Geoscientists offer new explanation for formation of monsoons

July 22nd, 2008 - 12:17 pm ICT by ANI  

Washington, July 22 (ANI): Geoscientists at the California Institute of Technology have come up with a new explanation for the formation of monsoons, proposing an overhaul of a theory about the cause of the seasonal pattern of heavy winds and rainfall that essentially had held firm for more than 300 years.

The traditional idea of monsoon formation was developed in 1686 by English astronomer and mathematician Edmond Halley.

In Halleys model, monsoons are viewed as giant sea-breeze circulations, driven by the differences in heat capacities between land and ocean surfaces that, upon heating by sunlight, lead to temperature differences between warmer land and cooler ocean surfaces - for example, between the Indian subcontinent and the oceans surrounding it.

These circulations form overturning cells, with air flowing across the equator toward the warmer land surface in the summer hemisphere, rising there, flowing back toward and across the equator aloft, and sinking in the winter hemisphere, explained Tapio Schneider, associate professor of environmental science and engineering at Caltech.

A different explanation has been offered by Schneider and Simona Bordoni of the National Center for Atmospheric Research in Boulder, Colorado.

The duo used a computer-generated, water-covered, hypothetical earth (an aquaplanet) to simulate monsoon formation and found that differences in heat capacities between land and sea were not necessary.

Monsoons arise instead because of an interaction between the tropical circulation and large-scale turbulent eddies generated in the atmosphere in middle latitudes.

These eddies, which can span more than 300 miles across, form the familiar systems that govern the weather in middle latitudes.

According to Schneider, the eddies are basically large waves, which crash into the tropical circulation. They break, much like water waves on the beach, and modify the circulation as a result of the breaking.

There are feedbacks between the circulation, the wind pattern associated with it in the upper atmosphere, and the propagation characteristics of the waves, which make it possible for the circulation to change rapidly, he said.

This can quickly generate the characteristic high surface winds and heavy rainfall of the monsoon.

These feedbacks provide one possible explanation for the rapidity of monsoon onset, which had been a long-standing conundrum in the traditional view of monsoons, because substantial differences between land and sea temperatures can only develop slowly through heating by sunlight, said Bordoni.

Although the results wont immediately produce better forecasts of impending monsoons, according to Schneider, In the long run, a better understanding of monsoons may lead to better predictions with semi-empirical models. (ANI)

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