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IRI ENSO Forecast

April 2021 Quick Look

Published: April 8, 2021

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)

Use the navigation menu on the right to navigate to the different forecast sections

In mid-March, SSTs in the east-central Pacific are roughly 0.3 degree C below average, and the evolution of most key atmospheric variables are consistent with weakening La Niña conditions. A large majority of the model forecasts predict SSTs to return to near-normal during spring, though a La Niña advisory remains in effect for now. The new official CPC/IRI outlook issued earlier this month is similar to these model forecasts, calling for a likely transition in Apr-May-Jun. A La Niña advisory remains in effect.

Figures 1 and 3 (the official ENSO probability forecast and the objective model-based 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

IRI ENSO Forecast

CPC/IRI ENSO Update

Published: April 8, 2021

El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued jointly by the Climate Prediction Center/NCEP/NWS and the International Research Institute for Climate and Society

ENSO Alert System Status: La Niña Advisory

Synopsis: A transition from La Niña to ENSO-Neutral is likely in the next month or so, with an 80% chance of ENSO-neutral during May-July 2021. 

La Niña continued during March, reflected by negative sea surface temperatures (SST) anomalies, which extended across much of the equatorial Pacific Ocean (Fig. 1). SST anomalies weakened but continue to oscillate week-to-week in most of the Niño index regions, particularly in the eastern Pacific Ocean (Fig. 2). With the exception of Niño-1+2, the latest weekly Niño index values were at or near -0.5ºC.  Sub-surface ocean temperatures also weakened during the month, with the integrated anomalies averaged between the 180-100°W becoming positive during the middle of the month (Fig. 3). Currently, negative subsurface anomalies are present from the surface to approximately ~100m below the surface only in the eastern Pacific between 110°W and 80°W (Fig. 4). Low-level easterly wind anomalies are present but weak across the equatorial Pacific, and are most notable in the far western Pacific. Upper-level wind anomalies were westerly across the most of the tropical Pacific.  The suppression of tropical convection over the western and central Pacific persisted during March, although the enhancement of rainfall around the Philippines and Indonesia weakened (Fig. 5). The Southern Oscillation and Equatorial Southern Oscillation were weakly positive in March.  Overall, the coupled ocean-atmosphere system is consistent with a weakening La Niña.

Most of the models in the IRI/CPC plume predict a transition to ENSO-neutral during the Northern Hemisphere spring 2021 (Fig. 6). The forecaster consensus agrees that a transition is imminent, with a 50-50% chance of La Niña or ENSO-neutral for the March-May average, and then predicts ENSO-neutral to continue at least through the Northern Hemisphere summer.  In part, due to the uncertainty in predictions made at this time of year, the forecast for the Northern Hemisphere Fall 2021 remains lower confidence with a 40-50% chance of either La Niña or ENSO-Neutral, with a small chance for El Niño.  In summary, a transition from La Niña to ENSO-Neutral is likely in the next month or so, with ENSO-neutral favored with a ~80% chance during May-July 2021 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).

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 13 May 2021. 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.



CPC/IRI Official Probabilistic ENSO Forecasts

Season La Niña Neutral El Niño
MAM 2021 50% 50% 0%
AMJ 2021 21% 79% 0%
MJJ 2021 15% 81% 4%
JJA 2021 20% 68% 12%
JAS 2021 30% 57% 13%
ASO 2021 37% 50% 13%
SON 2021 41% 46% 13%
OND 2021 46% 41% 13%
NDJ 2022 47% 40% 13%

Please refer to our licensing agreement for permission to use IRI ENSO materials. The CPC/IRI materials are not included in this licensing.

IRI ENSO Forecast

IRI Technical ENSO Update

Published: March 19, 2021

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. During ENSO events, OISSTv2 often shows stronger anomalies than ERSSTv5, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv5 may tend to be cooler than OISSTv2, 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 and so, half of the time, is based on a slightly older period and does not account as much for the slow warming trend in the tropical Pacific SST. In February 2021, both datasets were updated using the 1991-2020 climatology period.

Recent and Current Conditions

In mid-March 2021, SSTs are just slightly below average in the NINO3.4 region, although the tropical Pacific has exhibited La Niña conditions since August 2020. The SST anomaly for NINO3.4 during the Dec-Feb season was -1.01 C, and for the month of February it was -0.93 C, suggesting a continued, but weakening La Niña conditions that peaked in late October/early November 2020. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the 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 less. The climatological probabilities for La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly anomaly in the NINO3.4 region was -0.3 C, which is in the ENSO-neutral range; however, there exists discernible week-to-week variability in the strength of the SST anomalies, and again the 3-month average NINO3.4 index has not yet become ENSO-neutral. Many of the key atmospheric variables, indicative of La Niña conditions, have also weakened discernibly over the past month, such as the traditional and equatorial Southern Oscillation Indices, which have dropped considerably but remain positive. The low-level easterly wind anomalies are weakened further in February and are visible now mainly in the western Pacific; upper level wind anomalies remain westerly across much of the tropical Pacific but are less coherent. Anomalously dry conditions that were observed over the west-central part of the basin, continue to weaken and break up, as have the wetter than normal anomalies over the Maritime Continent. For the ocean, subsurface temperature anomalies in the central and eastern equatorial Pacific are quite reduced in mid-March. Warm sub-surface temperature anomalies (deepened thermocline anomalies) have been building in the western Pacific, and they are now moving eastward along the equator. In summary, current monthly conditions still indicate a borderline moderate La Niña that is weakening. A La Niña advisory remains in effect.

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 most recent official diagnosis and outlook was issued approximately one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it states that La Niña conditions are present and are most likely to continue through boreal winter, with a 98% chance for Feb-Apr, falling to 59% for Mar-May. A transition to neutral conditions during the Apr-Jun season is suggested with a 62% probability.

The latest set of model ENSO predictions from mid-March is now available in the IRI/CPC ENSO prediction plume. These are used to assess the probabilities of the three possible ENSO conditions by using the average value of the NINO3.4 SST anomaly predictions from all models on the plume, equally weighted. A standard Gaussian error is imposed over that average forecast, and its width is 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. Using this method, chances for La Niña are 54% for the Mar-May season, while chances for ENSO-neutral are 46%. ENSO-neutral conditions become most likely by Apr-Jun, starting at 66%. Later in the year, the collection of models suggests that the cool side of ENSO-neutral is most likely. However, it is typically difficult to accurately foresee ENSO conditions for the latter part of the year through the boreal spring prediction barrier. El Niño probabilities are less than 10% until Jun-Aug, then rise to around 20% during boreal fall. A plot of the probabilities generated from this most recent IRI/CPC ENSO prediction plume using the multi-model mean and the Gaussian standard error method summarizes the model consensus out to about 10 months into the future.

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 probabilities derived from the models on the IRI/CPC plume describe a very slim chance for El Niño conditions throughout most of the forecast period, and a preference for La Niña conditions relative to neutral conditions during the current season, Mar-May. By the Apr-Jun season, neutral becomes the more likely outcome through the remaining forecast periods, through the forecasts favor the cool side of the ENSO-neutral category. This scenario has been stable over the last couple months of forecasts.

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 that the models may have missed, are not considered. This approach is purely objective. Those issues are taken into account in the official outlooks, which are generated and issued early in the month by CPC and IRI, and which will include some human judgment in combination with the model guidance.

Please refer to our licensing agreement for permission to use IRI ENSO materials. The CPC/IRI materials are not included in this licensing.

IRI ENSO Forecast

IRI/CPC Model-Based Probabilistic ENSO Forecast

Published: March 19, 2021

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/CPC Mid-Month Model-Based ENSO Forecast Probabilities

Season La Niña Neutral El Niño
MAM 2021 54% 46% 0%
AMJ 2021 34% 66% 0%
MJJ 2021 27% 69% 4%
JJA 2021 27% 62% 11%
JAS 2021 30% 56% 14%
ASO 2021 36% 49% 15%
SON 2021 40% 44% 16%
OND 2021 35% 42% 23%
NDJ 2021 36% 40% 24%

IRI ENSO Forecast

CPC/IRI Official Probabilistic ENSO Forecast

Published: April 8, 2021

The official CPC/IRI 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/IRI 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.



CPC/IRI Official Probabilistic ENSO Forecasts

Season La Niña Neutral El Niño
MAM 2021 50% 50% 0%
AMJ 2021 21% 79% 0%
MJJ 2021 15% 81% 4%
JJA 2021 20% 68% 12%
JAS 2021 30% 57% 13%
ASO 2021 37% 50% 13%
SON 2021 41% 46% 13%
OND 2021 46% 41% 13%
NDJ 2022 47% 40% 13%

Please refer to our licensing agreement for permission to use IRI ENSO materials. The CPC/IRI materials are not included in this licensing.

IRI ENSO Forecast

IRI/CPC ENSO Predictions Plume

Published: March 19, 2021

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.


Because of occasional data corrections and late model runs following the time of ENSO product issuance, the data shown in the ENSO forecast table and the ENSO plume graph may not always match. The best source of the ENSO forecast data is http://iri.columbia.edu/~forecast/ensofcst/Data/ensofcst_ALLtoMMYY where MM is the month number and YY is the year.


Seasons (2021 – 2021)
Model MAM AMJ MJJ JJA JAS ASO SON OND NDJ
Dynamical Models
NASA GMAO -1.58 -2.10 -2.35 -2.34 -2.27 -2.28 -2.34
NCEP CFSv2 -0.53 -0.44 -0.38 -0.35 -0.38 -0.45 -0.48
JMA -0.48 -0.26 -0.07 0.07 0.11
BCC_CSM11m -0.16 0.36 0.87 1.21 1.31 1.27 1.21 1.21 1.27
SAUDI-KAU -0.41 -0.15 0.05 0.14 0.18 0.21 0.25 0.33 0.46
LDEO -0.12 0.08 0.26 0.41 0.46 0.45 0.51 0.56 0.42
AUS-ACCESS -0.37 -0.17 -0.07 -0.03
ECMWF -0.43 -0.20 -0.01 0.09 0.10
UKMO -0.35 -0.10 0.07 0.15
KMA SNU -0.50 -0.31 -0.09 0.15 0.37 0.58 0.70 0.78 0.80
IOCAS ICM -0.22 -0.20 -0.13 0.00 0.09 0.10 0.03 -0.04 -0.09
COLA CCSM4 -0.42 -0.40 -0.52 -0.81 -1.15 -1.45 -1.63 -1.75 -1.73
MetFRANCE -0.88 -0.87 -0.70 -0.41 -0.28 -0.28 -0.40
CS-IRI-MM -0.43 -0.14 0.04 0.11 0.03 -0.10
GFDL SPEAR -0.37 -0.09 0.12 0.18 0.08 -0.11 -0.31 -0.49 -0.56
CMC CANSIP -0.61 -0.49 -0.36 -0.29 -0.30 -0.40 -0.53 -0.63 -0.66
Average, Dynamical models -0.49 -0.34 -0.20 -0.11 -0.12 -0.20 -0.27 -0.00 -0.01
Statistical Models
NTU CODA -0.65 -0.70 -0.86 -0.79 -0.80 -0.90 -1.10 -1.17 -1.39
BCC_RZDM -0.53 -0.42 -0.37 -0.35 -0.38 -0.45 -0.50 -0.57 -0.67
CPC MRKOV -0.99 -0.86 -0.76 -0.69 -0.65 -0.60 -0.56 -0.47 -0.32
CPC CA -0.35 -0.11 0.08 0.15 0.08 0.05 0.07 0.19 0.27
CSU CLIPR -0.50 -0.40 -0.29 -0.19 -0.24 -0.29 -0.34 -0.35 -0.37
IAP-NN -0.69 -0.52 -0.34 -0.20 -0.07 0.03 0.12 0.17 0.20
FSU REGR -0.53 -0.33 -0.20 -0.14 -0.13 -0.09 -0.04 0.02 0.09
UCLA-TCD -0.38 -0.19 -0.08 -0.03 -0.05 -0.12 -0.19 -0.24 -0.26
Average, Statistical models -0.58 -0.44 -0.35 -0.28 -0.28 -0.30 -0.32 -0.30 -0.31
Average, All models -0.52 -0.38 -0.25 -0.17 -0.18 -0.24 -0.29 -0.15 -0.16

Discussion of Current Forecasts

Most of the models in the set of dynamical and statistical model predictions issued during mid-January 2021 show La Niña SST conditions likely to persist until the Mar-May season, with most transitioning to neutral during boreal spring.  In the most recent week, the SST anomaly in the NINO3.4 region was -1.2 C, indicative of moderate La Niña strength, and -1.05 C for the month of January. As of mid-February the subsurface water temperatures in the eastern equatorial Pacific remain below-average, having been reinforced by the action of the easterly wind anomalies, as warm anomalies shift into the far western equatorial Pacific.

A majority of the dynamical and statistical models predict at least weak La Niña conditions for the Feb-Apr season, decreasing to about 55% by Mar-May and below 50% thereafter. Objective model-based La Niña probabilities are 79% for Feb-Apr, dropping to about 40% by Apr-Jun. Thereafter, neutral conditions become the most likely at 62% confidence in Apr-Jun, and then decaying towards climatological odds, according to this suite of models. Based on the multi-model mean prediction, and the expected skill of the models by start time and lead time, the probabilities (X100) for La Niña, neutral and El Niño conditions (using -0.5C and 0.5C thresholds) over the coming 9 seasons are:

IRI/CPC Mid-Month Model-Based ENSO Forecast Probabilities

Season La Niña Neutral El Niño
MAM 2021 54% 46% 0%
AMJ 2021 34% 66% 0%
MJJ 2021 27% 69% 4%
JJA 2021 27% 62% 11%
JAS 2021 30% 56% 14%
ASO 2021 36% 49% 15%
SON 2021 40% 44% 16%
OND 2021 35% 42% 23%
NDJ 2021 36% 40% 24%

Summary of forecasts issued over last 22 months

The following plots show 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 also 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. The first plot shows forecasts for dynamical models, the second for statistical models, and the third for all models. For less difficult readability, forecasts are shown to a maximum of only the first five lead times. Below the third plot, we provide a mechanism for highlighting the forecasts of one model at a time against a background of more lightly colored lines for all other models.


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.

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 1971-2000 period as the base period, or a period not very different from it.

Please refer to our licensing agreement for permission to use IRI ENSO materials. The CPC/IRI materials are not included in this licensing.

IRI ENSO Forecast

Forecast Probability Distribution Based on the IRI/CPC ENSO Prediction Plume

Models not available yet for Apr, 2021