El Niño and La Niña

El Niño and La Niña are naturally occurring phenomena that typically occur every 3-5 years. The phenomena result from the interaction between the atmosphere and the ocean surface in the tropical Pacific, affecting the Pacific Ocean’s normal conditions. At normal conditions, trade winds blow from east to west, taking warm water in the same direction. As the warm water moves west, upwelling causes cold water from the deep to rise to the surface. El Niño and La Niña can break these normal patterns, resulting in massive global weather effects. El Niño and La Niña are Spanish names meaning “the boy” and “the girl,” respectively.

During El Niño, the trade winds weaken. Instead of warm water moving west, it is pushed back east toward South America’s west coast, causing an accumulation of humid, warm air. El Niño is associated with high air surface pressure in the tropical western Pacific and can significantly affect the weather. The warmer waters move the Pacific jet stream south from its neutral position, causing northern U.S. and Canada areas to be dryer and warmer than usual.  Convection above the warmer surface waters increases precipitation, causing increased rainfall in South America, especially Ecuador and northern Peru. Such excessive rainfall may lead to flooding, erosion, and destruction of infrastructure. El niño results in below-average rainfall in places such as India, which may lead to severe droughts. El Niño also affects marine populations. Because there is no upwelling of cold, nutrient-rich water, there is reduced phytoplankton, leading to food scarcity for the fish populations. One of the major and intense El Niños happened in 1997-1998, leading to drought conditions in Indonesia, Malaysia, and the Philippines. There were very heavy rains and severe flooding in areas such as Peru. In the United States, California experienced increased winter rainfall while the Midwest experienced record-breaking warm temperatures.

A La Niña event sometimes follows an El Niño event. During Lanina, the trade winds intensify. There is a greater push of warm water toward Asia, and upwelling increases off the coast of South America, causing more cold, nutrient-rich water that supports marine life to rise to the surface. La Niña is characterized by lower-than-normal air pressure over the western Pacific, leading to increased rainfall in areas such as Southeast Asia and Australia. Drier-than-normal conditions are observed along the west coast of tropical South America and the United States Gulf Coast as fewer rain clouds form. Cold waters in the Pacific Ocean result in a northward shift of the jet stream, leading to dry conditions in the southern United States and increased rainfall and flooding in the Pacific Northwest and Canada. In a La Niña, winter temperatures are warmer than normal in the South and cooler than normal in the North. La Niña is also known to lead to a more severe hurricane season.