A La Ni?a pattern occurs when surface temperatures of the central and eastern Pacific Ocean are cooler than normal in the tropics. This is usually the result of air and water current patterns, and it occurs every few years. This lower temperature affects weather patterns throughout the world, although the primary effects occur in North and South America.
Just as a La Ni?a event is the opposite of an El Ni?o event, the effects of the La Ni?a are essentially the opposite of El Ni?o's impact on the climate. During a La Ni?a year, the jet stream draws moisture north, producing wetter than normal conditions across the Pacific Northwest and Midwest. Meanwhile, the southern part of the United States receives less rainfall than normal. La Ni?a years tend to produce colder and snowier winters in Canada while bringing drought to the western part of South America and more rain to Brazil. The change in the jet stream also calms the air over the tropical portions of the Atlantic Ocean, encouraging the development of hurricanes.
The global current patterns that affect temperatures in the Pacific usually cause the region to oscillate between warm and cold temperatures every few years. Not every El Ni?o is followed by a La Ni?a, however. In some cases, the pattern shifts to neutral, not especially warm or cold, before repeating itself.