IRI ENSO Forecast
IRI Technical ENSO Update
Published: February 20, 2014
Recent and Current Conditions
The SST anomaly in the Nino3.4 region in recent weeks has been near the borderline between neutral and weak Niña, through mid-February 2014. For January 2014 the Nino3.4 SST anomaly was -0.51 C, indicative of weak La Niña conditions, and for November-January it was -0.18 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 -0.5 C, which is about the same as the -0.51 C observed in January. The recent borderline weak La Niña conditions may easily be a short-term fluctuation and cannot by themselves be considered the onset of a La Niña episode.
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 high likelihood of neutral ENSO conditions enduring through northern winter and into spring 2014, with probabilities of El Niño or La Niña each 40% or less until May-Jul 2014 after which El Niño probabilities rise to nearly 50% by late summer 2014. The latest set of model ENSO predictions, from mid-February, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are in the neutral to borderline La Niña range. Anomalies are above average in the western part of the basin and somewhat below average in the east-central part of the basin (including the Nino3.4 region). Subsurface temperature anomalies across the central and eastern equatorial Pacific had become below average during January, but during late January and the first half of February have reversed to being slightly above average. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), became somewhat as would be found during La Niña during January, but this pattern weakened during the first half of February. The low-level zonal winds have been approximately average across much of the basin, although slightly enhanced westerlies have been observed at upper levels of the atmosphere in a portion of the east-central tropical Pacific. Anomalous convection (as measured by OLR) has been negative in the central and west-central tropical Pacific, and positive in the far western part of the basin and in Indonesia. Together, these features reflect ENSO-neutral to borderline weak La Niña conditions. During January a westerly wind event was observed in the western tropical Pacific, and that event generated a downwelling Kelvin wave that is currently moving eastward along the thermocline in the east-central Pacific and is partly responsible for the recent return to above average subsurface temperatures in the central and eastern equatorial Pacific.
As of mid-February, 4% of the dynamical or statistical models models predicts La Niña SST conditions for the Feb-Apr 2014 season, none predicts El Niño conditions, and 96% 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 May-Jul 2014 season, among models that do use subsurface temperature information, 68% predicts ENSO-neutral SSTs, 32% predicts El Niño conditions and none predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions is above 80% for Jan-Mar through Apr-Jun 2014, is 68% for May-Jul 2014, and is not far from 50% Jun-Aug through Oct-Dec 2014 at the end of the forecast period. Probabilities for El Niño are below 20% through Apr-Jun 2014, rise to 32% for May-Jul, and settle in the 45-55% range for most of the periods from Jun-Aug through Oct-Dec. No model predicts La Niña conditions for any of the 3-month periods between Mar-May and Oct-Dec.
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 10% for Feb-Apr 2014, decreasing to just below 10% through the end of the forecast period in Oct-Dec 2014. Model probabilities for neutral ENSO conditions are above 70% for Feb-Apr through Apr-Jun 2014, 55% for May-Jul 2014, and between 45% and 50% for Jun-Aug through Oct-Dec 2014 at the end of the forecast period. Probabilities for El Niño are below 10% through Mar-May 2014, rise to 21% for Apr-Jun, and settle to 44-46% for Jun-Aug through Oct-Dec. It is clear that the models collectively favor neutral ENSO conditions into northern spring 2014; then by Jun-Aug El Niño probabilities become more competitive with ENSO-neutral probabilities, being nearly equally likely for Jun-Aug through Oct-Dec 2014. 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, maintenance of neutral ENSO conditions into northern spring 2014. A definite possibility for El Niño development is seen starting in Jun-Aug 2014, but the objective model-based probabilities for El Niño still remain slightly below 50% for Jun-Aug through Oct-Dec 2014. The uncertainty will diminish as we progress through the northern spring predictability barrier in the coming few months. 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 |
| FMA 2014 |
10% |
90% |
~0% |
| MAM 2014 |
8% |
87% |
5% |
| AMJ 2014 |
6% |
73% |
21% |
| MJJ 2014 |
8% |
55% |
37% |
| JJA 2014 |
8% |
48% |
44% |
| JAS 2014 |
9% |
47% |
44% |
| ASO 2014 |
9% |
45% |
46% |
| SON 2014 |
9% |
46% |
45% |
| OND 2014 |
9% |
46% |
45% |
