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Vinson Kurian

Thiruvananthapuram, Oct 22 The northeast monsoon has set in over the coasts of Tamil Nadu and adjoining States, the India Meteorological Department (IMD) announced on Monday.

This happened side by side with the southwest monsoon leaving the entire landmass and both the peninsular seas in one fell swoop. The IMD has forecast fairly widespread rainfall for south peninsular India for the next four days.

Isolated heavy falls have been forecast over Tamil Nadu, Kerala, south coastal Andhra Pradesh and Rayalaseema.

What promises to significantly drive up the rainfall is the ‘low’ forecast to develop over the Bay of Bengal soon. A preparatory cyclonic circulation has been doing the rounds over the South Andaman Sea for some time now.

Almost simultaneously, a ‘low’ is predicted to develop over the southwest Arabian Sea (away from the Indian coast), which could go on to become a severe weather system, as per the assessment of the European Centre for Medium-Range Weather Forecasts (ECMWF).

The Bay ‘low’ will keep pace, but may not intensify beyond the level of a depression before crossing the Tamil Nadu coast by the weekend, the ECMWF forecasts seemed to suggest.

In its projections, the US National Centres for Environmental Prediction (NCEP) said that the wet cover for coastal Tamil Nadu, the extreme south peninsula and Kerala would sustain at least until November 5.

Northeast monsoon is the major period of rainfall activity over south peninsula, particularly in the eastern half comprising the meteorological subdivisions of coastal Andhra Pradesh, Rayalaseema and Tamil Nadu-Pondicherry.

For Tamil Nadu, this is the main rainy season accounting for about 48 per cent of the annual rainfall. Coastal districts of the State get nearly 60 per cent of the annual rainfall and the interior districts get about 40-50 per cent.

Though the southwest monsoon is the principal rainy season for interior Karnataka, Kerala and Lakshadweep, rainfall continues till December in these sub-divisions, thanks largely to the northeast monsoon.

The increase in rainfall activity over Andhra-Tamil Nadu coasts around the middle of October is generally taken to mark the “setting in of northeast monsoon.”

Normal date of onset is around October 20 with a deviation of about a week on either side. India does not have a formal forecast system in place for the northeast monsoon.

There is a crucial difference in the rainfall pattern under the two monsoon systems. Rains are assured over the coast and comparatively less so in the interior during southwest monsoon, while the reverse becomes true in northeast monsoon.

Rain variability is high along the coast, while it pours in the interior during northeast monsoon in what presents an enduring paradox. This should also explain why the spatial and temporal distribution of southwest monsoon leaves, as is often the case, much to be desired.

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