December 2024 ENSO update: party time, excellent

There’s a 59% chance that weak La Niña conditions will develop shortly. This is very similar to last month’s estimate, just applied to November–January. It’s true; if you read last month’s post, you can pretty much carry that information over to this month. However, we have lots of fun sciency details to talk about this month, so stick around!

The office holiday party

La Niña, the cool phase of the El Niño/Southern Oscillation (ENSO), is a coupled ocean-atmosphere pattern in the tropical Pacific Ocean. To qualify as La Niña conditions, we need to see (1) surface water in the tropical Pacific that is at least 0.5 °C (just shy of 1˚F) cooler than the long-term average (long-term=1991–2020) and (2) evidence of changes in the Walker circulation, the atmospheric circulation over the tropical Pacific. This evidence includes stronger upper-level and near-surface winds (the trade winds), more rain than average over Indonesia, and less rain over the central Pacific.

It’s important to have both the ocean and the atmosphere showing changes, because there are feedbacks between them (this is the “coupled” part) that help La Niña grow and stick around for several months. When La Niña (or El Niño, can’t forget him) are present, they change global atmospheric circulation in known ways, allowing us a window into potential seasonal temperature and rain/snow patterns.

Cocktails with your old friends

Speaking of the tropical ocean and atmosphere—where are they now? Our key monitoring index, the temperature of the surface water in the Niño-3.4 region, is still running just a little cooler than the long-term average. According to our most reliable long-term dataset, ERSSTv5, the November index was -0.2 °C. While below average, this does not exceed the La Niña threshold of -0.5 °C.

November 2024 sea surface temperature compared to the 1985-1993 average (details on climatology from Coral Reef Watch). The box indicates the location of the Niño-3.4 ENSO-monitoring region in the tropical Pacific. The surface of the east-central tropical Pacific is slightly below average temperature, but much of the global ocean remains warmer than average. NOAA Climate.gov image from Data Snapshots.

Global ocean temperatures have been running way above average for more than a year now, and as you can see from the map above, November was no exception.

Meanwhile, looking up, we see an atmosphere that is showing signs of a Niña-ish pattern. In November, the trade winds were stronger than average, upper-level winds were also stronger, and the tropical Pacific was much less rainy than average. I’ll have more details on this in a few paragraphs.

Dance club

Let’s look at that Niño-3.4 sea surface temperature compared to all the La Niña events since 1950. As you can see below, the Niño-3.4 Index decreased sharply after the peak of last winter’s El Niño, but kind of stalled out in the spring and has been solidly in ENSO-neutral territory for months now.

How sea surface temperatures in the Niño-3.4 region of the tropical Pacific changed over the course of all La Niña events since 1950 (gray lines) and 2024 (black line). This shows the traditional calculation for Niño-3.4, the monthly temperature compared to the most recent 30-year average (1991–2020 for the 2024 line). By this measure, the La Niña threshold has not been crossed, and ENSO is still neutral. Climate.gov graph, based on data from Michelle L’Heureux from CPC using ERSSTv5.

However, that lit-up global ocean we see in the map above may be getting up in ENSO’s grill. Over the past few months, we’ve talked about the Relative Niño-3.4 Index, which compares the Niño-3.4 region to the rest of the tropical oceans. When you take the traditional Niño-3.4 and subtract the tropical average ocean surface temperature, you find a Relative Niño-3.4 Index that dips past the La Niña threshold (see footnote for additional details on Relative Niño-3.4 calculations). In short, the traditional Niño-3.4 says no La Niña yet; the relative index would say we’re already there.

How sea surface temperatures in the Niño-3.4 region of the tropical Pacific changed over the course of all La Niña events since 1950 (gray lines) and 2024 (black line), based on the relative Niño-3.4 calculation. Here, the monthly temperature is compared to the most recent 30-year average, but then the tropical average ocean surface temperature is subtracted, to account for global ocean warmth. By this measure, the La Niña threshold of 0.5 °C has been crossed. The relative Niño-3.4 index is not our official metric, though, and it needs more research. Climate.gov graph, based on data from Michelle L’Heureux. 

With that in mind, Michelle investigated some measurements of the atmospheric component of La Niña. Specifically, she graphed the Equatorial Southern Oscillation and the amount of clouds in the central tropical Pacific. The Equatorial Southern Oscillation compares the surface pressure in the eastern equatorial Pacific to the western. When it’s positive, that means the western pressure is weaker than average and the eastern pressure is stronger than average, indicative of a stronger Walker circulation—La Niña’s signature.

Clouds are estimated with satellite observations of outgoing longwave radiation, or “OLR” for short. Very cold surfaces, like the top of a deep thunderstorm cloud, emit less OLR than a warmer surface, like a cloud-free ocean. Therefore, more OLR generally means fewer clouds. Fewer clouds in the central tropical Pacific is also a La Niña signature move.

We don’t use these monthly atmospheric indexes for declaring ENSO events, because they are much more variable (they jump, jump around) than the ocean index. You can see this in how zig-zaggy the lines are in the below graphs compared to the above. They are very useful for understanding how conditions are evolving, though.

Two ways of looking at the atmospheric conditions in the tropical Pacific: the Equatorial Southern Oscillation (left) and cloudiness in the central Pacific (right). The colored lines show 2024, while the gray lines are every La Niña on record. Both measurements provide evidence that the Walker circulation is stronger than average, a La Niña atmospheric signature. Climate.gov graph, based on data from Michelle L’Heureux.

When Michelle graphed these two atmospheric indexes, she found that they both looked pretty darn La Niña-y. (In these graphs, higher numbers are more like La Niña). In fact, the OLR from November 2024 ranks higher than any previous La Niña! However, these numbers do change a lot from month to month because of other subseasonal patterns like the Madden-Julian Oscillation (which was active), so it could bounce back down into the mosh pit next month.

Afterparty

So what do we take home from all this? The atmosphere looks like La Niña, and has for a while, but the ocean doesn’t, at least by our traditional sea surface temperature measures. Forecasters still think it’s likely that the traditional Niño-3.4 Index will cross the threshold soon, in part helped along by the strong trade winds, which cool the surface and keep warm water piled up in the far western Pacific.

Out of the three climate possibilities—La Niña, El Niño, and neutral—forecasts say that La Niña conditions are the most likely for the November–January season (blue bar over the NDJ label, 59% chance). NOAA Climate Prediction Center image.

But even if we do declare a La Niña Advisory soon, it will very likely be a weak event at most. Check out Nat’s recent post for the implications of a weak La Niña on North American winter forecasts.

This is very much a developing story—you’re reading about scientific development and discovery in real time. Our official ENSO metrics may not describe ENSO quite as well in the context of the much-above-average global ocean temperatures we’ve seen over the past year, but we don’t know yet if the relative Niño-3.4 Index is going to consistently describe ENSO better into the future. We need more research to better understand what is happening.

What we do know is that the ENSO Blog is going to keep you up to date and make sure you’re never late to the party!

Footnote

After you subtract the tropical average (20°S–20°N) sea surface temperature anomalies, the difference has lower variance than the original SST anomalies. This is why the computation of relative Niño-3.4 also has a variance adjustment where you multiply by a scaling factor (ratio of the standard deviation of the SST anomaly with the standard deviation of the difference index). If you want to see how this really works, here is some github code to compute relative ONI using observations. In the relative SST map above, the standard deviation of SST anomaly at each grid box is used instead of the standard deviation of the Niño-3.4 index.