IRI ENSO Forecast

IRI Technical ENSO Update

Published: September 20, 2021

Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv5. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv5 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 ERSSTv5, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv5 may tend to be cooler than OISSTv2, because ERSSTv5 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. In February 2021, both datasets were updated using the 1991-2020 climatology period.

Recent and Current Conditions

In mid-September 2021, SSTs remained close to average, across much of the equatorial Pacific Ocean. The August SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) was -0.44 C, and for Jun-Aug it was -0.3 C, while the most recent weekly anomaly was -0.4 C, still within the ENSO-neutral territory. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the SST anomaly in the NINO3.4 region exceed 0.5 C. Similarly, for La Niña, the anomaly must be -0.5 C or colder. 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 evolution of some key atmospheric variables is also consistent with ENSO-neutral conditions. The traditional and equatorial Southern Oscillation Indices that were in the near-neutral range last month, have now increased to levels outside neutral, similar to values seen during mild La Niña conditions. The Trade Winds near the surface were close to average across most of the eastern Pacific, but were stronger than average to the west of the international date line. The upper-level, westerly wind anomalies that would accompany a large-scale response to La Niña conditions were not noticeable. During August, the sub-surface SST showed cool anomalies in the eastern equatorial Pacific, while weak warm anomalies were observed to the west of the Date line.

In summary, the equatorial Pacific region currently remains in ENSO-neutral conditions. CPC announced a La Niña Watch in July 2021, and this Watch is still in effect for Sep 2021.

Expected Conditions

Note – Only models that produce a new ENSO prediction every month are considered in this statement.

What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued on 09 September 2021 in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it states that La Niña is favored during the coming Northern Hemisphere winter and into the early spring.

The latest set of model ENSO predictions from mid-September is now available in the IRI/CPC ENSO prediction plume. These are used to assess the probabilities of the three possible ENSO conditions by using the average value of the NINO3.4 SST anomaly predictions from all models on the plume, equally weighted. Currently, however, the NASA-GEOS model is not factored into the probabilistic update, even though it appears on the plume-of-models graphic. A standard Gaussian error is imposed over that average forecast, and its width is 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.

Using this method, chances for transitioning from current ENSO-neutral to La Niña is 60% for the Sep-Nov season, while chances for ENSO-neutral are 40%. During the Northern Hemisphere winter, the majority of models, as well as multi-model mean, suggest that tropical Pacific temperatures may cool to La Niña levels. Both dynamical and statistical models agree well this time on the likelihood of a second La Niña. ENSO-neutral again becomes the most likely outcome in 2022, with probabilities exceeding 50% from Feb-Apr onward.  El Niño probabilities start at 1% in Nov-Jan and slowly rise to around 20% by the late spring of 2022. 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.

Caution is advised in interpreting the forecast distribution from the Gaussian standard error as the actual probabilities, due to differing biases and performance of the different models. 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. At longer leads, the skill of the models degrades, and uncertainty in skill must be convolved with the uncertainties from initial conditions and differing model physics, which leads 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.

In summary, the probabilities derived from the models on the IRI/CPC plume indicate that La Niña is the most likely outcome through boreal winter – possibly extending into the spring of 2022. The likelihood for El Niño development remains very low during the entire forecast period.

A caution regarding the model-based ENSO plume predictions released mid-month, is that factors such as known specific model biases and recent changes in the tropical Pacific that the models may have missed, are not considered. This approach is purely objective. Those issues are taken into account in the official outlooks, which are generated and issued early in the month by CPC and IRI, and which will include some human judgment in combination with the model guidance.

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Discussion of Current Forecasts

Most of the models in the set of dynamical and statistical model predictions issued during mid-September 2021 show below-average SST conditions likely to cool further in coming months. During the Sep-Nov through Dec-Feb seasons, La Niña conditions are favored over ENSO-neutral. For Jan-Mar, the odds are more equal. This re-emergence of La Niña is predicted to potentially persist long enough to constitute a La Niña event. In 2022 late spring ENSO-neutral is again predicted as the most likely outcome (60-70%). In the most recent week, the SST anomaly in the NINO3.4 region was -0.4 C, indicative of ENSO-neutral, and -0.44 C for the month of August. As of mid-September, the subsurface water temperatures in the eastern equatorial Pacific are below-average, while above-average temperatures exist to the west; in both cases the temperature anomalies are relatively weak.

Dynamical and statistical models show coherent La Niña conditions with 60% likelihood for the Sep-Nov season, increasing to nearly 70% at the end of the year, and then decreasing again in early 2022. La Niña conditions is more likely than ENSO-neutral during rest of boreal autumn and winter months. Based on the multi-model mean prediction, and the expected skill of the models by start time and lead time, the probabilities (X100) for La Niña, neutral and El Niño conditions (using -0.5 C and 0.5 C thresholds) over the coming 9 seasons are:

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