What is El Niño and how is it influenced by climate change?

El Niño and La Niña are two phases of the naturally occurring climate phenomenon called the El Niño–Southern Oscillation (ENSO), which leads to the most dramatic year-to-year variation of Earth’s climate. El Niño is characterised by warmer global temperatures, while La Niña years are typically cooler.

In neutral years that are neither El Niño nor La Niña, strong Pacific winds along the equator push warm water from east to west, leading to an upwelling of deeper cooler waters in the eastern Pacific while warm water gathers in the western Pacific. The warm water heats up the air above the western Pacific, which in turn regulates weather patterns by driving atmospheric circulation. However, every few years (two to seven on average) the Pacific winds ease, reducing the upwelling of cooler waters in the east and causing sea surface temperatures to rise and become consistent across the Pacific Ocean.[1] These conditions are known as El Niño.

During the El Niño phase, the warmer temperatures in the eastern Pacific Ocean cause changes in atmospheric circulation patterns which strongly influence global weather patterns. El Niño impacts are usually felt for around 6-12 months and are strongest along the equator on either side of the Pacific.[2]

In Australia and Southeast Asia, hotter and drier conditions raise the risk of drought, heatwaves, and wildfires.[3] On the west coast of South America, wetter conditions result in heavy rainfall and increased flood risk.[4] The effects are less extreme elsewhere, but El Niño can induce shifts in the position of winds from the west in the northern hemisphere known as the polar front jet stream. By pushing this jet stream southward, El Niño can bring about heavy rainfall in the southern United States and increased temperatures in the northern United States and Canada.[5] In Europe, the effects are comparatively much milder but can drive colder winters and stronger heat waves in summer.[6]

La Niña has a less dramatic effect on extreme weather events, though it can still influence global weather patterns. During La Niña years, the easterly winds across the Pacific Ocean become stronger. This cools ocean temperatures in the eastern Pacific while pushing warmer waters further to the western Pacific, leading to increased rainfall and risks of cyclones in Pacific islands, Australia, and New Zealand.[7] Conversely, drier conditions and droughts are observed along the west coast of South America and the Gulf Coast of the United States.[8]

There is a high level of uncertainty regarding the likelihood of an ENSO event occurring and the magnitude of that event[9] [10]. To monitor ENSO, researchers routinely measure sea surface temperatures, and oceanic pressure using satellite imagery and a system of buoys positioned across the Pacific Ocean[11] [12]. Using these measurements researchers calculate indices and provide forecasts and warnings about the potential impacts and timings of El Niño and La Niña. These forecasts help governments and communities prepare for extreme weather events.

Key to monitoring ENSO is a stretch of the Pacific Ocean at the equator known as ‘Niño 3.4’ which is regularly monitored to calculate the Oceanic Niño Index (ONI), a 3-month running mean of sea surface temperatures compared to long-term averages. El Niño and La Niña events are defined when the ONI index exceeds +/- 0.5^oC for a period of five months or more.[13]

References:

[1] Timmermann, A., et al. (2018). El Niño–southern oscillation complexity. Nature, 559, 535-545. https://doi.org/10.1038/s41586-018-0252-6

[2] WMO. (2023). World Meteorological Organization declares onset of El Niño conditions. https://public.wmo.int/en/media/press-release/world-meteorological-organization-declares-onset-of-el-niño-conditions

[3]Australian Government Bureau of Meteorology. (2021). What is El Niño and how does it impact Australia? http://www.bom.gov.au/climate/updates/articles/a008-el-nino-and-australia.shtml

[4] 4. Cai, W., et al. (2020). Climate impacts of the El Niño–Southern Oscillation on South America. Nature Reviews Earth & Environment, 1, 215-231. https://doi.org/10.1038/s43017-020-0040-3

[5] Halpert, M. (2014). United States El Niño Impacts. National Oceanic and Atmospheric Administration. https://www.climate.gov/news-features/blogs/enso/united-states-el-niño-impacts-0

[6] Brönnimann, S. (2007). Impact of El Niño–Southern Oscillation on European climate. Reviews of Geophysics, 45, RG3003. https://doi.org/10.1029/2006RG000199

[7] Kuleshov, Y., et al. (2014). Extreme Weather and Climate Events and Their Impacts on Island Countries in the Western Pacific: Cyclones, Floods and Droughts. Atmospheric and Climate Sciences, 4, 51441. https://doi.org/10.4236/acs.2014.45071

[8] McPhaden, M.J. (2002). El Niño and La Niña: causes and global consequences. Encyclopedia of global environmental change, 1, 353-370. https://www.pmel.noaa.gov/gtmba/files/PDF/pubs/ElNinoLaNina.pdf

[9] Timmermann, A., et al. (2018). El Niño–southern oscillation complexity. Nature, 559, 535-545. https://doi.org/10.1038/s41586-018-0252-6

[10] Capotondi, A. (2023). Simplifying climate complexity. Nature Geoscience, 16, 280-281. https://doi.org/10.1038/s41561-023-01161-y

[11] National Oceanic and Atmospheric Administration. (2023). El Niño/Southern Oscillation (ENSO). https://www.ncei.noaa.gov/access/monitoring/enso/

[12] Voosen, P. (2023). Critical El Niño monitor gets an upgrade. Science, 380, 445-445. 10.1126/science.adi5370

[13] National Oceanic and Atmospheric Administration. (2023). El Niño/Southern Oscillation (ENSO). https://www.ncei.noaa.gov/access/monitoring/enso/

The relationship between climate change and stronger El Niño is a topic of ongoing scientific research. While there is no definitive agreement in the scientific community, recent studies suggest that global heating may be leading to stronger El Niño events.

One recent study found that current sea surface temperature extremes driven by El Niño have intensified by around 10% compared to pre-1960 levels.[1] This builds on previous studies which predicted that the frequency of extreme El Niño events could double over the next century due to faster surface warming of the eastern Pacific Ocean brought on by global temperature rises.[2]

However, it can take up to a decade for the El Niño-Southern Oscillation (ENSO) to shift from El Niño to La Niña and back again, and the intensity of La Niña and El Niño events can vary substantially.[3]

This variable nature of ENSO means it is difficult for scientists to confidently connect the intensity of ENSO phases with climate change. However, the Intergovernmental Panel on Climate Change predicts the frequency of strong El Niño and La Niña events is likely to increase throughout the next century.[4]

Many scientists believe the effects of climate change will be intensified by El Niño because increased global temperatures are themselves associated with increased extreme weather events [5] [6]. Therefore, the coinciding effects of both elevated temperatures and El Niño events are predicted to induce record breaking spikes in global temperatures[7], and a further increased probability that catastrophic weather conditions will occur.[8]

The last strong El Niño in 2015/2016 coincided with the hottest year on record and resulted in widespread droughts, wildfires, floods, and agricultural disruption, leading to ecosystem instability in some regions[9] [10]. Estimates from the UN suggest that this El Niño phase significantly impacted the lives of over 60 million people via socioeconomic damage, migration, disease outbreaks and food insecurity.[11] The extreme impact of this ‘super’ El Niño is widely linked to a strong El Niño event on top of ongoing global warming caused by human activity, such as burning fossil fuels.[12]

Record-breaking global temperatures have been observed even during La Niña years, demonstrating the long-term warming trend driven by climate change. For example, the La Niña year in 2022 was ranked as the fifth warmest year on record.[13]

On 4 July 2023, the World Meteorological Organization announced the onset of an El Niño phase and estimated that there is a 90% chance El Niño will continue at moderate or high strength up to the end of the year. A variety of El Niño indices, including the Oceanic Niño Index (ONI), also predict with high certainty that an extended El Niño period with extreme weather conditions will last well into 2024.[14] Global average temperatures have already reached record-breaking levels in 2023[15] and with a strong El Niño phase, it is likely the world will experience climatic instability equal to or greater to 2016, with widespread floods and droughts predicted to cause vast economic damage and food insecurity.[16] [17] 

References:

[1] Cai, W., et al. 2023. Anthropogenic impacts on twentieth-century ENSO variability changes. Nature Reviews Earth & Environment, 4, 407-418. https://doi.org/10.1038/s43017-023-00427-8

[2] Cai, W., et al. 2014. Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change, 4, 111-116. https://doi.org/10.1038/nclimate2100

[3] Klingsman, N., Will, K. (2018). El Niño 2018-2019: Historical impact analysis, in Strengthening Resilience and response to crises. UK Government: Department for International Development. https://www.dai.com/uploads/DFID_El%20Nino_2018_Historical%20Impact%20Analysis%20(University%20of%20Reading).pdf

[4] Masson-Delmotte, V., et al. (2021). Climate change 2021: the physical science basis. Contribution of working group I to the sixth assessment report of the intergovernmental panel on climate change, 2021. 2. https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FrontMatter.pdf

[5] Diffenbaugh, N.S., et al. (2017). Quantifying the influence of global warming on unprecedented extreme climate events. Proceedings of the National Academy of Sciences, 114, 4881-4886. https://doi.org/10.1073/pnas.1618082114

[6] Knutson, T.R., et al. (2021). Climate change is probably increasing the intensity of tropical cyclones. Critical Issues in Climate Change Science, Science Brief Review, 4570334. https://news.sciencebrief.org/cyclones-mar2021/

[7] WMO (2023). WMO Global Annual to Decadal Climate Update. Target years: 2023-2027. https://library.wmo.int/index.php?lvl=notice_display&id=22272

[8] Rifai, S. W., Li, S., & Malhi, Y. (2019). Coupling of El Niño events and long-term warming leads to pervasive climate extremes in the terrestrial tropics. Environmental Research Letters, 14, 105002. https://doi.org/10.1088/1748-9326/ab402f

[9] Santoso, A., McPhaden, M.J. & Cai, W. (2017). The Defining Characteristics of ENSO Extremes and the Strong 2015/2016 El Niño. Reviews of Geophysics, 55, 1079-1129. https://doi.org/10.1002/2017RG000560

[10] Field, R. D., Van Der Werf, G. R., Fanin, T., Fetzer, E. J., Fuller, R., Jethva, H., … & Worden, H. M. (2016). Indonesian fire activity and smoke pollution in 2015 show persistent nonlinear sensitivity to El Niño-induced drought. Proceedings of the National Academy of Sciences, 113, 9204-9209. https://doi.org/10.1073/pnas.1524888113

[11] OCHA. (2016). El Niño: Overview of Impact, Projected Humanitarian Needs and Response. https://reliefweb.int/report/world/el-ni-o-overview-impact-projected-humanitarian-needs-and-response-16-august-2016

[12] Funk, C.C. (2021). Climate Change Made the 2015–2016 El Niño More Extreme, in Drought, Flood, Fire: How Climate Change Contributes to Catastrophes. Cambridge University Press, 186-211.

[13] NASA. (2022). NASA Says 2022 Fifth Warmest Year on Record, Warming Trend Continues. https://www.nasa.gov/press-release/nasa-says-2022-fifth-warmest-year-on-record-warming-trend-continues

[14] NOAA. (2023). El Niño/Southern Oscillation (ENSO) diagnostic discussion. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml

[15]  NOAA. (2023). OISST V2.1. Climate reanalyzer.org, University of Maine, Climate Change Institute. https://climatereanalyzer.org/clim/sst_daily/

[16] Economist Intelligence (2023). El Niño: South and South-east Asia’s 2023 wildcard. https://www.eiu.com/n/el-nino-south-and-south-east-asias-2023-wild-card/

[17] FAO. (2023). Threat of El Niño looms, FAO prepares anticipatory actions with Members and partners. https://www.fao.org/documents/card/en/c/cc5749en