Sea-surface temperatures in the tropical Pacific Ocean have continued to rise extremely quickly this summer. This region that meteorologists monitor for the El Nino Southern Oscillation (ENSO) has seen steadily increasing temperatures dating back to the beginning of the year and show no signs of slowing down. What has been forecast as a potentially historic event earlier this year is becoming more likely as we move toward the fall season.

Models have been noting the potential for a developing El Nino, when sea-surface temperatures are at least 0.5 degrees Celsius above normal, since the beginning of last winter. The American CFS climate model has been rather consistent on its potential for a "super" El Nino (temperatures of at least 2.0 C above normal) since that time, and has only increased its forecast throughout the first half of the year.

Other global climate models have caught up over the course of this summer. Though some had been less enthusiastic about an historic event in the spring, all of the members of the International Multi-Model Ensemble (IMME) have caught up to the CFS, agreeing that this year's El Nino event, which will likely peak in November or December, is likely to be the strongest on record. The only member of the IMME that is forecasting a peak of less than 3.0 C above normal comes from the French member, which arrives at a peak of about 2.9 C above normal. All others, the American CFS, European ECMWF, UK Met Office, German DWD, and Canadian CMCC models have peaks above 3.0 C. The CFS remains the most aggressive with a peak of over 4.0 C in the traditional Oceanic Nino Index (ONI). Another important climate model that is not included in the IMME, the Australian ACCESS model, has a peak *relative ONI of 3.5 C above normal in the December-to-January timeframe.

*Note: The standard for calculating ONI has changed this year from the traditional method -- which simply calculates the anomaly above or below the long-term average -- to the relative method -- which calculates the anomaly from the global mean sea-surface temperature. Doing so changes the historical record of the strongest El Ninos and La Ninas. For this piece, the traditional ONI is being used to maintain historical perspective.

To put this current projection in perspective, the strongest El Nino in our record was in 2015, when three-month average ocean temperatures peaked at 2.6 C above normal. Projections from all the major global climate models exceed this peak, indicating that this year's El Nino is almost certain to break this record, possibly by the end of September. The latest three-month average for May through July was 1.4 C above normal. On a weekly basis, sea-surface temperatures have just eclipsed the historic mark, sitting at 2.6 C above normal. Six more weeks at the current pace of warming would eclipse the record. It should be noted that this record only extends back to 1950 and the most reliable data comes after the advent of satellite monitoring in the 1960s.

There are no current indicators that would point to this warming slowing down in the near future as ocean temperatures below the surface remain historically warm, in the 4-7 C above normal range, and winds in the equatorial Pacific remain weak.

This historic event will affect the weather in the U.S. during the next several months and into 2027. In general, it should mean an increase in the jet stream across the southern U.S., especially heading into the winter season. That should mean increased rainfall from California to the Carolinas and likely up the East Coast as well. For most of the U.S. and southern Canada, strong El Ninos correlate with extremely warm winters resulting in fewer cold snaps and in general less snowfall. However, we have not had an event of this magnitude before in our record, and so our model of El Nino's impact may be different than what actually occurs. Even when El Nino is the main driver of weather across the globe, the pattern can and will change from time to time, offering different scenarios than those described above. However, it may also be more likely that the background El Nino pattern is stronger than in previous El Ninos, making the above scenario even more likely. Tracking this historic event will be interesting to watch.

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John Baranick can be reached at john.baranick@dtn.com

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