IRI ENSO Forecast
IRI Technical ENSO Update
Published: May 20, 2019
Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv4. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv4 is used for retrospective official ENSO diagnosis because it is more homogeneous over time, allowing for more accurate comparisons among ENSO events that are years apart. During ENSO events, OISSTv2 often shows stronger anomalies than ERSSTv4, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv4 may tend to be cooler than OISSTv2, because ERSSTv4 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is updated every 10 years and so, half of the time, is based on a slightly older period and does not account as much for the slow warming trend in the tropical Pacific SST.
Recent and Current Conditions
In mid-May 2019, weak El Niño SST conditions were observed in the NINO3.4 region. The April SST anomaly was 0.82 C, in the weak El Niño range, and for February-April it was 0.84 C, also indicative of a weak El Niño. 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 anomaly in the Nino3.4 region was 0.5 C, indicating minimal weak El Niño conditions. Since late January some important atmospheric variables became El Niño-like, including on-and-off westerly low-level zonal wind anomalies and above-average convection near the dateline. In the latest month or two the wind anomalies have weakened to near average (although a new spell of westerly anomalies just appeared in mid-May), while the convection near the dateline has continued. The coupling of the atmosphere to the oceanic conditions is still just adequate to support a weak El Niño. The subsurface temperature anomalies across the eastern equatorial Pacific have been above-average for the past 12 months, but have weakened markedly to just slightly above average in the last month. The waters at depth are still well above average close to the surface in the eastern Pacific, but renewed westerly low-level wind anomalies are needed to reinforce the positive heat content anomaly to allow the weak El Niño to last longer than just one or two more months.
What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued approximately one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it gave a 70% chance for El Niño for this northern summer season, dropping to 55-60% for fall season. An El Niño advisory is in effect. The latest set of model ENSO predictions, from mid-May, now available in the IRI/CPC ENSO prediction plume, is next discussed. As of mid-May, 85% of the dynamical or statistical models predict El Niño conditions for the May-Jul season. After May-Jul, the percentage of models forecasting El Niño decreases, dropping to 73% for Jun-Aug and then hovering mainly in the 60-69% range for the longer-lead seasons out to Jan-Mar 2020. No model predicts La Niña for any season.
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 near 0% for May-Jul and Jun-Aug, rising to 9% by Sep-Nov and to near 10% for the farther-out seasons out to Jan-Mar 2020. Probabilities for neutral conditions begin at 20% for May-Jul and rise to the 30-39% range throughout the rest of the forecast seasons out to Jan-Mar 2020. Probabilities for El Niño begin at 80% for May-Jul, thereafter declining to 60% for Jul-Sep and hovering in the 55-59% range for the remainder of the seasons out to Jan-Mar 2020. The failure to drop below 50% throughout 2019 suggests a possibility for a two-year El Niño event, but at this time, even in the second half of the northern spring predictability barrier, there is considerable uncertainty in this possibility. 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.
In summary, the probabilities derived from the models on the IRI/CPC plume describe, on average, a tilt of the odds toward El Niño conditions from May-Jun through Jul-Sep 2019, becoming weaker but still at least 55% through the final Jan-Mar 2020 season. Probabilities for La Niña are close to zero through Jul-Sep. 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 early next month by CPC and IRI, which will include some human judgment in combination with the model guidance.
IRI/CPC Mid-Month Model-Based ENSO Forecast Probabilities