La Niña is defined as cooler than normal sea-surface temperatures (SSTs) in the tropical Pacific ocean that impact global weather patterns. This deviation from normal SSTs can have large scale impacts, not only on ocean processes, but also on the atmosphere.
How does this large pool of cool water impact the atmosphere?
In normal, non-La Niña conditions, the trade winds blow toward the west across the tropical Pacific Ocean. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper layers. This cold water is nutrient-rich, supporting a diverse marine ecosystem and major fisheries. Rainfall is found in rising air over the warmest water in the western Pacific, and the east Pacific is relatively dry. In a normal year, the atmosphere typically responds with rising air over the western Pacific and sinking air over the eastern Pacific, as can be seen in this diagram.
During La Niña, the trade winds strengthen in the central and eastern Pacific leading to increased upwelling along the South American coast. This cooler pool of water then spreads westward, due to the increased trade winds until it spreads across much of the eastern Pacific Ocean near the equator. The normally cool water located off the South American coast becomes increasingly cooler as the upwelling strengthens, and more sub-surface ocean water is brought to the surface. This leads to the cool SSTs that were shown at the figure at the top of this webpage, with the strongest deviations from normal just off the South American coastline and along the equator. The atmospheric response to this pattern, with much higher SSTs in the west, and much cooler ones over the eastern Pacific, is a strengthening of the normal pattern previously discussed. Rainfall and rising motion are enhanced over the warm water over the western Pacific, with associated flooding in Indonesia and Australia. The sinking motion over the eastern Pacific is strengthened further, leading to a supression of rainfall and dry conditions over portions of South America. This large change in the circulation pattern forces changes in weather across regions far removed from the tropical Pacific, such as Arizona.
How did it get the name La Niña?
La Niña means The Little Girl in Spanish. This name was chosen as the opposite of El Niño, the much more famous event. Both La Niña and El Niño are extreme phases of a naturally occurring climate cycle referred to as El Niño/ Southern Oscillation.
What are the global impacts of La Niña?
As mentioned, La Niña has impacts that are felt not only near the equator, but far removed from the equator. A large area of wetter than normal conditions are found in some of the normally wet regions of the world such as Indonesia and Australia. Places that are normally dry, like western South America and the southwest United States tend to be even more dry. For the United States, this band of drier than normal conditions extends across the entire southern portion of the country. Western Canada and Alaska tend to experience a cooler than normal winter as well. The northwest United States tends to experience wet and cool conditions throughout much of a La Niña winter.
How often do La Niña events occur?
La Niña events tend to occur on average about every 4 to 7 years and typically last for a year. However, La Niña events can last for two or even three years on occasion. For a list of La Niña (cold) events, please go to the Climate Prediction Center List of Cold and Warm Episodes by Season.
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