El Niño–Southern Oscillation: The Pacific Climate Engine That Can Reshape Weather Worldwide

The El Niño–Southern Oscillation, often shortened to ENSO, is one of the most influential climate patterns on Earth. It begins in the tropical Pacific Ocean, but its effects can reach far beyond the water where it forms, influencing rainfall, drought, storms, heat, agriculture, fisheries, transportation, energy demand, and disaster planning across continents.

At its core, ENSO is a recurring shift between warmer, cooler, and neutral conditions in the equatorial Pacific. Its warm phase is known as El Niño. Its cool phase is La Niña. Between them are neutral periods, when the Pacific is not strongly tilted toward either extreme. But when the system changes, the atmosphere responds — and that response can alter weather patterns worldwide.

Recent satellite observations have again placed ENSO in the spotlight. A large swell of warmer, higher water has been detected moving across the Pacific toward South America, a signal that an El Niño may develop later in the year. The development matters because El Niño events can bring heavy precipitation to some regions, rainfall deficits to others, and broader changes that affect daily life and commerce around the world.

A Climate Pattern Born in the Pacific

ENSO is driven by the interaction between the ocean and atmosphere over the equatorial Pacific. Under typical conditions, trade winds blow from east to west, pushing warm surface water toward the western Pacific. This allows cooler, nutrient-rich water to rise near the coast of South America, supporting marine ecosystems and fisheries.

During El Niño, those trade winds weaken or shift. Warm water that is normally concentrated in the western Pacific begins to move eastward. As the central and eastern Pacific warms, atmospheric circulation patterns change. The jet stream can shift, storm tracks can move, and regions far from the Pacific can experience unusual rainfall, heat, dryness, or cold.

That ocean-atmosphere link is what makes ENSO so powerful. It is not simply a patch of warm water. It is a planetary-scale climate signal.

The Warm Wave Scientists Are Watching

Satellite data from Sentinel-6 Michael Freilich, a sea level satellite launched in 2020 by NASA and European partners and led by ESA for the E.U. Copernicus Programme, has shown a broad swell of warm water arriving in the Pacific Ocean off the coast of South America.

The satellite measures and maps ocean height across the entire ocean every 10 days, down to fractions of an inch. That precision matters because warmer water expands. When sea level rises in a particular region, it can indicate that warmer water is building beneath or near the surface.

In this case, the satellite has been tracking warm Kelvin waves — pulses of warmer, higher water that travel eastward along the equator. These waves often appear before El Niño events. They usually form after brief periods when winds over the far western equatorial Pacific shift from the usual easterly direction to westerlies. When that shift combines with a broader weakening of easterly winds along the equator, warm water and higher sea levels gather in the western Pacific, then move east over several weeks.

By mid-May, satellite measurements showed seas around Peru more than 5.9 inches, or 15 centimeters, higher than long-term averages. That is a notable signal because El Niño develops when multiple Kelvin waves appear over several months and warm water accumulates off Colombia, Ecuador, and Peru.

Why Kelvin Waves Matter

Kelvin waves are important because they help move heat across the Pacific. When they travel eastward, they can deliver warm water toward the eastern Pacific, where El Niño conditions are monitored closely.

A single Kelvin wave does not guarantee a major El Niño. The question is whether more waves follow, whether trade winds continue to weaken, and whether the atmosphere reinforces the ocean warming. ENSO is a coupled system, meaning the ocean and atmosphere must work together for a full event to develop.

The latest observations suggest that the ocean is sending a clear signal. But the final strength of any El Niño depends on how the pattern evolves over the coming months.

Josh Willis, a sea level researcher at NASA’s Jet Propulsion Laboratory in Southern California and project scientist for Sentinel-6 Michael Freilich, described the current development cautiously but clearly: “While this year’s event started a bit later than the big El Niños of 2015 and 1997, it’s beginning to catch up,” he said. “We’ll see how big it gets.”

That uncertainty is central to ENSO forecasting. Scientists can identify early signs, but the magnitude of the event becomes clearer only as ocean and atmospheric conditions reinforce — or fail to reinforce — each other.

El Niño’s Long History and Its Name

El Niño has been recognized for centuries by people whose livelihoods depended on the Pacific Ocean.

Fishermen in the 1600s coined the name El Niño — Spanish for “the boy,” a reference to the birth of baby Jesus — because the warming tended to intensify around Christmastime. The warmer waters disrupted fish availability, meaning fishermen often caught fewer fish.

What began as a regional observation is now understood as part of a global climate system. Modern satellites, ocean buoys, climate models, and atmospheric measurements have transformed ENSO from a local fishing concern into one of the most closely watched climate patterns in the world.

How El Niño Changes Weather Patterns

El Niño affects weather by warming sea surface temperatures in the central and eastern Pacific. That warming changes atmospheric circulation and can shift the jet stream, which influences where storms develop and where they travel.

The consequences are not the same everywhere. Some regions may see heavy rain or snow, while others experience heat, dryness, or drought. The strength of the event also matters. Modest El Niño events tend to produce impacts closer to the tropical Pacific. Strong events can extend their influence much farther.

The events that began in 2018 and 2023 were more modest, with impacts such as drought and flooding mostly seen in and around the tropical Pacific. Larger El Niños, such as the 2015–2016 event, can reach more broadly, including drought in Africa and flooding in California.

That global reach is why ENSO is monitored by meteorological agencies, disaster planners, farmers, water managers, energy companies, insurers, shipping interests, and public health officials.

A Possible “Super” El Niño?

Forecasters have also discussed the possibility that a very strong El Niño — sometimes informally called a “super” El Niño — could develop.

The strength of El Niño is determined by how far above normal sea surface temperatures peak. According to the provided regional weather information, if sea surface temperature averages rise by 1.5 to 2 degrees above normal, the event is considered strong. If the average rises higher than 2 degrees above normal, it is considered very strong, or unofficially a “super” El Niño.

As of mid-May, there was an 82% chance that El Niño would occur before July, according to the National Weather Service Climate Prediction Center. There was about a 33% chance of a very strong, or “super,” El Niño.

“The uncertainty lies not in if an El Niño will form, but how strong it will ultimately be,” Matthew Sittel, assistant state climatologist at Kansas State University, said.

That distinction is important. A weak El Niño may influence seasonal forecasts, but a strong or very strong event can become a major global climate driver.

Why Impacts Differ From Place to Place

El Niño does not produce one uniform result. Its influence depends on geography, timing, ocean temperatures, atmospheric feedbacks, and regional weather patterns already in place.

Kansas offers a useful example of the uncertainty. During El Niño, the state can sit between competing climate influences. National Weather Service Wichita meteorologist Andy Kleinsasser explained that Kansas often finds itself in the “battleground” between warmer and drier weather more typical of the northern part of the country and cooler, wetter weather more common in southern states.

“Some El Niños, especially the stronger El Niños, we can more often than not get in on some of that cooler and wetter conditions, like hopefully this one coming up,” he said. “Whereas if it’s a weaker El Niño, it’s really anyone’s guess what’s going to win out … so given that this is a strong El Niño, there’s a better chance that it’s going to exert its influence with cooler and wetter conditions in Kansas.”

Christopher Redmond, a meteorologist with the K-State Weather Data Library, also described Kansas as a “battleground” state, especially for Wichita and central Kansas.

“Historically during the summer months, Pacific moisture is streamed into the southwestern US through hurricanes and tropical storms,” he said. “This moisture tends to push eastward into the High Plains and western Kansas, resulting in an increase of moisture and cool anomalies during the summer months. However, further east, high pressure tends to dominate, leading to warm and dry conditions more likely for eastern Kansas. This would again place the Wichita and central Kansas region in the ‘battleground’ of uncertain conditions.”

This example captures a broader truth about ENSO: it tilts the odds, but it does not write a day-by-day forecast. Local outcomes still depend on regional systems, seasonal timing, and how strong the El Niño becomes.

Agriculture, Water, Fisheries and Commerce

The importance of ENSO extends well beyond meteorology. It can affect the foundations of economic activity.

For farmers, rainfall shifts can determine planting decisions, crop stress, pasture conditions, irrigation demand, and harvest outcomes. In drought-affected regions, El Niño-driven rainfall can offer relief, but too much rain can also bring flooding, soil erosion, disease pressure, and logistical disruption.

For fisheries, warmer eastern Pacific waters can reduce the upwelling of nutrient-rich water, affecting fish populations and marine food chains. That effect is part of El Niño’s historical identity, because fishing communities first noticed the pattern through reduced catches.

For businesses, ENSO can influence transportation, commodity prices, energy demand, insurance risk, and infrastructure planning. Heavy precipitation can disrupt roads, ports, and supply chains. Drought can strain hydropower generation, water supplies, and food production. Heat anomalies can increase cooling demand, while cooler and wetter patterns can shift heating, construction, and retail activity.

El Niño is therefore not only a climate phenomenon. It is also a planning challenge for governments, companies, and communities.

Satellites Have Changed ENSO Monitoring

Modern ENSO monitoring depends heavily on satellites. Sentinel-6 Michael Freilich is especially important because it is the current official reference satellite for global sea level measurements.

Its mission continues a legacy that began in 1992 with the TOPEX/Poseidon satellite. Since then, a series of successors has carried forward continuous global sea level monitoring. The latest satellite, Sentinel-6B, launched in November 2025 and is expected to take over for Sentinel-6 Michael Freilich by the end of 2026.

This continuity matters because ENSO is best understood through long-term observation. Scientists need to compare current sea levels, temperatures, and circulation patterns against decades of data. The more precise and consistent the record, the better forecasters can interpret early signals.

“NASA’s observation of El Niño uses sea level satellites like Sentinel-6 Michael Freilich to track massive Kelvin waves as they cross the Pacific, capture changes in Earth’s ocean thermodynamics, improve forecasts of weather extremes, and help communities prepare for potential coastal hazards,” said Nadya Vinogradova Shiffer, lead program scientist at NASA Headquarters in Washington. “Stay tuned as more ocean stories continue to unfold.”

When El Niño Usually Peaks

El Niño events usually peak between November and January. That means early signs in spring or early summer may not reveal the full scale of the event for several months.

The timing is crucial for seasonal planning. Governments and agencies may begin preparing for possible droughts, floods, heat, or storm-track changes before the event reaches its peak. Farmers may adjust expectations for rainfall. Water managers may review reservoir strategies. Emergency planners may consider flood or drought readiness.

But forecasts must remain flexible. ENSO can strengthen, stall, or evolve differently than expected. Every event has its own structure.

“Every El Niño is different,” said JPL sea level researcher Severine Fournier, deputy project scientist for Sentinel-6 Michael Freilich. “But they almost always make for a hot year and big changes in rainfall in parts of the globe.”

The Public Should Watch the Trend, Not Just the Label

For the general public, the term “El Niño” can sound like a single forecast. It is not. It is a climate pattern that changes the probability of certain conditions.

A strong El Niño may increase the likelihood of wetter weather in some regions and drier weather in others, but it does not guarantee that every affected region will experience the same outcome. Seasonal averages may shift even while individual weeks still bring contrasting weather.

That is why forecasters emphasize strength, timing, and regional context. A developing El Niño is important, but its practical meaning depends on where a person lives, what season it is, and how the ocean-atmosphere pattern evolves.

Looking Ahead

The coming months will be critical. Scientists will watch whether additional Kelvin waves cross the Pacific, whether warm water continues to accumulate off South America, whether trade winds remain weakened, and whether the atmosphere begins responding in ways that reinforce El Niño.

If the event becomes strong or very strong, the effects could be felt across weather-sensitive sectors worldwide. Some regions may prepare for heavier precipitation and flooding risks. Others may brace for drought, heat, or disrupted rainfall seasons. Coastal areas may pay attention to higher sea levels and storm-related hazards.

What is already clear is that ENSO remains one of the world’s most consequential climate patterns. It begins with winds and warm water across the Pacific, but its reach can extend into farms, fisheries, cities, markets, power grids, and households.

El Niño–Southern Oscillation is not just a scientific term. It is a reminder that the ocean and atmosphere are deeply connected — and that a wave of warm water crossing the Pacific can become a global story.