The Hindu Business Line : ‘Classic monsoon onset’ not seen until June

Thiruvananthapuram, May 25 True to forecasts, the ‘low’ originating in the southeast Arabian Sea raced away from India’s southwest coast and was parked over southeast and adjoining east-central Arabian Sea on Sunday afternoon. The cyclonic circulation over Lakshadweep has merged with the system.

International models do not see a ‘classic onset of monsoon’ until the first week of June, since the ongoing thundershower activity along and off the coast is forecast to fade with the anomalous movement of the ‘low.’

An India Meteorological Department (IMD) update on Sunday too said that the rainfall would gradually lift over the next two days.

WET JUNE

Consensus now veers round to the prospect of intense rainfall becoming established in the Arabian Sea during the first two weeks of June. What goes to support this outlook is an active phase of the Madden-Julian Oscillation (MJO) wave seen ahead.

The MJO is an anomalous rainfall pattern transiting the equatorial region and is planetary in scale. It features an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean.

The anomalous rainfall is set off first over the western Indian Ocean, and is tracked easily as it propagates over the very warm ocean waters of the western and central tropical Pacific.

It begins to fade out as it moves over the cooler eastern Pacific, only to reappear over the tropical Atlantic and Indian Oceans.

The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30-60 days, and is in some ways responsible for the `break monsoon’ in which seasonal rainfall dries up over Indian mainland.

The Climate Prediction Centre (CPC) of the US National Weather Centre also sees an increased chance for above normal rainfall for sections of equatorial Africa and the western Indian Ocean during this period. It mentioned the unravelling of the MJO as being responsible for the emerging wet conditions.

CYCLONE TALK

Forecasters and model projections are still at variance over the issue of the Arabian Sea ‘low’ intensifying to cyclonic strength in the face of westerlies persisting with their run-ins towards northwest India.

At times, they sweep the north Arabian Sea where, according to the European Centre for Medium-Range Weather Forecasts (ECMWF), the ‘low’ was initially forecast to undergo a calibrated growth to a possible cyclone. But this is now increasingly becoming untenable, especially in view of arrival of a fresh westerly system in the next few days.

Satellite pictures on Sunday showed convective clouds gathering off the east African coast in the build-up to what is seen as a stronger ‘low’ in the west Arabian Sea. Its further growth and movement will bear some watching, according to forecasters.

The US Navy’s Joint Typhoon Warning Centre (JTWC) assessed as ‘poor’ the potential for development of a significant tropical cyclone within the next 24 hours since convection was yet to fully build and organise around the newly-defined low-level circulation centre.

Meanwhile, isolated thunder squalls and hailstorm have been forecast over Jammu and Kashmir, Himachal Pradesh, Punjab, north Haryana, Jharkhand, Bihar, West Bengal and Sikkim over the next two days.

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