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El Nino and the Southern Oscillation

Normally, the coastal water off Peru and the equatorial regions of South America are rich in seafood, especially in anchovies. But, once every few years, a warm current appears off the coast, which reduces, dramatically, the yield of fish. Normally, the equatorial regions of South American coast are under the influence of the easterly trade winds. These winds drive a northward flowing cold ocean current. This is the Humboldt or Peru Current. It flows northward to about 5º S before turning westwards. But, once in a few years the trade winds suddenly weaken and even reverse their direction. The Humboldt Current then weakens and there is little or no upwelling off Peru. The surface waters of the Pacific basin tend to pile up near the western sector of the basin in normal years. The sea levels near Indonesia, for example, are nearly a metre higher than over Peru. But in an El Nino year they become nearly equal. And, there is a corresponding reduction in pressure over the southern Pacific because the easterly trades are weaker. This in turn influences the Southern Oscillation. El Nino means ‘The Infant’ and refers to Infant Jesus, because the phenomenon occurs around the advent of Christmas.
A great deal of investigation based on analysis of past data, remains to be done before firm conclusions are reached. There have been years of deficient monsoon rainfall, which were not El Nino years. This suggests that there are other factors, apart from an El Nino, which lead to poor monsoons over India. The relationship between the two is not very firm, but generally an El Nino is associated with a poor or indifferent monsoon.

The Southern Oscillation was discovered by Sir Gilbert Walker over seventy years ago. It is an oscillatory pattern of weather between the Pacific and the Indian Ocean extending from Africa and Australia. Sir Gilbert discovered that when the pressures tended to be high over the southern Pacific they tended to be low over the Indian Ocean. As pressures are inversely related to rainfall, this suggests that when low pressures prevail over the Indian Ocean in the winter months, the chances are that the coming monsoon will be good in terms of rainfall. This fact is still utilized in long-range prediction of monsoon rainfall. The Southern Oscillation has a period varying from two to five years; consequently, pressure departures (deviations from the mean value) and their trends are better predictors than the absolute values of pressure.

In a sense the Southern oscillation is another version of the Walker Cell. Dr.Bjerknes, a well-known meteorologist from Norway, explains the Walker Cell as a feedback from the oceans. He suggests that the main drive for the Walker Cell comes from a difference in the temperature of the sea surface between Indonesia (warm) and the eastern Pacific (cold). Cold coastal waters of the eastern Pacific are generated by upwelling, that is, the replacement of surface waters by the colder water from greater depths. The Indonesian sector being warm in terms of sea surface temperature induces greater cloudiness, which, in turn, cuts off the input of solar radiation, thereby reducing the drive for the Walker circulation. But, as the drive behind the Walker Cell weakens, the coastal waters of the eastern pacific become warmer because of lesser upwelling. The sea surface temperature rises, thereby reducing the temperature difference between the Indonesian sector and the east pacific still further. In this manner, the Walker Cell passes from a phase of strong intensity to one of weak circulation until, the pulsating nature of the circulation reverses the role of Indonesia and the eastern Pacific after two to five years.

This is a broad and simple explanation in which many gaps remain to be filled in, but there appears to be an increasing body of evidence that points to the fact that convection over Indonesia and the eastern Pacific are important for understanding the dynamics of the Asian Summer and Winter Monsoons. The pressure difference between Tahiti 17.5º S, 149.6º W in French Polynesia and Darwin 124º S, 130.4º E in northern Australia is often taken to represent the intensity of the Southern Oscillation. It is called the Southern Oscillation Index (SOI). A negative SOI suggests a weak monsoon.

Excerpt from ‘The Monsoon’ by Dr.P.K.Das, former Director General of the National Meteorological Service of the Government of India. Published by National Book Trust, India. Price: Rs.75/=


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