DEEP-SEA drilling has confirmed that monsoons - the summer onslaught of rains around the Indian Ocean - have been a feature of the region's climate for millions of years.
The extreme height and persistent snow cover of the Himalayas cause marked seasonal differences in heating between the mountains and the Indian Ocean.
In summer, strong southwesterlies blow as warm air rises above the Indian subcontinent. In winter, as the land mass cools but the ocean remains warm, a reversal in wind direction influences the circulation of ocean waters. This, in turn, affects plankton productivity, which depends on the upwelling of nutrient-rich water.
Researchers at Cambridge University in England and Edinburgh University in Scotland who are working with the international Ocean Drilling Program say they have identified evidence in sediment core from under the Indian Ocean that shows changes in plankton productivity and wind velocity.
The scientists found that the strength of the monsoon varies in phase with changes in the Earth's orbit around the sun. These changes affect the intensity of incoming solar radiation.
This process has been continuing ever since the first monsoon began with the upthrust of the Himalayas, 10 million years ago.