More snow cover means less rain

A recent study reveals how the snow cover in Asia and Europe controls the intensity of the Indian monsoon.

By Anjani Khanna
Published: Monday 15 March 1993

-- MONSOON modeller A D Vernekar and his colleagues at the University of Maryland have worked out how the snow cover in Europe and Asia, especially in the Tibetan plateau, affects the Indian monsoon.

Using what he calls a COLA (Centre for Ocean-Land-Atmosphere Interactions) global circulation model, Vernekar simulated the global atmospheric circulations from February to September for two years -- one that saw heavy snow and the other, light snow. The scientists found the monsoon to be good, with heavy rains all over India, when the snow cover in the two continents was light, and weak when the snow cover was heavy.

Scientists have known for a long time that the temperature gradient between the equator (0o), which is usually warmer, and the 35o North parallel, which is cooler, reverses when the monsoon builds up. Through June, July and August, the temperature at the 35o North parallel, which coincides with the 4,000 metre high Tibetan plateau, is higher than that at the equator. It was this difference in temperature that was believed to induce the monsoon winds.

However, scientists have discovered since that the extent of temperature difference varies according to the depth of the snow cover in Europe and Asia. When the snow cover is heavy, the sun's heat is used to melt the snow and dry the soil below it. Consequently, the temperature in the Tibetan plateau does not rise as much as it would have if the snow cover had been light. The more the difference in temperature, the stronger the monsoon winds. These, in turn, induce greater evaporation and result in more moisture in the air.

Scientists found that a heavy snow cover results in a weak but prolonged monsoon because the absence of dense clouds causes more solar radiation to reach the earth's surface and maintains the reversed North-South temperature gradient for a longer period.

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