ENSO

El Niño Southern Oscillation (ENSO)

Description

File:LaNina.png

Walker Circulation

Spanish for "the child", El Niño originally referred to the warming of the waters off the coast of Peru around Christmas (summer in the southern hemisphere). The warm water brings with it rain and warmer temperatures and promotes vegetative growth upon the otherwise dry landscape. The warm water, once entrenched, can block normal upwelling off the coast of Peru, and organisms that depend on the normally nutrient-rich waters suffer. One half of what is called the Southern Oscillation (La Niña is the other half), El Niño is now understood to be a major player in the world's weather.

Normally, the waters off the coast of Peru are fed by the cold, nutrient-rich Peru Current, and the area is characterized by high atmospheric pressure. This high atmospheric pressure drives easterly trade winds west towards Australia along the equator. The trade winds push surface water west, and even cooler, more nutrient-rich water is brought to the surface through upwelling. Along its journey towards Australia, the water warms, and, by the time it reaches the tropical western Pacific, the warm water fuels atmospheric convection. This convection spills northward and helps to transfer heat energy from the equator toward the pole. This movement of air and water is known as the Walker Circulation.

During El Niño years, the warm water off the coast of Peru mix more fully with the normally cooler waters, and as a result, the easterly trade winds relax. This allows the warm water and convective power of the western tropical Pacific to slide eastward. If this convection moves far enough to the east, atmospheric pressure off the coast of Australia can rise and cause the trade winds to relax or even blow to the east, enhancing the El Niño event. Strong El Niño years are often marked by flooding in the western United States, droughts in Australia, and colder than normal temperatures in Europe.