Is snow a rainkiller?

FOR TWO decades snowfall was one of the parameters Indian meteorologists used to predict monsoon in the subcontinent.

They believed there was an inverse relation between snowfall in the Himalaya and monsoon; the wider the snow cover, the weaker the rainfall. But since 2007 the India Meteorological Department has dismissed this teleconnection citing paucity of data; satellite data are available since 1979 and ground observations are difficult in the mountainous terrain.

Andy Turner of the University of Reading in the UK recently created a computer model that establishes this connection between monsoon in India and snowfall in the Himalaya and Tibetan plateau the preceding winter and spring. One of the factors that initiates summer monsoon in India is the temperature gradient from north to south, Turner explained. Solar heating in late spring heats the Indian subcontinent, making it warmer than the Indian Ocean. It also warms the Tibetan plateau that acts as an elevated heat source. This drives southwest monsoon winds towards the Indian landmass.

The snow-monsoon teleconnection works by altering this temperature gradient. Turner’s model shows the dominant effect of the Himalaya and Tibetan plateau snow on monsoon is because of albedo, the reflectivity of snow. Increased snow cover over the Himalaya and Tibetan plateau reflects more solar radiation, resulting in less than normal warming of the land surface there. Consequently, the temperature gradient decreases and monsoon winds weaken. This means they bring less moisture to India and don’t penetrate as far north.

Turner said though this was only a modelling study, the scientists did not use any direct observation to examine the correlation. The study was published online on April 11 in Climate Dynamics. M Rajeevan, scientist working with the National Atmospheric Research Laboratory in Tirupati, Andhra Pradesh, said the Himalayan snowfallmonsoon teleconnection can still not be used as a predictive tool: the data on snowfall is not reliable as global warming has melted snow.

Turner pointed out the main motivation for the study was to explore the relative role of different regions of snowcover on the Indian monsoon. Some earlier studies had suggested that it is the spring/winter snow in northern Europe which is crucial. “However, here it is clear that the only obvious mechanism exists from the Himalayan region, and no teleconnection is found with northern Europe,” Turner said.

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