May 2024 ENSO Discussion
May 2024 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.
Note: This month, a new deep learning-based ensemble forecast model (TONGJI-ML) for ENSO prediction has been added to the IRI-ENSO Plume. This model is developed by the climate research team at Tongji University, China. Further details can be found here: https://iri.columbia.edu/our-expertise/climate/enso/enso-prediction-models/
Recent and Current Conditions
The SST anomaly for NINO3.4 during the Feb-Apr 2024 season was +1.19 °C, and for the month of April 2024 it was +0.81 °C. The most recent weekly (week centered on 08 May 2024) anomaly in the NINO3.4 region was +0.30 °C, indicating that the El Niño conditions are now rapidly declining in the tropical Pacific. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the monthly 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 colder.
The oceanic and atmospheric conditions across the tropical Pacific indicate the continued decline of the El Niño event. As of May 16, 2024, the 30-day Southern Oscillation Index stands within the ENSO-neutral range, with a value of -3.0, while the equatorial Southern Oscillation Index registers a value of 0.6 for the month of April 2024. The Trade winds (at 850 hPa) are now normal across the tropical Pacific, while the upper-level winds (at 200 hPa) are also close to average over the equatorial Pacific. In the equatorial Pacific Ocean, negative subsurface temperature anomalies have intensified and extended further westward across the Pacific Ocean, persisting both below 50 meters and near the surface east of 140 W. Warm subsurface temperature anomalies (~ +1 °C) remain prevalent above 50 meters depth between 160E to 160W. These observed conditions in the coupled ocean-atmosphere system indicate a weak El Niño state that is diminishing, and a future transition to ENSO-neutral conditions.
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 El Niño/Southern Oscillation (ENSO) Diagnostic Discussion released on 09 May 2024 by the Climate Prediction Center/NCEP/NWS continued the current El Niño advisory, and issued a La Niña watch with 49% likelihood for Jun-Aug, increasing to 69% in Jul-Sep 2024.
The latest set of ENSO prediction models from mid-May 2024 is now available in the IRI ENSO prediction plume. These are used to assess the probabilities of the three ENSO categories 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 averaged forecast, with 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.
According to the ENSO forecast issued by the IRI, ENSO-neutral conditions are forecasted for next three overlapping 3-month seasons with 89% likelihood in May-Jul, 71% in Jul-Sep, and 54% in Jul-Sep, 2024. In Aug-Oct 2024, La Niña onset is most likely though the probability for that is just 49%, while the continuation of ENSO-neutral is estimated at 44%. La Niña then remains the dominant category for the remainder of the forecast period, with probabilities of 55% in Sep-Nov, 61% in Oct-Dec, 62% in Nov-Jan, 58% in Dec-Feb, and 54% in Jan-Mar, 2025. A plot of the probabilitiessummarizes the forecast evolution. The climatological probabilities for La Niña, ENSO-neutral, and El Niño conditions vary seasonally, and are shown by the lines on the plot, and are given 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 forecast indicates continued ENSO-neutral conditions until Jul-Sep, 2024. The forecast shows La Niña onset in Aug-Oct (49%), while the continuation of ENSO-neutral conditions is estimated with 44% chances. La Niña then remains the most likely category until the end of the forecast period in Jan-Mar 2025.
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 CPC’s official outlooks, which are issued early in the month, and which will include some human judgment in combination with the model guidance.