Monday, April 27, 2015

Monsoon concerns

Monsoon concerns

In contrast to last year, the initial outlook for the southwest monsoon looks hardly promising. According to the first-stage forecast issued on April 22 by the India Meteorological Department, the southwest monsoon seasonal rainfall is likely to be 93 per cent of the long-period average with a margin of error of 5 per cent. 

For the June-September season, both the deficient (less than 90 per cent of long-period average, or LPA) and below-normal (between 90 and 96 per cent of the long-period average) categories have a nearly equal probability of 33 and 35 per cent respectively. 

The forecast probability of both deficient and below-normal categories is double the climatological probability, which is based on how the monsoon fared in previous years. While the chances of excess rainfall occurring are non-existent, initial indications are that the monsoon this year will be subnormal or deficient. 

However, the initial forecast made in April cannot be the basis for arriving at any firm conclusions; at best, it may serve as a pointer. For instance, as seen last year the El Niño conditions over the Pacific did not develop into a phenomenon that was strong enough to retard the southwest monsoon over the country and fizzled out swiftly. 

Of late, El Niño characteristics seem to change quickly. Hence, a true picture will emerge only by the end of May or in early June. The June forecasts will also be more comprehensive in terms of monthly rainfall over the country and seasonal rainfall over the four regions.

The El Niño conditions currently prevailing over the Pacific Ocean are stronger compared with last year. These have been particularly notable in the central Pacific Ocean. An increase of 0.5° C in the sea surface temperature has been recorded in the Pacific region, and in all likelihood the warming will increase and mature during the monsoon period. 

These have already been factored in to arrive at 93 per cent of the LPA with a margin of error of 5 per cent. A strong El Niño can play an important role, but it is just one of the factors that could affect rainfall. As witnessed in 1997, a positive Indian Ocean Dipole (IOD) has the potential to largely offset the adverse influence of even a strong El Niño and ensure above-average rains across India. 

Though slightly negative IOD conditions are now indicated, these can be largely ignored. Unlike in the case of El Niño, the IOD prediction is far from good; the Indian Ocean processes and how they develop are not quite well understood. 

Hence, the initial indications are that the IOD may neither support the impact of El Niño nor neutralise it. As in the case of El Niño, a better picture of the IOD will emerge only from the next round of the forecast.

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