ENSO

El Niño Southern Oscillation (ENSO)

Description

File:LaNina.png

Walker Circulation

Spanish for "the child", El Niño refers 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, if it becomes 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. Strong El Niño years are often marked by flooding in the western United States, droughts in Australia, and colder than normal temperatures in Europe.

How it Works

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, aided by the Coriolis Effect, and helps to transfer heat energy from the equator toward the pole. As the water moves northward away from the equator, it cools, and westerly trade winds develop at higher latitudes. 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. With weaker trade winds, not as much surface water is pushed westward, and the water in the eastern Pacific continues to warm, spawning increased convection in the eastern Pacific. Slacker trade winds increase surface pressure in the western Pacific and allow sea surface temperatures off the coast of Australia and Indonesia to rise above normal.

Complementing (or perhaps complicating) the atmospheric processes discussed above are coupled, oscillating oceanic waves called Kelvin waves and Rossby waves. Rossby waves transport cooler water and move westward along with the trade winds, while Kelvin waves transport warm water and move eastward. Rossby waves eventually hit the western boundary (coasts of Australia and Indonesia), and then rebound as a Kelvin wave headed for South America. Due, once again, to the Coriolis Effect, Kelvin waves travel about 3 times faster than Rossby waves. During an El Niño event, the slackening trade winds allow the eastward Kelvin wave to transport larger amount of warmer water, which, in turn, reinforce the abnormally high amount of warm water in the eastern Pacific. If Kelvin waves and Rossby waves travelled at the same speed, it would be difficult for more normal (non-El Niño) conditions to return. It is this variance in the speeds of the oceanic waves that help break the feedback loop.