Thiruvananthapuram, Sept. 8
FOR the first time during the ongoing monsoon season, the Bay of Bengal is tipped to host a cyclonic circulation migrating in from the South China Sea, signalling hope for the rain-starved north and northwest regions of the country.
South China Sea-born systems have special significance for the Indian summer monsoon as they normally deliver what they promise despite the disposition to stray, says Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF).
Systems moving west-northwest alone can cross into the Indian Ocean and onward into the Bay. But in the present instance, all others have been tracking a north-northwest course in the west Pacific/South China Sea, giving the Indian Ocean the slip, explains Dr Gupta.
An ideally behaving westerly system is more potent than in situ monsoon systems that develop over the Bay, since it would have already settled into a desirable pattern by the time it crosses into the territorial waters and can feed on whatever little the less-warmer waters of the Bay can afford. This feed will help the circulation thrive and intensify into a low, even a depression, by the time it settles firmly into position over the Bay. According to Dr Gupta, model predictions suggest that the incoming circulation will concentrate over the Bay by Monday next (September 12) off the Andhra-Orissa coast.
But the subsequent course the circulation will take is beyond the best forecasts. The villain of the piece is the wind pattern during September, the month when monsoon is known to enter into a transition period prior to signing off from the mainland.
If the steering winds in the high altitudes are predominantly westerlies, the monsoon circulation and associated rain belt will be oriented to the west-northwest benefiting the northwest regions of the country. Taking a call on what direction the winds will take by Monday is difficult at this point of time, says Dr Gupta.
One other relieving feature during the last few days has been the fact that parts of the west and northwest, where the monsoon is thought to have withdrawn from, have been receiving scattered to isolated rain. This, according to Dr Gupta, is the result of the feeble interaction between the incoming western disturbances and upper air cyclonic circulations propagating from north Madhya Pradesh.
In its update, the NCMRWF said fairly widespread rains with isolated heavy falls have occurred over the coastal areas of Andhra Pradesh, Orissa, Karnataka, Konkan, Goa and Lakshadweep during the 24 hours ending Thursday morning. Scattered to moderate rains have also occurred over interior Andhra Pradesh, Madhya Maharashtra and Gujarat.
Wednesday's low-pressure area over west central Bay of Bengal, off coastal Andhra Pradesh, has persisted. Associated cyclonic circulation extended up to the mid-tropospheric levels and is expected to move in a northwesterly direction. Under the influence of this system, fairly widespread rain with heavy falls is likely over Andhra Pradesh and Orissa during the next 2-3 days.
The western disturbance as a cyclonic circulation persisted over central Pakistan and adjoining northwest Rajasthan. Under its influence, scattered rains may occur over the hilly region of northwest India while it will be isolated over the plains during the next 48 hours. The other upper air cyclonic circulation over east central Arabian Sea has become less marked