What Is El Nino?

During El Nino, the surface of the Pacific Ocean off the coast of South American (brown region at right) is warmer (red) as cool water below (blue) does not upwell effectively. Click to see how it's different during non-El Nino times. CREDIT: NOAA

El Niño is marked by warmer water in the Pacific off the coast of South America. It alters weather patterns in the United States and around the world.

El Niño was originally recognized by fishermen off the coast of South America. Today, climate experts track it with ocean buoys and satellite data. In Spanish, El Niño means "the little boy" or "Christ child." This name was used for the tendency of the phenomenon to arrive around Christmas. The cool sister to El Niño is La Niña, which means "the little girl."

Here's how it works:

What happens when El Niño is not present:

In normal, non-El Niño conditions (top panel of schematic diagram), the trade winds blow toward the west across the tropical Pacific, away from South America.

These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1-2 feet (about 0.5 meter) higher at Indonesia than at Ecuador (in South America).

The sea-surface temperature is about 8 degrees Celsius higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems and major fisheries.

When El Niño kicks in:

During El Niño, the trade winds relax in the central and western Pacific. Surface water temperatures off South America warm up, because there is less upwelling of the cold water from below to cool the surface. This cuts off the supply of nutrients, resulting in a drastic decline in the food chain, including commercial fisheries in this region.

Among the known effects of El Niño:

• Increased rainfall across the southern tier of the United States and in Peru, which has caused destructive flooding.

• Throttles hurricane formation in the Atlantic Ocean by pumping energy high into the atmosphere and fueling wind currents that cross the Americas and shear the tops off some Atlantic storms before they can fully develop.

• Drought in the West Pacific, sometimes associated with devastating brush fires in Australia.

In recent years, El Niño has been blamed for just about everything. Mapping yearly changes in rainfall around the globe, the Tropical Rainfall Measuring Mission (TRMM) satellite showed in 2004 that El Niño is the main driving force for rainfall amounts in different locations.