Understanding the drought impact of El Niño on the global agricultural areas
El Niño is local warming of surface waters that takes place in the entire equatorial zone of the central and eastern Pacific Ocean of the Peruvian coast and which affects the atmospheric circulation worldwide (Kiladis and Diaz, 1989). It usually peaks around Christmas, hence the name of the phenomenon: El Niño is Spanish for Christ Child. La Niña refers to the cold equivalent of El Niño. It is a recurrent weather phenomenon that takes place approximately every two to seven years and usually lasts between 12 and 18 months (CPC, 2005). An El Niño event is defined by a high Oceanic Niño Index (ONI), which is based on Sea Surface Temperature (SST) departures from average in the region in the central equatorial Pacific. An El Niño episode is associated with persistent warmer than average sea surface temperatures and consistent changes in wind and rainfall patterns (Figure 1) (Ropelewski and Halpert, 1992; IRI, 2013). Despite their periodic and recurrent manifestations, El Niño episodes do not have a deterministic trend1 with fixed occurrence periods and a constant intensity. As a result, stochastic models2 have been developed to predict the onset and the intensity of El Niño episodes. However, while the accuracy of these models in predicting the onset of an El Niño episode is fairly high, the intensity is much more difficult to predict due to random atmospheric disturbances that may dampen or amplify the intensity of an El Niño occurrence and thus its impact on weather patterns (CPC, 2005).
The Southern Oscillation is an East-West balancing movement of air masses between the Pacific and the Indo-Australian areas. It is associated (roughly synchronized) with typical wind patterns and El Niño, and measured by the Southern Oscillation Index (SOI) (Parker, 1983). 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). Although there is no perfect correlation between El Niño and the Southern Oscillation as regards minor variations, large negative values of the SOI are associated with warm events.
Although FAO does not have a mandate in the geophysical aspects of the El Niño phenomenon, yet the Organization’s interests focus in the impacts on agriculture and,
consequently, food security of the extreme negative/positive climatic events that can be triggered by El Niño (FAO, 2013).
El Niño conditions are closely monitored by major meteorological institutes and forecasts are updated accordingly. The early warning provided by climate scientists each year enables governments throughout the region to discuss and implement El Niño-related contingency plans. This short paper describes the impact that El Niño has had on major crop production areas, focusing on cereals. To assess the impact of drought, the FAO Agriculture Stress Index (ASI) is used as an indicator. ASI is based on remote sensing data that highlight anomalous vegetation growth and potential drought conditions in arable land during a given cropping season (Rojas et al., 2011).
The main threat to crops is drought. However, El Niño could produce other climatic impacts, including flash floods or intense hurricanes that could influence the crop season, disrupting agricultural activities and damaging crops. ASI cannot assess the negative impact of flash floods or hurricanes, but only the positive ones, if any, due to the increase of water availability after intense rains.