83rd Annual

Tuesday, 11 February 2003: 11:30 AM
Snow cover, soil moisture, and the Asian summer monsoon
Alan Robock, Rutgers University, New Brunswick, NJ; and M. Mu, K. Y. Vinnikov, and D. A. Robinson
Prediction of the strength of the Indian summer monsoon and of the amount of precipitation generated is of importance to more than a billion people. Blanford more than a century ago suggested that Himalayan snow cover in the preceding winter could be used as a predictor of the monsoon, but subsequent investigations failed to clearly describe the mechanisms involved. Does snow cover produce its effect through induced or related atmospheric circulation, or is the effect a direct one on local radiative or hydrological phenomena, such as soil moisture? Or is snow cover just an indicator of other circulation and temperature anomalies, and with no direct physical influence on the monsoon? There have been many climate model simulations addressed to this issue, but few examinations of actual observations of snow and soil moisture. Here, using updated actual observations of snow cover, soil moisture, and atmospheric circulation, as well as long climate model simulations, we examine this topic.

Using updated actual observations of snow cover, soil moisture, and atmospheric circulation, we find that strong Indian summer monsoon precipitation is preceded by warmer than normal temperatures (and reduced snow cover) over Europe and North America in the previous winter and over western Asia in the previous spring, but only for the periods 1870-1895 and 1950-1995. This relationship has now disappeared. The European temperature anomalies are related to the positive phase of the North Atlantic Oscillation. After snow melts in the spring in Asia, soil moisture has a memory of one month at most in the upper 10 cm layer, and less than two months in the upper 1 m layer. Therefore, soil moisture cannot prolong a snow cover anomaly signal long enough to influence surface temperatures and the monsoon and there is no obvious relationship between soil moisture and the monsoon. By examining the same relationships in climate models, both in long control runs and in anthropogenically-forced transient runs, we conclude that the snow cover/monsoon relationship occurs by chance. The recent strong relationship is not a manifestation of global warming. Snow cover is a not a robust predictor of the monsoon.

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