The Hindu Business Line : Rain belt wobble along Kerala, south TN latitude

The inter tropical convergence zone (ITCZ) of the equatorial band of low-pressure and convective cloudiness is constantly shifting positions within a narrow band along the Kerala and south Tamil Nadu latitudes, producing locally excessive rainfall.

The ITCZ has been locked into a position anomalously south to the 13N (Chennai and southwest Bay of Bengal) Latitude, where it is normally known to create intense weather events and put the northeast monsoon on an autopilot.

MEAN POSITION

The farther the ITCZ anchors itself lies from the Equator, the more likely the development of a weather disturbance. Thus, during the period June to September (southwest monsoon), the ITCZ reaches its farthest north and sets up cyclones over the North Pacific and North Atlantic Oceans.

They also form over the Arabian Sea and Bay of Bengal, but only in the weeks before and after the southwest monsoon. Wherever cyclones occur, they need a sea-surface temperature of at least 26.5°C.

TROUGH IN SOUTH

Locally, the ITCZ moves south with the withdrawing southwest monsoon and runs into the south-southeast to north-northeast trough associated with the advancing northeast monsoon in the peninsula. This seasonal trough will also decide the limit to which the ITCZ can move to the north.

BROAD FEATURES

Mr K. Santhosh, Director, Met Office, Thiruvananthapuram, said that two broad northeast monsoon features, troughs of low pressure over the southwest Bay of Bengal and over the Lakshadweep islands, have been established.

"We are still witnessing the movement of a series of easterly waves, which establish themselves over the southwest Bay in the mornings and cross over the peninsula to enter the adjoining Arabia Sea in the afternoons. The amount of precipitation is high or low based on prevailing wind speeds," he said.

WIND SPEEDS

The wind speeds average in the region of 15 knots currently but sudden and likely bursts into 30 knots create most ideal conditions for heavy precipitation over sea and adjoining land.

Going forward, they can also set up a seasonal `low' or depression in the Bay.

NO SIGN OF `LOW'

According to Mr Santhosh, there were still no signs of a `low' brewing.

But the odd easterly wave has also been known to descend to suitable heights and set up a weather system. An India Meteorological Department IMD) forecast said rain or thundershowers are likely at many places over Tamil Nadu, Pondicherry, Kerala and Lakshadweep. Outlook for the next 48 hours said that the ongoing rainfall activity is likely to continue over south peninsula.

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