La Nia flees

Indian monsoon may lose out

Published: Friday 15 August 2003

the chances of 2003 being a La Nia year have become remote, if scientists are to be believed. This is a big disappointment for India, as the phenomenon of the unusual upwelling of cool waters in the equatorial Pacific Ocean is associated with heavy monsoons in the country. Since 1901, all the 18 La Nia years with the exception of one have been marked by heavy rainfall all across the country.

A cooling was evident in the eastern equatorial Pacific since January this year, raising the hopes of a La Nia event, but the trend has reversed since early June. "It stopped mid-way," says M Rajeevan, director of long-range forecasting of the India Meteorological Department (imd), Pune.

A key element in monitoring La Nia is the sub-surface temperature gradient in the Pacific Ocean. The surface water of the ocean is usually warm and the deeper water cold. In between exists a narrow band, called the thermocline, where temperatures fall rapidly. As a La Nia develops, the thermocline becomes shallower, due to an upward surge of cool water.

Negative sea-surface temperature anomalies (read: cooling) have weakened across the central and eastern equatorial Pacific during June, resulting in near normal thermocline in all La Nia regions. A notice circulated by the us-based Climate Prediction Centre as latest as July 10, 2003, said that current atmospheric and oceanic conditions do not support the development of La Nia in the next few months. Several atmospheric indices linked to La Nia such as Southern Oscillation Index, which is derived from the pressure differences at Tahiti and Darwin (Australia), also do not support its development.

The dim chances of a La Nina year have prompted imd to be cautious while updating its southwest monsoon forecast in July 2003. However, it asserts that this year's rainfall is going to be 98 per cent of the long period average of 88 centimetres, rather than the 96 per cent predicted in April 2003.

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