Overview of the ENSO System

Monitoring ENSO

The Southern Oscillation Index

The Southern Oscillation Index (SOI), is a measure of the difference in surface air pressure between Darwin, Australia and Tahiti, and is the index of longest record. It dates back to the beginning of the 20th century when it was first realized by Sir Gilbert Walker that there was a large scale pattern in surface air pressure which extended over the entire tropical Pacific region. A drawback to this index is that it is based on the pressure at two points and therefore can easily be affected by local weather disturbances making it somewhat "noisy" when viewed on a month-to-month basis. In order for the index to be more representative of larger scale fluctuations in pressure, it is common to present the SOI averaged over a 5 month period. Generally, the SOI is negative during El Nino, and positive during La Nina.

Sea-Surface Temperature Indices

The TAO Array

In recent decades, indices based on sea surface temperature have come into common usage because satellites and an observing network of buoys in the equatorial Pacific now allow for collection real time, high quality data. The network of buoys referred to is the Tropical Atmosphere Ocean (TAO) Array. The array consists of a number of moored buoys distributed along the equator that measure both surface conditions in the atmosphere and the surface and subsurface temperatures in the ocean. Figure1 below indicates the locations of the moored buoys in the TAO Array. 

Figure 1. The TAO Array Picture courtesy of the Pacific Marine Environmental Laboratory (PMEL)

The NINO Regions

Indices based on sea surface temperature (or, more often, its departure from the long-term average) are those obtained by simply taking the average value over some specified region of the ocean. There are several regions of the tropical Pacific Ocean that have been highlighted as being important for monitoring and  identifying El Niño and La Niña. The most common ones are the NINO regions (see also Figure 2 below):

• NINO1+2 (0-10S, 80-90W). The region that typically warms first when an El Niño event develops.
• NINO3 (5S-5N; 150W-90W). The region of the tropical Pacific that has the largest variability in sea-surface temperature on El Niño time scales.
• NINO3.4 (5S-5N; 170W-120W). The region that has large variability on El Niño time scales, and that is closer (than NINO3) to the region where changes in local sea-surface temperature are important for shifting the large region of rainfall typically located in the far western Pacific.
• NINO4 (5S-5N: 160E-150W). The region where changes of sea-surface temperature lead to total values around 27.5C, which is thought to be an important threshold in producing rainfall

Figure 2. The NINO Regions The thin grey line in the center of the plot is the equator.

If the concern regarding El Niño and La Niña is the subsequent effect of that tropical Pacific variability on the climate in a particular region, then one index may be more useful than the others. For widespread global climate variability, NINO3.4 is generally preferred, because the sea surface temperature variability in this region has the strongest effect on shifting rainfall in the western Pacific. And in turn, shifting the location of rainfall from the western to central Pacific modifies greatly where the location of the heating that drives the majority of the global atmospheric circulation.

Examples

Values of the NINO3.4 sea surface temperature index and the SOI (5-month average) for the period November 1981 to December 2001 are shown in Figure 3 below. The units of the NINO 3.4 Index are degrees Celsius. Note how the NINO 3.4 Index and the Southern Oscillation Index (SOI) tend to vary with opposite signs.

Figure 3(a). Nino 3.4 Sea Surface Temperature Index (departure from average) Figure 3(b). The Southern Oscillation Index (Tahiti - Darwin)