IRI ENSO Forecast
IRI Technical ENSO Update
Published: February 19, 2015
Recent and Current Conditions
The SST anomaly in the NINO3.4 region has been at the weak El Niño level from mid-October through mid-February. For January the average NINO3.4 SST anomaly was 0.53 C, indicative of weak Niño conditions, and for Nov-Jan it was 0.72 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, in the lower portion of the category of weak El Niño. Accompanying this SST has been an atmospheric pattern with mainly weak indications of an El Niño-like pattern–weak westerly low-level wind anomalies and, recently, weak positive anomalies of convection near the dateline. During the last four weeks, however, these atmospheric indicators have become closer to those expected during a weak El Niño. For example, the Southern Oscillation Index (SOI) has been somewhat indicative of weak El Niño lately, with values between -0.5 and -1.0.
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 50-60% likelihood for El Niño conditions during the rest of northern winter and early spring, followed by a greater than 50% chance for ENSO-neutral beginning around April. The latest set of model ENSO predictions, from mid-January, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are in the lower portion of the weak El Niño cagtegory. Subsurface temperature anomalies across the eastern equatorial Pacific had returned to neutral from above average in January, but have returned to a somewhat above average level again in the recent few weeks. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI) has been somewhat negative (between -0.5 and -1.0) over the last month, suggestive of borderline to weak El Niño conditions. Other atmospheric parameters continue to reflect warm-neutral to weak El Niño conditions. Noticeable but mainly weak anomalous low-level westerlies have reappeared during the last several weeks, and are inducing a downwelling Kelvin wave that might reinforce some warming at the surface in the coming few months. Upper level anomalous easterly anomalies have been observed over part of the equatorial Pacific, indicative of El Niño. More importantly, anomalous convection (as measured by OLR) had been near or below average near and eastward of the dateline, but in the most recent weeks has shown some signs of enhancement. Together, the oceanic and atmospheric features reflect a warm-neutral to borderline El Niño condition, with weak El Niño status for the SST during the last four months.
As of mid-February, none of the dynamical or statistical models models predicts La Niña SST conditions for the initial Feb-Apr 2015 season, 44% predicts El Niño conditions, and 56% 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 2015 season, among models that do use subsurface temperature information, 55% predicts El Niño SST conditions, 45% predicts ENSO-neutral conditions and none predicts La Niña conditions. For all model types, the probabilities for El Niño are 44% for Mar-May through May-Jul, and between 48% and 56% between Jun-Aug and Oct-Dec. No model predicts La Niña conditions for any of the 3-month periods between Feb-Apr and Oct-Dec 2015. The season having highest probability for El Niño SST conditions is Oct-Dec, when that probability is 56%.
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 Feb-Apr 2015 through Apr-Jun, rising to 6% by Jul-Sep and up to 17% by Oct-Dec. Model probabilities for neutral ENSO conditions are 53% for the initial period of Feb-Apr 2015, 54% for the next running period of Mar-May 2015, and drop through the 40s during Apr-Jun and May-Jul and into the 30s from Jun-Aug through Oct-Dec. Probabilities for El Niño are 47% for Feb-Apr 2015, 46% for Mar-May, and slowly increase to the 50s during the remainder of the forecast seasons, peaking at 60% for Jun-Aug. The models collectively favor El Niño over other ENSO conditions by the largest margin during Jun-Aug, but that margin is not so large (60% vs 40%). 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, neutral ENSO to borderline El Niño conditions for the Feb-Apr and Mar-May seasons, with redevelopment of El Niño conditions suggested during the Apr-Jun through Oct-Dec seasons. Thus, the consensus of model predictions calls for a brief dissipation of the recent weak El Niño SST levels, followed by a return to weak El Niño levels by early northern summer. 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 March 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 2015 |
~0% |
53% |
47% |
MAM 2015 |
~0% |
53% |
47% |
AMJ 2015 |
~0% |
46% |
54% |
MJJ 2015 |
1% |
41% |
58% |
JJA 2015 |
3% |
36% |
61% |
JAS 2015 |
6% |
37% |
57% |
ASO 2015 |
9% |
35% |
56% |
SON 2015 |
13% |
34% |
53% |
OND 2015 |
17% |
32% |
51% |