Friday, 30 April 2010

Monsoons went erratic 18,000 years ago

Syed Akbar
Hyderabad, April 30: Erratic rainfall during present day monsoons is not a new phenomenon, but a result of climatic changes that occurred in the Arabian sea some 18,000 years ago.

The Arabian sea, from which emerge the monsoons bringing in rainfall to the Indian sub-continent, turned warmer by 4 degrees Celsius about 18 millennia ago, leading to reduced rainfall in the region. The phenomenon continues till day causing erratic monsoons.

Researchers at the National Institute of Oceanography and the National Centre for Antarctic and Ocean Research have reconstructed the rainfall viability over thousands of years to study what went wrong with the monsoon system, since the Indian sub-continent largely depends on the monsoon for rainfall.

The joint research team of NIO-NCAOR has found that the Arabian sea was 4 degrees C cooler during 18,000 years ago, while the general belief is that the sea was cooler by just 2 degrees C. "In our study we have demonstrated that Arabian sea was 4 degrees C cooler during 18 kilo annum (18,000 years), which had a strong bearing in the initiation of monsoon rainfall in the Asian sub-continent," Dr Pothuri Divakar Naidu of NIO, Goa.

He told this reporter that the reconstruction of rainfall viability over thousands of years reveal that from 10.5 to 3.5 ka (10,500 years to 3,500 years ago) there was good amount of rainfall in the subcontinent. "But from about 3,500 years onwards descending phase of monsoon rainfall started due to arid climatic conditions in the Indian  subcontinent," Dr Divakar pointed out.

The team studied variations in sea surface temperature, sea water and salinity in the Arabian sea. They reconstructed the atmospheric model for the past 68,000 years using a sediment core from the Eastern Arabian Sea to understand the changes in evaporation and precipitation associated with the monsoon system.

The southwest monsoon emanates from the temperature and pressure gradients that occur between the Indian Ocean and Tibet Plateau during summer. The upper ocean heat supplies the necessary evaporation and atmospheric moisture transport during this phenomenon. Thus, a strong link exists between ocean dynamics and the atmospheric heat and moisture transfer behind the monsoon

"The socio-economic and agriculture development of southeast Asia depend on the monsoon precipitation from June through September,
which contributes about 70 to 90 per cent of the annual precipitation in the region. Therefore, an understanding of the monsoon system beyond historical records is desired," he said.

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