The Hindu Business Line : `Below-par heating of plains delaying onset of monsoon'

The heating of the Central and Northwest Indian plains is one, says Dr M. Rajeevan, Director-Forecasting, India Meteorological Department (IMD), Pune. "This year, the heating was below normal," Dr Rajeevan told Business Line.

Given the onset dynamics, it would appear that the heat wave now stalking parts of the country is `too mild and too late.'

The second factor is the cross-equatorial flow, which normally brings in moisture from the Indian Ocean over the Southwest of the peninsula. The wind speeds embedded in the system were below normal this time.

The cross-equatorial flow is known to decide the fate of the Indian summer monsoon. One of the main drawbacks of the failed 2002 monsoon was the skewed nature of this flow, despite having been strong enough.

The third factor is the anomaly associated with the arrival of the monsoon over Southeast Asia. There is an established pattern in which the onset takes place, one after the other, over Southeast Asia and India. It is associated with an intra-seasonal mode in the atmosphere. The pattern is poorly developed this year, said Dr Rajeevan.

The first rains of the monsoon occur over Burma and Thailand by mid-May and then progress generally to the Northwest, so that by mid-June, rains advance over all of India and Pakistan.

"But still, the monsoon onset process is very complex. What we have given in terms of onset forecast is based on the large-scale atmospheric features. The onset of rains over Kerala depends on many other factors," Dr Rajeevan said.

During any one season, the dates of the commencement of the monsoon in a particular location are quite variable, according to researchers. Furthermore, the onset can be very rapid.

It can be recognised by the rapid acceleration of southerly winds in the Western Indian Ocean in early June. Winds accelerate from weak northerlies to strong southerlies within a week.

The onset also appears to have many definitions, varying from a gradual increase in humidity and the commencement of precipitation to the sudden establishment of a `monsoon vortex' in the Northern Indian Ocean.

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