The Hindu Business Line : Indian Ocean dipole favours good monsoon

Thiruvananthapuram May 2 A positive Indian Ocean Dipole (IOD), the seasonal seesawing of sea surface temperatures in the Indian Ocean, is forecast to combine with an emerging La Nina event in the equatorial Pacific in a rare virtuoso performance with implications for the Indian monsoon.

A positive IOD is characterised by the anomalous cooling of surface temperatures in the southeast equatorial Indian Ocean (around Indonesia) and a corresponding warming in the west equatorial Indian Ocean (along east Africa). Warm seas drive convection and bring about better precipitation.

RARE EVENT

"The expected combination of a La Nina and a positive IOD is very rare," according to Dr Toshio Yamagata of the Department of Earth and Planetary Science at The University of Tokyo.

Writing to Business Line, Dr Yamagata, whose team of researchers first identified the IOD phenomenon, said: "We expect a good (even too good) monsoon in South Asia and a very hot summer in east Asia."

Dr Yamagata's coupled global circulation model (GCM) is predicting a positive IOD for year 2007. But then, the preceding year (2006) was also IOD positive. "Normally, IOD shows a biennial character. So this sequence is also very rare. Our prediction for further evolving La Nina is solid, too," he added.

The correlation between the IOD and a concurrent Indian monsoon has been showing an increasing trend, according to researchers.

The high correlation observed in the early sixties can be attributed to intense IOD events during 1961, 1963, and 1967, which strongly influenced the concurrent monsoon.

But there were not many strong IOD events till the 1980s and the correlation values were very weak during this period. But the return of intense IOD events in the 1980s and 1990s helped re-establish the correlation.

When the IOD event occurs in the absence of an intense El Nino/La Nina, it can strongly influence the season's rainfall, as in 1961. The La Nina event evolving this year has not yet shown signs of rapid intensification either.

`LOW' MAY INTENSIFY

Meanwhile, southeast Bay of Bengal is hosting a `low' as predicted, which may become more marked, according to a forecast by India Meteorological Department on Wednesday. Model projections suggest a north-northeast track for the `low', which would make it spin away from the peninsular coast.

It's likely that the `low' may proceed to impact the Bangladesh/Myanmar coast. In any case, widespread rainfall has been indicated for the Andaman Islands, which fall within its immediate footprint, for the next 48 hours.

DUST STORMS

A shift in the prevailing wind pattern in north and northeast India, where easterlies have become predominant, is expected to spin up thunder squalls/dust storms in West Bengal, Sikkim, Jharkhand, Bihar, west Uttar Pradesh, Delhi and Haryana over the next 48 hours.

This will bring a brief respite from the searing heat these regions have been experiencing during the past week or so.

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