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Ocean activity is key controller of summer monsoons

Each summer, a climatic shift brings persistent wind and rain to much of Southeast Asia, in the form of a seasonal monsoon. The general cause of the monsoon is understood to be an increasing temperature difference between the warming land and the comparatively cool ocean. But for the most part, the strength and timing of the monsoon, on which millions of farmers depend each year, is incredibly difficult to predict. Now, National Science Foundation-funded scientists have found that an interplay between atmospheric winds and the ocean waters south of India has a major influence over the strength and timing of the South Asian monsoon. Their results show that as the summertime sun heats up the Indian subcontinent, it also kicks up strong winds that sweep across the Indian Ocean and up over the South Asian land mass. As these winds drive northward, they also push ocean waters southward, much like a runner pushing against a treadmill's belt. The researchers found these south-flowing waters act to transport heat along with them, cooling the ocean and, in effect, increasing the temperature gradient between the land and sea. They say this ocean heat transporting mechanism may be a new knob in controlling the seasonal South Asian monsoon, as well as other monsoon systems around the world.

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