ENSO – El Niño Southern Oscillation
El Niño is one of three cyclical phases of the Pacific Ocean. Generally the El Niño phase occurs every five to seven years. The neutral phase is defined by average sea surface temperatures and heights, along with typical weather patterns.
La Niña is the positive or cold phase of the Pacific. When this occurs, sea surface temperatures, from near the South American coast westward into the equatorial Pacific, will drop below normal, while abnormally high air pressure keeps the atmosphere stable. Southern California will typically see below-normal rainfall during La Niña phases. In fact, a large chunk of our current four-year drought played out during a La Niña event. Meanwhile, the western Pacific experiences abnormally low air pressure and stormy weather.
The negative or warm phase of the Pacific is called El Niño. During El Niño, the equatorial sea surface temperatures across the eastern Pacific, particularly off the South American coast, climb well above average. Abnormally low pressure dominates the eastern Pacific while abnormally high pressure is in place across Indonesia. For Southern California, strong El Niño phases generally produce above normal rainfall.
El Niño was the name fishermen gave to the warm waters that sometimes developed off the coast of Peru. The phenomenon typically happened around Christmas time and hence the name El Niño, or “the Christ Child.” The immediate effect of El Niño, as observed by the fishermen, was that anchovies did not care for the warmer waters and fled leaving the fishermen high and dry.
So What Is El Niño?
El Niño is a phenomenon of the ocean-atmosphere system in the tropical Pacific. El Niño displaces a huge volume of warm water from the western Pacific moving it to the eastern Pacific. This disruption has important consequences for weather and climate around the globe.
When El Niño occurs, a large portion of the eastern Pacific waters, near the equator, can warm up to 9 degrees above normal. As stated previously, El Niño is part of the ENSO (El Niño Southern Oscillation) cycle of the Pacific Ocean. In the normal phase of the Pacific, westerlies blow along the equator and literally push warm surface waters toward Indonesia. Cooler waters are subsequently drawn from the Polar Regions along the coast of North and South America toward the equator. The warm pool of water over the western Pacific produces an unstable atmosphere with rains and storms. While the cooler waters of the eastern Pacific support a stable atmosphere. This is one reason Southern California has a relatively dry climate.
El Niño starts when the westerly trade winds weaken, collapse and/or reverse directions. This allows the massive pool of warm water, residing near Indonesia, to move east along the equator toward South America. The bigger picture shows that interactions of the ocean and the atmosphere are systemically related. During El Niño, it is not only the anchovy harvest disrupted, but also weather patterns. That large pool of warm waters acts as a giant furnace, heating billions of gallons of water (moisture) into the atmosphere every day. Shifting this furnace east from Indonesia to South America disrupts jet streams (upper level currents of air), which move weather systems around the globe, and enhances storm systems.
El Niño is not a single storm like Hurricane Katrina. Nor is it a single tornado or severe thunderstorm. We cannot say that a single storm is El Niño. Rather, we might see a series of storms over a period of days producing above normal rainfall totals. For example, in 1983 a 12-day period of stormy weather, from Feb. 24 until March 7 dumped 9.28 inches of rain. We would say that this abnormal pattern of storms with excessive rainfall was fueled by or enhanced by El Niño.
This influence occurs because of teleconnections. These are large scale links of atmospheric phenomenon. For example, during El Niño, Australia and Indonesia experiences drier than normal precipitation, while the Southern United States typically has above-normal rainfall. Another teleconnection sends stronger winds from the Pacific across the Gulf of Mexico and then out into the Atlantic Ocean. These winds are hurricane killers, as they disrupt the circulation of tropical systems.