Come April, it is time for an important ritual in the run up to the monsoon — the Indian Meteorological Department's forecast of how the rainy season, which still sets the pace for so much of the country's economy, will turn out. This early in the game — the rains typically set in between the end of May and early June — and without a scientifically reliable crystal ball, the IMD has an unenviable task. The monsoon is a hugely complex phenomenon, and scientists are still in the process of deciphering how the ocean and atmosphere interact to decide its progress. Without such understanding, attempting to predict how the rains will fare can look rather like soothsaying. The IMD's forecast for this year's monsoon used a statistical model to estimate the probabilities for various outcomes. It predicted an 88 per cent probability of the monsoon turning out to be ‘normal' in the sense that scientists generally use the term (meaning a season when the nationwide rainfall is between 90 per cent and 110 per cent of the long period average). Rainfall data for over a hundred years shows that seven years out of ten turn out that way. The IMD forecast would, therefore, appear to indicate enhanced odds for a ‘normal' monsoon.

But that prediction needs to be taken with some degree of caution. How the sea surface temperatures of the equatorial Pacific, especially in the central part of the ocean, evolve in the coming months can impact the monsoon. Several coupled models, which attempt to simulate processes in the oceans and atmosphere, are already suggesting that the central Pacific will warm and a weak El Nino could develop as a result. An El Nino often adversely impacts the monsoon. At present, those models do not show rainfall deficits over India. But that could change as time goes on. However, it is noteworthy that even if the monsoon does turn out to be ‘normal,' the IMD's probabilistic forecast already indicates that it is likely to be towards the lower end of that range. The category it terms as ‘below normal', with the monsoon rains between 90 per cent and 96 per cent of the long period average, has a probability of 24 per cent. That is higher than its climatological probability of 17 per cent. How the monsoon fares will also depend on what happens in the Indian Ocean. There are some indications that this ocean too may not favour a good monsoon. Such uncertainties over the fate of a monsoon should diminish as scientists gain more insights into the phenomenon and consequently are able to improve models that simulate it. One hopes that the National Monsoon Mission, which seeks to do just this and has now received governmental approval, succeeds — and soon.

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