IRI ENSO Forecast
IRI Technical ENSO Update
Published: February 19, 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 Nov–Jan 2023 season was +1.89 °C, and for the month of Jan 2024 it was +1.78 °C. The most recent weekly (week centered on 14 Feb 2024) anomaly in the NINO3.4 region was +1.50 °C, indicating that the tropical Pacific is experiencing moderate-strong El Niño conditions but which have started to decline. 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.
Oceanic and atmospheric conditions over the tropical Pacific are consistent with an ongoing El Niño event, though atmospheric indicators are quite weak as compared to past El Niño events of similar magnitude. The traditional Southern Oscillation Index is in ENSO-neutral range (as of January 2023, the last recorded index value was 3.7). Simultaneously, the equatorial Southern Oscillation Index registers a value of -1.0 for the same month. While the equatorial Southern Oscillation Index suggests El Niño conditions, its present value does not align with the characteristics of a strong event. Indonesia was characterized by positive anomalies in Outgoing Longwave Radiation (OLR), indicating suppressed convection and reduced precipitation. Conversely, negative OLR anomalies, denoting enhanced convection and increased precipitation, were observed near the Date Line, extending into the eastern Pacific just north of the equator. The Trade winds (at 850 hPa) were normal across most of the tropical Pacific, while easterly anomalies characterized the upper-level winds (at 200 hPa) over the equatorial Pacific. Throughout the equatorial Pacific Ocean, there is a widespread presence of warm temperature anomalies, covering the central to eastern Pacific region from surface to 50 meters in depth. Concurrently, negative subsurface temperature anomalies have further intensified and expanded westward across the Pacific Ocean, persisting at depths below 50 meters. Together, the current observed state of the coupled ocean-atmosphere system reflects ongoing El Niño conditions that are gradually declining.
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 on 08 February 2024 by the Climate Prediction Center/NCEP/NWS released the El Niño advisory, and also issued a La Niña watch (55% chance) for Jun-Aug, 2024.
The latest set of ENSO prediction models from mid-Feb 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.
The forecast from the IRI for ENSO suggests a strong probability of El Niño conditions continuing over the next few months, followed by a shift to ENSO-neutral conditions in the boreal spring, susequently transitioning to La Niña during boreal summer. In particular, El Niño is the dominant category, with values of 83% during Mar-May, 2024. Thereafter, there is a rapid decrease in the probability of El Niño and a transition to ENSO-neutral is likely in Apr-Jun (72%). The ENSO-neutral category remains dominant during May-Jul (71%). During Jun-Aug, ENSO-neutral (46%) and La Niña (49%) are almost equally likely. La Niña then becomes the dominant category for the remainder of the forecast period, with probabilities of 65% in Jul-Sep, 71% in Aug-Oct, 68% in Sep-Nov, and 69% in Oct-Dec, 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 Feb-Apr 2024 shows a very high probability of continued El Niño conditions. The chances of a transition to ENSO-neutral conditions then rapidly increases to about 72% in Apr-Jun, and remains so for May-Jul 2024. La Niña becomes the most likely category in Jul-Sep through the end of the forecast period in Oct-Dec 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.