April 2025 ENSO update: La Niña has ended

After just a few months of La Niña conditions, the tropical Pacific is now ENSO-neutral, and forecasters expect neutral to continue through the Northern Hemisphere summer. Neutral is also the most likely state through the fall (greater than 50% chance).

What is ENSO? What does neutral even mean?

ENSO stands for “El Niño/Southern Oscillation,” a pattern of changes in the temperature of the ocean surface and atmospheric circulation of the tropical Pacific region. La Niña means the surface water is cooler than average, the trade winds are stronger, and the central equatorial Pacific receives less rain. On the other hand, El Niño is represented by warmer surface water, weaker trade winds, and more rain in the central and sometimes eastern Pacific.

Both phases of ENSO change global weather and climate patterns, including temperature and rain/snow patterns, hurricanes, tornadoes, and many other impacts. Since El Niño and La Niña can be predicted months in advance, their known changes to global climate allow us an early picture of upcoming seasonal patterns. “ENSO-neutral” means that neither El Niño nor La Niña are in effect, and global seasonal conditions are less predictable (for more on what neutral means, check out the second half of this older blog post).

A very brief La Niña

How do we know that La Niña has ended? Our primary metric for ENSO is the surface temperature of the ocean in the Niño-3.4 region of the east-central Pacific. The surface temperature in this region is compared to the long-term average, providing us with an index that measures how much the water is warmer or cooler than average. Long-term is currently 1991–2020 (more on that later!). The La Niña threshold is a Niño-3.4 index of at least -0.5 °C (-0.9 °F). With the March 2025 Niño-3.4 index measuring -0.01 °C (i.e., nearly identical to the 1991-2020 average) according to our most reliable sea surface temperature dataset (ERSSTv5), we can say with confidence that La Niña conditions have ended.

2-year history of sea surface temperatures in the Niño-3.4 region of the tropical Pacific for all La Nina events since 1950 (gray lines) and the recent (2024-25) event (purple line). After dipping into La Niña for a few months, the Niño-3.4 index returned to near average in March 2025. Graph by Emily Becker, based on monthly Niño-3.4 index data from CPC using ERSSTv5.

Last month, forecasters expected neutral conditions would develop very soon, and indeed they did. The combination of below-average temperatures weakening in the central Pacific and the westward expansion of very warm water in the far eastern Pacific helped to dissipate the cooler surface of La Niña.

This animation shows weekly sea surface temperatures in the Pacific Ocean compared to average for late January through early April 2025. Orange and red areas were warmer than average; blue areas were cooler than average. La Niña's cooler-than-average sea surface temperatures in the key ENSO-monitoring region of the tropical Pacific (outlined with black box) have dissipated in recent weeks. NOAA Climate.gov animation, based on Coral Reef Watch Data and maps from NOAA View. 

Over the past few months, there has been a substantial amount of cooler-than-average water under the surface of the equatorial Pacific, but this has also waned in recent weeks. The subsurface provides a source of fuel to the surface, so the marked decrease in cooler subsurface water is another indication that La Niña has ended.

ENSO is a coupled ocean-atmosphere system, meaning characteristic changes are required in both the ocean and the overlying atmospheric circulation to qualify as La Niña or El Niño. During La Niña, the Walker circulation—the overturning loop in the atmosphere over the tropical Pacific—gets amped up, signified by the stronger trade winds, more storms over Indonesia, and a less rainy central Pacific region. These atmospheric signs of La Niña were still around in March, but without the ocean surface temperature component, the overall system qualifies as ENSO-neutral.

A matter of time

As mentioned above, our definition of “long-term” is currently the 1991–2020 average. The use of a 30-year average for seasonal climate prediction is the World Meteorological Organization standard. For most prediction purposes, the average period is updated every 10 years, but for ENSO prediction, we update it every 5 years, to try to account for changes in the tropical Pacific Ocean.

Thirty-year average periods that NOAA is using to calculate the strength of historic El Niño and La Niña events. Climate.gov figure from ERSSTv5 data, based on CPC original.

In addition, the historical ENSO table uses a centered averaging period, e.g., the individual years in 1986­–1990 are compared to the 30-year average 1971–2000, and the years in 1991–1995 are compared to 1976–2005. Check out CPC’s webpage on the “rolling” climatology approach.

The reason for mentioning this detail is that this past winter does not qualify as a La Niña event in our historical table, because it did not last 5 consecutive rolling seasons (we call any 3-month-average a season) exceeding the -0.5 °C threshold. However, in 2036, this past winter will be near the middle of a 30-year period (2006–2035), which is very likely to be warmer than the 1991–2020 average. Thus, with the benefit of another decade, this year’s small La Niña may be cooler when compared to that warmer averaging period. So, in 2036, the 2024–25 winter may end up colored blue, and be considered a formal La Niña, in the historical table.

This complexity is one of the reasons we have been mentioning relative sea surface temperature anomalies, which are the sea surface temperature differences from average in a particular location, like the Niño-3.4 region, relative to the entire tropical average sea surface temperature. By using relative sea surface temperature measures, we can skip calculating all of these rolling climatologies.  For the relative index, we only need to use a single climatology over the entire historical record.  As we have mentioned in recent blog posts (here and here and here), the relative Niño-3.4 index has also been better than the more traditional indexes at describing the intensity of the expected atmospheric response to ENSO during this past La Niña. 

The outlook for the rest of 2025

ENSO-neutral is likely through the summer. Chances for El Niño or La Niña increase later in the year, with La Niña chances about double those of El Niño, but neutral is still the highest probability through the early winter.

Out of the three climate possibilities—La Niña, El Niño, and neutral—forecasts say that ENSO-neutral conditions (gray bars) are most likely for the Northern Hemisphere spring and summer. Looking out to the fall, neutral is still the most likely, but chances for either La Niña (blue bars) or El Niño (red bars) are increasing. NOAA Climate Prediction Center image.  

This outlook is largely based on computer climate model forecasts. The consensus of the North American Multi-Model Ensemble, a collection of climate models, also predicts neutral, but there is a substantial range of potential outcomes for later in the year.

Line graph showing observed and predicted temperatures (black line) in the key ENSO-monitoring region of the tropical Pacific from spring 2025 through the fall. The gray shading shows the range of temperatures predicted by individual models that are part of the North American Multi Model Ensemble (NMME, for short). By late summer, the shading spans from El Niño to La Niña, showing the range of potential ENSO states. However, the majority of model forecasts are concentrated in the neutral range, meaning the highest chance is that the temperature in the Niño-3.4 region of the tropical Pacific will be near average. NOAA Climate.gov image, based on data provided by Climate Prediction Center. 

Forecasts made in the spring are known to be less successful than forecasts made in the rest of the year, an effect called the “spring predictability barrier.” We don’t have a clear understanding of why forecasts are worse this time of year, but one potential culprit is that ENSO tends to be changing phase (e.g., going from La Niña to neutral). For lots more detail on the spring predictability barrier, check out this post and this one on Seasoned Chaos on the topic.

As spring turns to summer, our crystal ball should become clearer. For now, we’ll bid La Niña adieu and bide our time in neutral.