The Hindu Business Line : Westerly trough seen salvaging monsoon flows

Thiruvananthapuram May 25 Weathermen are hoping that any prospective buzz in the Bay of Bengal may not rise beyond a threshold to scare away the Arabian Sea arm of monsoon, which is making a repeat bid to approach the Kerala coast.

Tropical Cyclone Akash in the Bay and its distant cousin Typhoon Yutu, a category-4 storm in the Pacific, had connived to stop the monsoon on its tracks while seeking to hit the home stretch a few days back for what was widely seen as an earlier-than-usual onset.

Indications are that a prevailing westerly trough, which had played its part in sending the monsoon current into disarray, will play a proactive role and, in fact, "excite" the monsoon flow to get going this time round, says Dr K. J. Ramesh of the Department of Science and Technology. Westerly troughs can at times work to the advantage of a monsoon in the making, he said.

Organised rainfall activity is expected to begin along the Kerala coast from Monday/Tuesday. This will happen in tandem with the westerly trough delving deeper into the south, unravelling a series of thunderstorm and heavy rainfall events over the peninsula.

Latent heat from associated condensation will cause the air to warm, raising its moisture-carrying capacity, and stimulate a progressively strengthening monsoon current to make splash along the west coast. The rains are seen sustaining for 3-4 days in the initial burst, before getting subdued briefly.

But the real onset of monsoon (as different from the event taking place largely at the behest of the westerly trough) will take place around June 2/3, when flows will expectedly peak as part of the `push effect' from the southern hemisphere.

Interestingly, Tamil Nadu, a rain shadow region from the monsoon point of view, is seen benefiting from the first few rain showers. This is so because the westerly trough will keep moving to the east with its rain-head in tow, unleashing thunderstorms to this part of the peninsula as well.

But the rains will dry up as the real monsoon current closes in from behind, barges into the Western Ghats and gets lifted to condense as rain. This rain belt (in contrast with the one generated by the upper level westerly trough in the earlier instance) will travel north only, since the windward side of the West Ghats acts as a natural barrier preventing the flows from spilling into the eastern side.

Projections from the US National Centres for Environmental Prediction (NCEP) suggests that the realised rainfall along the west coast will range from 100- to 150 per cent of the average during week from May 25 to June 1. But this will be between 400- to 500 per cent along the east coast (Tamil Nadu and parts of Andhra Pradesh).

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