Anyone who has ever tried to predict something months away—the weather, the stock market, the seasonal flu—knows what a tricky business it is. Even a hint of how things might turn out is an advantage. For seasonal climate prediction, that advantage comes from the El Niño–Southern Oscillation (ENSO), a pattern of coupled anomalies in tropical Pacific sea-surface temperature and atmospheric circulation that recurs approximately every two to seven years (see the article by J. David Neelin and Mojib Latif, Physics Today, December 1998, page 32). With two active phases, El Niño (warmer-than-average tropical Pacific surface) and La Niña (cooler-than-average), ENSO alters global atmospheric circulation in known ways, providing months in advance a means to predict potential seasonal precipitation and temperature, tropical cyclones, coastal flooding, and other impacts.
At the time of this writing, El Niño has developed in the tropical Pacific and is predicted to last through the Northern Hemisphere winter. This follows three years of La Niña conditions and is the first El Niño in five years; the last strong El Niño event was in 2015–16. La Niña events often persist through consecutive winters, whereas El Niño more commonly lasts for one year, flanked by either neutral conditions or La Niña.
We have an increasingly confident understanding of how ENSO works and of its impacts across the world. But many questions remain, including how ENSO and its effect on global weather and climate will change as the planet warms.
A seasonal, coupled atmosphere–ocean system
In the early 20th century, the English physicist Gilbert Walker observed that atmospheric pressure recordings in Tahiti and Darwin, Australia, were related: When Tahiti’s air pressure was stronger than average, Darwin’s was weaker, and vice versa. It was several decades before climate scientists made a connection between this “Southern Oscillation” and a pattern of warmer-than-average ocean water that had been observed off the Pacific coast of South America by Peruvian fishermen in the 19th century. Because the warm pattern peaked around Christmas, the Peruvians called the phenomenon El Niño.
Our understanding of the mechanisms involved has advanced substantially since then. The first signs of a developing El Niño often occur beneath the ocean’s surface. Under average conditions, winds blow from the east to the west across the surface of the tropical Pacific. Those trade winds keep the surface in the eastern and central Pacific slightly colder through evaporative cooling. They also pile up warm water on the western side of the Pacific basin, which leads to an upwelling of cold water in the eastern Pacific. Occasionally, however, the trade winds weaken, allowing the warmer water in the western basin to slide eastward under the surface. Known as a downwelling equatorial Kelvin wave, the disturbance propagates along the equator, and subsurface water temperature increases in the central and eastern Pacific.
The trade winds are part of an overturning atmospheric circulation cell called the Walker circulation that spans the Pacific, coupled to the warm water in the west and relatively cooler water in the east. The Walker circulation consists of rising air over the very warm waters of the far western Pacific, west-to-east winds aloft, descending motion over the cooler eastern Pacific, and the trade winds moving to the west.
During an El Niño event, warmer conditions in the central and eastern Pacific increase convection there, weakening the Walker circulation. The trade winds slow, allowing the surface to warm further and reinforce El Niño. The feedback mechanism provided by this ocean–atmosphere coupling is critical to maintaining and strengthening an ENSO event.
The strength of an ENSO event is measured by the magnitude of the sea-surface temperature anomaly in the equatorial Pacific: the greater the departure of the temperature from the long-term average, the stronger the event. The peak anomaly usually occurs in the November to January period. In the US, a common metric for ENSO strength is the average sea-surface temperature anomaly in the Niño-3.4 region of the equatorial Pacific, a box from 170° E–120° E and 5° S–5° N. (Other countries use other regions in the equatorial Pacific for their primary El Niño monitoring.) When this anomaly, currently defined as the departure from the 1991–2020 average, reaches or exceeds 0.5 °C, the first requirement for El Niño conditions has been met. Informally, we consider an anomaly in excess of 1.0 °C an event of moderate strength, and 1.5 °C is the threshold for a strong event.
The stronger the event, the more likely that ENSO will alter global atmospheric circulation and seasonal climate in predictable ways. The four strongest El Niño events on record have notched Niño-3.4 anomalies in excess of 2.0 °C. Based largely on the confident forecasts of global climate computer models and the currently warm tropical Pacific surface, NOAA believes there is a good chance that, at its peak, the current El Niño will be a strong event.
An anomaly of 2.0 °C in the Niño-3.4 region contains approximately 100 quintillion joules of extra heat energy, roughly equivalent to the average energy consumption of the US, in just the top two meters of the ocean. Considering that the warmer-than-average area can extend more than 100 meters below the surface, it becomes clear that this is a lot of extra heat—enough to change global atmospheric circulation.
One of the most immediate El Niño–related changes to global climate is drier, warmer conditions and increased fire risk in Indonesia as convection there weakens. Other effects are felt much farther from the immediate ENSO region because of the changes to global atmospheric circulation patterns. The strong convection over the anomalously warm ocean in the east-central tropical Pacific transfers heat from the surface to the upper-level atmosphere. The increased upper-level heating intensifies the Hadley circulation, the average circulation pattern that moves heat from the tropics toward the poles and leads to the jet streams. During El Niño, the Pacific jet stream is extended eastward and intensified, tending to bring more storms to the southern tier of the US.
Another consequence of the El Niño–altered Walker circulation is increased wind shear—the change in wind velocities with height—over the tropical Atlantic. Increased rising air over the east-central Pacific spreads out when it gets high up in the atmosphere, leading to stronger-than-average upper-level winds over the Atlantic, which blow from west to east. Those stronger upper-level winds contrast with the east-to-west near-surface winds, and thus shear increases. Higher shear can tilt storms and tear them apart. This year, however, the tropical storm fuel provided by the extremely warm Atlantic may well act against the storm-dampening effect of any El Niño–related increase in shear.
El Niño also plays an important role in the global average temperature. Past El Niño years, usually defined as the calendar year that starts with El Niño conditions (e.g., 2016), have registered the warmest global average temperatures of their decades, whereas La Niña years tend to be the coolest. Recent La Niña years, however, have been warmer than El Niño years from earlier decades—ENSO merely temporarily enhances or damps the global warming trend. Despite starting with a La Niña, 2023 will surely be one of the warmest years in history.
ENSO in a warming world
Coral records and other proxies tell us that ENSO has occurred for thousands of years, and we are very confident that the oscillation will continue to operate in the future, regardless of other global climate changes. That’s where our confidence ends and the questions begin. Studies have suggested that in the future climate ENSO could be weaker or stronger, more or less frequent, and so on. The most recent Intergovernmental Panel on Climate Change report concludes that there is no consensus among climate models as to how ENSO will be affected by climate change.
Beyond potential changes in ENSO itself are changes in its far-reaching effects, which are already taking place in a warmer, wetter world. If El Niño tends to bring storms to a certain region, such as the southeastern US, and those storms are carrying more water due to climate change, impacts could be more extreme. This is a particularly active area of research.
For physical scientists, the combination of fluid dynamics, air–sea interaction, and teleconnections makes the study of El Niño irresistible. But the general public is interested in El Niño as well. A few weeks ago, a journalist asked me why I think this is so. I replied that most people are curious about how the natural world works, and El Niño is a singular phenomenon that allows a window into complex subjects like oceanography, atmospheric circulation, and meteorology, one that connects the world through a natural pattern. Or perhaps people want an early heads-up for potential winter weather patterns. Or maybe it’s just the celebrity factor: How many natural phenomena have been impersonated by Chris Farley on Saturday Night Live?
Emily Becker is the lead writer for NOAA’s climate.gov ENSO Blog and associate director of the University of Miami’s Cooperative Institute for Marine and Atmospheric Studies.