Overview of the ENSO System
Monitoring of El Niño and La Niña requires observations from both the atmosphere and oceans. These observations are often summarized in terms of various atmospheric and oceanic indices. Listed here are some of the more common indices in use.
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.
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.
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
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.
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.