March 2025 ENSO update: neutral conditions expected soon

La Niña conditions persisted through February, but forecasters expect ENSO-neutral conditions to develop in the next month and persist through the Northern Hemisphere summer.

ENSO

La Niña is the cool phase of the El Niño/Southern Oscillation, or ENSO, a pattern of changes in the tropical Pacific Ocean and the atmospheric circulation over the tropical Pacific that persists for many months. La Niña’s signature is cooler-than-average surface water in the east-central tropical Pacific, stronger-than-average trade winds, more rain and clouds over Indonesia, and drier conditions over the central Pacific. El Niño is the opposite, with warmer-than-average surface water and weaker atmospheric circulation, while ENSO-neutral conditions are close to average. 

ENSO can be predicted months in advance, and it changes global atmospheric patterns in known ways. This means studying ENSO provides an early assessment for potential weather and climate risks such as tornado and hurricane seasons, drought and flooding rains, extreme temperature, global crop yields and disease outbreaks, and even financial markets.

Recent ocean and atmosphere conditions

Our primary metric for ENSO is the surface temperature of the east-central tropical Pacific Ocean in what we call the Niño-3.4 region. Specifically, the temperature anomaly in this region—the departure from the long-term average, where long-term is currently 1991–2020. According to our longest, most reliable sea surface temperature dataset, ERSSTv5, the anomaly in February 2025 was -0.6 °C (a fraction below -1.0 ˚F), just slightly exceeding the La Niña threshold of -0.5 °C.

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 2024-25 (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 was crossed in December 2024, but La Niña remains weak. Climate.gov graph, based on data from Michelle L’Heureux from CPC using ERSSTv5.

The atmospheric component of ENSO is represented by changes in the Walker circulation, an overturning loop in the atmosphere over the tropical Pacific. Hallmarks of the Walker circulation include rising air, clouds, and rain over Indonesia; west-to-east winds high up in the atmosphere; sinking air over the eastern Pacific; and the near-surface, east-to-west trade winds. During La Niña conditions, this circulation is strengthened—more clouds and rain over Indonesia, less over the cooler central and eastern Pacific, and stronger winds. During El Niño, the Walker circulation is weakened.

Three phases of the tropical Pacific atmospheric circulation pattern: El Niño, La Niña, and neutral. During El Niño, the engine driving tropical circulation shifts to the east, while during La Niña, it's stronger than average. Figure by climate.gov.

We don’t have official indexes for the atmospheric component of ENSO, because the atmosphere is highly variable, and patterns can change week to week, or even day to day, potentially obscuring ENSO’s seasonal signal. Also, while a lot of research has gone into creating long, climate-quality sea surface temperature records, that's trickier for atmospheric observations over the tropical Pacific Ocean, because satellite information is required to cover the entire area. 

However, we do closely follow some atmospheric metrics to understand the strength of ENSO conditions. The Equatorial Southern Oscillation Index compares the surface air pressure in the far western Pacific to that in the eastern Pacific; when the index is positive, it tells us that the Walker circulation is stronger than average. The cloudiness over the central tropical Pacific is another indicator of La Niña, since La Niña’s cooler-than-average water means less rising air, leading to fewer clouds.

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 2024–25, while the gray lines are every La Niña on record since 1979. Both measurements provide evidence that the Walker circulation is stronger than average, a La Niña atmospheric signature, and among the strongest on the record. Climate.gov graph, based on data from Michelle L’Heureux.

In February, the Equatorial Southern Oscillation Index decreased from January but remained quite strong, indicating that the tropical atmosphere is not yet ready to give up on this La Niña. Cloudiness in the central tropical Pacific was close to the lowest we’ve seen among all past La Niña events in the satellite era. Despite the variability mentioned above, shown in how much the lines on these graphs jump from month to month, it’s clear that the Walker circulation is strong.

Taken together, the ocean-atmosphere system in the tropical Pacific continued to represent La Niña conditions in February. The ocean component was weaker than the atmosphere, an unusual mismatch that has been present throughout the past several months. Also present for the past several months is an unusual pattern of overall ocean warmth.

February 2025 sea surface temperature compared to the 1985-1993 average (details on climatology from Coral Reef Watch). 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.

A possible way to make sense of this unusual situation is to isolate the conditions in the tropical Pacific from the tropics-wide ocean warmth using the Relative Niño-3.4 index. The Relative Niño-3.4 reveals that the cool temperature anomaly in the tropical Pacific is pretty strong relative to the rest of the tropical oceans (see footnote for details). That contrast might explain why the atmosphere has responded as strongly as it has to such a weak-seeming cool anomaly. (Michelle blogged about this previously.) Recent research has suggested the Relative Niño-3.4 index could better account for ENSO and its impacts than the traditional metric, and more research and exploration is ongoing.

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, La Niña conditions are stronger than by the traditional measure. 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. 

One more comment about the map above—the eastern Pacific is also notably warm.  The Peruvian committee in charge of monitoring ENSO, called “ENFEN,” closely monitors the sea surface temperature in this region. If warm anomalies persist, it is likely to be classified as a “coastal El Niño,” which can cause higher air temperatures and heavy rain in Peru and Ecuador.

The outlook

Our official metric, the Niño-3.4 index, has moved closer to average in recent weeks, and forecasters expect neutral oceanic conditions will develop in the next month. Even if the Walker circulation remains strong, both components must be present to qualify as La Niña conditions because it is the feedback loop between the atmosphere and the ocean that allows La Niña to persist long enough to influence the seasonal climate. There’s a 75% chance that the February–April average will be ENSO-neutral, and neutral is the most likely state through the summer.

Out of the three climate possibilities—La Niña, El Niño, and neutral—forecasts say that ENSO-neutral conditions are most likely for the February–April period (gray bar). Looking out to the Northern Hemisphere fall, neutral and La Niña (blue bar) are neck-and-neck, with the odds of El Niño (red bar) about 1-in-5. NOAA Climate Prediction Center image.  

Looking toward the fall, the chances of La Niña increase. The October–December average has a tie between neutral and La Niña, with a lower chance of El Niño, around 1-in-5. ENSO forecasts made in the spring are subject to the spring predictability barrier, meaning they have lower accuracy than those made during other times of the year. Tom and the climate.gov team put together some nice graphics that show the effect of this barrier in a blog post last month (based on the analysis from Azhar Ehsan).  As we move forward this year, we expect the signal to become clearer. Keep following us, and we’ll keep you in the loop.

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. You can also find monthly Niño-3.4 and seasonal (RONI) relative indices on the CPC webpage.