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Here’s to a good monsoon

The south-west monsoon is approaching. The India Meteorological Department (IMD) has said the monsoon could set in over Kerala around May 29. In IMD’s first long-range forecast for the coming monsoon, issued in April, its scientists predicted that nationwide rainfall during the season is likely to be 99 per cent of the long-term average, with an error bar of five percentage points — a “near normal” monsoon. About 33 per cent of south-west monsoons during 1901-2005 fall in this category, wherein the total rainfall during the season is between 96 per cent and 104 per cent of the long-term average. The IMD’s prediction is based on a statistical model developed by its scientists. The model was used to make an operational forecast for the first time in 2007 and it was way off. Against the 93 per cent rainfall predicted by June-end, the monsoon finished with above-average rains of 105 per cent.

By way of explanation, it is said that a La Nina, an unusual cooling of surface waters of the eastern and central Pacific that is generally associated with bountiful rains over India, developed rapidly by August 2007. In addition, a pattern of temperature changes in the waters of the Indian Ocean known as a “positive Indian Ocean Dipole,” which too is associated with plentiful rain, developed by September 2007. It is only rarely that the two events occur about the same time; the last time it happened was in 1967. The statistical model, which relies on meteorological parameters during the winter and pre-monsoon seasons, was unable to capture in advance the positive difference these two events would make to the monsoon. Moreover, IMD scientists note, the predictors used in their model are well correlated with rainfall patterns over north-west and central India, which are responsible for much of the year-to-year variability of countrywide rainfall. In any event, a statistical model must be evaluated over a period, not judged by its performance in one year. There is little reason at present to fear that the model’s prediction of a near normal monsoon this year will be incorrect. The Indian economy was once described as “a gamble on the monsoon.” That may no longer be the case, but the rains do have a huge impact on the economy. As Prime Minister Manmohan Singh noted recently, good rains in the coming weeks could help moderate the inflation the country is experiencing.

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