What Is La Nina?

Meet El Nino's Cool Little Sister

Lost at sea
La Nina is associated with cooler-than-normal sea surface temperatures. gremlin / Getty Images

Spanish for "little girl," La Niña is the name given to the large-scale cooling of sea surface temperatures across the central and equatorial Pacific Ocean. It is one part of the larger and naturally occurring ocean-atmosphere phenomenon known as the El Niño/Southern Oscillation or ENSO (pronounced "en-so") cycle.

La Niña conditions recur every 3 to 7 years and typically last from 9 to 12 months up to 2 years.

One of the strongest La Niña episodes on record was that of 1988-1989 when ocean temperatures fell as much as 7 degrees Fahrenheit below normal. The last La Niña episode occurred from 2010-2012. NOAA scientists expect La Niña conditions to return again during late 2016.

La Niña vs. El Niño

A La Niña event is the opposite of an El Niño event. Waters in the equatorial regions of the Pacific Ocean are unseasonably cool. The cooler waters affect the atmosphere above the ocean, causing significant changes in climate, though usually not as significant as the changes that occur during an El Niño. In fact, the positive effects on the fishing industry make La Niña less of a news items than an El Niño event.

Both La Niña and El Niño events tend to develop during the Northern Hemisphere spring (March to June), peak during late fall and winter (November to February), then weaken the following spring into summer (March to June).

What Causes La Niña Events?

You can think of La Niña (and El Niño) events as water sloshing in a bathtub. Water in the equatorial regions follow the patterns of the trade winds. Surface currents are then formed from the winds. Winds always blow from areas of high pressure to low pressure; The steeper the gradient difference in the pressure, the faster the winds will move from highs to lows.

Off the coast of South America, changes in air pressure during a La Niña event cause winds to increase in intensity. Normally, winds blow from the eastern Pacific to the warmer western Pacific. The winds create the surface currents that literally blow the top layer of water of the ocean westward. This is the production of surface currents. As the warmer water is "moved" out of the way by the wind, colder waters are exposed to the surface off the western coast of South America. These waters, though more dense, carry important nutrients from deeper ocean depths. The colder waters are important to fishing industries and the nutrient cycling of the ocean.

How Are La Niña Years Different?

During a La Niña year the trade winds are unusually strong, leading to increased movement of water towards the western Pacific. Much like a giant fan blowing across the equator, the surface currents that form carry even more of the warmer waters westward. This creates a situation where the waters in the east are abnormally cold and the waters in the west are abnormally warm. Because of the interactions between the temperature of the ocean and the lowest air layers, climate is affected worldwide. Temperatures in the ocean affect the air above it, therefore creating shifts in climate that can have both regional and global consequences.

How La Niña Affects Weather and Climate

Rain clouds form as a result of the lifting of warm, moist air. Without the air getting its warmth from the ocean, the air above the ocean is abnormally cool above the eastern Pacific. This prevents the formation of rain, often needed in these areas of the world. At the same time, the waters in the west are very warm, leading to increased humidity and warmer atmospheric temperatures. The air rises and the number and intensity of rain storms increase in the western Pacific. As the air in these regional locations changes, so too does the pattern of circulation in the atmosphere, thereby affecting climate worldwide.

Monsoon seasons will be more intense in La Niña years, while the western equatorial portions of South America may be in drought conditions. In the United States, the states of Washington and Oregon may see increased precipitation while portions of California, Nevada, and Colorado may see drier conditions.

Edited by Tiffany Means