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September 2024 Quick Look

Published: September 12, 2024

A monthly summary of the status of El Niño, La Niña, and the Southern Oscillation, or ENSO, based on the NINO3.4 index (120-170W, 5S-5N)

As of mid-August 2024, ENSO-neutral conditions persist in the western equatorial Pacific, and oceanic and atmospheric indicators also align with an ENSO-neutral state. The IRI ENSO prediction plume forecasts ENSO-neutral conditions for Aug-Oct, and Sep-Nov, 2024. Borderline La Niña conditions are forecasted during Oct-Dec, and Nov-Jan, but with very weakly elevated probabilities. ENSO-neutral conditions subsequently re-emerge as the most likely during the boreal winter and spring of 2025.

Similar to the most recent official CPC ENSO Outlook (issued on August 08, 2024), the objective IRI model-based ENSO outlook forecasts indicate ENSO-neutral conditions for Aug-Oct 2024. However, there is a notable difference between the probability numbers in the early-month CPC and mid-month IRI ENSO forecasts. The CPC ENSO Outlook predicts a clear preference for La Niña onset in Sep-Nov (66% chances) with continuation until Jan-Mar, 2025 (57% chances), and then a return to ENSO-neutral state during boreal spring of 2025. In contrast, the objective IRI ENSO forecasts show late, weak and temporary La Niña conditions in Oct-Dec, and Nov-Jan. The forecasts then show a return to ENSO-neutral conditions that remain dominant during the boreal winter and spring of 2025.

Figures 1 and 3 (the official CPC ENSO probability forecast and the objective model-based IRI ENSO probability forecast, respectively) are often quite similar. However, occasionally they may differ noticeably. There can be several reasons for differences. One possible reason is that the human forecasters, using their experience and judgment, may disagree to some degree with the models, which may have known biases. Another reason is related to the fact that the models are not run at the same time that the forecasters make their assessment, so that the starting ENSO conditions may be slightly different between the two times. The charts on this Quick Look page are updated at two different times of the month, so that between the second and the third Thursday of the month, the official forecast (Fig. 1) has just been updated, while the model-based forecasts (Figs. 3 and 4) are still from the third Thursday of the previous month. On the other hand, from the third Thursday of the month until the second Thursday of the next month, the model-based forecasts are more recently updated, while the official forecasts remain from the second Thursday of the current month.
Click on the for more information on each figure.

Historically Speaking

    El Niño and La Niña events tend to develop during the period Apr-Jun and they
  • Tend to reach their maximum strength during October - February
  • Typically persist for 9-12 months, though occasionally persisting for up to 2 years
  • Typically recur every 2 to 7 years

ENSO Forecast

CPC ENSO Update

Published: September 12, 2024

El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued by the Climate Prediction Center/NCEP/NWS

ENSO Alert System Status: La Niña Watch

Synopsis: La Niña is favored to emerge in September-November (71% chance) and is expected to persist through January-March 2025.

ENSO-neutral continued during August 2024, with near-average sea surface temperatures (SSTs) observed across most of the equatorial Pacific Ocean (Fig. 1). The weekly Niño indices did not change substantially during the month, with the latest weekly index values varying between +0.2°C (Niño-4) and -0.4°C (Niño-1+2; Fig. 2). Below-average subsurface temperatures were also similar to those in early August (area-averaged index in Fig. 3). Negative temperature anomalies continued to dominate across most of the subsurface equatorial Pacific Ocean (Fig. 4). Low-level wind anomalies were easterly over most of the equatorial Pacific, and upper-level wind anomalies were easterly over the east-central Pacific.  Convection was slightly enhanced over parts of Indonesia and was near average near the Date Line (Fig. 5). Both the Southern Oscillation index and the equatorial Southern Oscillation indices were positive.  Collectively, the coupled ocean-atmosphere system reflected ENSO-neutral.

The IRI plume predicts a weak and a short duration La Niña, as indicated by the Niño-3.4 index values less than -0.5°C (Fig. 6). This month, the team relies more on the latest North American Multi-Model Ensemble (NMME) guidance, which predicts La Niña to emerge in the next couple of months and continue through the Northern Hemisphere winter.  The continuation of negative subsurface temperatures and enhanced low-level easterly wind anomalies supports the formation of a weak La Niña.  A weaker La Niña implies that it would be less likely to result in conventional winter impacts, though predictable signals could still influence the forecast guidance (e.g., CPC’s seasonal outlooks).  In summary, La Niña is favored to emerge in September-November (71% chance) and is expected to persist through January-March 2025 (Fig. 7).

This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site (El Niño/La Niña Current Conditions and Expert Discussions). Additional perspectives and analysis are also available in an ENSO blog. A probabilistic strength forecast is available here. The next ENSO Diagnostics Discussion is scheduled for 10 October 2024. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message to: ncep.list.enso-update@noaa.gov.

Season La Niña Neutral El Niño
ASO 38 62 0
SON 71 29 0
OND 81 19 0
NDJ 83 17 0
DJF 77 22 1
JFM 63 35 2
FMA 47 49 4
MAM 33 61 6
AMJ 22 66 12

IRI ENSO Forecast

IRI Technical ENSO Update

Published: August 19, 2024

Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv5. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv5 is used for retrospective official ENSO diagnosis because it is more homogeneous over time, allowing for more accurate comparisons among ENSO events that are years apart. These two products may differ, particularly during ENSO events. The difference between the two datasets may be as much as 0.5 °C. Additionally in some years, the ERSSTv5 may tend to be cooler than OISSTv2 in the context of warming trends, because ERSSTv5 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is updated every 10 years. In February 2021, both datasets were updated to reflect the 1991-2020 climatology period.

Recent and Current Conditions

The SST anomaly in the NINO3.4 region during the May-Jul 2024 season was +0.25 °C, and for the month of July 2024 it was +0.21 °C. The most recent weekly (week centered on 07 August 2024) of the NINO3.4 index was 0.0 °C. These values indicate that the tropical Pacific is in the ENSO-neutral state. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the monthly SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) exceed +0.5 °C. Similarly, for La Niña, the anomaly must be -0.5 °C or colder.

Oceanic and atmospheric conditions across the tropical Pacific are indicative of ENSO-neutral conditions. The Equatorial Southern Oscillation Index (defined by the standardized sea-level pressure difference between the eastern equatorial Pacific and Indonesia) was within the ENSO-neutral range for the month of July, 2024. The trade winds (at 850 hPa) are normal across the central-eastern equatorial tropical Pacific, while the upper-level winds (at 200 hPa) are also close to average over the equatorial Pacific. Colder waters have persisted at depths of 75–200 meters in the central Pacific sub-surface, with temperatures between 1 and 5 °C below the long-term average. Meanwhile, surface cooling in the far eastern Pacific showed a slight reduction. Together, these observed conditions in the coupled ocean-atmosphere system indicate an ongoing ENSO-neutral conditions in the equatorial Pacific.

Expected Conditions:
Note – Only models that produce a new ENSO prediction every month are considered in this statement.
What is the outlook for the ENSO status going forward?

The El Niño/Southern Oscillation (ENSO) Diagnostic Discussion released on 08 August 2024 by the Climate Prediction Center/NCEP/NWS issued a continuation of ENSO-neutral conditions in the tropical Pacific for the next several months, along with a La Niña watch with 66% likelihood for La Niña onset in Sep-Nov, 2024.

 The latest set of ENSO prediction models from mid-August 2024 is now available in the IRI ENSO prediction plume. These are used to assess the probabilities of the three ENSO categories by using the average value of the NINO3.4 SST anomaly predictions from all models in the plume, equally weighted. A standard Gaussian error is imposed over that averaged forecast, with its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill results in a relatively narrower error distribution, while low skill results in an error distribution with width approaching that of the historical observed distribution.

According to the ENSO forecast issued by the IRI, ENSO-neutral conditions are forecasted to continue during Aug-Oct (70% chances), and Sep-Nov (55% chances), while chances for La Niña are 28%, and 41% respectively. In Oct-Dec 2024 (48%), and Nov-Jan 2025 (49%) La Niña onset is slightly favored as compared to the continuation of ENSO-neutral conditions, which is estimated at 47% and 45% respectively. ENSO-neutral then remains the most-likely category for the rest of the forecasts period with probabilities of 47% in Dec-Feb, 52% in Jan-Mar, 62% in Feb-Apr, 72% in Mar-May, and 71% in Apr-Jun, 2025. During the winter and spring of 2025, the La Niña is estimated within the range of 15% to 46%. The probability of El Niño remains very low throughout the forecast period, (less than 10%).  A plot of the probabilities summarizes the forecast evolution. The climatological probabilities for La Niña, ENSO-neutral and El Niño conditions vary seasonally, are shown by the lines on the plot, and are given in a table at the bottom of this page for each 3-month season.

Caution is advised in interpreting the forecast distribution from the Gaussian standard error as the actual probabilities, due to differing biases and performance of the different models. In particular, this approach considers only the mean of the predictions, and not the total range across the models, nor the ensemble range within individual models. At longer leads, the skill of the models degrades, and uncertainty in skill must be convolved with the uncertainties from initial conditions and differing model physics, which leads to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution.

 In summary, the forecast indicates continued ENSO-neutral conditions until Sep-Nov, 2024. The forecast then shows borderline La Niña conditions emerging during Oct-Dec, and Nov-Jan with probabilities of 48% and 49% respectively, while the continuation of ENSO-neutral state is estimated to be 47% and 45% respectively. ENSO-neutral then becomes again the most likely category for the rest of the forecasts period with of probabilities 72-47%, while the probability for La Niña becomes quite weak (15%-46%).

A caution regarding the model-based ENSO plume predictions (released mid-month) is that factors such as known specific model biases and recent changes in the tropical Pacific the models may have missed, are not considered. This approach is purely objective. Those issues are taken into account in CPC’s official outlooks, which are issued early in the month, and which will include some human judgment in combination with the model guidance.

IRI ENSO Forecast Histogram Image
Season La Niña Neutral El Niño
ASO 28 70 2
SON 41 55 4
OND 48 47 5
NDJ 49 45 6
DJF 46 47 7
JFM 41 52 7
FMA 32 62 6
MAM 18 72 10
AMJ 15 71 14

ENSO Forecast

IRI Model-Based Probabilistic ENSO Forecast

Published: August 19, 2024

A purely objective ENSO probability forecast, based on regression, using as input the model predictions from the plume of dynamical and statistical forecasts shown in the ENSO Predictions Plume. Each of the forecasts is weighted equally. It is updated near or just after the middle of the month, using forecasts from the plume models that are run in the first half of the month. It does not use any human interpretation or judgment. This is updated on the third Thursday of the month.

IRI ENSO Forecast Histogram Image

Season La Niña Neutral El Niño
ASO 28 70 2
SON 41 55 4
OND 48 47 5
NDJ 49 45 6
DJF 46 47 7
JFM 41 52 7
FMA 32 62 6
MAM 18 72 10
AMJ 15 71 14

ENSO Forecast

CPC Official Probabilistic ENSO Forecast

Published: September 12, 2024

The official CPC ENSO probability forecast, based on a consensus of CPC and IRI forecasters. It is updated during the first half of the month, in association with the official CPC ENSO Diagnostic Discussion. It is based on observational and predictive information from early in the month and from the previous month. It uses human judgment in addition to model output, while the forecast shown in the Model-Based Probabilistic ENSO Forecast relies solely on model output. This is updated on the second Thursday of every month.

NOAA?CPC ENSO Forecast Image
NOAA/CPC ENSO Forecast Graphic, courtesy of NOAA/CPC
Season La Niña Neutral El Niño
ASO 38 62 0
SON 71 29 0
OND 81 19 0
NDJ 83 17 0
DJF 77 22 1
JFM 63 35 2
FMA 47 49 4
MAM 33 61 6
AMJ 22 66 12

ENSO Forecast

IRI ENSO Predictions Plume

Published: August 19, 2024

Note on interpreting model forecasts

The following graph and table show forecasts made by dynamical and statistical models for SST in the Nino 3.4 region for nine overlapping 3-month periods. Note that the expected skills of the models, based on historical performance, are not equal to one another. The skills also generally decrease as the lead time increases. Thirdly, forecasts made at some times of the year generally have higher skill than forecasts made at other times of the year--namely, they are better when made between June and December than when they are made between February and May. Differences among the forecasts of the models reflect both differences in model design, and actual uncertainty in the forecast of the possible future SST scenario.

Interactive Chart

You can highlight a specific model by hovering over it either on the chart or the legend. Selecting An item on the legend will toggle the visibility of the model on the page. You can also select DYN MODELS or STAT MODELS to toggle them all at once. Clicking on the "burger" menu above the legend will give you options to download the image or expand to full screen. If you have any feedback on this new feature, please let us know at webmaster@iri.columbia.edu.

List of Models Used

Forecast SST Anomalies (deg C) in the Nino 3.4 Region

Seasons (2024 – 2025)
Model ASO SON OND NDJ DJF JFM FMA MAM AMJ
Dynamical Models
AUS-ACCESS -0.20 -0.23 -0.17 -0.07
BCC_CSM11m -0.73 -0.67 -0.51 -0.33 -0.14 0.08 0.31 0.53 0.77
CMC CANSIP -0.35 -0.54 -0.63 -0.61 -0.54 -0.46 -0.40 -0.36 -0.26
COLA CCSM4 -0.52 -0.72 -0.87 -0.92 -0.88 -0.70 -0.39 -0.12 0.02
CS-IRI-MM -0.24 -0.33 -0.44 -0.52 -0.45 -0.26
DWD -0.15 -0.25 -0.30 -0.32
ECMWF -0.16 -0.21 -0.24 -0.29 -0.28
GFDL SPEAR -0.26 -0.41 -0.56 -0.60 -0.48 -0.24 -0.01 0.15 0.27
IOCAS ICM 0.05 -0.16 -0.48 -0.72 -0.85 -0.90 -0.86 -0.78 -0.76
JMA -0.30 -0.47 -0.54 -0.56 -0.47
KMA -0.70 -0.90 -1.00 -1.00
LDEO 0.08 0.05 -0.02 -0.01 0.08 0.24 0.34 0.42 0.52
MetFRANCE -0.14 -0.21 -0.20 -0.14 -0.04
NASA GMAO -0.92 -1.29 -1.41 -1.44 -1.42 -1.29 -1.07
NCEP CFSv2 -0.64 -1.05 -1.27 -1.39 -1.26 -0.99 -0.63
UKMO -0.59 -0.82 -0.91 -0.94
Average, Dynamical models -0.361 -0.513 -0.597 -0.616 -0.561 -0.502 -0.338 -0.027 0.093
Statistical Models
BCC_RZDM -0.06 -0.15 -0.30 -0.36 -0.36 -0.31 -0.27 -0.33 -0.40
CPC CA -0.50 -0.54 -0.52 -0.56 -0.64 -0.61 -0.43 -0.21 -0.07
CPC MRKOV -0.38 -0.38 -0.34 -0.24 -0.14 -0.08 -0.06 -0.03 0.01
CSU CLIPR -0.04 -0.06 -0.08 -0.10 -0.14 -0.19 -0.23 -0.16 -0.09
IAP-NN 0.12 0.12 0.14 0.17 0.23 0.31 0.40 0.48 0.52
NTU CODA -0.01 -0.05 -0.11 -0.18 -0.16 -0.16 -0.11 -0.12 -0.25
TONGJI-ML -0.26 -0.42 -0.55 -0.62 -0.52 -0.32 -0.17
UCLA-TCD 0.05 -0.10 -0.20 -0.24 -0.23 -0.17 -0.08 0.04 0.18
UW PSL-CSLIM -0.04 -0.12 -0.23 -0.37 -0.50 -0.55 -0.54 -0.46 -0.37
UW PSL-LIM -0.21 -0.34 -0.42 -0.44 -0.43 -0.43 -0.45 -0.45 -0.42
Average, Statistical models -0.133 -0.204 -0.260 -0.294 -0.289 -0.250 -0.194 -0.138 -0.099
Average, All models -0.273 -0.394 -0.467 -0.492 -0.437 -0.369 -0.258 -0.094 -0.022

Discussion of Current Forecasts

The IRI-Plume indicates ENSO-neutral conditions during Aug-Oct (70% chances) and Sep-Nov (55% chances) 2024. The multimodel mean of statistical models show ENSO-neutral conditions during the entire forecasts period, however, the multimodel mean of dynamical models show borderline La Niña conditions during boreal winter of 2025, and then a return to ENSO-neutral conditions during boreal spring of 2025. Based on the multi-model mean prediction, and the expected skill of the models by start time and lead time, the probabilities (in %) for La Niña, ENSO-neutral and El Niño conditions (using -0.5 °C and 0.5 °C thresholds) over the coming 9 seasons are:

Season La Niña Neutral El Niño
ASO 28 70 2
SON 41 55 4
OND 48 47 5
NDJ 49 45 6
DJF 46 47 7
JFM 41 52 7
FMA 32 62 6
MAM 18 72 10
AMJ 15 71 14

Summary of forecasts issued over last 22 months

The following interactive plot shows the model forecasts issued not only from the current month (as in the plot above), but also from the 21 months previous to this month. The observations are shown up to the most recently completed 3-month period. The plots allow comparison of plumes from the previous start times, or examination of the forecast behavior of a given model over time.
Hovering over any single model will highlight that particular model in the chart.
Clicking a particular model will hide/show that model in the chart.
At the bottom of the plot, you can select which models to show in the chart: all the models, the dynamical models only, or the statistical models only.

Notes on the data 

Only models producing forecasts on a monthly basis are included. This means that some models whose forecasts appear in the Experimental Long-Lead Forecast Bulletin (produced by COLA) do not appear in the table.

Once an IRI ENSO probability forecast has been published, the results stand even if a model reports an error and changes their data. When this happens we will update the plume with the model's correct values even though our forecast hasn't changed. What this means is that our forecast is always the same, but the underlying data may be different from what we based our forecast on.

The SST anomaly forecasts are for the 3-month periods shown, and are for the Nino 3.4 region (120-170W, 5N-5S). Often, the anomalies are provided directly in a graph or a table by the respective forecasting centers for the Nino 3.4 region. In some cases, however, they are given for 1-month periods, for 3-month periods that skip some of the periods in the above table, and/or only for a region (or regions) other than Nino 3.4. In these cases, the following means are used to obtain the needed anomalies for the table:

  • Temporal averaging
  • Linear temporal interpolation
  • Visual averaging of values on a contoured map

The anomalies shown are those with respect to the base period used to define the normals, which vary among the groups producing model forecasts. They have not been adjusted to anomalies with respect to a common base period. Discrepancies among the climatological SST resulting from differing base periods may be as high as a quarter of a degree C in the worst cases. Forecasters are encouraged to use the standard 1991-2020 period as the base period, or a period not very different from it.

Historical SST Anomalies Image

ENSO Forecast

Forecast Probability Distribution Based on the IRI ENSO Prediction Plume

Published: August 19, 2024

The plots on this page show predictions of seasonal (3-month average) sea surface temperature (SST) anomaly in the Niño3.4 region in the east-central tropical Pacific (5°N-5°S, 120°-170°W), covering the nine overlapping seasons beginning with the current month. The predictions are based on the large (20+) set of dynamical and statistical models in the plume of model ENSO predictions.

  • Model Based Prediction Percentiles Image

    Figure 5

    Predictions of ENSO are probabilistic. The ensemble mean prediction is only a best single guess. On either side of that prediction, there is a substantial uncertainty distribution, or error tolerance. The second plot (Figure 2) shows the estimated probability distribution of the predictions, showing a set of percentiles within that distribution for each lead time. The distribution is modeled as a normal (Gaussian) distribution, so that the overall mean forecast represents the center, or 50 percentile, in the distribution. The overall mean is formed using equal weighting among all models. On either side, other percentile values are shown symmetrically, ranging from 1 to 99 and including some intermediate percentiles (5 and 95, 15 and 85, and 25 and 75). The plot enables a user to estimate the probability of the Niño3.4 SST anomaly to be greater or less than some critical value, or within some interval. If, for example, the 85 percentile falls at 1.8° C above average, the probability of the SST exceeding 1.8° C can be estimated at 15%. Probabilities for exceeding or not exceeding values not exactly on percentile line can be roughly interpolated by eye. The overall width of the probability distribution is derived from the historical skill of the hindcasts of the models, from 1982 to present, for the specific forecast start time and lead time. This method of defining the probability distribution represents one of two general approaches, the other approach being a direct counting of ensemble members within each of the percentile bands. This second approach assumes that the ensemble spreads of the models are true representations of the uncertainty. Individual model spreads have often been found to be somwehate narrower than they should be, although in multi-model ensembles this tendency has been shown to be milder or even eliminated.

  • Model Based Prediction Distribution Image

    Figure 6

    Figure 6, sometimes called a spaghetti diagram, shows synthetically generated prediction scenarios that are equally likely. Here, 100 scenarios are shown; any number can be generated for such a diagram. Each scenario is produced using a random number generator, combined with knowledge of the mean forecast and its uncertainty, as well as the amount of persistence of anomalies. The degree of persistence of anomalies is based on the correlation of prediction errors from one lead time to another. In other words, the individual lines are designed to show the correct amount of persistence as expected in nature, rather than jumping around more randomly from one lead time to the next. The uncertainty and persistence statistics are based on the set of 7 NMME (North American Multimodel Ensemble) models, as it is assumed that these statistics are approximately applicable to all of the models. Sometimes the “spaghetti density” may appear asymmetric about the mean of all the forecasts or outside of the 85 and 15 percentile lines. This is purely sampling variability, and would not occur if many thousands of such lines were plotted. But with that many lines, most of the plot would be too crowded to get a sense of the behavior of the lines near the center of the distribution. The main purpose of the diagram is to serve users who want to assess realistic individual scenarios of ENSO behavior rather than statistical summaries of the forecast like the percentiles shown in the second plot.

The CPC ENSO forecast is released at 9am (Eastern Time) on the second Thursday of each month.

The IRI ENSO forecast is released on the 19th of each month. If the 19th falls on a weekend or holiday, it is released on the closest business day.

All data from this website is covered under the Creative Commons Attribution 4.0 License. When citing IRI ENSO images or data, please use "Images [or Data] provided by The International Research Institute for Climate and Society, Columbia University Climate School", with a link to https://iri.columbia.edu/ENSO.