The Hindu Business Line : Monsoon back to ‘active phase’ over coastal TN

Thiruvananthapuram, Dec. 16 The prevailing easterly wave has helped swing back the northeast monsoon into ‘active phase’ over coastal Tamil Nadu as heavy to very heavy rains lashed the region for the second day on Tuesday.

The Chennai Met Centre said in its update that overnight rainfall was recorded at many places over coastal Tamil Nadu and a few places over the interior.

The official northeast monsoon season may have ended in November but easterly waves get organised in December and even in January with implications for south Tamil Nadu and adjoining Kerala.

This is attributed to the ‘homecoming’ of the Inter Tropical Convergence Zone (ITCZ) from the northern hemisphere to the Equator, explained Dr Akhilesh Gupta, Senior Adviser to the Department of Science and Technology.

ITCZ is the band of low-pressure encircling the globe and is the principal atmospheric feature driving weather in the tropics. It tracks the movement of the Sun to both the hemispheres to set up the monsoons and other regional weather systems.

The ITCZ presides over the June-to-September southwest monsoon over land (India and East Asia) and tracks the Sun to get back to its moorings over the equatorial ocean waters. The northeast monsoon (and sometime later the Australian monsoon) gets triggered on this ‘home stretch’ from October onwards.

Unlike the elaborate transition from one monsoon to the other involving wholesale change in wind direction and shifting of seasonal anti-cyclone bearings, the breaking winter just gets woven into the northeast monsoon fabric — and hence, is also known by the apt alias of the winter monsoon.

WAVES AND CYCLONES

Even as the ITCZ lies supine around the Equator, it has been known to get accentuated at times over the open waters of the equatorial Indian Ocean giving rise to easterly waves and even the rare cyclones in December and January.

As per the tropical cyclone atlas for India, the storm activity is least, even nil, during February only.

In this context, Tamil Nadu and Kerala continuing to witness easterly wave activity does not surprise, Dr Gupta said. This is underlined by the latest update from the Climate Prediction Centre of the US National Weather Services indicating wet weather for Sri Lanka and south peninsular India from a wet trail leading to the Australian monsoon.

Meanwhile in the north and northwest, the rainy and icy weather thrown up by a prevailing western disturbance is forecast to continue for the next four days, according to the India Meteorological Department. The scattered to fairly widespread precipitation will be in evidence mostly from Wednesday through Friday.

The southerly winds and cloudy conditions will continue to maintain the minimum temperatures at the elevated levels. Rain or snow is likely at many places over Jammu and Kashmir and at a few places over Himachal Pradesh and Uttarakhand.

Rain or thundershowers are likely at a few places over Rajasthan and isolated over Gujarat.

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