April 2002

El Niño situation and prospects

Agrometeorology Group,
Environment and Natural Resources Service
FAO Research, Extension and Training Division

18 March 2002

El Niño¹ is a coupled atmospheric and oceanic phenomenon; predictions are based on several independent observations, including the following:

• the Southern Oscillation Index (SOI), based on the atmospheric pressure differences between Darwin (Australia) and Tahiti (the SOI is low during El Niño phases). El Niño is often referred to as El Niño-Southern Oscillation (or ENSO). The SOI is illustrated in figure 1;
• satellite-derived measurements of the level (sea level, or elevation) of the Pacific Ocean (during El Niño phases, warm water piles up against western central America, resulting in lower elevations along the coast of SE Asia, higher elevations along S. America)
• Sea-surface temperatures (SST), directly observed by satellites. Figure 2 illustrates the current SST anomalies.

Indications for a warm episode in the Tropical Pacific (or El Niño proper) were first noted in August 2001. With oscillations, the evolution towards a warm episode in the tropical Pacific continued until February 2002. Several of the atmospheric indices indicate that El Niño/ Southern Oscillation (ESN) conditions have not developed to the point that guarantees sustained growth of the event.

To estimate future ENSO conditions, a common practice is to compare different reputable models that currently represent the state-of-the-art as far as dynamical and statistical ENSO prediction systems are concerned; they are useful guidance up to about nine months ahead.

Of 11 models, five show a transition to warm conditions between about May and September 2002. However, at both 5 months (July) and 8 months lead (October), the majority of models favour Neutral conditions. Furthermore, the amount of warming shown across the various predictions is not as strong as that shown last month, and it is in no way comparable to the latest strong 1997-98 event.

For the time being, the interpretation of El Niño indicators thus remains mixed. Several leading meteorological services (e.g. the US National Oceanic and Atmospheric Administration -NOAA- and the Australian Bureau of Meteorology -BOM-) caution that it is still too early to predict the magnitude and the duration of the potential 2002-2003 El Niño.

Fig. 1: Normalised Southern Oscillation Index from 1997 to February 2002. The red curve corresponds to current monthly SOI up to February 2002. The dotted black line indicates 5- months running averages. Compare current tendency and values with the latest strong 1997-98 El Niño.

Fig. 2: Current (February 2002) Sea Surface Temperature (SST) anomalies, compared with the long-term average. Note a slight warm anomaly along the South-American coast (Colombia, Ecuador, and Peru).

Endnotes

¹For a general description of the El Niño phenomenon, please refer to the FAO website: An el Niño primer

For the latest El Niño updates, please see:

• El Niño fact files