IRI ENSO Forecast
IRI Technical ENSO Update
Published: March 16, 2017
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 often shows stronger anomalies than ERSSTv4, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv4 may tend to be cooler than OISSTv2, because ERSSTv4 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is based on a slightly older period and does not account as much for the slow warming trend in the tropical Pacific SST.
Recent and Current Conditions
In March 2017, the NINO3.4 SST anomaly remained in the neutral range, after having been at a borderline or weak La Niña levels during much of the second half of 2016. For January the SST anomaly was 0.14 C, and for Dec-Feb it was -0.20 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 anomaly in the Nino3.4 region was -0.2, at an ENSO-neutral level. The SST farther east has been maintaining above-average levels for several months, making for a coastal El Niño during the rainy season in coastal Ecuador and northern Peru. Most of the pertinent atmospheric variables have also been assuming neutral patterns, with the exception of the convection anomalies in the central and western tropical Pacific, which have continued to suggest borderline La Niña condtions. The lower-level trade winds and upper level westerly winds have been largely near-average, although there are still weakly enhanced trade winds in the west-central part of the basin. The Southern Oscillation Index (SOI) has been near-average during the last couple of months. Subsurface temperature anomalies across the eastern equatorial Pacific have increased to near to slightly above average. Overall, given the SST and the atmospheric conditions, an ENSO-neutral diagnosis is in order.
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 stated that ENSO-neutral is the most likely condition through northern spring 2017, with increasing chances (50 to 55%) for El Niño development during the summer or fall time frame. The latest set of model ENSO predictions, from mid-March, now available in the IRI/CPC ENSO prediction plume, is discussed below. Those predictions suggest that the SST is most likely to be in the ENSO-neutral range for March-May but with an increased likelihood for El Niño development from summer onward.
As of mid-March, 68% of the dynamical or statistical models predicts neutral ENSO conditions for the initial Mar-May 2017 season, while 32% predicts El Niño conditions and 0% predicts La Niña conditions. 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 Jun-Aug 2017 season, among models that do use subsurface temperature information, no model predicts La Niña conditions, 72% predicts El Niño conditions, while 28% predicts neutral ENSO. For all model types, the probabilities for La Niña are 6% or less for for all predicted seasons from Mar-May 2017 through Nov-Jan 2016-17. The probability for El Niño conditions exceeds 50% beginning with Apr-June and lasting throughout the rest of 2017. The chances for El Niño rises to about 55% for Apr-Jun, 65% for May-Jul, between 70% and 75% for Jun-Aug and Jul-Sep, and approximately 80% from Aug-Oct through the final season of Nov-Jan 2017-18. Chances for neutral ENSO conditions drops from near 70% for Mar-May to below 50% thereafter, hovering between 10% and 30% from Jun-Aug through Nov-Jan 2017-18.
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 less than 10% from Mar-May through Oct-Dec 2017, increasing slightly to 11% for Nov-Jan 2017-18. Probabilities for ENSO-neutral are 77% for Mar-May 2017, falling steadily to 40% by May-Jul, and down to near 30% from Jul-Aug through Nov-Jan 2017-18. Probabilities for El Niño are 23% for Mar-May, rise to 45-50% by Apr-Jun and to approximately 60-68% for May-Jul through the final season of Nov-Jan 2017-18. 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.
In summary, the probabilities derived from the models on the IRI/CPC plume describe, on average, a likelihood for neutral ENSO conditions for Mar-May 2017, and more weakly for Apr-Jun. Beginning with May-Jul, El Niño becomes more likely than neutral through the final season of Nov-Jan 2017-18. Although most likely, the chances for El Niño are not overwhelming, reaching approximately two-thirds or slightly higher from Jun-Aug through Nov-Jan 2017-18. Chances for La Niña are very low throughout the forecast period. 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 April 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/CPC Mid-Month Model-Based ENSO Forecast Probabilities
Season |
La Niña |
Neutral |
El Niño |
MAM 2017 |
0% |
77% |
23% |
AMJ 2017 |
0% |
53% |
47% |
MJJ 2017 |
1% |
40% |
59% |
JJA 2017 |
2% |
32% |
66% |
JAS 2017 |
3% |
29% |
68% |
ASO 2017 |
4% |
29% |
67% |
SON 2017 |
6% |
28% |
66% |
OND 2017 |
9% |
29% |
62% |
NDJ 2017 |
11% |
29% |
60% |