22 July 2008
Monsoon Formation Theory Gets Overhaul
by Kate Melville
Geoscientists from the California Institute of Technology have proposed a new explanation for the formation of monsoons, overhauling a traditional theory that had held firm for more than 300 years.
The old idea of monsoon formation was developed in 1686 by English astronomer and mathematician Edmond Halley (mainly known for his eponymous comet). In Halley's 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.
A different explanation is offered by atmospheric researchers Tapio Schneider and Simona Bordoni, who used a computer-generated, water-covered, hypothetical earth to simulate monsoon formation and found that differences in heat capacities between land and sea were not necessary.
They say that monsoons arise instead because of an interaction between the tropical circulation and large-scale turbulent eddies generated in the atmosphere in middle latitudes. According to their theory, these eddies, which can span more than 300 miles across, form the familiar systems that govern the weather in middle latitudes.
The eddies, Schneider says, 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." 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," added Bordoni.
Although the results won't immediately produce better forecasts of impending monsoons; "in the long run, a better understanding of monsoons may lead to better predictions with semi-empirical models, but much more work needs to be done," concluded Schneider.
Source: California Institute of Technology