In a normal year, the trade winds blow across the Pacific Ocean near the equator from east
to west (in the direction of Southeast Asia), piling up warm water in the western Pacific
Ocean and actually raising sea levels there by half a meter. Meanwhile, along the western
coast of South America, the Peru Current carries cold water northward, and cold, nutrient-
rich waters upwell from the deep sea. When the Peru Current nears the equator, it flows
westward across the Pacific Ocean with the trade winds.
Figure 17.29: Normal conditions in the Southern Oscillation have a low pressure region in
the western Pacific Ocean and warm water (shown in red) building up there as well. Notice
that continents are shown in brown in the image. North and South America are on the right
in this image. ( 2 )
When water temperature reaches around 28°C (82°F), the trade winds weaken or reverse
direction and blow east (towards South America). An El Niño cycle has begun (Figure
17.30). Warm water is dragged back across the Pacific Ocean, heating the central Pacific
Ocean and the surface waters off the west coast of South America. With warm, low density
water at the surface, no upwelling occurs along the coast of South America. Without up-
welling, nutrients are scarce and plankton populations decline. Since plankton form the base
of the food web, fish cannot find food, and fish numbers decrease as well. All the animals
that eat fish, including birds and humans are affected by the decline in fish.
El Niño events change global climate when circulation patterns in the atmosphere and oceans
change. Some regions receive more than average rainfall, including the west coast of North
and South America; the southern United States; and Western Europe. Drought occurs
in other parts of South America, the western Pacific, southern and northern Africa, and
southern Europe.
An El Nino cycle lasts one to two years and ends when the warm mass of central Pacific
water has moved eastward once more. Normal circulation patterns resume, but sometimes