ENSO and El Niño are among the planet’s most influential natural climate patterns. What are they, how do they work, and what do they mean for weather, climate, and local impacts?
By Jason Klein
Diagram of the El Niño Southern Oscillation, courtesy of Internet Geography
El Niño is once again a major climate story, and forecasters expect it to strengthen as the year goes on, eventually becoming one of the strongest El Niños on record. While the El Niño pattern originates in the tropical Pacific, its effects can reach much farther, influencing weather, storms, and temperatures across the United States and around the world. As climate change pushes global temperatures higher, El Niño can add another layer of volatility on top of the warming trend.
What is El Niño?
El Niño is the warm phase of ENSO, the El Niño-Southern Oscillation, a recurring climate pattern in the tropical Pacific. Under normal conditions, trade winds—the constant easterly winds that circle the Earth near the equator—push warm water westward toward Asia while cold water rises near South America through upwelling from greater depths. During El Niño, those trade winds weaken or reverse, causing warm water to spread eastward, raising sea-surface temperatures in the eastern Pacific. La Niña is the opposite phase, with cooler-than-normal waters in the eastern Pacific and stronger trade winds. El Niño and La Niña tend to alternate every two to seven years, with El Niño happening more frequently than La Niña.
How it forms
Scientists do not fully know what triggers every El Niño event, but the pattern usually begins when shifts in pressure across the tropical Pacific weaken the normal east-to-west winds. NOAA’s Climate Prediction Center says El Niño is already underway and is expected to persist through the Northern Hemisphere winter of 2026-27. In practical terms, that means the ocean is already setting up atmospheric changes that can influence weather well beyond the equator.
Learn More: The connection between weather and climate
Weather is the short-term state of the atmosphere. It describes what is happening today, this week, or over the next few days. Climate is the long-term pattern of weather in a place, measured over many years. A cold snap, a heat wave, or a heavy rainstorm is weather. The usual seasonal conditions that a region experiences is climate.
The two are connected, but they are not the same. Climate helps shape the range of weather that is possible, while individual weather events are the day-to-day changes within that range. That is why El Niño can influence weather patterns for a season, and why climate change can make certain types of weather more likely or intense over time.
Why does El Niño change weather?
The reason El Niño is so consequential is that the tropical Pacific holds enormous amounts of heat energy, meaning changes there can reorganize atmospheric circulation on a large scale. Coupling a strong El Niño with the long-term warming trend increases the probability that we’ll see a new record global mean temperature. El Niño also alters the jet stream—the narrow bands of strong winds that form when warm air masses meet cold air masses—which helps explain why some regions become warmer or wetter while others become cooler or drier. In the U.S., it often increases the odds of a wetter, stormier winter in the South, while parts of the North tend to experience warmer and drier conditions. These alterations in weather generally reverse during La Niña events.
What does El Niño mean in a warming climate?
Dr. Michael Mann, Professor of Earth & Environmental Science and Director of the Penn Center for Science, Sustainability and the Media, said that the most important point to remember is that El Niño can amplify a background climate trend that is already being driven by human emissions. “For one thing, it will almost certainly yield a new global temperature record,” he said, pointing to research showing that the 2024 temperature record was helped by a very strong El Niño event layered onto long-term warming. He also warned against treating El Niño like a simple planet-wide heating effect: “The biggest misconception perhaps is that El Niño leads to warming over the entire planet, in a manner similar to greenhouse warming from human carbon emissions.”. Instead, he said, the main impact in many places comes through shifts in the jet stream. Mann also noted that El Niño’s effects are not uniformly bad, since El Niño years tend to bring fewer Atlantic hurricanes and, in some regions, less drought than La Niña years. ENSO events can be incredibly diverse, making prediction difficult, but important.
What does El Niño mean for Philadelphia and the Northeast?
For Philadelphia and the Northeast, the message is less about a single outcome than about changing odds. Dave Epstein, an experienced meteorologist and horticulturalist, said, “As we are entering a strong El Niño, the weather around the globe, including in the Northeast, will be impacted,” and added that this region generally sees “a warmer and wetter winter” during these events. He said that shift can start to show up in the second half of summer and into autumn, with more humidity and more rainfall, and that the coming El Niño may help end a drought that has persisted for several months. Epstein also warned that wetter, more humid conditions can increase fungal pressure and threaten crops, especially tomatoes, with late blight. These weather conditions do not mean constant heavy rain, but rather more frequent precipitation events. In the Northeast, that kind of pattern can increase flooding risk, especially if rain comes in intense bursts or repeated storms that saturate the ground.
Epstein’s advice for gardeners and homeowners was practical: watch for disease as conditions turn wetter and take precautionary measures rather than waiting for problems to appear. He also emphasized that El Niño does not eliminate winter weather hazards. “That does not mean we’re not going to see a big snowstorm,” he said, though he expects deep cold to be less persistent than during the previous winter. In other words, El Niño changes the baseline, but it does not erase normal winter conditions.
Forecasts and uncertainty
El Niño is tracked with a wide network of satellites, buoys, ships, and sea-level measurements that feed into climate models. NOAA’s Climate Prediction Center issues regular ENSO outlooks based on that data, giving forecasters a way to estimate the odds of El Niño months in advance. Still, no two El Niño events are identical, and the atmosphere and ocean do not always respond the same way from one event to the next. That is why the same El Niño pattern can produce different local outcomes, even when the large-scale ocean signal looks similar.
Closing perspective
El Niño is a reminder that climate patterns are interconnected across distance and time. A shift in the tropical Pacific can influence storms in the Atlantic, rainfall in the Northeast, and even the odds of a global temperature record. As most climate scientists put it, the event becomes most important when it arrives on top of the long-term warming trend already reshaping the planet. A strong El Niño is coming, but its effects are likely to be felt in different ways depending on where you live. To learn more about ENSO monitoring, forecasting, and global impacts, and to get monthly updates, check out Climate.gov’s ENSO Blog.
Jason Klein is a Penn Climate student assistant who graduated from Penn’s MES program in May 2026.