Occasional major disruptions occur to the ocean current systems. These cause widespread changes to climate and sea level conditions especially in the tropics. The best known of these are the El Niño southern oscillation events. Under normal conditions, the cold, nutrient-rich Peru current flows northward along the western coast of South America (see Figure 1.6). This is accompanied by a coastal upwelling of nutrients caused by southerly winds. As a result, these waters are some of the most productive in the world. Huge numbers of anchovetta and sardine feed in the plankton-rich waters supporting a massive fishery as well as great numbers of seabirds and other wildlife.

Every 7 to 10 years or so, the trade winds cease to blow in their normal pattern from the east or south-east and warm equatorial water is blown in by abnormal winds from the west. In simple terms, pressure gradients in the west and east Pacific are reversed, causing a reversal of the trade winds and equatorial currents. A huge area of warm water is created and upwelling ceases thus reducing the supply of nutrients to the surface waters (Wuethrich, 1995). The increased water temperature kills many of the cold-water organisms normally present and, along with the lack of nutrients, leads to a dramatic reduction in primary production and a subsequent collapse of pelagic fish stocks. This has effects right up the food chain. Even the famous Galapagos iguanas suffer due to a reduction in the production of their seaweed food (Barber and Chavez, 1983).

The most extreme El Niño this century occurred in 1982-83. It started in July 1982 and continued until October 1983. Surface temperatures increased by 5°C and sea levels rose by as much as 22 cm in the Galapagos and 10 cm as far north as San Diego in California. Greatly increased rainfall caused extensive flooding in Peru and Ecuador. The exact mechanism is still not understood but two recent long-term programmes, TOGA, the Tropical Ocean Global Atmosphere, and WOCE (see Section 1.3.5) should one day allow scientists to model how the system works. This will help in the prediction of future El Niño southern oscillation events. Recently, there appears to have been an increase in the frequency of El Niños and this has led to considerable speculation on whether this could be due to global warming (see Chapter 10).

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