The following animations are created from the data included in the CD-ROM: (images are generated by GrADS)
Regional Sea Surface Temperature Anomalies & Wind Fields (quicktime movie, 8.9 mb)
Regional Precipitation Anomalies & Wind Fields (quicktime movie, 8.9 mb)
NOTE: These are large movies, so be sure your browser memory is set with a high enough threshold.
The ENSO system is a semi-periodic oscillator whose major parameters include the sea surface temperature and pressure, the surface wind, and the upper (warm water) layer thickness (ULT). Roughly every three to seven years, an El Nino event develops which lasts for several months and includes a general warming of the Central and Eastern tropical Pacific. The strongest El Nino recorded occurred in 1982-1983. At present (December 1997) a warming event is developing that some predict will be just as strong. This event can have severe impacts on the local (Eastern Pacific) fishing industry as well as global impacts, both economically and climatologically.
An El Nino occurs when the easterly trade winds in the tropical Pacific dwindle or reverse direction. This occurs in sequence with a longitudinal shift in the tropical Pacific surface pressure, which is one of the phases in the Southern Oscillation. During non-El Nino periods the surface pressure in the Western Pacific is lower than in the Eastern Pacific. As a consequence the winds blow from east to west across the Pacific, pushing the heated surface water to the west, allowing cooler water to upwell in the east, and resulting in higher sea surface levels in the Western Pacific than in the Eastern Pacific. During an El Nino event the trade winds dissipate, and in sever El Nino events reverse directions. The change in trade winds results in warmer waters from the Western Pacific migrating eastward, balancing the sea surface level between the Western and Eastern Pacific. As the warm waters migrate, the cool nutrient-rich waters normally found along the coast of South America, are replaced with warmer, nutrient-depleted waters. This change in Ocean temperatures affects precipitation and wind patterns throughout the tropical Pacific and beyond. It also causes a reduction in marine fish and plant life in the Eastern Pacific.
This series of events also help trigger off-equatorial baroclinic Rossby waves in the upper layer of the ocean which slowly travel across the Pacific. The reflection of these waves from the western land boundary and their subsequent arrival back in the eastern equatorial Pacific act to thin the El Nino warm water layer there and help halt the El Nino (Graham and White, 1988; Philander, 1990). The opposite of an El Nino event is called La Niña, and is characterized by a cooling event in the tropical Eastern Pacific, resulting in the return of strong easterly trade winds, that flow from regions of high to low pressure. These easterly trade winds deepen the original cooling in the eastern Pacific. La Niña occurs in sequence with a decrease in the Western Pacific sea surface pressure and an increase in the Eastern Pacific sea surface pressure. This is the other phase in the Southern Oscillation. The reasons why the amplitudes and periods of El Nino and La Niña events vary as they do are still being investigated.
These multiple impacts on the global climate can show up in the global economy.
The SOI is a measure of the phase and amplitude of the Southern Oscillation which describes the seesaw in surface patterns between the Eastern and Western Pacific regions. Because Tahiti and Darwin are located near opposite poles of the oscillation, the pressure difference (Tahiti minus Darwin) is widely used as an index of the phase and amplitude of the Southern Oscillation. The El Nino events show large negative SOI deviations, while the La Nina events show positive SOI deviations.
Sea Surface Temperature (SST) changes associated with ENSO are Pacific-wide, and possibly global in extent. The appearance of warm water over the equatorial central and eastern Pacific is associated with anomalous weather fluctuations elsewhere in the tropics and the extra-tropics, due in part to the influence of SST on the large-scale atmospheric circulation. Therefore, SST is the most important parameter for ENSO monitoring, and is the key predictor in all ENSO forecast schemes.
During ENSO, large scale SST changes are accompanied by changes in the distribution of atmospheric heat sources and sinks in the form of latent heat released in tropical rainfall. Extratropical precipitation patterns may also be shifted through teleconnections.
One principal link between ENSO-related changes in the atmospheric circulation and those of the ocean circulation is the sea surface wind stress. Therefore, the wind fields are also important for ENSO monitoring.
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Last update:Wed Oct 1 12:30:42 EDT 1997