Sometimes, but not always, El Niño conditions give way to the other extreme of the El Niño-Southern Oscillation (ENSO) cycle. This cold counterpart to El Niño is known as La Niña.
During non-El Niño years, low surface pressures the concentrate over the equatorial western Pacific. Surface pressures tend to be higher in the eastern equatorial Pacific. This contrast in pressure drives the trade winds, the prevailing large-scale surface winds that blow from east to west.
During La Niña years, unusually strong trade winds contribute to colder than normal surface waters over the eastern tropical Pacific and warmer than normal surface waters in the western tropical Pacific.
The La Niña of the winter 2010-2011 was one of the strongest events in the past half century.
Climatic impacts will spread from the western Pacific to the west coast of North America. La Niña typically affects weather patterns in the Northwest. The winter of 2011-2012 see a continuation of a weaker La Niña causing a cooler than normal, but wet weather here, stormy, colder, and more above normal snowfall east of the Rockies, a slightly wetter than normal Southwest and a drier than normal Southeast.