National Weather Service
An El Niño and La Niña are temporary changes in the climate of the Pacific Ocean. They can be seen in measurements of the sea surface temperature in the region around the equator. Although the changes in sea temperatures may seem small, they can have huge effects on the world’s climate.
La Niña is essentially the opposite of El Niño. The name La Niña (“the girl child”) was coined to represent the opposite of El Niño (“the boy child”). The terms El Viejo and anti-El Niño are also used.
In normal, non-El Niño conditions, the trade winds blow towards the west across the tropical Pacific. Warmer water near the surface of the ocean, heated by the sun, is blown to the west by the trade winds. Warmer water “piles up” in the west Pacific (the sea surface is about one and one half feet higher at Indonesia than at Ecuador) as cooler water rises from deep in the ocean in the east Pacific to replace the water that was moved away by the winds. The sea surface temperature is about 8 degrees C higher in the west, with cooler temperatures in the east Pacific due to upwelling of colder water from deep in the ocean. Rainfall develops much more frequently in rising air over warm water so normal rainfall amounts are higher over the west Pacific and the east Pacific area is relatively dry.
During El Niño, the trade winds relax in the Pacific. This leads to warmer water temperatures in the eastern and, especially, central Pacific because upwelling is reduced. As the pool of warmer water moves eastward, the areas of best rainfall development also move to the east, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of this heat source (the warmest water) results in large changes in the global atmospheric circulation, which in turn forces changes in weather in regions far removed from the tropical Pacific. El Niño episodes typically occur every 3 to 5 years.
La Niña, also called El Viejo, is essentially the opposite of El Niño. La Niña exists when cooler than usual ocean temperatures occur in the eastern Pacific. La Niña occurs almost as often as El Niño, as the two are stages of the same larger phenomenon.
Stronger than usual trade winds, which blow from east to west across the Pacific Ocean, define La Niña. As warmer water near the surface of the ocean is blown to the west, cooler water rises from deep in the ocean in the east Pacific to replace water that was moved away. The colder water cools the overlying air which hampers the formation of clouds and tropical thunderstorms in central and eastern areas of the Pacific Ocean. This suppression of rain-producing clouds leads to dry conditions from near the International Date Line east to South America.
La Niña is characterized by unusually cold ocean temperatures in the equatorial Pacific, as compared to El Niño, which is characterized by unusually warm ocean temperatures in the equatorial Pacific. Like El Niño, La Niña occurs every 3 to 5 years.
So how does it affect hurricane formation? Hurricane formation requires that winds be fairly uniform throughout the atmosphere. In other words, hurricanes cannot form if the vertical wind shear is too high.
El Niño produces stronger westerly wind at upper levels of the atmosphere across the tropical Atlantic than in normal non-El Niño seasons. This increases the total vertical wind shear, basically shearing the tops from developing storms before a healthy circulation can form. El Niño events generally suppress Atlantic hurricane activity so fewer hurricanes than normal form in the Atlantic during August to October, the peak of Atlantic hurricane season.
During La Niña, westerly winds high in the atmosphere weaken. This results in an expanded area of low vertical wind shear, allowing more Atlantic hurricanes to develop during La Niña events. La Niña increases the number of hurricanes that develop and allows stronger hurricanes to form.
The chances for the continental U.S. and the Caribbean Islands to experience a hurricane increase substantially during La Niña and decrease during El Niño.
El Niño and La Niña also influence where Atlantic hurricanes form. During La Niña, more hurricanes form in the deep Tropics from weather disturbances that originate over North Africa. These systems have a much greater likelihood of becoming major hurricanes, and of eventually reaching the U.S. and the Caribbean Islands.
