A Periodic, Seasonal Visitor to Planet Earth
Every three to seven years, El Niño occurs in the Pacific Ocean. El Niño means "the (Christ) Child" because it commonly appears at Christmas time. El Niño begins when water warms up by more than 1°F along the equator in the eastern Pacific and off the coast of Peru. Nutrients normally found in the cold waters of the area disappear. Underwater plants called Phytoplankton which use these nutrients, just like plants on land use fertilizer, grow slowly or die. This leaves less food for small animals called zooplankton. Small fish and other creatures who eat the zooplankton leave the area or starve to death. (See Fisheries for more information on the effects of El Niño.) A few months after the water warms up in the eastern tropical Pacific, weather patterns around the world begin to change. To understand these changes, let's look at what happens along the equator in the Pacific.
Normally, the trade-winds blow strongly from east to west across the tropical Pacific. This flow of the winds from east to west is part of the Walker Circulation, named after Sir Gilbert Walker who studied atmospheric circulation in the 1920's. Walker circulation forces the warm surface water to the west along the equator. This produces a big pool of water hot as 86°F or 30°C in the western equatorial Pacific. The warm pool is shown in red in the diagram on the left below.
As the winds blow along the equator, the Coriolis force causes water to flow northward away from the Equator in the northern hemisphere and southward in the southern hemisphere. The water pushed away from the equator must be replaced by water pulled up from deeper in the ocean, a process called upwelling. In the west, upwelling brings up warm water. In the east, upwelling brings up nutrient-rich water as cold as 73°F or 23°C, causing the ocean to teem with life. The cold area is shown in green in the diagram.
El Nino is set in motion when the trade-winds in the western Pacific, which normally blow quite vigorously, slow down or reverse their direction. The weak winds can no longer hold the warm pool in the west. So the warm nutrient-poor water in the west surges back along the equator towards South America.
Warm water heats the air. The warm air rises and produces heavy rains shown by clouds in the diagram. As El Niño develops, the rain follows the warm water eastward into the central equatorial pacific. Because rain drives the Walker circulation, the circulation changes. Soon the changed Walker circulation causes a change in weather all over the world. (Visit Role of the Ocean in Weather.) If you imagine the world as a room, with warm water in the ocean acting like a heater, you can see how moving the heater could make parts of the room that were once cold to warm up. Moving this heater also makes parts of the room that were once warm to cool off.
Eventually, in one to two years, normal air pressure patterns and the trade-winds go back to normal. Warm water is once again pushed to the western Pacific, and cold upwelled water returns in the east. This back-and-forth reversal of air pressure in the equatorial Pacific is called the Southern Oscillation. When related to El Niño it is called the El Niño-Southern Oscillation (ENSO) for short. (To enhance your understanding, check out our web page on Currents.)
El Niños can last from six to eighteen months.