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Environment

December 2002

El Niņo situation and prospects

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

3 December 2002

The major monitoring systems (IRI, the International Research Institute for Climate Prediction, BOM, the Australian Bureau of Meteorology, NOAA/CPC, the US Climate Prediction Center) confirm that there is a virtually 100% probability of El Niņo conditions to persist for the remainder of 2002 and into early 2003. The forecasts are based on both Pacific Ocean sea surface temperature and the associated atmospheric circulation pattern, in particular a reversal of the trade winds.

Most of the computer predictions are now in favour of neutral conditions in five months' time, indicating neutral conditions in April 2003.

It is most likely that this will remain a moderate El Niņo, which is significantly weaker than the 1997-98 event.

Seasonal forecasts for January-February-March indicate typical patterns correlated to the El Niņo event.

Overview

The Southern Oscillation Index, a measure of El Niņo strength based on atmospheric measurements, has shown a steady tendency to negative values since March 2002 (figure 1) showing two negative peak values in May and August. The negative persistence has confirmed the likelihood of an El Niņo event which should persist until March-April 2003.

Figure 1: recent variations of the Southern Oscillation Index. The graph compares the current season (black) with the averages of 10 strongest El Niņo and La Niņa on record (Based on data available on the website of the Australian Bureau of Meteorology, up to 30 November 2002.

As indicated in earlier bulletins, it is a common practice to compare several models that currently represent the state-of-the-art for ENSO predictions; they are considered to provide reliable guidance up to about eight months ahead.

Based on Australian Bureau of Meteorology (BOM) sources, of 12 models examined in Table 1, 25% foresee warm conditions1 for April 2003, while only 17% indicate that the warm conditions should persist at least until July. The remaining models predict neutral conditions, while one model points at cool conditions.

According to Table 1, there is a possibility of a strong weakening of the warm conditions between April and July 2003, which may be regarded as a tendency to return to normal (neutral) conditions. One model indicates the possibility to have a "cold" phase starting in July, thus corresponding to La Niņa conditions.

Tab. 1: Comparison of ENSO model outputs and forecasts for the current season, based on information provided by the Australian Bureau of Meteorology.

The probability for a very strong El Niņo (e.g. 1997-98 or 1982-83) is low, and of a low strength El Niņo (e.g. 1977-78 or 1979-80) is also low. It is most likely that this will remain a moderate El Niņo, which is significantly weaker than the 1997-98 event. The associated climate effects in most regions are expected to be weaker than those associated with the 1997-98 El Niņo, but may nonetheless be substantial in some regions.

The Southern Oscillation Index is used to produce seasonal forecasts up to three to six months in advance. Figures 2 and 3 shows seasonal forecast, for December-January-February 2003 and for January-February-March 2003 respectively, of the deviation from normal of precipitation in Africa as provided by ECMWF (European Center for Medium-Range Weather Forecasts).

Fig. 2: Seasonal forecast for December2002-January-February 2003 of the mean precipitation anomaly in Africa. Provided by the European Center for Medium-Range Weather Forecasts (Reading, UK).

Fig. 3: Seasonal forecast for January-February-March 2003 of the mean precipitation anomaly in Africa. Provided by the European Center for Medium-Range Weather Forecasts (Reading, UK).

1The forecast refers to an area of the central Pacific Ocean known as "Nino3", which is usually taken as a reference. "Warm" conditions correspond to 0.8 C above average (+0.8 C) while "cold" corresponds to 0.8 C below average (-0.8 C). Values between -0.8 C and +0.8 C are referred to as "neutral". Other areas may display stronger or weaker anomalies. Sea surface temperatures in the central equatorial Pacific currently remain up to 2°C warmer than average in places.



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