South-West monsoon will be normal: First forecast

India is set to have a ‘normal' South-West monsoon this year, with rainfall during June-September forecast at 98 per cent of the long period average (LPA) of 893 mm for the country as a whole.

The India Meteorological Department's (IMD) first-stage Long Range Forecast, released here on Tuesday, has pegged the probability of ‘deficient' rains (below 90 per cent of LPA) over the four-month monsoon season at just 6 per cent and 30 per cent for ‘below normal' (90-96 per cent of LPA).

On the other hand, the likelihood of a ‘normal' monsoon — wherein rainfall would be 96-104 per cent of the LPA — is as high as 53 per cent, according to the IMD's ‘Statistical Ensemble Forecasting' Model. The respective probabilities for ‘above normal' (104-110 per cent) and ‘excess' (110 per cent-plus) are reckoned at only 10 per cent and one per cent.

“Quantitatively, the monsoon season rainfall is likely to be 98 per cent of the LPA with a model error of plus or minus five per cent,” the IMD Director-General, Dr Ajit Tyagi told presspersons.

The agency's forecast is largely based on the assumption of the current ‘La Nina' conditions — a counterpart to ‘El Nino' associated with monsoon failure — continuing till June and weakening thereafter to a ‘neutral' phase.

Dr Tyagi, however, did not rule out La Nina conditions turning straightaway into El Nino without passing through an intermediate ‘ENSO-neutral' stage. “We have had instances of this earlier in 1964 and 1971,” he noted.

El Nino and La Nina, collectively known as ENSO, basically involve unusual spurts (as in the former) or dips (as in the latter) in sea surface temperatures across the tropical Pacific Ocean.

In 2009 — which recorded a poor monsoon — El Nino conditions originated in June and peaked in December, before reaching ENSO-neutral level towards May 2010.

Subsequently, from June, weak La Nina conditions emerged and attained moderate-to-strong status from mid-August to early February this year. The result: Bountiful rains and an extended winter, contributing to a farm rebound.

Right now, La Nina is assessed to be of moderate-to-weak strength and the hope is it does not directly graduate to El Nino, as was the case in 1964 and 1971.

“The climate forecasts prepared at this time of the year has large uncertainty,” Dr Tyagi admitted. The IMD will issue a forecast update in June, which will also have separate rainfall predictions for July and August and for the country's four homogenous regions.

In the model used for IMD's current forecast, two of the five parameters – ‘North Atlantic Sea Surface Temperature (December-January)' and ‘Equatorial South Indian Ocean Sea Surface Temperature (February-March)' – have been considered favourable.

The other three parameters — ‘East Asia Mean Sea Level Pressure (February-March)', ‘North West Europe Land Surface Air Temperature (January)' and ‘Equatorial Pacific Warm Water Volume' — are seen as neutral, translating into an overall ‘normal' monsoon forecast.

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