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2016 April Quick Look

Published: April 21, 2016

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)

During mid-April 2016 the positive tropical Pacific SST anomaly was weakening, now indicating only a moderate strength El Niño. All atmospheric variables continue to support the El Niño pattern, but at reduced strength. This includes weakened trade winds and excess rainfall in the east-central tropical Pacific, extending eastward to a lesser extent than last month. Most ENSO prediction models indicate continued weakening El Niño conditions during the rest of the northern spring season, returning to neutral by late spring or early summer 2016, with La Niña development likely by fall.

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: April 14, 2016

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

ENSO Alert System Status: El Niño Advisory/La Niña Watch

Synopsis: A transition to ENSO-neutral is likely during late Northern Hemisphere spring or early summer 2016, with an increasing chance of La Niña during the second half of the year

Sea surface temperature (SST) anomalies were between 1.0° and 1.5°C across most of the central and eastern equatorial Pacific Ocean during early April (Fig. 1), having weakened appreciably over the last month. The latest weekly values for all of the Niño indices dropped to below 1.5°C (Fig. 2). The subsurface temperature anomaly in the central and eastern Pacific decreased to negative values (Fig. 3) in association with a significant expansion of below-average temperatures at depth (Fig. 4). Low-level westerly wind anomalies and upper-level easterly wind anomalies weakened compared to February. The equatorial Southern Oscillation Index (SOI) remained negative but weakened, while the traditional SOI was near zero. Enhanced convection continued over the central tropical Pacific but weakened east of the Date Line, and was suppressed over northern Indonesia and the Philippines (Fig. 5). Collectively, these anomalies reflect a weakening El Niño.

Nearly all models predict further weakening of El Niño, with a transition to ENSO-neutral likely during late spring or early summer 2016 (Fig. 6). Then, the chance of La Niña increases during the late summer or early fall. The official forecast is consistent with the model forecasts, also supported by a historical tendency for La Niña to follow strong El Niño events. A transition to ENSO-neutral is likely during late Northern Hemisphere spring or early summer 2016, with an increasing chance of La Niña during the second half of the year (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). Forecasts are also updated monthly in the Forecast Forum section of CPC’s Climate Diagnostics Bulletin. Additional perspectives and analysis are also available in an ENSO blog.

The next ENSO Diagnostics Discussion is scheduled for 12 May 2016. 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.

Climate Prediction Center
National Centers for Environmental Prediction
NOAA/National Weather Service
College Park, MD 20740

CPC/IRI Early-Month Official ENSO Forecast Probabilities

Season La Niña Neutral El Niño
MAM 2016 ~0% 6% 94%
AMJ 2016 4% 40% 56%
MJJ 2016 21% 56% 23%
JJA 2016 45% 47% 8%
JAS 2016 58% 36% 6%
ASO 2016 65% 32% 3%
SON 2016 69% 28% 3%
OND 2016 70% 27% 3%
NDJ 2016 71% 26% 3%

IRI ENSO Forecast

IRI Technical ENSO Update

Published: April 21, 2016

Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv4. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv4 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 usually shows stronger anomalies than ERSSTv4, and during very strong events the two datasets may differ by as much as 0.5 C.

Recent and Current Conditions

After having been at a strong El Niño level since around mid-July 2015, the weekly SST anomaly in the NINO3.4 region recently dropped below 1.5 C during early April. For March 2016 the average NINO3.4 SST anomaly was 1.68 C, indicative of strong El Niño conditions, and for Jan-Mar it was 2.23 C. 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 SST anomaly in the Nino3.4 region was 1.3 C, in the category of moderate El Niño. Accompanying this SST has been a continuing but weakening El Niño-like atmospheric pattern, including westerly low-level wind anomalies and positive anomalies of convection near and just east of the dateline. The Southern Oscillation Index (SOI) and the equatorial SOI have also been negative, but less strongly so than in recent months, indicating continuing but weakening El Niño conditions.

Expected Conditions

What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it called for this strong El Niño to continue weakening, and return to neutral by late spring or early summer 2016, with about a 65% possibility for La Niña development by fall and 70% for the existence of La Niña during mid-fall. The latest set of model ENSO predictions, from mid-April, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, while the most recent Nino3.4 SST anomalies are still in the moderate El Niño category, subsurface temperature anomalies across the eastern equatorial Pacific have cooled substantially over the last few months and are now somewhat below average. With these below-average subsurface temperatures, the SST is poised to continue retreating toward average and then likely below average in the coming few months.  In the atmosphere, the basin-wide sea level pressure anomaly pattern (e.g. the SOI) has been at moderate El Niño levels, no longer at strong levels, and with some fairly large week-to-week variations.  Anomalous convection (as measured by OLR) has been above average near and somewhat east of the dateline, but more weakly than seen in the previous months, particularly the portion to the east of the dateline. Together, the oceanic and atmospheric features reflect continuing but weakening El Niño conditions for late March through mid-April. By early May, the Niño3.4 anomaly will likely be below 1 C, in the weak category, and later in May it could decrease to the ENSO-neutral range.

As of mid-April, 76% of the dynamical or statistical models models predicts El Niño SST conditions, and 24% predicts neutral conditions for the initial Apr-Jun 2016 season. At lead times of 3 or more months into the future, statistical and dynamical models that incorporate information about the ocean’s observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. For the Jul-Sep 2016 season, among models that do use subsurface temperature information, 50% predicts ENSO-neutral conditions, and 50% predicts La Niña conditions. For all model types, the probabilities for El Niño are greater than 50% only for Apr-Jun, and drop to about 30% by May-Jul, 20% for Jun-Aug, and less than 10% for the remainder of the seasons through Dec-Feb 2016-17.  No model predicts La Niña conditions until May-Jul 2016, when the chance becomes just 12%, but the chances rise to just over 30% by Jun-Aug and to 50% or more beginning in Sep-Nov, reaching about 70% for Oct-Dec through Dec-Feb 2016-17. Chances for neutral ENSO conditions rise to 60% for May-Jul, and then fall back to about 45% for Aug-Oct and Sep-Nov, and then below 30% beginning in Oct-Dec through the remainder of the forecast period.

Note  – Only models that produce a new ENSO prediction every month are included in the above statement.

Caution is advised in interpreting the distribution of model predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading 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 from that taken verbatim from the raw model predictions.

An alternative way to assess the probabilities of the three possible ENSO conditions is more quantitatively precise and less vulnerable to sampling errors than the categorical tallying method used above. This alternative method uses the mean of the predictions of all models on the plume, equally weighted, and constructs a standard error function centered on that mean. The standard error is Gaussian in shape, and has 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. This method shows probabilities for La Niña at near-zero for Apr-Jun 2016, rising to near 50% by Jul-Sep and to near 60% from Oct-Dec 2016 through Dec-Feb 2016-17. Model probabilities for neutral ENSO conditions are about 25% for Apr-Jun, rise to at least 60% for May-Jul and Jun-Aug, and then drop to 40% or less for Aug-Oct through Dec-Feb 2016-17. Probabilities for El Niño are about 75% for Apr-Jun 2016, near 20% for May-Jul, and 10% or less from Jun-Aug through the final season of Dec-Feb 2016-17.  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. The same cautions mentioned above for the distributional count of model predictions apply to this Gaussian standard error method of inferring probabilities, due to differing model biases and skills. 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.

The probabilities derived from the models on the IRI/CPC plume describe, on average, rapidly decreasing likelihood for El Niño conditions from the initially high levels of the initial Apr-Jun 2016 season. The models indicate a return to neutral ENSO by early summer, and then suggest a likelihood for La Nina development by autumn 2016. A caution regarding this latest set of model-based ENSO plume predictions, is that factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the next official outlook to be generated and issued in early October by CPC and IRI, which will include some human judgement in combination with the model guidance.

Climatological Probabilities

Season La Niña Neutral El Niño
DJF 36% 30% 34%
JFM 34% 38% 28%
FMA 28% 49% 23%
MAM 23% 56% 21%
AMJ 21% 58% 21%
MJJ 21% 56% 23%
JJA 23% 54% 23%
JAS 25% 51% 24%
ASO 26% 47% 27%
SON 29% 39% 32%
OND 32% 33% 35%
NDJ 35% 29% 36%

 

IRI ENSO Forecast Histogram Image

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

Season La Niña Neutral El Niño
AMJ 2016 ~0% 24% 76%
MJJ 2016 5% 76% 19%
JJA 2016 31% 61% 8%
JAS 2016 47% 47% 6%
ASO 2016 52% 40% 8%
SON 2016 55% 35% 10%
OND 2016 59% 31% 10%
NDJ 2016 60% 30% 10%
DJF 2016 59% 32% 9%

ENSO Forecast

IRI Model-Based Probabilistic ENSO Forecast

Published: April 21, 2016

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

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

Season La Niña Neutral El Niño
AMJ 2016 ~0% 24% 76%
MJJ 2016 5% 76% 19%
JJA 2016 31% 61% 8%
JAS 2016 47% 47% 6%
ASO 2016 52% 40% 8%
SON 2016 55% 35% 10%
OND 2016 59% 31% 10%
NDJ 2016 60% 30% 10%
DJF 2016 59% 32% 9%

ENSO Forecast

CPC Official Probabilistic ENSO Forecast

Published: April 14, 2016

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

CPC/IRI Early-Month Official ENSO Forecast Probabilities

Season La Niña Neutral El Niño
MAM 2016 ~0% 6% 94%
AMJ 2016 4% 40% 56%
MJJ 2016 21% 56% 23%
JJA 2016 45% 47% 8%
JAS 2016 58% 36% 6%
ASO 2016 65% 32% 3%
SON 2016 69% 28% 3%
OND 2016 70% 27% 3%
NDJ 2016 71% 26% 3%

ENSO Forecast

IRI ENSO Predictions Plume

Published: April 21, 2016

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 (2016-2016)
Model  AMJ MJJ JJA JAS ASO SON OND NDJ DJF
Dynamical models
NASA GMAO model 0.3 -0.6 -1.2 -1.5 -1.4 -1.3 -1.2
NCEP CFS version 2 0.6 -0.2 -0.9 -1.2 -1.4 -1.6 -1.8 -1.9
Japan Met. Agency model 0.8 0.2 -0.4 -0.7 -0.9
Scripps Inst. HCM -0.1 -0.8 -1.4 -1.8 -2.1 -2.3 -2.4 -2.4 -2.4
Lamont-Doherty model 1.1 0.8 0.6 0.5 0.5 0.6 0.6 0.6 0.5
POAMA (Austr) model 0.8 0.5 0.2 0 -0.1 -0.2 -0.3
ECMWF model 0.1 -0.5 -0.6 -0.5 -0.4
UKMO model 0.6 -0.3 -1 -1.2
KMA (Korea) SNU model 1 0.7 0.4 0.2 -0.1 -0.3 -0.6 -0.7 -0.9
IOCAS (China) Intermed. Coupled model 1 0.7 0.6 0.5 0.4 0.2 0.1 0.1 0.1
COLA CCSM4 model 0.4 -0.4 -1.1 -1.4 -1.6 -1.8 -2 -2.2 -2.1
Japan Frontier Coupled model 0.6 0.1 -0.2 -0.2 -0.2 -0.3 -0.5 -0.7 -0.8
CSIR-IRI 3-model MME 0.4 -0.1 -0.3 -0.3 -0.3 -0.5
GFDL CM2.1 Coupled Climate model 0.3 -0.5 -1.2 -1.3 -1.2 -1 -0.9 -0.8 -0.7
Canadian Coupled Fcst Sys 0.5 -0.2 -0.8 -1.2 -1.2 -1.2 -1.3 -1.3 -1.3
GFDL CM2.5 FLOR Coupled Climate model 0.9 0.2 -0.4 -0.6 -0.7 -0.7 -0.7 -0.7 -0.7
Average, dynamical models 0.6 0 -0.5 -0.7 -0.7 -0.8 -0.9 -1 -0.9
Statistical models
NCEP/CPC Markov model 0.9 0.7 0.6 0.4 0.3 0.3 0.2 0.2 0.1
NOAA/CDC Linear Inverse 0.7 0.4 0.1 -0.2 -0.3 -0.5 -0.6 -0.6 -0.6
NCEP/CPC Constructed Analog 0.6 0.2 -0.2 -0.4 -0.6 -0.7 -0.8 -0.9 -0.8
NCEP/CPC Can Cor Anal 1 0.6 0.1 -0.2 -0.4 -0.6 -0.7 -0.9 -1
Landsea/Knaff CLIPER 0.5 0.2 -0.2 -0.6 -0.8 -1 -1.2 -1.2 -1.1
Univ. BC Neural Network 0.5 0 -0.2 -0.4 -0.6 -0.6 -0.6 -0.5 -0.5
FSU Regression 0.7 0.3 0 -0.2 -0.3 -0.4 -0.6 -0.6 -0.6
TCD – UCLA 1.2 0.9 0.8 0.6 0.5 0.5 0.4 0.4 0.3
UNB/CWC Nonlinear PCA 1.2 0.9 0.6 0.2 -0.1 -0.3 -0.3 -0.3 -0.2
Average, statistical models 0.8 0.5 0.2 -0.1 -0.3 -0.4 -0.5 -0.5 -0.5
Average, all models 0.7 0.2 -0.2 -0.5 -0.5 -0.6 -0.7 -0.8 -0.7

Discussion of Current Forecasts

Most of the set of dynamical and statistical model predictions issued during late March and early April 2016 predict weakening El Niño SST conditions during April and May 2016, becoming neutral conditions by around June.  El Niño probabilities remain over 50% for the Apr-Jun season of 2016, but fall rapidly to about 20% by May-July and to less than 10% or lower thereafter.  In the most recent week, the SST anomaly in the Nino3.4 region was 1.3 C, reflecting moderate El Niño conditions in this weekly time scale, and 1.68 C for the month of March, still at a strong level.  All of the atmospheric variables continue to reflect El Niño, but less strongly than in previous months. This includes the lower and upper level wind anomalies, the Southern Oscillation Index and the pattern of anomalous convection. Based on the multi-model mean predictions, 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
AMJ 2016 ~0% 24% 76%
MJJ 2016 5% 76% 19%
JJA 2016 31% 61% 8%
JAS 2016 47% 47% 6%
ASO 2016 52% 40% 8%
SON 2016 55% 35% 10%
OND 2016 59% 31% 10%
NDJ 2016 60% 30% 10%
DJF 2016 59% 32% 9%

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: April 21, 2016

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.