El-Nino Conditions

globe showing an El NIno event.El Nino happens when weakening trade winds (which sometimes even reverse direction) allow the warmer water from the western Pacific to flow toward the east. This flattens out the sea level, builds up warm surface water off the coast of South America, and increases the temperature of the water in the eastern Pacific.

An El Nino condition results from weakened trade winds in the western Pacific Ocean near Indonesia, allowing piled-up warm water to flow toward South America.

The deeper, warmer water in the east limits the amount of nutrient-rich deep water normally surfaced by the upwelling process. Since fish can no longer access this rich food source, many of them die off. This is why these conditions are called "El Nino", or "the Christ Child", which is what Peruvian fisherman call the particularly bad fishing period around December. More importantly, the different water temperatures tend to change the weather of the region.

What happens to the ocean also affects the atmosphere. Tropical thunderstorms are fueled by hot, humid air over the oceans. The hotter the air, the stronger and bigger the thunderstorms. As the Pacific's warmest water spreads eastward, the biggest thunderstorms move with it. If you look on a map, you will see that suddenly islands like Tahiti, normally tropical paradises, experience massive storms.

El Nino can have impacts on weather at various locations around the globe. Off the east coast of southern Africa, drought conditions often occur. In countries such as Zimbabwe, the effects of drought can be devastating.

The clouds and rainstorms associated with warm ocean waters also shift toward the east. Thus, rains which normally would fall over the tropical rain forests of Indonesia start falling over the deserts of Peru, causing forest fires and drought in the western Pacific and flooding in South America. Moreover the Earth's atmosphere reponds to the heating of El-Nino by producing patterns of high and low pressure which can have a profound impact on weather far away from the equatorial Pacific. For instance, higher temperatures in western Canada and the upper plains of the United States, colder temperatures in the southern United States. The east coast of southern Africa often experiences drought during El Nino.

Before and after
diagram of the trade winds.
Weakening Winds
Wind patterns leading up to an El Nino condition in 1994 show the trade winds blowing strongly (longer arrows) westward toward New Guinea in April, weaken in July, and reversing direction in October.

Movie of the El Nino Event.
The Global Conveyor Belt
This movie, taken from "Images of Earth and Space: The Role of Visualization in NASA Science" video tape, shows an El Nino Event in progress. Warm water (in red) moves from the western to the eastern Pacific. (QT 1.8 MB, 30 sec.)

Atmospheric pressure diagram.
Ocean "Storms"
Disruption of the upper level pressure pattern associated with eastern displacement of warm water (red) and associated rain clouds (gray) during an El Nino event.

Updated: January 22, 2003