume and areal extent. It is the most variable season too. Year-to-year variability of snow volume is more prominent than that of snow area. The increase in snow cover area is succeeded by the increase in snow volume during growing progression of snow cover, whereas in decaying phase, the decrease in snow volume is followed by the decrease in snow cover area. The large interannual snow cover anomalies over the Tibetan Plateau are the most striking feature. The seasonal progress of snow cover in the Tibetan Plateau is opposite to that in the lowlands. It is characterized by a rapid grow and a slow decline. All generated time series of snow volume, snow cover area, and number of snow covered days by using SMMR snow charts, NOAA snow charts,and ground station data demostrated that the periods with above normal snow cover have included the late 1970s and mid 1980s, while the mid 1970s and early 1980s were on the low side. The years with greatest snow volume and snow cover extent in western China were 1976/1977, 1977/1978,and 1985/1986,while winters with the least snows were 1973/1974, 1984/1985. There is a clear indication of long term trend in 30-year records over the Tibetan Plateau, increasing with the global warming. Time evolution for the recent 30-year events suggests that both excessive Tibetan snow cover and poor Indian summer monsoon rainfall are mainly result from the global anomalies of oceanic and atmospheric pattern referred as the El-Nino-Southern Oscillation. The eastward excursion of the West Pacific warm pool leads to a poor Indian summer monsoons. The wintertime atmospheric anomalies and the northern hemisphere teleconnection with ENSO events bring more snowfall on the Tibetan Plateau. Empirical and modeling studies indicate that snow cover over the Tibetan Plateau plays a significant role in seasonal atmospheric conditions. Our study fails to confirm the widely-accepted view. Perhaps, the fact is that the dearth of snow cover limits the snow-albedo effect and snow-hydrological effect on climate. The implication of these results for the Ice Age issues are also briefly discussed.