| South-East Asia's Environmental Future: The Search for Sustainability (1993) |
|Part II - Climatic change and variability|
|7. Enso, drought and flooding rain in south-east Asia|
The El Niño-southern oscillation
Variations in climate from year to year appear at first glance to be random. Examination of historical data, however, reveals a coherent global pattern of oceanic and atmospheric fluctuations called the Southern Oscillation. Extreme anomalies in this pattern involve dislocations of rainfall distribution in the Tropics, bringing drought to some regions and torrential rains to others (Ropelewski and Halpert, 1987, 1989). These anomalies typically last about a year. Related anomalies of the atmospheric circulation extend high into the atmosphere and polewards into the temperate zones, especially in the southern hemisphere.
Some major changes in the ocean currents and temperatures are also related to the Oscillation. The best known of these is the El Niño, a marked temperature increase that occurs every few years in the eastern Equatorial Pacific with catastrophic effects on marine ecosystems along the west coast of the Americas. Because El Niño usually occurs with an extreme anomaly in the Oscillation, the two phenomena are often referred to jointly as 'El Niño-Southern Oscillation' or 'ENSO'. Periods with very warm sea surface temperatures (SSTs) in the eastern Equatorial Pacific, and the global pattern of climatic anomalies that usually accompany this warm water, are referred to as El Niño events. A major El Niño event occurred in 1982-3 with severe droughts in Australia, Indonesia, parts of Africa, and India. A schematic of the atmospheric circulation during an El Niño is provided in Figure 7.1.
During the other extreme of the Oscillation, the eastern Equatorial Pacific is cold (a phenomenon now called 'La Nina') and heavy rainfall and flooding is observed over the areas usually affected by drought during El Niño events. (See Figure 7.2 for a schematic depiction of a La Nina.) Heavy rainfall in India, Africa and Australia during 1988 was associated with a La Nina event. The dislocations in rainfall distribution associated with El Niño and La Nina events mean that the areas affected tend to have more variable rainfall than is the case elsewhere.
ENSO is the result of interactions between the tropical oceans (especially the Pacific) and the atmosphere. The detailed form of this interaction is yet to be determined. Major research programmes aimed at modelling it are under way. Models of the Equatorial Pacific Ocean and the atmosphere, apparently capable of reproducing and predicting some aspects of the phenomenon, have been developed (Barrett et al., 1988). A model capable of a realistic simulation of the complete phenomenon, however, has yet to be developed.
There is often confusion about the terminology used in discussions. El Niño events are just one extreme of the quasi-cyclic ENSO phenomenon. La Nina events are the other extreme. These are illustrated in Figure 7.3 which shows monthly values of the Southern Oscillation Index (SOI). This index, which reflects the behaviour of ENSO, is the standardized difference in pressure between Tahiti and Darwin. El Niño events occur when the SOI is at large negative values; La Nina events occur at large positive values. The SOI fluctuates quasi-periodically; the nature of this fluctuation is discussed below.