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Gazing into a cloudy crystal ball

N. Gopal Raj

The India Meteorological Department's current prediction of below-average rain during the south-west monsoon could change.

ON APRIL 24, the India Meteorological Department (IMD) issued its long-range forecast for the coming south-west monsoon, predicting that though the country would receive below-average rain, a drought was unlikely.

For the forecast, the IMD depended on a statistical model using the relationship of eight parameters measuring atmosphere, ocean, and land conditions in various parts of the globe with the Indian monsoon. This year, four of those parameters are unfavourable for the Indian monsoon.

The IMD model predicts that the nationwide rainfall from June 1 to September 30 (the period of the south-west monsoon) would be 93 per cent of its long-term average. The seven per cent deficit would still classify the monsoon as "normal," which is defined as one where the nationwide rainfall is within 10 per cent of the long-term average.

Other statistical models at the Indian Institute of Tropical Meteorology (IITM) in Pune and the Indian Institute of Science (IISc) had also indicated similar deficits in rainfall, according to M. Rajeevan, Director of the IMD's National Climate Centre at Pune.

As the IMD forecast has a possible error of five percentage points, it does not exclude the possibility of a drought. However, using another statistical model, the IMD has computed the probability of a drought to be just 22 per cent.

The IMD, along with scientists at the IISc, has been trying out a dynamical model developed at the Experimental Climate Prediction Center in the United States. Experimental runs with this model, which attempts to simulate processes in the atmosphere with predefined ocean conditions, have suggested above-average rain in the coming monsoon.

La Nina effect

The model was most probably reacting to conditions in the Pacific Ocean, according to Ravi Nanjundiah of the IISc's Centre for Atmospheric and Oceanic Sciences. La Nina conditions, characterised by waters of the Pacific becoming unusually cool, had developed in the latter half of 2005. A La Nina was generally favourable for a good monsoon. Although the La Nina was rapidly dissipating, the central Pacific was still cooler than normal during March.

Dynamical models at the European Centre for Medium-Range Weather Forecasts and the National Centers for Environmental Prediction in the United States have also been showing normal or slightly excess rain during the monsoon months. As these models take into account how the atmosphere and ocean influence one another, they ought to be better able to simulate how conditions in the Pacific evolve and what that might do to the Indian monsoon.

But a transition from La Nina conditions is difficult to predict, says Dr. Nanjundiah. Most predictions, using a variety of methodologies, currently foresee near-neutral conditions prevailing in the Pacific during the monsoon.

An El Nino, when the Pacific becomes abnormally warm, was considered unlikely. An El Nino often affects the Indian monsoon adversely.

A statistical model at the IITM that relies wholly on sea-surface temperatures is also predicting slight excess rainfall in the coming monsoon.

The sea-surface temperature in the Pacific is not among the parameters used in the statistical model for the IMD's current monsoon prediction. It will, however, be one of the two additional parameters used in the updated forecast that the IMD expects to issue in early July. The IMD's current prediction for the monsoon could change, agrees Dr. Rajeevan.

Till the mid-1970s, there was a strong correlation between El Nino and the Indian monsoon, with droughts usually occurring in El Nino years. That correlation has since diminished considerably and 1997, a year with one of the strongest El Ninos of the last century, enjoyed a monsoon with above average rainfall.

The loss of that correlation has made it much more difficult for statistical and dynamical models to predict the monsoon's outcome, says K. Krishna Kumar of IITM. The steady warming of the waters of the Indian Ocean over the past two to three decades might have played a role in the breakdown of that correlation besides influencing the year-to-year variability of the monsoon, he told The Hindu.

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