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Normally, there is low atmospheric pressure over northern Australia and Indonesia, while there is high pressure over the eastern Pacific Ocean. The difference in air pressure drives the trade winds from east to west across the Pacific. The trade winds hold water on the surface in the western Pacific so it gets hot. The warm, surface waters bring rain to northern Australia and Indonesia. Meanwhile, cold deep water is drawn up to the surface along the coast of Peru and Ecuador bringing nutrients from the depths that support large anchovy stocks, making this region one of the world’s most productive fisheries. Every two to seven years, this air pressure pattern reverses. The usually low atmospheric pressure over Australia is replaced by high pressure. Over the central Pacific and along the South American coast, the air pressure drops, slackening the westward trade winds. The easing winds and influx of warm water in the east stop the upwelling of cool, nutrient rich water along the South American coast. These unusual conditions were first noticed around Christmas near Peru and were called El Nino, which is Spanish for ‘the boy child’. The effects of El Nino are far-reaching. Along the South American coast, fisheries collapse and the warm moist air brings torrential rains and flooding. In contrast, there are droughts and bushfires in Australia, Indonesia and other south-east Asian countries. With global warming, it is possible that El Nino events could be more frequent and intense.
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