The El Niño phenomenon dramatically affects the weather in many parts of the world. It is therefore important to predict its appearance. Various climate models, seasonal forecasting models, ocean-atmosphere coupled models, and statistical models attempt to predict El Niño as a part of interannual climate variability. Predicting El Niño has been possible only since the 1980s, when the power of computers became sufficient to cover very complicated large-scale ocean-atmosphere interactions.
El Niño were observed as early as the 1600s. More systematic study began at the end of the 19th century, when Peruvian geographers noted unusual oceanic and climatic phenomena occurring periodically along the Peru coast. They noticed that eastern Pacific warming was sometimes very strong. In the 1920s The British scientist Sir Gilbert Walker empirically identified that some notable climate anomalies—changes in atmospheric pressure and circulation—happen around the world every few years. He invented the term for those climate oscillations, "the Southern Oscillation."
According to the World Meteorological Organization, the 1997-'98 El Niño was a major factor in 1997s record high temperatures. The estimated average surface temperature for land and sea worldwide was 0.8°F higher than the 1961-1990 average of 61.7°F. According to the National Oceanic and Atmospheric Administration (NOAA), 1998 has set all-time highs of global land and ocean surface temperatures, above record high levels in 1997. In 1998 the mean temperature was1.2°F (0.7°C) above the long-term (since 1880) mean of 56.9°F (13.8°C).
The impact of the 1997/8 El Niño has been felt in many parts of the world: Droughts have occurred in the Western Pacific Islands and Indonesia as well as in Mexico and Central America. In Indonesia drought caused uncontrollable forest fires and floods, while warm weather led to a bad fisheries season in Peru, and extreme rainfall and mud slides in southern California. Corals in the Pacific Ocean were bleached by warmer than average water, and shipping through the Panama Canal was restricted by below-average rainfall.
The prevalent surface winds across the equatorial Pacific ocean are easterly trade winds. These drag warm surface water away from the coast of Peru and cause colder deep ocean water to come to the surface (so-called "upwelling"). Upwelling causes thethermocline (the zone at the top part of the ocean in which temperature decreases rapidly with depth) to be much shallower in the Eastern Pacific than in the western. Trade winds and the equatorial upwelling maintain warm sea surface temperatures at the western equatorial Pacific and cold surface temperatures in the east. When trade winds weaken, the equatorial upwelling decreases, the thermocline gets deeper, the ocean surface along the coast of South America becomes warmer, and trade winds weaken even more. This in turn causes surface waters in the eastern Pacific to became even warmer and so on. This mechanism is known as the Bjerknes hypothesis and represents an onset of El Niño.
The questions remain: what stops warming in the eastern Pacific and why do El Niño events last approximately 12-18 months? The widely accepted (but not unique!) explanation is the delayed oscillator hypothesis.
During the warming event in the eastern Pacific, the thermocline deepens along the equator and rises in the regions about 3 to 8 latitude degrees from the equator. These off-equator thermocline anomalies have little effect on the ocean surface temperature, but they propagate westward under the ocean surface as so-called Rossby waves, with a speed of about 0.8 m/s. When Rossby waves reach the Indonesian archipelago they are reflected back as another type of the ocean underwater waves, namely equatorial Kelvin waves. Because of the deep thermocline in the western Pacific, the arrival of the Rossby signal does not affect surface temperature. Kelvin waves are much faster than Rossby waves and propagate eastward along the equator with an approximately 3 m/s speed as a shallower thermocline anomaly. When Kelvin waves reach the equatorial Eastern Pacific, they move the thermocline (and, therefore, cold deep waters) in this region even closer to the surface, cool the ocean's surface and terminate the warm El-Nino event.
El Niño is a warm phase of the interannual climate oscillation called El Niño Southern Oscillation (ENSO) event, an example of large-scale ocean-atmosphere interaction, and is characterized by large-scale warming of the surface tropical Pacific Ocean. El Niño events occur every 3-6 years, last 9-12 months, sometimes even up to 18 months, and have a big impact on world weather.
The major impacts of El Niño are temperature anomalies, changes in precipitation variability, floods and droughts throughout the world.
El Niño events happen irregularly and are hard to predict. However many numerical climate models predicted the last few El Niño events successfully. El Niño forecasting is becoming more and more reliable with our improving knowledge of the phenomenon's nature, with the help of more and more powerful computers, and with the operational El Niño Southern Oscillation observation system.
El Niño forecasting is especially important for tropical countries where El Niño impacts are the strongest.
