What is El Nino?

What is an El Niño?



El Niño is a phenomenon that has been observed and recorded at least since the 1600's off the coast of Peru. Its abnormally warm waters occasionally appeared around Christmas so the Peruvian fishermen dubbed it El Niño de Navidad, spanish for the Christ child.

schematic of the tropical Pacific

The warm waters of El Niño are a part of the event called the El Niño Southern Oscillation, which encompasses the central Pacific region. Normally a warm pool of ocean water builds up in the western equatorial Pacific Ocean, kept at bay from the trade winds, blowing east to west. The easterly trade winds are driven by a surface pressure pattern of lower pressure in the western Pacific and higher pressure in the eastern Pacific. When this pressure pattern weakens, the trade winds slacken or even reverse! This causes the warmer water to propagate eastward via Kelvin Waves.

The warmer water causes the thermocline to sink, which causes the upwelling to occur at a deeper level in the ocean. This is critical at the western coasts of the Americas since without the upwelling of nutrient rich bottom water the fish population is greatly reduced. Convection is driven by the warming of the surface air over the warm water; as the water propagates eastward, so does the heavy rains caused by the convection. The shift in the convection causes a change in the global weather pattern, causing drought, floods, storms, and other weather anomalies in many area of the world.

3d schematic shows increased tropical convection in an El Nino


Image of El Niņo conditions from "What is an El Niņo?" TAO Project at NOAA/Pacific Marine Environmental Laboratory.

Within one to two years after the initiation of the El Niño, the normal pressure gradients and trade winds set up again, and cold nutrient rich water again upwells off the coast of Peru. This reversal of air pressure in the equatorial Pacific is called the Southern Oscillation, it occurs along with El Niño, so the whole shebang is properly called El Niño Southern Oscillation or ENSO, for short. This roughly occurs every 4-7 years.

Also part of the El Niño Southern Oscillation is its cold counterpart, La Niña, also called the cold event or El Viejo. It is marked by cold water propagating to the west and strong trade winds. The weather patterns associated with La Niña are not necessarily equal and opposite to the El Niño weather patterns.

The following graphic shows the sea surface temperature distribution in the Pacific Ocean for an El Niño.
El Niño
Dec. 1997
 warmer than average SST distribution is associated with El Nino
     
color scale

This graphic was taken from the El Niño theme page (NOAA)

A good way to understand the changes from a normal scenario to an El Niño or a La Niña is to see all three illustrations.