El Niño–Southern Oscillation

Changes to temperature and precipitation during El Niño (left) and La Niña (right). The top two maps are for December to February, the bottom two are for June to August.[1]

El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variation in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO is not predictable. 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 oscillation, 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.[2][3] 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.[4] El Niño events can be more intense but La Niña events may repeat and last longer. El Niño events, on average, reduced Panama Canal Water Times–contrary to belief [5].

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. Weaker easterly trade winds result in a surge of warm surface waters to the east and reduced ocean upwelling on the equator. In turn, this leads to warmer sea surface temperatures (called El Niño), a weaker Walker circulation (an east-west overturning circulation in the atmosphere) and even weaker trade winds. Ultimately the warm waters in the western tropical Pacific are depleted enough so that conditions return to normal. The exact mechanisms that cause the oscillation are unclear and are being studied.

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.[6] 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.[7][8] 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.[9] Therefore, the relative frequency of El Niño compared to La Niña events can affect global temperature trends on timescales of around ten years.[10] The countries most affected by ENSO are developing countries that are bordering the Pacific Ocean and are dependent on agriculture and fishing.

In climate change science, ENSO is known as one of the internal climate variability phenomena.[11]: 23  Future trends in ENSO due to climate change are uncertain,[12] although climate change exacerbates the effects of droughts and floods. The IPCC Sixth Assessment Report summarized the scientific knowledge in 2021 for 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".[11]: 113  The scientific consensus is also that "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".[11]: 114 

  1. ^ Wald, Lucien (2021). "Definitions of time: from year to second". Fundamentals of solar radiation. Boca Raton: CRC Press. ISBN 978-0-367-72588-4.
  2. ^ Climate Prediction Center (2005-12-19). "Frequently Asked Questions about El Niño and La Niña". National Centers for Environmental Prediction. Archived from the original on 2009-08-27. Retrieved 2009-07-17.
  3. ^ Trenberth, K.E.; P.D. Jones; P. Ambenje; R. Bojariu; D. Easterling; A. Klein Tank; D. Parker; F. Rahimzadeh; J.A. Renwick; M. Rusticucci; B. Soden; P. Zhai. "Observations: Surface and Atmospheric Climate Change". In Solomon, S.; D. Qin; M. Manning; et al. (eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 235–336. Archived from the original on 2017-09-24. Retrieved 2014-06-30.
  4. ^ "El Niño, La Niña and the Southern Oscillation". MetOffice. Archived from the original on 2023-10-27. Retrieved 2015-08-18.
  5. ^ Fuentes, Gabriel; Munim, Ziaul Haque (2025). "Climate influence on Panama Canal operations: Predicting canal water times with integrated environmental and operational data". Transportation Research Part E: Logistics and Transportation Review. 203 104319. doi:10.1016/j.tre.2025.104319.
  6. ^ Cite error: The named reference December 2014 EU2 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference NIWA El Niño/La Niña was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference How Much Do El Niño was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference :1 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference :2 was invoked but never defined (see the help page).
  11. ^ a b c IPCC, 2021: Climate Change 2021: The Physical Science Basis Archived 2023-12-08 at the Wayback Machine. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Archived 2023-05-26 at the Wayback Machine [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp. doi:10.1017/9781009157896.
  12. ^ Collins, Mat; An, Soon-Il; Cai, Wenju; Ganachaud, Alexandre; Guilyardi, Eric; Jin, Fei-Fei; Jochum, Markus; Lengaigne, Matthieu; Power, Scott; Timmermann, Axel; Vecchi, Gabe; Wittenberg, Andrew (23 May 2010). "The impact of global warming on the tropical Pacific Ocean and El Niño". Nature Geoscience. 3 (6): 391–397. Bibcode:2010NatGe...3..391C. doi:10.1038/ngeo868. Archived from the original on 14 September 2019. Retrieved 10 January 2019.
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