El Niño and the Southern Oscillation, also known as ENSO is a periodic fluctuation in sea surface temperature (El Niño) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean. El Niño is so termed because it generally reaches full strength toward the end of the year, and early Christian inhabitants of western equatorial South America equated the warm water current and the resulting impacts with their holiday celebrating the birth of Jesus (known as El Niño (Literally, El Niño translates to "The Boy Child" and is a reference to Jesus as a baby.) in Spanish).
The Southern Oscillation describes a bimodal (Having two peaks (or modes).) variation in sea level barometric pressure (The force exerted by the atmosphere at sea level (zero meters elevation) as measured by a barometer.) as measured by a barometer.) between observation stations at Darwin, Australia and Tahiti. It is quantified in the Southern Oscillation Index (SOI), which is a standardized (Adjusted so values from samples with different properties (e.g., months) can be compared to each other) difference between the two barometric pressures. Normally, lower pressure over Darwin and higher pressure over Tahiti encourages a circulation of air from east to west, drawing warm surface water westward and bringing precipitation to Australia and the western Pacific. When the pressure difference weakens, which is strongly coincidental with El Niño conditions, parts of the western Pacific, such as Australia experience severe drought, while across the ocean, heavy precipitation can bring flooding to the west coast of equatorial South America.
Although the exact initiating causes of an ENSO warm or cool event are not fully understood, the two components of ENSO – sea surface temperature and atmospheric pressure are strongly related. During an El Niño event, the easterly trade winds converging across the equatorial Pacific weaken. This in turn slows the ocean current that draws surface water away from the western coast of South America and reduces the upwelling of cold, nutrient–rich water from the deeper ocean, flattening out the thermocline (A zone beneath the ocean surface at which the surface water transitions to deep water, and a marked decrease in water temperature occurs. In both the surface (or mixed) layer and in the deep water, temperature is relatively constant with depth. Within the thermocline, water temperature quickly decreases from surface water temperature to deep water temperature.) and allowing warm surface water to buld in the eastern part of the basin.
The strengthening and weakening of the trade winds (Winds in the tropics generally flow in an east to west direction (easterlies). So called because their consistency facilitated transoceanic sailing and commerce.) is a function of changes in the pressure gradient (A change in air pressure over a distance. The stronger the gradient, the faster the air will flow from the high pressure to low.) of the atmosphere over the tropical Pacific. Ironically, the warming of the sea surface works to decrease the atmospheric pressure above it by transfering more heat to the atmosphere and making it more buoyant (As the air is heated it expands, becoming less dense than the air above it.) . So, in summary, the pressure gradient affects the sea surface temperatures, and the sea surface temperatures affect the pressure gradient.
The connection between the Southern Oscillation and precipitation is also manifest in the quantity of long–wave (e.g., infrared (Wavelength of radiation longer than visible light and associated with the "heat" given off by a body.) ) radiation leaving the atmosphere. Under clear skies, a great deal of the long–wave radiation released into the atmosphere from the surface can escape into space. Under cloudy skies, some of this radiation is prevented from escaping. Satellites are able to measure the amount of long–wave radiation reaching space, and from these observations, the relative amount of convection (The transfer of energy by moving the heated molecule from one place to another – also the rising of heated air which forms cumuliform clouds and results in precipitation.) in different parts of the basin can be estimated.
Monitoring of ENSO conditions primarily focuses on sea surface temperature (SST) anomalies (Variations from an average or other statistical reference value.) in 4 geographic regions of the equatorial Pacific (see image to the right). SST anomalies equal to or greater than 0.5°C (0.9°F) in the Niño 3.4 region (comprising portions of Niño regions 3 and 4, from 170°E to 120°W longitude) are indicative of ENSO warm phase (El Niño) conditions, while anomalies less than or equal to –0.5°C (–0.9°F) are associated with cool phase (La Niña) conditions. Niño 3.4 SST anomalies are averaged over the three months ending with the current month, and that value is called the Oceanic Niño Index (ONI). If the ONI exhibits warm or cool phase conditions for at least five consecutive values, it officially becomes an El Niño or La Niña event.
In addition, the thermal expansion (Heating an uncontained fluid or gas results in an increase in its volume as it attempts to maintain a constant pressure. In the ocean, since only the surface is unbounded, the expansion increases the sea level.) of the warming water in the eastern part of the basin measurably raises sea level in these regions, and this change in sea level can be measured by satellite sensors. Therefore, variations in sea level are good indicators of the presence of an El Niño. During an El Niño, sea level in the eastern Pacific is well above average, while during a La Niña, the increased flow of cold deep water to the surface acts to lower the sea level.
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