Figure 17.30: In El Niño conditions, the trade winds weaken or reverse directions. Warm
water moves eastward across the Pacific Ocean and piles up against South America. ( 30 )
they are quicker and more energetic. This pattern, with unusually cold water in the eastern
Pacific Ocean, is called La Niña (Figure17.31). El Niño events take place every three to
seven years but vary in their strength.
Figure 17.31: During a La Niña, ocean temperatures along the coast of South America are
colder than normal (instead of warmer, as in El Niño) and cold water reaches farther into
the western Pacific than normal. As in a normal year, trade winds moving from east to west
and warm water piles up in the western Pacific Ocean. ( 20 )
Other important oscillations are smaller and have a local, rather than global, effect. The