La Nina Definition, Causes, and Impacts

Spanish for little girl, La Nià ±a 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 Nià ±o/Southern Oscillation or ENSO (pronounced en-so) cycle. La Nià ±a conditions recur every 3 to 7 years and typically last from 9 to 12 months up to 2 years. One of the strongest La Nià ±a episodes on record was that of 1988-1989 when ocean temperatures fell as much as 7 F below normal. The last La Nià ±a episode occurred during late 2016, and some evidence of La Nià ±a was seen in January of 2018. La Nià ±a vs. El Nià ±o A La Nià ±a event is the opposite of an El Nià ±o 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 Nià ±o. In fact, the positive effects on the fishing industry make La Nià ±a less of a news item than an El Nià ±o event. Both La Nià ±a and El Nià ±o 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). El  Nià ±o (meaning the Christ child) earned its name because of its usual appearance around Christmas time. What Causes La Nià ±a Events You can think of La Nià ±a (and El Nià ±o) events as water sloshing in a bathtub. Water in the equatorial regions follows the patterns of the trade winds. Surface currents are then formed by 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 Nià ±a 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. 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 carry important nutrients from deeper ocean depths. The colder waters are important to fishing industries and the nutrient cycling of the ocean. How La Nià ±a Years Differ During a La Nià ±a 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, the climate is affected worldwide. Temperatures in the ocean affect the air above it, creating shifts in climate that can have both regional and global consequences. How La Nià ±a Affects Weather and Climate Rain clouds form as a result of the lifting of warm, moist air. When the air doesnt get 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 rainstorms 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 Nià ±a 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.