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The Pacific and the Indian monsoon

N. Gopal Raj

The warming that has been occurring in the western and central Pacific, even if it does not develop into a full-fledged El Nino, is still cause for worry.

— Photo: Arunangsu Roy Chowdhury

Monsoon clouds over the Vidyasagar Setu on the Hooghly in Kolkata.

IN ITS updated seasonal forecast, the India Meteorological Department has reiterated its earlier prediction that the current monsoon was likely to be "normal" despite the deficient rain the country would receive.

Using a statistical model, the IMD had predicted in April that the rain this monsoon would fall short of the long-term average by seven per cent. If that prediction is correct, the current monsoon would still be a "normal" one. A monsoon is deemed to be "normal" when the nationwide rainfall between June 1 and September 30 is within 10 per cent of the long-term average. A monsoon is said to have ended as a drought only when the rainfall deficit crosses 10 per cent. As the IMD's prediction had an error band of five percentage points, its April forecast did not preclude the possibility of the monsoon slipping into a drought. The IMD, however, computed the probability of a drought at only 22 per cent.

In its update issued on June 30, the IMD pretty much reiterated its earlier forecast. After incorporating additional data in its statistical model, the Met Department put the rainfall deficit that was likely during the current monsoon at eight per cent with an error band of four percentage points.

Taking into account also the heavy rain in late May after the monsoon set in several days early, the IMD says the rainfall received from onset till June 28 was deficient by only eight per cent. But as monsoon activity petered out after the first week of June, the deficit rises to 21 per cent when only rainfall since June 1 is taken into account.

Although poor rainfall in June does not presage a failure of the monsoon, it does mean that rainfall during July will be particularly important. In its updated forecast, the IMD has predicted that July rainfall could fall short of the long-term average for the month by only three per cent. It put an error band of nine percentage points on the forecast.

The monthly rainfall data for the years from 1871 to 2003 indicate that when rainfall during June and July taken together is deficient by more than 10 per cent, then chances of the monsoon ending as a drought are greatly increased.

Numerical weather prediction models run by the National Centre for Medium Range Weather Forecasting near Delhi, the U.K. Met Office, the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) in the United States reportedly show that the monsoon will be active over the country during the first 10 days of July.

The ECMWF and NCEP numerical models indicate a weakened cloud band in the Indian Ocean south of the equator, right below India, in the coming months, notes Ravi Nanjundiah of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science. This weakening ought to lead to strengthening of the cloud band north of the equator, producing enhanced cloud formation and rain over India. But that does not guarantee plentiful rainfall during the remaining months of the monsoon, he cautions.

Worrying sign

One worrying sign is the warming that has been occurring in the western and central Pacific. Since the end of May this year, surface waters of the equatorial Pacific from east of Indonesia to well beyond the International Date Line have warmed by more than 0.5 degrees Centigrade above the average. Most forecasts, however, suggest that the warming is unlikely to develop into a full-fledged El Nino. The abnormal warming of the equatorial Pacific that is characteristic of an El Nino has often gone hand-in-hand with failures of the Indian monsoon.

But even if an El Nino does not develop, published research shows that warmer-than-usual surface and sub-surface waters in the western and central Pacific can lead to more typhoons forming there. Such typhoon activity can draw away the moisture-laden winds necessary to sustain the monsoon over India. Every monsoon is made of active spells when most of the country receives copious rain interspersed with periods when the monsoon is weak. However, prolonged "breaks" in the monsoon, when much of the country gets hardly any rain, can lead to a drought as happened in 2002, which saw one of the worst droughts in the last 100 years.

After examining daily rainfall data for the last 50 years, two researchers at the Cochin University of Science and Technology found that active spells of the monsoon were becoming shorter while the weak spells were lengthening. The days when the monsoon was strong had decreased by over 77 per cent between 1950 and 2002 and weak spells had increased in duration by about 45 per cent, observed P.V. Joseph and Anu Simon in a paper published last year.

Will the warming of the waters of the Pacific Ocean lead to long periods of poor rain this time? The answer to that might well determine the outcome of the current monsoon.

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