La Nina is an atmospheric interaction between the ocean/air interface. Temperatures in the ocean affect the air above it, therefore creating shifts in climate that can have both regional and global consequences.
La Nina Background
La Nina is a Spanish name meaning little girl. Essentially, a La Nina event is the opposite of an El Nino 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 Nino. In fact, the positive effects on the fishing industry make La Nina less of a news items than an El Nino event.
The frequency of La Nina events depends on a number of contributing factors. One can think of both El Nino and La Nina events (part of the larger El Nino Southern Oscillation or ENSO) 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.
What Causes a La Nina Event?
Off the coast of South America, changes in air pressure during a La Nina 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 Nina Years Different?
During a La Nina 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 in locations worldwide.
What Are the Effects of La Nina?
Rain clouds form as a result of the uplift 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 a La Nina year, while the western equatorial portions of South America may be in drought conditions. In the United States, Washington and Oregon may see increased precipitation while portions of California, Nevada, and Colorado may see drier conditions.
Why Does the Ocean Oscillate
The oscillation of warm to cold and back again across the Pacific is part of the El Nino Southern Oscillation. The pattern usually cycles every three to five years. Sometimes an El Nino will be backed up by neutral conditions; Sometimes an El Nino will be followed by the colder La Nina conditions. Though a La Nina event can last for two years, the typical time period is between 9 and 12 months, on average.