What is the El Nino?news
27 March 2007

The last severe El Nino took place between 1997 and 1999, causing massive forest fires in South East Asia and more than $20 billion of damage worldwide. What exactly is this devastating phenomenon? El Niño is a disturbance of the ocean-atmosphere system in the tropical Pacific, which causes widespread disruptions in global weather conditions.

Among these disruptions are increased rainfall across different continents (the southern parts of the US in the North American continent and Peru in the South American continent), that cause destructive flooding and drought in the west Pacific and extreme dryness in Australia (in the Australian continent) sometimes causing devastating brush fires. Observations of conditions in the tropical Pacific are essential for the predicting short-term climate variations ranging from a few months to a year.

Since the highest incidence of solar radiation occurs near the equator, measuring what heat is doing in the equator is a reliable index of what it is doing elsewhere. As warming water moves, it can send strong signals, indicating that some parts of the world will be drier, wetter, colder or warmer. If rainfall is heavier, water may pool, increasing the chances of water-borne parasitic infections. Some fish also follow ocean temperature, and this information is essential for sustaining healthy fisheries.

To gather data on current and wind patterns and temperature, a network of buoys is used in the Pacific equatorial belt. These buoys transmit data every day, which is available in real time to researchers and forecasters around the world.

In normal, non-El Niño conditions, trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 0.5 meter higher at Indonesia in the Asian continent than at Ecuador in the north-western tip of South America.
The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling (the deeper layers of cold waters being brought up to the surface by strong seasonal winds). This cold water is nutrient-rich, and supports diverse marine ecosystems and fish life. Rainfall is found in the rising air over the warmest water, and the east Pacific is relatively dry.

During El Niño trade winds relax in the central and western Pacific, leading to a depression of the thermocline (a condition where the water layer is warmer on top than at the bottom) compared to the eastern Pacific, with an elevation (of the thermocline) in the west.

So what? Consider this: in 1982-1983, the 17-degree isotherm (the connecting point in the water layers having the same temperature at a given time) dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone (the upper layer of the ocean that allows enough light to penetrate to support photosynthetic, or green, plants.)

The result was a rise in sea surface temperature and a drastic decline in primary productivity, which adversely affected higher nutrition levels of the food chain, including commercial fisheries in this region. The displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

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What is the El Nino?