IRI ENSO Forecast
IRI Technical ENSO Update
Published: June 19, 2014
Recent and Current Conditions
The SST anomaly in the Nino3.4 region in recent weeks has been near the borderline of neutral and El Nino during the mid-May to mid-June period, 2014. For May the Nino3.4 SST anomaly was 0.45 C, indicative of neutral conditions but very close to the borderline of El Nino, and for Mar-May it was 0.16 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, which is about the same as the 0.45 C observed in May, close to the borderline of an El Niño condition if it were to persist. The trend is an upward one for Mar-May to May. However, although the SST has been near the borderline of weak El Niño conditions, the atmospheric participation in an El Niño-like pattern has been absent to very weak at best.
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 summer 2014, with probabilities of El Niño rising to 70% by Jun-Aug 2014, and to approximately 80% by northern autumn 2014. The latest set of model ENSO predictions, from mid-June, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are near the borderline of neutral and weak El Niño, and have been that way for several weeks. Positive anomalies maximize near the dateline and also in the far eastern part of the Pacific basin, with relatively weaker anomalies in between where the Nino3.4 region is located. Subsurface temperature anomalies across the eastern equatorial Pacific are above average levels, due to a downwelling Kelvin wave triggered by two westerly wind events in the western tropical Pacific during the Jan-Mar period. These anomalies at depth have been surfacing in the far eastern part of the basin, so that the integrated heat content has been slowly declining over the last couple of months. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI) has been close to average recently. The low-level zonal winds and the upper level winds have also been close to average in recent weeks. Anomalous convection (as measured by OLR) has been positive somewhat to the west of the dateline, but near average near and east of the dateline. These atmospheric conditions are not suggestive of El Niño despite the SST being near the borderline of El Niño. Together, the oceanic and atmospheric features continue to reflect neutral ENSO conditions that lean toward the borderline of weak El Niño. The ocean-atmosphere coupling needs to appear as the onset of El Niño conditions likely gets underway over the course of the coming northern summer season.
As of mid-June, none of the dynamical or statistical models models predicts La Niña SST conditions for the Jun-Aug 2014 season, 71% predicts El Niño conditions, and 29% indicates neutral ENSO for the initial Jun-Aug 2014 period. 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 Sep-Nov 2014 season, among models that do use subsurface temperature information, 10% predicts ENSO-neutral SSTs, 90% predicts El Niño conditions and none predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions is below 25% between Aug-Oct 2014 and Jan-Mar 2015, being between 30% and 39% during Jun-Aug and Jul-Sep, and again at the end of the forecast period in Feb-Apr 2015. Probabilities for El Niño rise to more than 75% during the very same times, Aug-Oct 2014 to Jan-Mar 2015. Probabilities for El Niño fall to about 60% by Feb-Apr 2015. No model predicts La Niña conditions for any of the 3-month periods between Jun-Aug 2014 and Feb-Apr 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 no higher than 1% for any period between Jun-Aug 2014 through Feb-Apr 2015. Model probabilities for neutral ENSO conditions are 39% for the initial period of Jun-Aug 2014, 35% for the next running period of Jul-Sep, and the hover between 22% and 30% through the northern summer and fall 2014, rising again to 32% for Dec-Feb 2014-15 and to 42% for Feb-Apr 2015. Probabilities for El Niño are 61% for Jun-Aug 2014, rise to near 65% for Jul-Sep, 69% for Aug-Oct, and to 78% for Oct-Dec 2014 and Nov-Jan 2014-15. The models collectively favor El Niño over other ENSO conditions by a clear margin between Jun-Aug 2014 and Jan-Mar 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 Jun-Aug 2014, as the objective model-based probabilities for El Niño exceed those for neutral ENSO by more than a small margin between that season and Jan-Mar 2015. The consensus of model predictions calls for a weak to moderate El Niño event. 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 |
JJA 2014 |
~0% |
39% |
61% |
JAS 2014 |
~0% |
35% |
65% |
ASO 2014 |
1% |
30% |
69% |
SON 2014 |
~0% |
26% |
74% |
OND 2014 |
~0% |
22% |
78% |
NDJ 2014 |
~0% |
22% |
78% |
DJF 2014 |
~0% |
26% |
74% |
JFM 2015 |
1% |
32% |
67% |
FMA 2015 |
1% |
42% |
57% |