Pacific cyclone and monsoon onset

India Meteorological Department (IMD) on Monday evening said that scattered rain or thundershowers would unfold over Lakshadweep, coastal Karnataka, Kerala and Andaman and Nicobar Islands until Wednesday and increase thereafter.

An extended outlook valid until Saturday said that the rains would become fairly widespread over extreme south peninsula and Andaman and Nicobar Islands.

This clearly is thought to be a clear indicator of the approaching onset of southwest monsoon, being facilitated by a raging tropical cyclone, Songda, in the northwest Pacific. The cyclone system is part of the larger Asian monsoon system that covers the northwest Pacific, South China Sea, the Bay of Bengal, equatorial Indian Ocean and the Arabian Sea.

Purely coincidental, formation of this cyclone would have some impact on the onset phase of the Indian monsoon, though seen largely beneficial in the preliminary analysis.

Songda is, however, is forecast by most models as intensifying into a super cyclone (of Category-4 strength and above) and racing away to north-northeast off the Philippines towards East China Sea and beyond.

All northwest Pacific cyclone systems tracking in east-northeast direction are considered inimical to the interests of the Indian monsoon since they would also wake away a lot of moisture with them.

The London-based Tropical Storm Risk group said that Songda might reach super cyclone status by Saturday.

This would mean that once the onset of monsoon over India's southwest coast is facilitated, Songda would divert moisture across the Bay of Bengal into Southeast Asia and pull the same towards itself.

In the process, the onset of monsoon over Andaman and Nicobar Islands and Sri Lanka might take place one after the other. The normal timeline for the Andaman Sea onset if May 15 to 20.

Some other models, including Roundy-Albany and the Taiwanese Central Weather Bureau, still see the possibility of a separate system developing in the Arabian Sea and clambering up north towards Mumbai-Gujarat region.

This is expected to happen during the first week of June, these models indicated, after the monsoon has set in over the Kerala coast.

Meanwhile, updated forecasts from both the International Research Institute (IRI) at Columbia University and the European Centre for Medium-Range Weather Forecasting (ECMWF) seemed to indicate a largely normal monsoon for the country during the impending season.

LA NINA OUTLOOK

Interestingly, the IRI has joined a group of Japanese researchers at the Tokyo-based Regional Institute for Global Change in maintaining a watch for the possibility of a return of La Nina conditions in the equatorial east Pacific.

The IRI said in its latest update that the moderate to strong La Nina conditions observed between mid-August 2010 and early February 2011 has weakened during March and April, and dissipated to “neutral” conditions as of mid-May 2011.

For the May-July season currently in progress, it assessed that there is an approximately a 24 per cent probability for the equatorial east Pacific to return to La Nina conditions and a 63 per cent probability for remaining in neutral conditions.

LOW PROBABILITY

There is only a low (13 per cent) probability for the development of El Nino conditions, which are not considered ideal for the Indian monsoon