March 2022 ENSO Discussion

March 2022 ENSO Discussion

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. These two products may differ, particularly during ENSO events. The difference between the two datasets may be as much as 0.5 C. Additionally in some years, the ERSSTv5 may tend to be cooler than OISSTv2 in the context of warming trends, 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. In February 2021, both datasets were updated to reflect the 1991-2020 climatology period.

Recent and Current Conditions

The SST anomaly for NINO3.4 during the Dec-Feb season was -0.86 C, and for the month of February it was -0.71 C. The most recent weekly (09 Mar 2022) anomaly in the NINO3.4 region was -1.0 C, suggesting a reinforcement of the current weak La Niña conditions. 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.

Many of the key atmospheric variables remain indicative of La Niña conditions, such as the traditional and equatorial Southern Oscillation Indices, which strengthened in February 2022. The low-level easterly wind anomalies are slightly above average and upper-level wind anomalies remain westerly across the tropical Pacific, and anomalously dry conditions have been observed around the date line. For the ocean, subsurface temperatures anomalies in the western equatorial Pacific are above average, while below normal subsurface temperatures are located in the central Pacific (between 170W to 120W). In summary, tropical Pacific atmospheric and oceanic conditions are consistent with La Niña. A La Niña advisory is still in place.

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 10 March 2022 in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI. It indicates a continuation of the current La Niña event during the Northern Hemisphere summer (53% chance in Jun-Aug 2022).

The latest set of model ENSO predictions from mid-March 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 in the plume, equally weighted. 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 La Niña are 64% for the Apr-Jun season, while chances for ENSO-neutral are 36%. Going forward, probabilities for La Niña decrease to 46% by May-Jul, and remain 30-40% for rest of the forecast period. A transition from La Niña to ENSO-neutral is forecast during May-Jul (53% chance), which remains the most likely category until Jul-Sep 2022; however, probabilities remain in the low 50s (near their climatological probabilities), indicative of large uncertainty in the forecast. El Niño probabilities remain very low during the forecast period. A plot of the probabilities summarizes the forecast evolution. The climatological probabilities for La Niña, ENSO-neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season.

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 in the IRI/CPC plume indicate a high preference for the continuation of a weak La Niña during Apr-Jun of 2022, after which ENSO-neutral conditions becomes the most likely outcome for next three seasons (May-Jul, Jun-Aug, and Jul-Sep), with near-equal odds of La Niña or ENSO-neutral for rest of the forecast period. The likelihood for El Niño development remains very low during the 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.