India may lose one of its crucial lifelines -- the south west monsoon, which brings rains across the country during the summers, in the next 150 years, warns a new study by Indian Institute of Tropical Meteorology.

The phenomenon is the result of global warming which has led to increase in the rate of temperature rise over Arabian Sea, researchers say.

This rise is reducing temperature difference between land and sea, known as Temperature Gradient (TG), responsible for attracting rain causing winds from Arabian Sea towards Indian mainland.

"For climatology, 30 years variations are considered. The decreasing trend (of TG) is highly significant and in another 150 years or so, it may tend to zero," lead author S M Bawiskar, a scientist at the Pune-based institute, told PTI.

Once the gradient becomes zero, Mr. Bawiskar warns, the monsoon winds will be replaced by dry easterly winds thus disturbing the flow of south-west monsoon.

He found that average temperature over Arabian Sea was 18.77 degrees Celsius during 1948-77 but it has increased by 0.87 degrees to touch 19.64 degrees during 1979-2008.

The results published in the Journal of Earth System Science show that rate of temperature increase over sea are higher than land which has narrowed the Gradient by 1.11 degrees during the said period.

The below normal rains during the current season are also an indication of the changes taking place in the temperature over the Arabian sea.

The Indian Meteorology Department has shown a 23 per cent reduced rain activity during the present season, which may be an indication of this phenomenon.

"With decreasing TG, monsoon current over Arabian sea would become weak. This will lead to reduced rainfall activity over Indian Peninsula. The break like circulation will prevail for a longer period of time, which we are already experiencing during this monsoon," Mr. Bawiskar said.

Global warming, which is increasing the average temperature across the planet, is also behind the increasing temperature over the Arabian sea, the study says.

During monsoon, Arabian sea and Indian peninsula are under the grip of westerlies (winds coming from west) in the lower troposphere.

These winds are mainly responsible for bringing rains over the Indian land mass and are established during the monsoon due to the Temperature Gradient.

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