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Vinson Kurian

Thiruvananthapuram , Sept. 19

THE International Research Institute for Climate Prediction (IRI) at Columbia University has forecast "enhanced probabilities for above normal precipitation" for southern India and Sri Lanka during October - December this year, which coincides with the north-east monsoon season in Kerala and Tamil Nadu.

The Institute has gone ahead predicting "extended wet episodes" for these regions during December 2003-February 2004 and January-March 2004 as well.

The IRI forecasts come as whiff of fresh air for Kerala, which has suffered a second successive failure of the southwest monsoon. As per statistics available till September 17, the shortfall was estimated to be 22 per cent. Last year, the shortfall during the season was 35 per cent for the whole season.

Of relevance in the preparation of the IRI outlook is a high likelihood that neutral El Nino-Southern Oscillation (ENSO) conditions, observed since mid-June, will continue through the forecast periods. Such neutral ENSO conditions are indicated in the sea surface temperature (SST) predictions on which these climate forecasts are based.

Warmer than average SSTs continue to dominate much of the tropical Western Pacific and Indian Ocean. These are predicted to continue, but gradually weaken, during the forecast period.

Studies by several scientists from India and abroad have shown that above a threshold temperature of 27 degree Celsius, it has been found that warmer ocean surface temperatures produce more rain bearing clouds till a temperature of about 30 degree Celsius.

The southwest monsoon period from June to September is the main rainy season for most parts of India where rainfall during this season varies from 65 to 95 per cent of the annual. During October, the winds in the lowest few kilometres of the atmosphere over peninsular India and the Indian seas reverse their direction and begin to blow from the northeast.

This is the genesis of the northeast monsoon that gives rainfall to Tamil Nadu, Kerala and parts of Andhra Pradesh and Karnataka up till and through December.

According to Dr P. V. Joseph, climate expert and former Director of India Meteorological Department (IMD), a study of the northeast monsoon for the period 1901 to 1990 has shown that its mean date of onset over coastal Tamil Nadu was October 20.

The earliest date of onset has been October 5 in 1943 and the most delayed one on November 11 in 1915. (The mean date of onset of southwest monsoon over Kerala is June 1 and the extreme dates are May 11 in 1918 and June 18 in 1972).

However, for rainfall accounting purposes, the four-month period June1 to September 30 is taken as the southwest monsoon and the period October 1 to December 31 as the northeast monsoon.

Southwest and northeast monsoons contribute 68 and 17 per cent respectively of the annual rainfall in Kerala, whereas these figures for Tamil Nadu are 33 per cent and 49 per cent. These are the only two States where the northeast monsoon can wipe away a part of the rainfall deficiency they already may have suffered during the preceding southwest monsoon season.

The northeast monsoon, which is also known as the winter monsoon, is an important source of water for south peninsular India and Sri Lanka during October-December. While the summer monsoon accounts for most of the annual rainfall over a large part of India, the southeast peninsular Indian regional falls under a rain-shadow area during this season.

Therefore, this region critically depends on the northeast monsoon to supplement the inadequate summer monsoon rainfall. In Tamil Nadu, which is at the core of the northeast monsoon region, nearly 50 per cent of the annual rainfall is received during the northeast monsoon season.

Experts are of the view that seasonal prediction of northeast monsoon rainfall, particularly over the core regions of Tamil Nadu and Kerala, has considerable application for decision making in agriculture and water resource sectors.

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