EL NIÑO-SOUTHERN OSCILLATION (ENSO)
The El Niño is a large-scale abnormal warming of surface waters in the central and eastern Pacific Ocean off the Peruvian coast, coupled with changes in the atmosphere that affect weather patterns across much of the Pacific Basin. These include a negative value of the Southern Oscillation Index (SOI), which measures the difference in atmospheric pressure between the Eastern and Western Pacific, as well as sustained weakening of winds and increased cloudiness over the tropical Pacific. El Niño is the oceanic component, while the Southern Oscillation is the atmospheric one. This combination gives rise to the term ENSO (El Niño Southern Oscillation). El Niño appears every 2 to 7 years, with different intensity and duration and usually peaks around Christmas, hence the name of the phenomenon: El Niño (Spanish for Christ Child). Important changes in temperatures and precipitation patterns are often noticed during El Niño, having a positive or negative impact on agriculture. The overall changes in the ocean surface temperatures caused by El Niño also affect marine fisheries, particularly in the eastern Pacific. However, the particular character of the impact differs quite markedly from one event to another, even with similar changes and patterns in the Pacific Ocean. Therefore, no precise quantitative association between the occurrence of El Niño and changes in agricultural production have been established and it is difficult to forecast precisely the impact of El Niño. The impact on agriculture will decisively depend on the relative timing of the El Niño and the crop calendar in a particular region. La Niña refers to the "cold" equivalent of El Niño.
The oldest El Niño recorded dates back to 1578, when torrential rains and floods devastated crops in northern Peru. More recently, El Niño event in 1982/83 resulted in severe flooding and drought in several parts of the world, as well as the decline of a number of fish stocks, and reportedly caused over U.S. $ 10 billion in weather-related damages. In 1991/92 El Niño resulted in a severe drought in Southern Africa. The last strong El Niño occurred in 1997/98, whit drought and floods in several areas of South America and South East Asia that had severe adverse effects on agricultural production and infrastructure.
By early December 2004 uncertainties continue to prevail about an El Niño event developing this year. While surface temperatures in the western to central Pacific have reached near El Niño thresholds for about three to four months, other important indicators such as the Southern Oscillation Index (SOI) has generally been weakly negative and marked atmospheric changes in the Pacific winds and cloudiness have failed to materialized. Furthermore, as the impact of El Niño is generally expected to occur between December 2004 and March 2005, the possibility of a fully-fledged episode in early 2005 gradually weakens. Nevertheless, even in the absence of strong indication of a clearly defined El Niño episode, continued warmer than average temperatures in the tropical Pacific may still impact on climatic patterns. Overall, conditions in the tropical Pacific are currently indicative of a weak El Niño. Based on the latest observations and forecasts, there are 60 percent of probabilities that weak El Niño conditions will prevail through early 2005. While the associated climatic effects in most regions are expected to be weak, these may be, nonetheless, significant at local level.
FAO will keep closely monitoring weather anomalies and assessing possible effects these may have on agricultural production in various parts of the world in order to warn about adverse situations developing and to enable preventive action.