There are several parameters that characterise the state of the Equatorial ocean and atmosphere: sea level height (SLH), sea surface temperature (SST), sea level pressure, surface winds, and ocean subsurface temperature. El Niño changes the expected behaviour of these parameters.
The El Niño phenomenon has a well-defined spatial pattern, and a more or less continuous time scale. The parameters mentioned above (SLH, SST, winds, etc.) are analysed using the measure of the ‘anomaly’, which is calculated by subtracting a standard average from the measure of interest. What is an anomaly? An anomaly is an unusual behaviour of a process. For example, in order to calculate the sea surface temperature anomaly for the month of December 1997, we must subtract the temperature average of all the months of December of a reference period (e.g. from the last twenty years) from the month of interest (i.e. December 1997). The result is expressed in positive or negative values, where zero is the ‘normal’ value. If the result is positive, the parameter is abnormally high. If the result is negative, then the parameter is abnormally low.
There are some indices which are often used to identify an El Niño phenomenon and quantify its intensity. The most common of these indices are the sea surface temperature anomalies averaged in specific areas. One of these areas is the so-called Niño 4 Region, bounded by 160°East-150°West and 5°S- 5°N. Another one is the Niño 3.4 Region, located at 5°S-5°N, 120°-170°W.
There is also another index commonly used to identify the El Niño pheneomenon, which is the Southern Oscillation Index (SOI). The SOI is the normalised difference between the pressure anomalies of Tahiti (18°, 150°W) and those of Darwin (12°S,131°E).
One conclusion extracted after the analysis of the indices is, for instance, that periods with a high Niño-3 index have a low SOI and vice versa.