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Thursday, Jun 16, 2005

Opinion

Opinion - Leader Page Articles

Which way will the monsoon go this year?

N. Gopal Raj

It is quite possible that June rainfall for the country as a whole will be deficient. But meteorologists say neither this nor a delayed onset indicates that the monsoon will turn out badly.

ON JUNE 5, the India Meteorological Department (IMD) declared that the monsoon had set in over Kerala. After the onset, the normally strong winds that bring rain-bearing clouds to India were weak and the monsoon's northward progression has been slow. So much so that many regions in central and northern India are still enduring severe heat wave conditions.

Between the delay in the setting in of the monsoon (June 1 is taken by IMD as the date when the monsoon onset ought to occur over Kerala) and its weakened state thereafter, the countrywide rainfall a week after the monsoon began was only half of what it ought to have been. It is quite possible that June rainfall for the country as a whole will be deficient. Meteorologists, however, point out that neither a delay in the onset nor poor rains in June indicate that the monsoon will turn out badly.

Despite its vagaries, the southwest monsoon is remarkably robust. Over the last 130 years, the monsoon rainfall has been normal in 70 per cent of the years. There was a drought (when the nationwide rainfall between June and September fell to less than 90 per cent of the long-term average) in only 16 per cent of the years. Excess rains (more than 10 per cent of the long-term average) occurred in 14 per cent of the years.

For over a century, scientists have been trying to understand the factors that influence the Indian monsoon and especially those that cause it to fail. In the early years of the last century, Sir Gilbert Walker, then Director-General of Observatories in India, noticed that droughts occurred in years when atmospheric pressure was abnormally high in Darwin in northern Australia and low on the Pacific island of Tahiti. The monsoon would be good when the reverse happened. Sir Gilbert called the see-sawing atmospheric pressure the `Southern Oscillation.' It was only much later that scientists discovered that the Southern Oscillation was the atmospheric component associated with changes in the temperature of the surface waters of the Pacific Ocean. Usually, the western Pacific (off Asia) is warm while the eastern Pacific near the coast of South America is relatively cool. But in years when there is an El Nino, the western Pacific cools while the surface waters of the central Pacific (around the international dateline) and eastern Pacific become warmer. There are also years when a La Nina occurs, with the western Pacific becoming excessively warm and the eastern Pacific turning unusually cold.

As the El Nino/La Nina and Southern Oscillation are coupled, they are usually referred to jointly as the `'El Nino/Southern Oscillation' (ENSO). ENSO events affect the seasonal climate on a global scale. These events have been shown to influence precipitation and temperature in many parts of Asia, Australia, Africa and the Americas. Major Indian droughts, such as those in 1965, 1972, 1979 and 1987, occurred in years with an El Nino while La Nina years usually have bountiful rains.

But the link between El Nino and the failure of the Indian monsoon broke down spectacularly in 1997 when there was a normal monsoon despite the strongest El Nino occurrence of the 20th century. In 2002, although the El Nino was quite weak, one of the worst droughts of the last century occurred. Last year too, the monsoon failed (although not quite as badly as in 2002) and once again there was only a mild El Nino in evidence. All this has made scientists wonder what other factors could be at play in wrecking the Indian monsoon.

There is evidence that the El Nino comes in two flavours and only one of themreduces Indian monsoon rainfall, says K. Krishna Kumar of the Indian Institute of Tropical Meteorology at Pune. A few years ago, he along with researchers in the United States published a much-quoted paper in Science, suggesting that global warming might have weakened the link between El Nino and the Indian monsoon.

Now, Dr. Krishna Kumar and scientists at the University of Colorado and Columbia University are finding that the temperature gradient of the El Nino might be vital in determining its effect on the Indian monsoon. In one flavour of El Nino, the surface waters of the central Pacific are colder than those of the eastern Pacific. This type of El Nino does not seem to have an adverse impact on the Indian monsoon. That was why 1997's strong El Nino did not produce a drought, he told The Hindu.

If, on the other hand, an El Nino occurs with the opposite temperature gradient — with the waters of the central Pacific warmer than the eastern Pacific — it seems to affect the monsoon badly. The events of 2002 and 2004 showed that this type of temperature gradient could have a disastrous impact even when the El Nino was rather weak, according to Dr. Krishna Kumar.

Sulochana Gadgil of the Indian Institute of Science has a different perspective. As in the Pacific, in the Indian Ocean too there is a see-saw ocean-atmosphere system at work, she says. The ocean component is called the Indian Ocean Dipole. During a positive Dipole event, the waters of the eastern Indian Ocean off Sumatra in Indonesia become colder than usual while the waters of the Arabian Sea near the Somali coast in Africa warm up; in a negative Dipole, the reverse happens. Positive Dipoles have been associated with increased monsoon rainfall and negative ones with reduced rain.

The atmospheric component of this system, characterised by anomalous wind patterns, has been given the name `Equatorial Indian Ocean Oscillation' (EQUINOO) by Prof. Gadgil and fellow scientists at the Institute's Centre for Atmospheric and Oceanic Sciences. Like the Indian Ocean Dipole, a positive EQUINOO is favourable for the monsoon and a negative one unfavourable. They argue that the effect of a strong El Nino in 1997 was offset by a positive EQUINOO and associated positive Dipole, and so the monsoon was normal. In 2002, on the other hand, although the El Nino was weak, its effect was enhanced by the strong negative EQUINOO. The same thing happened in 2004.

So how is the present monsoon likely to shape up?

Worrying signs

Although an El Nino can be predicted with reasonable accuracy some months in advance, methods of predicting the associated temperature gradient have still to be worked out, says Dr. Krishna Kumar. Nevertheless, the current sea surface temperatures in the Pacific show worrying signs. Although there is no El Nino and the neutral conditions are expected to persist in the coming months, there is a temperature gradient with a warmer central Pacific and a cooler eastern Pacific. That is similar to the pattern that obtained in 2002 and 2004, although weaker in scale. If that temperature anomaly strengthens, it could impact the current monsoon, he believes.

As for EQUINOO, the wind anomalies in the relevant regions of the Indian Ocean are close to normal at present, says Dr. Gadgil. Only by this month-end will it be possible to say whether EQUINOO is favourable or unfavourable to the monsoon. Moreover, in the absence of an El Nino or La Nina, the wind anomalies have to be substantial for EQUINOO to lead to droughts or excess rain. As there is only a low probability of El Nino developing in the coming months, the monsoon is unlikely to end in a drought unless a strong negative EQUINOO appears, she says.

The mechanisms controlling the monsoon are not still well understood, according to J. Srinivasan, also of IISc. In 2002, the monsoon progressed well in June, he points out. Then, there was a prolonged dry spell in July and the rainfall that month had an unprecedented 49 per cent deficit. Despite all the data available on atmospheric and ocean conditions during the monsoon as well as the many modelling studies done subsequently, no one has been able to pinpoint what caused the monsoon to weaken so abruptly in July, he says.

Likewise, in 1997, most meteorological parameters in May suggested that the monsoon would be below normal. In the first half of June, it did appear to go badly. Then suddenly the monsoon revived and the season ended with normal rains. What caused this switch is again not well understood, he says. "The monsoon is a complex system and until we understand the underlying mechanisms that strengthen and weaken it, predicting its outcome is not going to be easy," says Prof. Srinivasan.

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