El Niño–Southern Oscillation

El Niño–Southern Oscillation (ENSO) is a climate phenomenon that exhibits irregular quasi-periodic variation in winds and sea surface temperatures over the tropical Pacific Ocean. It affects the climate of much of the tropics and subtropics, and has links (teleconnections) to higher latitude regions of the world. The warming phase of the sea surface temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, which is coupled with the sea temperature change. El Niño is associated with higher than normal air sea level pressure over Indonesia, Australia and across the Indian Ocean to the Atlantic. La Niña has roughly the reverse pattern: high pressure over the central and eastern Pacific and lower pressure through much of the rest of the tropics and subtropics. The two phenomena last a year or so each and typically occur every two to seven years with varying intensity, with neutral periods of lower intensity interspersed. El Niño events can be more intense but La Niña events may repeat and last longer.

"El Niño" redirects here. For other uses, see El Niño (disambiguation).
Changes to temperature and precipitation during El Niño (left) and La Niña (right). The top two maps are for Northern hemisphere winter, the bottom two for summer.

A key mechanism of ENSO is the Bjerknes feedback (named after Jacob Bjerknes in 1969) in which the atmospheric changes alter the sea temperatures that in turn alter the atmospheric winds in a positive feedback. Relaxed easterly trade winds result in a surge of warm surface waters to the east and reduced ocean upwelling on the equator.  In turn that leads to warmer sea surface temperatures (El Niño), a weaker Walker circulation (an east-west overturning circulation in the atmosphere) and further relaxed trade winds. Ultimately the warm waters in the western tropical Pacific are depleted enough that conditions return to normal. The exact mechanisms that cause the oscillation remain under study.

Each country that monitors the ENSO has a different threshold for what constitutes an El Niño or La Niña event, which is tailored to their specific interests.

El Niño and La Niña affect the global climate and disrupt normal weather patterns, which as a result can lead to intense storms in some places and droughts in others. El Niño events cause short-term (approximately 1 year in length) spikes in global average surface temperature while La Niña events cause short term surface cooling. Therefore, the relative frequency of El Niño compared to La Niña events can affect global temperature trends on decadal timescales. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected.

In climate change science, ENSO is known as one of the internal climate variability phenomena.: 23  Future trends in ENSO due to climate change are uncertain, although climate change exacerbates the effects of droughts and floods. The IPCC Sixth Assessment Report summarized the state of the art of research in 2021 into the future of ENSO as follows: "In the long term, it is very likely that the precipitation variance related to El Niño–Southern Oscillation will increase": 113  and "It is very likely that rainfall variability related to changes in the strength and spatial extent of ENSO teleconnections will lead to significant changes at regional scale".: 114 

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