The El Niño phase of the El Niño-Southern Oscillation, predicted earlier this year by meteorological models, has begun. But how strong the warming and its impact get remains to be seen. University of Arizona Cooperative Extension Specialist Mike Crimmins, a climate science professor in the Department of Environmental Science, explains what El Niño is and how it might affect our weather in Aizona.

What are ENSO, El Niño and La Niña?

ENSO stands for El Niño–Southern Oscillation, and it is a natural pattern of warming and cooling of sea surface temperatures in the tropical Pacific Ocean along the equator. These temperature shifts also shift weather patterns in the atmosphere that can resonate around the globe. El Niño is the warm phase, when waters in the central and eastern equatorial Pacific are warmer than average, while La Niña is the cool phase. These ocean changes interact with tropical winds and rainfall patterns, which can shift storm tracks and seasonal climate conditions far from the Pacific.

Has El Niño started? How do we know?

Yes. As of the National Oceanic and Atmospheric Administration’s July 9 ENSO Diagnostic Discussion, an El Niño advisory is in effect, meaning El Niño conditions are present and expected to continue. NOAA determines this by looking for persistent warming in the central and eastern tropical Pacific along with an atmospheric response, including changes in winds, pressure and tropical rainfall patterns.

Is it as strong as predicted? What are the potential impacts for Arizona?

Imagen

The event is strengthening, and NOAA currently gives it an 81 percent chance of becoming a “very strong” El Niño during October–December. In June that chance was 61 percent. Current outlooks suggest El Niño will likely weaken rapidly in early spring 2027, though warmer-than-average tropical Pacific conditions may linger for several months. For Arizona, El Niño most often shifts the odds toward wetter cool-season conditions, especially from late fall through winter into early spring. Its relationship with the summer monsoon is weaker and less consistent. Because this event is expected to be strong, there is somewhat greater confidence in the tilt toward wetter-than-average conditions during the upcoming cool season, which is already reflected in NOAA-Climate Prediction Center seasonal precipitation outlooks for the Southwest U.S. We will know more about the event’s ultimate strength and likely Arizona impacts as fall ocean-atmosphere coupling becomes clearer and NOAA seasonal outlooks continue to be updated.

Is ENSO related to AMOC?

ENSO is centered in the tropical Pacific, while the Atlantic Meridional Overturning Circulation is a major Atlantic Ocean circulation system that moves heat, salt, carbon and nutrients. The AMOC is a very slow-moving ocean-atmosphere system that expresses changes on the order of tens to hundreds of years. They are separate systems, but the atmosphere and oceans connect them through global climate “teleconnections.” A major AMOC weakening or collapse could influence the tropical Pacific and ENSO behavior, but the size and direction of that influence remain an active research question.

There have been news stories lately that AMOC might be breaking down. How and when will we know? Will that affect ENSO?

Scientists will not know from a single year or single indicator whether the AMOC is undergoing a major long-term slowdown. They need sustained observations of ocean currents, temperature, salinity, sea level and related climate patterns over many years to separate long-term change from natural variability. Evidence from reconstructions, observations and models indicates that the AMOC has likely weakened over time and is expected to weaken further this century, but studies differ on whether recent changes signal an approaching collapse or a more gradual long-term slowdown. If AMOC weakened substantially, it could affect ENSO through changes in tropical atmospheric circulation, but that would be a longer-term influence and have little connection to the current El Niño event.

Is ENSO changing over time, and if so, how?

ENSO is now occurring within a warmer global ocean and atmosphere, so its impacts can play out in a warmer and more moisture-loaded climate system. There is evidence that extreme El Niño and La Niña events may become more frequent during the 21st century, but there is still uncertainty about exactly how ENSO frequency, strength and location will change. This is an active research area because ENSO is naturally variable and difficult for models to simulate perfectly.

How does University of Arizona or Cooperative Extension science contribute to understanding these systems?

University of Arizona Cooperative Extension helps connect large-scale climate patterns like ENSO to local impacts that matter for Arizona communities, agriculture, water, wildfire, rangelands and emergency management. The work we do in the Climate Science Applications Program and with the NOAA-RISA Climate Assessment for the Southwest translates NOAA forecasts, climate monitoring and university research into usable information for managers and the public. This regional interpretation is important because even a strong El Niño does not guarantee the same impacts everywhere, and local history, season, drought conditions and land-management context all matter. See tools like the ENSO-Precipitation Explorer to examine how seasonal precipitation varied with past events and check out the CLIMAS ENSO and ENSO Monsoon pages to learn more about El Niño and the Southwest U.S.