IRI ENSO Forecast
IRI Technical ENSO Update
Published: October 16, 2014
Recent and Current Conditions
The SST anomaly in the NINO3.4 region in recent weeks has slightly retreated from a borderline El Niño level, after having briefly reached that level around mid-September. For September the average NINO3.4 SST anomaly was 0.45 C, indicative of warm but still neutral conditions, and for Jul-Sep it was 0.28 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 0.4 C, very near the 0.45 C observed in September. Although the SST had been near the borderline of weak El Niño conditions in May and June, the atmospheric participation in an El Niño-like pattern had been weak or absent. Around mid-September, both ocean and atmosphere were suggestive of borderline or weak El Niño conditions, but that situation failed to lead to a sustained weak El Niño event, and current conditions fall short of the required thresholds both in the ocean and in the overlying atmosphere.
Expected Conditions
What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it called for a likelihood for a transition from neutral ENSO conditions to El Niño conditions during the fall of 2014 now in progress, with probabilities of El Niño rising to 67% for Nov-Jan 2014-15. The latest set of model ENSO predictions, from mid-October, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are just short of the borderline 0.5C threshold for weak El Niño after having reached that level for a couple of weeks in mid-September. Subsurface temperature anomalies across the eastern equatorial Pacific have remained somewhat above average during the last month. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI) was around the borderline of weak Niño in mid-September but has recently weakened to the neutral range. Weak anomalous low-level westerlies have appeared at times during September and early October but have not been sustained. Upper level anomalous westerlies have also been observed during some periods over the last couple of months, but are currently weak. Anomalous convection (as measured by OLR) has been near average near and eastward of the dateline, and slightly suppressed in some portions of the area west of the dateline while enhanced in some small portions of the far western tropical Pacific. Together, the oceanic and atmospheric features currently reflect a warmish but neutral ENSO condition.
As of mid-October, none of the dynamical or statistical models models predicts La Niña SST conditions for the initial Oct-Dec 2014 season, 70% predicts El Niño conditions, and 30% indicates neutral ENSO. 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 Jan-Mar 2015 season, among models that do use subsurface temperature information, 26% predicts ENSO-neutral SSTs, 74% predicts El Niño conditions and none predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions is 30% or below between Oct-Dec 2014 and Apr-Jun 2015. Probabilities for El Niño rise to 70% or higher between these same two seasons, peaking at 79% for Mar-May 2015. No model predicts La Niña conditions for any of the 3-month periods between Oct-Dec 2014 and Jun-Aug 2015.
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 near 0% from Oct-Dec 2014 through Apr-Jun 2015, rising to 8% by Jun-Aug. Model probabilities for neutral ENSO conditions are 35% for the initial period of Oct-Dec 2014, 34% for the next running period of Nov-Jan, reach a low of 33% for Dec-Feb 2014-15 through Feb-Apr 2015 and rise to 44% by Jun-Aug 2015. Probabilities for El Niño are 65% for Oct-Dec 2014, rise to 67% for Dec-Feb 2014-15 through Feb-Apr 2015, and decline to 48% for Jun-Aug 2015. The models collectively favor El Niño over other ENSO conditions by a sizable margin between Oct-Dec 2014 and Apr-Jun 2015. 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, a likely development of El Niño development during the Oct-Dec 2014 season currently in progress, enduring well into northern spring season of 2015. The consensus of model predictions calls for a weak El Niño event, although a moderate event or no event are also possible. A strong event appears very unlikely. 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 |
37% |
28% |
35% |
JFM |
34% |
37% |
29% |
FMA |
30% |
48% |
22% |
MAM |
26% |
54% |
20% |
AMJ |
24% |
54% |
22% |
MJJ |
25% |
51% |
24% |
JJA |
25% |
50% |
25% |
JAS |
27% |
46% |
27% |
ASO |
29% |
40% |
31% |
SON |
32% |
34% |
34% |
OND |
34% |
31% |
35% |
NDJ |
37% |
27% |
36% |
IRI/CPC Mid-Month Plume-Based ENSO Forecast Probabilities
Season |
La Niña |
Neutral |
El Niño |
OND 2014 |
~0% |
35% |
65% |
NDJ 2014 |
~0% |
34% |
66% |
DJF 2014 |
~0% |
33% |
67% |
JFM 2015 |
~0% |
33% |
67% |
FMA 2015 |
~0% |
33% |
67% |
MAM 2015 |
~0% |
35% |
65% |
AMJ 2015 |
~0% |
39% |
61% |
MJJ 2015 |
3% |
42% |
55% |
JJA 2015 |
8% |
44% |
48% |