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The South-West monsoon is expected to break over the Kerala coast two to three days earlier than usual, says P.V. Joseph, eminent monsoon meteorologist and researcher.
Joseph has been a former director of India Meteorological Department (IMD) and is professor emeritus in the atmospheric science department at Cochin University of Science and Technology.
CLOUD BAND
He told Business Line on Tuesday that the long-term mean date of onset of monsoon over Kerala was June 1. A cloud band with rain-bearing thunderclouds had formed over extreme South Bay of Bengal.
This cloud band was expected to intensify and move northwards and give light to moderate rain in Kerala, Tamil Nadu and south Bay of Bengal during the next few days.
This cloud band is called the ‘pre monsoon rain peak,’ which is an indicator of the date of onset of South-West monsoon over the Kerala coast.
BAY ‘WARM’
The pre-monsoon rain peak is reached when surface temperature of seawater 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.
Joseph’s pre-monsoon rain peak is among the key parameters used for its own forecast by IMD. He is often consulted by the department while finalising its forecast.
Joseph said that last week’s average of sea surface temperature measurements obtained from Indian satellite Kalpana-1 as on April 15 had shown that the Central Bay of Bengal is ‘very warm.’
Sea surface temperatures have measured to more than 30 degree Celsius, a pre-condition for the formation of the pre-monsoon rain peak.
PAST TREND
It is the western edge of this rain band that normally gives moderate rainfall over Kerala and Tamil Nadu during this season. The Bay Islands have already been getting rain.
It has been noticed in the past that once central Arabian Sea too warms up to 31-32 degree Celsius, a rain band would form over south Arabian Sea as well.
This rain band would intensify to bring about the monsoon onset over Kerala. While doing so, it would also induce the formation of the low-level jet stream of monsoon, a current of strong south-westerly winds.
vinson.kurian@thehindu.co.in
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