The onset of the South-West monsoon could be delayed by up to three days from the usual timeline of June 1, says PV Joseph, renowned researcher and scientist.

He attributed this to a lag in the warming of the south-central Bay of Bengal (south of Sri Lanka), a familiar track that the monsoon must tread before precipitating the onset along the Kerala coast.

IMD is expected to issue its initial forecast on the Monsoon and also make its observations on the emergence of El Nino on Thursday, reports our New Delhi Bureau.

No relevance

Delay in the onset has no relevance to the performance of the four-month-long annual rain season that ends in September.

But forecasts of an El Nino year have brought this year’s monsoon into focus.

Not all El Nino years have proved a drought year for India but studied caution is the watchword among weather watchers.

The warming of ocean waters during this time of the year sets up rain-bearing clouds over extreme south Bay of Bengal.

Sufficiently warmed waters alone can sustain the cloud band whose crucial presence over south Bay of Bengal indicates the calibrated build-up to the monsoon.

Movement of the band to the north towards the latitudes of peninsula is tracked closely to predict what Joseph calls the ‘pre-monsoon rain peak.’

Delayed ‘peak’

It is a term used to refer to the light to moderate rain activity over Kerala, Tamil Nadu and south Bay of Bengal usually breaking out around April 22 every year.

“For a normal monsoon onset on June 1, the pre-monsoon rain peak should have formed over Kerala latitudes during April 20-22,” Joseph explained to Business Line.

In the current year, this has been delayed by a few days which indicates that monsoon onset over Kerala is likely to be delayed by about three days, he said. The pre-monsoon rain peak is reached when surface temperature of seawaters in the Bay of Bengal off India’s east coast rises about 35-40 days before the monsoon, and clouds near the Equator move north.

This year, sea surface temperatures of the central Bay of Bengal began warming above the 30 degree Celsius-mark from April 18 only.

Key parameter

“Today, to the south of the warm Bay, a cloud band has formed covering the whole of south Bay of Bengal between the Equator to​ five degrees north (south of Sri Lanka) as depicted in morning satellite pictures,” Joseph explained.

This will now intensify and move north. It will reach the latitudes of Kerala by April 25-27 when Kerala and Bay of Bengal to its east will get fairly widespread rain.

The cloud band will constitute the pre monsoon rain peak, among key parameters that India Meteorological Department uses for its own forecast of the annual rain season.

Joseph has himself been a former director of IMD and professor emeritus in the atmospheric science department at Cochin University of Science and Technology.

(This article was published on April 22, 2014)
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