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Long monsoon tail on view as `easterly wave' approaches

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Vinson Kurian

Thiruvananthapuram, Dec. 12

THE `easterly wave' comprising convective clouds bracing to enter the Bay of Bengal waters might just signal the beginning of the end of a phenomenon in which a parade of such rain-bearing systems has rolled down from the South China Sea, as if on assembly line.

These westward propagating systems have fed on the warm waters of the Bay with a vengeance, growing rapidly to become `lows', depressions and even cyclones to roar their way across the east peninsular coast one after the other during this northeast monsoon.

There is still a mass of cloudiness left over Southeast Asia, but it could be the last vestiges of the cloudiness associated with the Inter Tropical Convergence Zone (ITCZ) bracing to migrate to the southern hemisphere, bringing the curtains down on a hyperactive season this year.

This seasonal migration of ITCZ to the north and south of the Equator is the key to monsoon behaviour on the Indian landmass. The cycle begins with the ITCZ migrating to the northern hemisphere around June, setting off the southwest monsoon.

On Monday, the National Centre for Medium Range Weather Forecasting (NCMRWF) maintained its outlook on the `easterly wave' impacting the already rain-battered Tamil Nadu coast by the weekend. Dr Akhilesh Gupta of the NCMRWF told Business Line that the wave had not showed up having crossed into the Bay waters. But it is expected to do it earliest by Tuesday.

It could proceed to set up a `low' over the Andaman Sea and the neighbourhood, where some convective cloudiness has already been noticed. A feeble trough of low may already have been generated in the process. The sea continues to be very warm in the area around 90 deg E, too. But prevailing atmosphere dynamics did not exactly presage the development of a severe weather system on the seas.

The most pronounced irritant was the wind shear, which had shown an increasing tendency over the Bay of Bengal. According to the University of Wisconsin-Madison, the wind shear values had turned unfavourable over the past 24 hours. This could interfere with the development of an organised and deep convection within the system. In its update on Monday, the NCMRWF said the upper air cyclonic circulation over south Tamil Nadu (remnant of `Fanoos') of the previous day had persisted and lay over Lakshadweep and adjoining southeast Arabian Sea. Fairly widespread rains are likely in Kerala and Lakshadweep and scattered in Tamil Nadu and Pondicherry. The rainfall activity may reduce after 24 hours. In its forecast, it said scattered to fairly widespread rains are expected over the Andaman and Nicobar Islands during the next three days. Isolated rains are likely in coastal Andhra Pradesh, Rayalaseema and south interior Karnataka.

Related Stories:
Land features mellow `Fanoos'

(This article was published in the Business Line print edition dated December 13, 2005)
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