What happens in the ocean during ENSO?
The Pacific Ocean is a huge mass of water which can control many climate features in its region, since changes in the ocean result in characteristic changes in the atmosphere which, in turn, alter climate and weather patterns across the globe.
During normal years (non-ENSO years) relatively cold water occurs along the west coast of South America , an effect increased by upwelling of cold water along the Peruvian coast. The cold water then flows westward along the equator to Australia and is heated by the tropical sun. These normal conditions make the western Pacific about 3°C to 8°C warmer than the eastern Pacific.
During La Nina years, the upwelling off the Peruvian coast is enhanced and the SSTs in the Nino regions become cooler than usual. During El Nino years, the area of warm water (usually over the western tropical Pacific near Australia ) cools down and the warm water is displaced eastward to the central Pacific. The upwelling off the Peruvian coast is suppressed and the SSTs in this region become warmer than usual.
Measuring the ocean
Sea-surface temperature (SST) anomalies are (amongst other) used to measure the state of the global oceans. The long-term mean for each location over the oceans is calculated from a long record of SST data for these specific locations. The mean is then subtracted from the current value. If the ocean is warmer than usual, it will have a higher SST value than the mean and therefore a positive anomaly. On the other hand, a colder-than-usual ocean surface will give rise to a negative anomaly. El Nino events are associated with positive SST anomalies, while La Nina events are associated with negative SST anomalies.
What happens in the atmosphere during ENSO?
The atmospheric circulation between high and low pressure regions in the tropical Pacific is known as the Walker Circulation. The easterly trade winds are part of the low-level component of the Walker circulation. During normal years (non-ENSO years), the trade winds move over the warmer sea bringing warm, moist air towards the Indonesian region. This moist air rises to high levels and travels eastward before sinking over the eastern Pacific Ocean . The rising air is associated with a region of low air pressure, towering cumulonimbus clouds and rain. High pressure and dry conditions accompany the sinking air. (Figure: Normal conditions in the Pacific Ocean and the atmosphere .)
During La Nina years, the Walker Circulation operates in the same way as described for normal years, but because of the larger area of colder water off the South American coast and the displacement of warmer water to the west, the atmospheric pattern also shifts accordingly. During El Nino years, the Walker Circulation is altered due to the changes in the Pacific Ocean . The lifting and sinking of air - and therefore rainy and dry conditions - move with the warmer and colder SSTs to form the pattern depicted in the figure Conditions in the Pacific Ocean and the atmosphere during El Nino .
Measuring the atmosphere
The Southern Oscillation Index (SOI) gives a simple measure of the strength and phase of the difference in sea-level pressure between Tahiti (in the mid-Pacific) and Darwin (in Australia ). This difference is given in terms of an index. The typical Walker circulation has an SOI close to zero, while a strong negative value usually indicates that the oscillation has entered an El Nino phase. A strong positive value usually indicates a La Nina phase.