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 have 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, we address these issues.

We find that strong Indian summer monsoon precipitation is preceded by warmer than normal temperatures over Europe and North America in the previous winter and over western Asia in the previous spring, but colder temperatures over Tibet. The European temperature anomalies are related to the positive phase of the North Atlantic Oscillation. Negative snow cover anomalies in Europe, previously shown to be correlated with strong monsoons, are produced by circulation and temperature anomalies. The snow-albedo feedback is always operating, but the snow by itself does not physically control the monsoon. Strong Indian summer monsoon precipitation is actually accompanied by higher than normal Tibetan snow cover in spring. Anomalous snow cover impacts on temperature are not prolonged by soil moisture feedbacks, and there is no obvious relationship between soil moisture and the monsoon. The mechanism by which winter Eurasian temperature anomalies persist for several months in order to be able to influence the strength of the monsoon still need to be examined.