What is the El Niño–Southern Oscillation?

Abstract

The El Niño–Southern Oscillation (ENSO) is a large-scale climatic phenomenon that originates in the tropical Pacific but affects global climate patterns. The warm phase is known as El Niño, and the cold phase, La Niña. El Niño occurs irregularly every 2 to 7 years and generally peaks during Northern Hemisphere winter. In a normal year, when ENSO is inactive, the equatorial Pacific trade winds blow from east to west. The winds push the warmer surface water towards the west, and colder water rises up from deeper in the ocean to replace it. This creates an east–west difference in sea-surface temperature (SST) and hence an east–west difference in sea level air pressure that maintains the trade winds and so drives a positive feedback loop (Figure 1(a)). During an El Niño year, the east–west SST difference weakens, the air pressure difference weakens, and the trade winds and their effects on the ocean weaken, so the eastern Pacific warms (Figure 1(b)). During a La Niña year, the opposite ­happens: the east–west difference in temperature strengthens, the pressure difference strengthens, and the trade winds and their effects on the ocean strengthen, so the east Pacific cools further (Figure 1(c)). Such changes in sea-surface temperature affect the atmosphere over vast areas, with both local and global repercussions. Locally, during the El Niño phase, the associated atmospheric circulation changes drive increased atmospheric convection and precipitation over the central and eastern Pacific, while rainfall is reduced over the western Pacific. These changes have remote impacts throughout the tropics but also at higher latitudes via large-scale motion of the atmosphere, especially when ENSO is at its strongest in winter. A network of ocean buoys measure sea surface temperature, ocean currents and the wind. Satellites also provide surface temperature and ocean current data, providing the ability to monitor ENSO in real time. El Niño months can be predicted in advance by measuring the heat content of the upper ocean, one of the precursors for the onset of El Niño, and then using climate models to predict its evolution. Although various criteria exist, an El Niño event is normally said to be underway when the sea surface temperature in the equatorial Pacific (officially called the ‘Niño 3.4’ region) rises by +0.5 degC above the historical average for at least three months in a row. The strongest impacts are experienced by those countries near the tropical Pacific origin of ENSO. Changes in surface temperature, wind and moisture affect rainfall intensity and patterns, which can lead to extreme events such as flooding and drought. During an El Niño event, Peru, Ecuador and southeastern parts of South America receive heavy rainfall. In northern Brazil, drier conditions or even drought may result. Indonesia, South Asia and parts of Australia are also more likely to experience drought during the El Niño phase. The change in weather patterns associated with El Niño can greatly impact the economy, in particular agriculture, water resources, fisheries and public health. ENSO also has a strong influence on the occurrence and intensity of tropical cyclones, and Atlantic hurricane activity weakens during El Niño but strengthens during La Niña. In the UK we experience the socio-economic impacts of an El Niño event, partly through increased food prices, and there are effects on the jet stream and European weather, especially in late winter, when El Niño increases the chances of cold snaps and La Niña increases the chances of wet and stormy conditions. El Niño releases heat to the atmosphere and increases subsequent global temperatures. For example, global average temperatures for 2016 were around 1.1 degC above pre-industrial values, and the strong El Niño episode of 2015/2016 partly contributed to this rise. However, researchers have concluded that the warming from this El Niño accounted for only about 0.2 degC of this overall figure. Because of the large event-to-event variability of El Niño, we don't have enough past years of observations to show how climate change may impact it. There is now some evidence that rainfall may increase during El Niño events in the future and that we may even see more very strong ENSO events, but these remain active research questions. Real-time observational monitoring of the tropical ocean and El Niño are available from NOAA at: https://www.pmel.noaa.gov/tao/drupal/disdel/ Met Office Hadley Centre forecasts of El Niño and global temperature can be found here: https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina Forecasts from other prediction models provided by the International Research Institute for Climate and Society are available here: https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/