The El Niño Southern Oscillation (ENSO) is a sporadic climate pattern that is caused by sea surface temperature anomalies in the central and eastern tropical Pacific Ocean. The ENSO is remarkable due to the far reaching impact it has on global weather patterns. In fact, ENSO has the ability to impact temperatures and rainfall globally. It is also significant because ENSO phases can last for several years, providing forecasters and climatologists with the ability to predict the severest weather conditions associated with ENSO many seasons in advance.
The ENSO has three main phases. When sea surface temperatures are anomalously warm in the central and eastern tropical Pacific Ocean over an extended period, ENSO is considered to be in a positive phase, known as El Niño. Conversely, when sea surface temperatures are persistently cooler than average, ENSO is in a negative phase, known as La Niña. Between these two phases, ENSO is classified as neutral.
During an El Niño event, the trade wind circulation in the Pacific is weakened and in some cases may reverse. Rainfall in south east Asia and Australia decreases, while the west coast of South America is likely to experience increased rainfall. Further afield, El Niño can strengthen the Atlantic trade winds and increase rainfall in the eastern United States and the Caribbean. Hurricanes are also impacted by El Niño; Atlantic hurricanes tend to be suppressed while Pacific hurricane activity is enhanced. El Niño events tend to be more frequent and more persistent than La Niña events.
A La Niña phase has an opposite effect on the global climate. Trade winds are enhanced in the Pacific and weakened in the Atlantic, while hurricanes are suppressed in the Pacific and enhanced in the Atlantic. Furthermore, eastern seaboards tend to have enhanced rainfall, while western seaboards are more likely to experience drought conditions.
While models and forecasts will incorporate ENSO effects into their predictions, an understanding of ENSO and its far reaching impact on the world’s weather can help to understand why the trade winds vary from one year to the next or why the Caribbean is wetter this year than last. ENSO phases are also used in seasonal hurricane forecasts. Currently (May 2020) the ENSO is in a neutral phase. According to the Australian Bureau of Meteorology ENSO analysis, the last strong ENSO event was an El Nino in 2015–2016 and the last strong La Nina was in 2010–2012.
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