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The South-West monsoon is likely to arrive on June 5 on the Kerala coast, four days later than normal and with a model error of plus or minus four days, the India Meteorological Department (IMD) announced on Friday. The normal date of onset along the country’s South-West coast is June 1.
The IMD has already forecast a normal monsoon for this year (96-104 per cent of the long-period average). This is predicated on the likely development of a friendly La Nina (alter-ego of El Nino) in the Equatorial Pacific, though, nearer home, cousin Indian Ocean Dipole (IOD) may stay negative. A record strong positive IOD had led to a surplus as well as an extended monsoon last year. The pre-monsoon season this year has so far seen normal to above normal rain for most parts of the country. Notable exceptions are Tamil Nadu, Saurashtra and Kutch and parts of North-East India.
Onset deciding factors
The IMD announcement came on a day when private forecaster Skymet Weather projected the onset date four days earlier, on May 28. Last month, an outlook from The Weather Company, an IBM Business, had said that the seasonal rains may arrive over mainland India along the Kerala coast on May 31, a timeline closer to the normal date of onset.
Mrutyunjay Mohapatra, Director-General, IMD, told BusinessLine the onset over Kerala is dependent on a number of large-scale (global) and regional factors. These include minimum temperatures over North-West India, pre-monsoon rainfall peak over the South Peninsula, cloud cover over the South China Sea and the South-West Pacific region, and lower-level winds over the South-East Indian Ocean (south of the Bay of Bengal) and upper-level winds over the East Equatorial Indian Ocean (south of Sri Lanka).
Storm development
The development of a low-pressure area over the South-East Bay of Bengal and its projected intensification with a movement to the North-North-East is also expected to influence the monsoon onset. According to DS Pai, Head, Climate Research and Services, IMD, Pune, a lot of kinetic energy is expended into this strong, single meteorological event that the seas need time to recoup over time. Thus, the monsoon will have to go back to the drawing board to be able to reorganise the flows and precipitate the onset.
An IMD press release said the monsoon arrives first over the South Andaman Sea, and then advances in a North-West direction across the Bay. As per the new normal dates of monsoon onset/progress, it enters the Andaman Sea around May 22. A persisting well-marked low-pressure area over the South-East Bay may concentrate into a depression on Friday itself and further intensify into a cyclone over the central parts of the South Bay by tomorrow (Saturday) evening.
Onset over the Andamans
This is also expected to make conditions favourable for the advance of the monsoon into the Andaman Sea, the Andaman & Nicobar Islands and some parts of the South-East Bay during the next two days. Past data suggest that there is no association of the onset over the Andaman Sea either with the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country. The IMD uses a modern, in-house statistical model that has a model error of +/- four days for calculating the onset date over Kerala.
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