IRI ENSO Forecast
IRI Technical ENSO Update
Published: November 20, 2023
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 Aug-Oct 2023 season was +1.47 °C, and for the month of Oct 2023 it was +1.59 °C. The most recent weekly (15 Nov 2023) anomaly in the NINO3.4 region was +1.90 °C, indicating that the tropical Pacific is experiencing strong El Niño conditions. 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 tropical Pacific atmospheric and oceanic anomalies are consistent with an ongoing El Niño event. For example, the traditional and equatorial Southern Oscillation Indices, are both in the El Niño range (as of 18 Nov 2023, the last observed value of the traditional Southern Oscillation Index was -8.4), low-level easterly winds are weaker than normal near the International Date Line, while near normal over the eastern tropical Pacific Ocean. Upper-level winds are weak easterly over the central-western equatorial Pacific. Above-normal cloudiness is observed over the central and western Pacific Ocean, together with below-normal convection over Indonesia. In the equatorial Pacific Ocean, subsurface temperatures are generally warmer than average, with the exception of a small and isolated but persistent region of anomalously cold subsurface temperatures around 130ºW-120ºW, at depth (50-150m). A recent intense westerly wind burst has triggered the initiation of another downwelling oceanic Kelvin wave, tending to further warm the subsurface waters between 170ºW-140ºW .
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 Nov 2023 by the Climate Prediction Center/NCEP/NWS issued an El Niño advisory.
The latest set of ENSO prediction models from mid-November 2023 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.
The IRI ENSO prediction plume indicates a very high likelihood of El Niño conditions persisting during the rest of 2023 and first quarter of 2024. Specifically, during the winter and spring of 2024, the probabilities of El Niño range from 100% to 90% (Dec-Feb: 100%, Jan-Mar: 100%, Feb-Apr: 98%, and Mar-May: 87%). Thereafter, there is a rapid decrease in the probability of El Niño (Apr-Jun: 58%, May-Jul: 32%, Jun-Aug: 22%, and Jul-Sep: 19%). The second most probable category throughout the forecast period is ENSO-neutral. ENSO-neutral becomes the most likely category during May-Jul (60%) and remains so during Jun-Aug (56%) and Jul-Sep (50%). The probability of redevelopment of La Niña is zero until the spring of 2024, increasing to 8% in May-Jul, 22% in Jun-Aug, and 31% in Jul-Sep, 2024.
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 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 for the winter and early spring shows a very high probability of continued El Niño conditions. The chances of a return to ENSO-neutral conditions then rapidly increases to about 58% in May-Jul 2024 and becomes the most probable category thereafter. The chances of La Niña redevelopment are zero until the boreal spring of 2024, but increase progressively, reaching climatological odds in Jul-Sep 2024.
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 which will include some human judgment in combination with the model guidance.