Itās that time again! And by āthat time,ā I mean the El NiƱo forecast update, of course. The chance that El NiƱoāthe warm phase of the El NiƱo Southern Oscillation (aka āENSOā) climate patternāwill continue through the winter is greater than 95%, so letās sharpen our pencils and get into the details of what that means for upcoming seasons.
- Related: NOAA July 2023 Climate Report: Record-High Temperatures, Devastating Floods Across the U.S.
Mathematics
Greater than 95% is a very strong chance! Forecastersā confidence that El NiƱo will continue is based on a few factors. First, the east-central tropical Pacific is quite warm. Specifically, our primary El NiƱo-monitoring metric, the NiƱo-3.4 Indexāthe average sea surface temperature in the NiƱo-3.4 region in the east-central tropical Pacificāwas 1.0 Ā°Celsius (about 2 Ā°Fahrenheit) warmer than the long-term average in July, according to our most reliable dataset, ERSSTv5. (Long-term = 1991ā2020.)
The three-month-average NiƱo-3.4 Index, the Oceanic NiƱo Index, was 0.8 Ā°C above the long-term mean for the MayĀāJuly average, the second three-month-period in a row above the El NiƱo threshold of 0.5 Ā°C. We need to see five consecutive three-month averages above this threshold before these periods will be considered a historical āEl NiƱo episodeā and colored red in our ENSO record). Two is a good start, especially with the 0.8 Ā°C recording from MayĀāJuly. If this El NiƱo were to collapse after hitting this high, dropping back below the threshold of this magnitude before next winter, it would be the first time in our historical record, dating back to 1950.
Social studies
El NiƱo is a coupled phenomenon, meaning the changes we see in the ocean surface temperature must be matched by changes in the atmospheric patterns above the tropical Pacific. The average atmospheric circulation over the tropical Pacific, the Walker circulation, is like a conveyor belt: rising air over the very warm far western Pacific, west-to-east winds high up in the atmosphere, descending air and dry conditions over the east-central Pacific, and returning east-to-west winds near the surfaceāthe trade winds.
During El NiƱo, the warmer east-central tropical Pacific Ocean surface leads to lower surface air pressure and more rising air, clouds, and rain over that region, weakening the overall circulation. The trade winds slow, and drier conditions and higher-than-average air pressure are observed over the western Pacific and Indonesia. The ocean-atmosphere coupling is both how El NiƱo perpetuates itself, as the atmospheric changes feed back into the oceanic changes, and how El NiƱo affects global weather and climate.
In July, we observed more rain and clouds over the central Pacific, with somewhat drier conditions in Indonesia and some reduced trade wind activity in the western Pacific. The Equatorial Southern Oscillation Index, which measures the relationship between surface air pressure across the Pacific, was -0.9 in July, indicating weaker pressure in the eastern Pacific and higher in the western. Taken together, these are all signs of the atmospheric component of El NiƱo, providing more confidence that the system is engaged and that these conditions will last through the winter.
Computer science
Our global climate models are predicting that the warmer-than-average Pacific Ocean conditions will not only last through the winter but continue to increase. There is a good chanceāapproximately 2 in 3āthat the peak Oceanic NiƱo Index this winter will match or exceed 1.5 Ā°C, our informal threshold for a āstrongā El NiƱo event. This is more confident than last month, in large part because the peak of this El NiƱo is one month closer, and, as I mentioned above, the surface is already 1.0 Ā°C warmer than average.
The sea surface temperature changes associated with El NiƱo events usually peak in NovemberāJanuary or thereabouts. Why? We still donāt know exactly why ENSOās seasonal cycle is timed the way it is, with most events peaking in the winter and decaying through the spring. There is still a lot to learn!
Geography
El NiƱoās most extensive impacts on global climate also occur during the winter and early spring. (Iām using Northern Hemisphere seasonal terms here.) Typical impacts include more rain and storms across the southern tier of the United States, southeastern South America, around the horn of Africa, and eastern Asia. Drier conditions are often found in DecemberāFebruary during El NiƱo through the Maritime continent/Indonesia, southeastern Africa, and northeastern South America. El NiƱo affects summer (JuneāAugust) climate, too, including drier conditions through the Caribbean, Indonesia, India, northern South America, parts of Central America, and eastern Australia.
El NiƱo is also known to interact with the Atlantic and Pacific hurricane seasons, with El NiƱo years tending to be less active in the Atlantic. In the hurricane season outlook released in May, NOAA scientists expected the potential suppressing effect of El NiƱo may be offset by the much warmer than average Atlantic Ocean surface, as warm water provides fuel for hurricanes. NOAAās update to the outlook will be released later today, so be sure to keep an eye out for that!
Experimental psychology
Speaking of the warm Atlantic, youāve probably heard about the recent unusually high global average temperatures. Some of the worldās oceans are extremely warm (North Atlantic, Southern Ocean, and so on), and there have been long-lasting heat waves across many land regions. El NiƱo is linked to higher global averages, although this El NiƱo is just getting going and canāt be blamed for all the heat events that have occurred already this year. (In fact, over North America, impacts tend to be very weak during the summer). Thereās a good chance, though, that it will contribute to (at the very least) a top-3 average temperature for 2023. In a guest post in June, Karin Gleason discussed how NOAA predicts the global average temperature.
But what are the computer climate models predicting? Michael Tippett of Columbia University, noted friend-of-the-blog, took a look at some of the predictions from the North American Multi-Model Ensemble. These graphs are a lot, so bear with me!
Each panel shows the global average temperature forecast from one model, relative to the āpre-industrialā periodāthat is, the increase in global average temperature since 1850. To see how the predictions vary from month-to-month, Mike has included forecasts made in April, May, June, July, and August of each year, starting in 2013. The most recent forecasts, from April, May, June, July, and August of 2023, are on the right-hand end, in colors.
The gray lines are forecasts from previous years. For most models, the forecasts extend out 12 monthsāfor example, the forecast made in June 2014 goes out to May 2015. The black line shows the observed global temperature, from the Hadley Centerās HADCRUT5 temperature record.Ā These forecasts have the same structure as our NiƱo-3.4 forecasts, but instead of predicting the average sea surface temperature in the NiƱo-3.4 region, theyāre predicting the average temperature over the entire globe. (For help visualizing what I mean, take a look at this animation showing NiƱo-3.4 forecasts from 2015ā16.)
The different models and the different monthly forecasts show that there is some variation in the predictions overall. Most models suggest that the global average temperature will substantially exceed that of early 2016, our last strong El NiƱo event, but not all of them. Month-to-month, the predicted average can shift around. You can also see that the 2016 record was pretty well predicted by the models. There are a lot of things to note in these graphs, but Iām running out of space here, so I leave you to your studies. What do you observe? Let us know in the comments!
This post first appeared on the climate.gov ENSO blog and was written by Emily Becker.