El Niño-Southern Oscillation (ENSO) Conditions

As of May 11, 2023, the NOAA National Centers for Environmental Prediction's (NCEP) El Niño-Southern Oscillation (ENSO) Alert System is at El Niño Watch. During April, above-average sea surface temperatures (SSTs) expanded slightly westward to the east-central equatorial Pacific Ocean. The latest weekly Niño-3.4 index value was +0.4°C, with the easternmost Niño-3 and Niño-1+2 regions at +0.8°C and +2.7°C, respectively. Area-averaged subsurface temperature anomalies continued to increase, reflecting widespread positive temperature anomalies below the surface of the equatorial Pacific. Low-level wind anomalies were westerly during mid-April, before switching back to easterly by the end of the month. Upper-level wind anomalies were westerly across most of the Pacific. Suppressed convection was observed over parts of Indonesia, and anomalies weakened near the International Date Line. While the warming near coastal South America remains striking, the basin-wide coupled ocean-atmosphere system remained consistent with ENSO-neutral.

The most recent IRI plume (pictured and discussed below) indicates El Niño is likely to form during the June-August 2023 season. It is expected to persist into the Northern Hemisphere winter. The combination of a forecasted third westerly wind event in mid-late May, and high levels of above-average oceanic heat content means that a potentially significant El Niño is on the horizon. While at least a weak El Niño is likely, the range of possibilities at the end of the year (November 2023-January 2024) includes an 80% chance of at least a moderate El Niño (Niño-3.4 ≥ 1.0°C) to about a 55% chance of a strong El Niño (Niño-3.4 ≥ 1.5°C). It also is possible that the tropical atmosphere does not couple with the ocean, and El Niño fails to materialize (5-10% chance).

In summary, a transition from ENSO-neutral is expected in the next couple of months. As the figure directly above indicates, there is a greater than 90% chance the El Niño will persist into the Northern Hemisphere winter.


The latest IRI ENSO prediction plume, pictured just above and published on May 19, 2023, shows forecasts made by a set of dynamical and statistical models for SST in the Niño-3.4 region, for nine overlapping three-month periods. The SST anomaly for Niño-3.4 during the February-April 2023 season was -0.08 °Celsius (°C), and for the month of April 2023 it was +0.19 °C. The most recent weekly (May 10, 2023) anomaly in the Niño-3.4 region was 0.5 °C, indicating that the tropical Pacific is experiencing developing El Niño conditions. The IRI's definition of El Niño, like the NOAA Climate Prediction Center's, requires that the SST anomaly in the Niño-3.4 region (5 °S-5 °N; 170 °W-120 °W) exceeds 0.5 °C. Similarly, for La Niña, the anomaly must be -0.5 °C or colder.

The key atmospheric variables currently indicate a developing El Niño. The traditional and equatorial Southern Oscillation Indices are in the El Niño range (as of May 16, 2023, the last observed value of the traditional Southern Oscillation Index was -7.1), and the low-level easterly winds have weakened in the central and western tropical Pacific. Upper-level wind anomalies remain westerly across the tropical Pacific, and near-normal cloudiness has been observed over the central and western Pacific Ocean. Since February 2023, there has been a significant increase in SSTs in the equatorial Pacific, particularly along the coast of South America (as of May 10, 2023, the SST anomaly in the Niño-1+2 region was +2.4 °C). This warming is due to warmer subsurface temperatures and a downwelling Kelvin wave, leading to the declaration of coastal El Niño conditions.

Per the IRI-ENSO plume, the forecasts indicate a high likelihood of El Niño. Specifically, during the boreal summer and autumn, the probabilities of El Niño range from 79% to 88%, as follows: June-August: 86%, July-September: 88%, August-October: 88%, September-November: 84%, and October-December: 79%. Thereafter, there is a slight decrease in the probability of El Niño during the upcoming boreal winter (November 2023-January 2024: 76%, December 2023-February 2024: 72%, and January-March 2024: 68%). The second most probable category throughout the forecast period is ENSO-neutral, with probabilities ranging from 12% to 29%. The redevelopment of La Niña is highly unlikely, with chances of less than 5%.

In summary, most of the models in the IRI ENSO-plume above now indicate the arrival of El Niño in June‚ÄďAugust 2023. El Niño remains the dominant category during the rest of the forecast period (68-88% chance). We will continue to provide updates as the forecasts change.


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