IRI ENSO Forecast
IRI Technical ENSO Update
Published: August 15, 2013
Recent and Current Conditions
The SST anomaly in the Nino3.4 region has been in the neutral range lately, through mid-August 2013. For July 2013 the Nino3.4 SST anomaly was -0.31 C, indicative of neutral ENSO conditions, and for April-June it was -0.26 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 -0.3 C, indicating neutral ENSO conditions in the tropical Pacific; this is about the same as the -0.31 C level observed in July.
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 the rest of the northern summer, the autumn, and into winter of 2013-14, with probabilities of El Niño or La Niña each less than 30% through that time. The latest set of model ENSO predictions, from mid-August, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are just slightly negative. The SST continues to be slightly above average in the far western part of the basin, but have been below average in the eastern quarter of the basin since May. Subsurface temperature anomalies across the equatorial Pacific have been mixed, but overall have averaged near to slightly above average since June. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), has been somewhat above average (indicative of somewhat cool ENSO conditions) but the low-level zonal winds have not been far from average across much of the basin. The upper level zonal winds lean toward enhanced westerlies, indicative of cool ENSO conditions. Anomalous convection (as measured by OLR) has generally been negative in the central and eastern tropical Pacific, and positive in the far western part of the basin. Together, these features reflect ENSO-neutral conditions, with a tendency toward the cool part of average. However, all fields taken collectively do not indicate weak La Niña conditions, but rather cool-neutral conditions.
As of mid-august, 17% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Aug-Oct 2013 season, 4% predicts El Niño conditions, and 78% 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 Nov-Jan season, among models that do use subsurface temperature information, 76% predicts ENSO-neutral SSTs, 6% predicts El Niño conditions and 18% predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions ranges from 65% (for Oct-Dec 2013) to 81% (for Dec-Feb 2013-14 and Mar-May 2014) through the end of the forecast period in northern spring 2014. (Note 1). 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 19% for Aug-Oct 2013, remaining near 20% through Nov-Jan and then between 10% and 17% from Dec-Feb 2013-14 to Apr-Jun 2014. Model probabilities for ENSO-neutral conditions are 80% for Aug-Oct 2013, and ranging between 72% and 76% through early 2014. Meanwhile, probabilities for El Niño are 1% for Aug-Oct 2013, 3% for Sep-Nov, and remain no higher than 10% through Jan-Mar 2014 and then rise to 32% for the final forecast period of Apr-Jun 2014. Clearly, the models collectively favor neutral ENSO conditions through to early 2014; La Niña is slightly favored over El Niño through early northern winter 2013-14. 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 24 or more models on the IRI/CPC plume describe, on average, maintenance of neutral ENSO conditions during the coming months, continuing through the northern winter of 2013-14. Some uncertainty exists, but it is not great because we are past the period typically most likely for new ENSO event evolution. The model forecast spread expresses that uncertainty, ranging between weak La Niña and weak El Niño conditions, even though the majority of the forecasts is in the neutral range. More statistical models call for development of weak La Niña than the small set of dynamical models calling for development of weak El Niño. 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 August by CPC and IRI, which will include some human judgement in combination with the model guidance.
Using the 0.5 C thresholds, the climatological probabilities of La Nina, neutral, and El Nino conditions for each 3-month season are as follows:
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% |
Note 1 – Only models that produce a new ENSO prediction every month are included in the above statement.
IRI/CPC Mid-Month Plume-Based ENSO Forecast Probabilities
| Season |
La Niña |
Neutral |
El Niño |
| ASO 2013 |
19% |
80% |
1% |
| SON 2013 |
23% |
74% |
3% |
| OND 2013 |
23% |
73% |
4% |
| NDJ 2014 |
18% |
76% |
6% |
| DJF 2014 |
17% |
76% |
7% |
| JFM 2014 |
14% |
76% |
10% |
| FMA 2014 |
11% |
72% |
17% |
| MAM 2014 |
11% |
66% |
23% |
| AMJ 2014 |
11% |
57% |
32% |
