Date:29/09/2007 URL: http://www.thehindu.com/2007/09/29/stories/2007092961781300.htm

Tamil Nadu

State may not get bountiful rains this monsoon

T. Ramakrishnan

Northeast monsoon is the main season for Tamil Nadu
80% of rainfall is usually recorded in October and November

CHENNAI: The State may not get bountiful rainfall during this year’s northeast monsoon.

This is the indication of a study of the Regional Meteorological Centre on the basis of the different parameters used to predict the monsoon. Normally, the monsoon sets in on October 20, with an allowance of seven days. The monsoon goes on till the end of December. Eighty per cent of the monsoon rainfall is usually recorded in October and November.

For Tamil Nadu the northeast monsoon is the main rainy season, accounting for about 48 per cent of the annual rainfall of around 100 cm. Coastal districts of the State get nearly 60 per cent of the annual rainfall and the interior districts get about 40 to 50 per cent of their annual rainfall during the monsoon. The meteorological centre does not issue an official long-range forecast of the monsoon.

The westerlies in April, tropical easterly jet (TEJ) stream and southern oscillation index (SOI) are the parameters taken into account while predicting the monsoon.

Though these parameters do not have a one-to-one relationship with the pattern of rainfall during the monsoon, they are among the factors that influence the behaviour of the monsoon, according to Y.E.A. Raj, Director of meteorology. For a bountiful monsoon, strong westerlies in April and weak TEJ stream in August and September should be prevalent. But, this year the westerlies in April were weak and the TEJ stream was strong.

The SOI should be negative for heavy rain in October and November. But, the index has remained positive. When the index remains positive, the equatorial trough, the major synoptic feature responsible for monsoon activity and which is constantly in motion from north to south during September-January, is pushed northwards affecting the rainfall in October and November.

A mild La Nina condition (decline in the Pacific Ocean sea-surface temperature off Peru’s coast along the equatorial belt) is also said to be prevailing.

Dr. Raj says that all these do not mean that the monsoon will be deficient. “In all likelihood, it may be near normal.”

Going by meteorological standards, the departure of plus or minus 19 per cent from the average rainfall of 48 cm is considered normal.