Neutral conditions

Under normal "neutral" conditions, the trade winds blow steadily from east to west across the tropical expanse of the Pacific Ocean. These vital winds are driven by a combination of warm air rising dynamically along the equator and the continuous rotation of the Earth. As they sweep westward, the trade winds push warm surface water from the coast of South America toward Asia.

To replace this displaced water, cold, deep ocean water is drawn upward from the ocean depths off the South American coastline in a vital ecological and oceanographic process known as upwelling. This cold, nutrient-rich water transforms the region into one of the most productive marine environments and fishing grounds on Earth. Peru’s anchovy fishery, for instance, relies entirely on these conditions and stands as the largest fishery by volume in the world.

Rainfall patterns across the Pacific mirror these underlying surface water temperatures. The pooling of warm waters in the western Pacific provides the energy needed to fuel heavy, persistent rainfall across Indonesia and the islands of southeast Asia. Conversely, the cooler waters dominating the eastern Pacific suppress local rainfall, bringing dry, arid conditions to the west coast of South America.

This stark contrast between warm and cold waters is closely linked to a massive seesaw in atmospheric air pressure differences across the Pacific basin. Known formally as the Southern Oscillation, this pattern is routinely tracked by meteorologists comparing air pressure readings on the Pacific island of Tahiti with those recorded in the northern Australian city of Darwin.

How an El Niño event develops

During an El Niño event, the dynamic balance of the Pacific shifts dramatically as the trade winds weaken—or occasionally reverse direction locally. This atmospheric breakdown allows the accumulated warm surface water to slowly spread back eastward toward South America, effectively shutting down or heavily suppressing ocean upwelling.

This disruption reduces the stark contrast in sea surface temperatures, which in turn weakens the air pressure differences across the equatorial Pacific. The weakening pressure gradients further reduce the trade winds, establishing a classic positive feedback loop that amplifies the entire event. As these warm waters shift eastward, global rainfall patterns are upended, typically bringing very wet, stormy conditions to the west coast of South America while leaving southeast Asia unusually dry. The profound shift in winds over the Pacific also triggers far-reaching knock-on impacts for weather systems around the entire globe.

How a La Niña event develops

In stark contrast, a La Niña event is characterized by the complete opposite atmospheric and oceanic behavior. Rather than weakening, the trade winds strengthen significantly, blowing harder and more consistently from east to west. This intensification pushes warm surface waters even deeper into the far western reaches of the tropical Pacific and drastically increases the upwelling of cold water along the South American coast.

Cooler-than-average surface waters extend much further west into the tropical Pacific, strengthening both the temperature and pressure gradients across the ocean basin. Ultimately, a La Niña event amplifies the typical rainfall patterns seen in a neutral year, bringing heavier, more frequent rainfall to southeast Asia while delivering notably cooler and drier conditions to the west coast of South America.

The history of El Niño

The climatic cycles of El Niño and La Niña are not modern anomalies; they have been actively shaping the Earth’s climate and ecosystems for millennia. Paleoclimatic research utilizing proxy data extracted from preserved ancient corals suggests that ENSO-like variability has existed for at least 130,000 years.

Historical human records reveal that Peruvian fisherfolk had already recognized El Niño events by at least the 1600s. They observed that periodically, a warm, southward-flowing ocean current would replace the normally cold, nutrient-rich waters off the coast of Peru. Because this shift was most noticeable around the Christmas holiday, they named the warm current "El Niño de Navidad," or the "Christ Child."

Centuries later, in 1926, British scientist Gilbert Walker formally coined the term "Southern Oscillation" to describe the periodic atmospheric pressure fluctuations between the eastern and western tropical Pacific. It wasn’t until the 1960s that Swedish-born meteorologist Jacob Bjerknes successfully connected the oceanic changes of El Niño with Walker’s atmospheric Southern Oscillation, cementing the unified concept of the ENSO phenomenon.

Defining the ‘strength’ of an event

Scientists identify and monitor emerging El Niño or La Niña events by continuously tracking sea surface temperatures using a vast network of ships, ocean buoys, and specialized satellites deployed across the Pacific Ocean. Typically, researchers focus heavily on the "Niño 3.4" region—a massive, rectangular section of the central Pacific covering more than six million square kilometers, centered directly on the equator. Other designated regions are also utilized, such as Niño 1+2 in the east and Niño 4 in the west, though Niño 3.4 remains the primary benchmark.

The traditional Oceanic Niño Index tracks how far conditions in the Niño 3.4 region depart from the long-term average using a three-month running average. Sustained index values reaching at least +0.5C indicate the onset of El Niño conditions, whereas values dropping to -0.5C signal that a La Niña event is developing. Anything falling between these thresholds is classified as neutral.

Different international scientific organizations maintain slightly distinct criteria for officially declaring that an El Niño or La Niña event is underway. The US National Oceanic and Atmospheric Administration, for example, requires these conditions to have persisted—or be explicitly expected to persist—for five consecutive overlapping three-month periods. NOAA’s formal criteria also factor in critical atmospheric indicators, including shifting trade winds and anomalous rainfall patterns.

The impacts of an El Niño event

The arrival of an El Niño event alters global temperatures and rainfall distribution, significantly escalating the risk of extreme weather events across numerous continents. Southeast Asia and South America bear some of the most immediate and profound impacts.

Across central and South America, El Niño typically ushers in warmer and drier weather across northern and tropical regions, while dumping heavy rainfall in the south. Consequently, nations such as Colombia, Venezuela, and northern Brazil face heightened risks of severe drought and destructive wildfires. Meanwhile, southern Brazil, central Chile, and northern Argentina frequently experience severe, damaging flooding. In southeast Asia and parts of Australia, El Niño systematically delivers drier conditions, sharply increasing the threat of prolonged droughts and wildfires.

In India, El Niño can drive extreme heat waves during the opening months of the year, followed by an increased risk of agricultural drought later in the season by weakening the vital summer monsoon. Other parts of Asia, including China and Japan, can similarly experience unusually warm conditions early in the year.

Across the African continent, El Niño raises the risk of dryness and drought in southern regions from December through February, while paradoxically triggering severe flooding in eastern nations. In North America, the phenomenon typically brings milder winter temperatures to northwestern Canada and parts of the United States, alongside an increase in winter storms and coastal flooding across southern US states and California.

Tropical storms

Because of its profound influence on atmospheric circulation and global sea surface temperatures, El Niño alters the formation, timing, and geographic distribution of tropical storms worldwide.

In North America and the Caribbean, El Niño typically leads to a quieter Atlantic hurricane season. This suppression occurs because El Niño alters atmospheric and ocean dynamics in a way that increases vertical wind shear across the tropical Atlantic and Caribbean seas. These powerful, shearing winds tear apart developing storm systems, effectively inhibiting the formation and intensification of hurricanes.

In other parts of North America, including southern US states and California, El Niño drives more intense winter storms by shifting the jet stream, the fast-flowing river of air high in the atmosphere that dictates mid-latitude weather patterns.

Conversely, the weakening of trade winds causes warm water to shift eastward across the Pacific, which relocates the primary zones of tropical storm formation. In Asia, this eastward shift means that typhoons are less likely to strike the Philippines and instead pose a greater threat to China, Japan, and South Korea. Because these storms have a much longer distance to travel before making landfall, they often have extended time to gather heat and moisture, significantly increasing their potential to become major, highly destructive weather events.

For vulnerable island nations in the central and eastern Pacific, such as Hawaii, Kiribati, and Tuvalu, these elevated sea surface temperatures bring an increased frequency of intense tropical storms and severe weather.

The impacts of a La Niña event

The global environmental footprint of a La Niña event on temperatures and rainfall acts as a near-mirror opposite to El Niño.

In central and South America, La Niña typically brings wetter-than-average weather to northern and tropical areas, sharply escalating flood risks in nations like Colombia, Venezuela, and northern Brazil. In the southern portion of the continent, however, La Niña triggers severe dry spells, heightening the threat of agricultural drought in countries such as southern Brazil and northern Argentina. Across southeast Asia and northeastern Australia, La Niña is reliably associated with well-above-average rainfall, frequently driving catastrophic flooding and deadly landslides.

In Africa, La Niña increases rainfall and flood risks across southern nations during the peak summer months of December through February, while simultaneously inflicting dry weather and heightened drought risks upon East Africa. In North America, La Niña frequently delivers colder and wetter winters to northwestern Canada and the northern United States, paired with notably drier and hotter winter conditions across the southern tier of the US and Mexico.

How El Niño affects global temperature

An active El Niño event typically provides a temporary, noticeable boost to average global temperatures. The weakening trade winds allow massive amounts of warm surface water to pool across the Pacific Ocean, releasing vast quantities of accumulated heat directly into the atmosphere.

As a general rule, a 1C temperature shift in the Niño 3.4 region is historically associated with roughly a 0.1C change in the global average temperature, operating with a time lag of between three and six months. Consequently, major El Niño events have historically contributed to record-breaking annual global temperatures, most notably in 1998, 2016, and 2024.

Conversely, during a La Niña event, strengthening trade winds promote robust upwelling of cold deep water in the eastern Pacific, exerting a temporary cooling influence on global temperatures.

While El Niño and La Niña remain the largest natural drivers of year-to-year variability in global temperatures, this influence is strictly temporary. Ongoing, human-caused global warming means that temperature records set during El Niño years have consistently been broken over time. In the ERA5 global temperature dataset, for instance, 1998 stood as a record-warm year during a strong El Niño, only to be surpassed by 2005, which was subsequently eclipsed by 2010, 2015, 2016, 2023, and 2024.

Removing the natural background noise of El Niño and La Niña from the global temperature record highlights how these events create sharp peaks and troughs from one year to the next. Ultimately, however, human-caused climate change remains the definitive driver behind long-term global temperature records.

The 2026-27 ‘super El Niño’

The Earth is currently experiencing an active El Niño event that began in June and is projected to persist well into 2027. Numerous complex meteorological forecasts suggest this particular event could develop into one of the strongest on record, triggering severe ecological and societal impacts globally. Because of these extraordinary projections, many media outlets and commentators have adopted the term "super El Niño," although meteorologists emphasize this remains an informal descriptor rather than an official scientific classification.

An analysis conducted by Carbon Brief utilizing forecasts from 14 international climate modeling groups tracking El Niño globally indicates that 96% of model runs predict this event will rank among the strongest in the modern observational record. The models suggest that peak sea surface temperature anomalies in the Niño 3.4 region will exceed the previous historical benchmark of 2.75C set during the legendary 2015-16 El Niño event.

How might climate change affect El Niño?

With a potent El Niño event underway, scientific interest has intensified regarding whether human-caused climate change is actively increasing the frequency or severity of such extreme events. The potential intersection of climate change and the ENSO cycle remains a subject of intense scientific debate.

The Intergovernmental Panel on Climate Change noted in its comprehensive climate science assessments that the overall intensity of El Niño, alongside the frequency of high-magnitude events, has generally increased since 1950 when compared to historical reconstructions stretching back centuries. However, paleoclimate proxy data derived from tree rings, sediment cores, and coral archives demonstrates that El Niño events have naturally varied widely in frequency and intensity over the past 11,000 years, complicating efforts to definitively attribute recent shifts entirely to human activity.

This inherent scientific uncertainty also makes projecting the future trajectory of El Niño a formidable challenge. While some modeling studies project that extreme El Niño events could increase in frequency under continued global warming scenarios, observational studies continue to refine our understanding. Research published in the journal Science examining centuries of sea surface temperature reconstructions from Galapagos coral samples indicates that the past century has indeed experienced a higher frequency of strong El Niño events than the millennium preceding the industrial revolution, underscoring the complex dynamics shaping the future of the Pacific climate engine.