To what extent does the Tibetan plateau influence the south-west monsoon?

Some 130 years ago, Sir H.F. Blanford, Chief Reporter of the newly-established India Meteorological Department (IMD), noticed that more Himalayan snow cover during the preceding winter presaged a poor monsoon. On that basis, IMD began issuing the first monsoon forecasts from 1882. But monsoon prediction was not so easily done and remains a difficult problem to this day.

Years later, the established view came to be that the Himalayas acted on the monsoon in two ways. The Tibetan plateau, heated up during summer and thereby established an atmospheric circulation that was conducive for the monsoon.

The vast mountain range also acted as a tall barrier, preventing cold, dry air in the northern latitudes from entering the subcontinent and subduing the warm, moisture-laden winds from the oceans that drive the monsoon.

In a paper published in the journal Nature in 2010, William Boos and Zhiming Kuang of Harvard University in the U.S argued that the Himalayas’ role as a barrier was the crucial factor for the monsoon.

Using a general circulation model that simulated what happened in the atmosphere, they found that even if the Tibetan plateau did not exist, the monsoon would be unaffected provided the Himalayas and adjacent mountain ranges were there to prevent intrusion of northern air.

However, Balaji Rajagopalan and Peter Molnar of the University of Colorado at Boulder in the U.S. have taken the view that the Tibetan plateau should not be written off so easily. The heating of the plateau correlated with monsoon rainfall but only during the early and late parts of the season, they observed in a paper published recently in Journal of Geophysical Research: Atmospheres.

As the vast Tibetan plateau, high up in the mountains, warmed during the summer months, it heated the air above, which then rose and created an area of low pressure, explained Dr. Rajagopalan. That belt of low pressure sucked in moisture from the oceans, thus initiating the monsoon.

The heating of the Tibetan plateau correlated well with rainfall over India from May 20 to June 15 when the monsoon was setting in. But then the correlation disappeared only to reappear again for rainfall between September 1 and October 15 when the monsoon was tailing off. “We don’t have a very good answer yet” about how the Tibetan plateau could be influencing the late stage of the monsoon, he said.

In an earlier paper, he and Dr. Molnar had noted that swings in the temperature of the tropical Pacific Ocean’s surface waters near the international dateline, known as the El Nino-Southern Oscillation (ENSO), also strongly influenced rainfall over central India and its west coast during the early and late phases of the monsoon.

With the Tibetan heating and ENSO acting independently of each other, the two factors taken together could have predictive value for rainfall in the monsoon’s early and late phases.

Preliminary results looked promising, Dr. Rajagopalan told this correspondent. Those two phases of the monsoon accounted for over one-third of the total rainfall during the entire season and was “nothing to be sneezed at.”

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