La Nina means “The Little Girl.” La Niña is sometimes called El Viejo (Old Man), anti-El Niño, or simply “a cold event” or “a cold episode”. A La Niña event is also sometimes called an anti-ENSO (anti-El Niño-Southern Oscillation) event.
La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, as compared to El Niño, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific.
La Nina Formation
Researchers discovered that during non-El Niño years, surface pressures tend to be low over the warm waters of the equatorial western Pacific as overlying warm moist air rises and then diverges aloft. Over the colder waters of the eastern equatorial Pacific, surface pressures tend to be higher as converging winds aloft contribute to the sinking of cool air. In much the same way as a ball rolls down a hill, air flows from high pressure in the east to low pressure in the west along this equatorial pressure gradient. This contrast in pressure is what drives the trade winds, the prevailing large-scale surface winds that blow from east to west. As these winds blow along the surface of the equatorial waters, there is a net transport of ocean water in a westward direction. As this occurs, cold, nutrient-rich water rises up (or up wells) along the coast of South America to replace the westward-moving surface water. This upwelling brings nutrients to the surface waters off the coast allowing the fish population living in these upper waters to thrive.
Global climate La Nina impacts tend to be opposite those of El Niño impacts. Globally, La Niña is characterized by wetter than normal conditions west of the equatorial central Pacific over northern Australia and Indonesia during the northern hemisphere winter, and over the Philippines during the northern hemisphere summer. Wetter than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern hemisphere winter season.
During the northern hemisphere summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India. Drier than normal conditions are observed along the west coast of tropical South America, and at subtropical latitudes of North America (Gulf Coast) and South America (southern Brazil to central Argentina) during their respective winter seasons. The last lengthy La Niña event was 1998-2001, which contributed to serious drought conditions in many sections of the western United States.
There are many scientists who are convinced that the increased intensity and frequency, now nearly every two to three years of El Niño and La Niña events in recent decades is the result of warmer ocean temperatures caused by global warming. Several years ago, scientists from NOAA explained that increasing global temperatures and increasing evaporation from land added moisture to the air, which intensified the storms and floods associated with El Niño.