As of July 11, 2024, the NOAA National Centers for Environmental Prediction's (NCEP) El Niño-Southern Oscillation (ENSO) Alert System has changed to La Niña Watch. ENSO-neutral conditions continued this past month, as indicated by the mostly near-average sea surface temperatures (SSTs) across the east-central and eastern equatorial Pacific Ocean. The most recent weekly Niño-3.4 index was +0.3°C, while SST anomalies remained cooler in the eastern Niño-3 region (-0.1°C) and warmer in the western Niño-4 region (+0.5°C). Below-average subsurface temperatures weakened during the past month, but negative anomalies still dominated the eastern half of the Pacific. Low-level wind anomalies were easterly over the western equatorial Pacific, and upper-level winds were westerly over the eastern Pacific. Convection was near average around Indonesia and the International Date Line. Collectively, the coupled ocean-atmosphere system reflected ENSO-neutral conditions.

Compared to the previous month, the most recent IRI plume delayed the emergence of La Niña to September-November 2024, with La Niña then persisting through Northern Hemisphere winter. The forecast team is also favoring a delayed development of La Niña again this month, but is anticipating the transition to occur earlier (i.e., August-October). This is, in part, supported by the continuation of below-average subsurface ocean temperatures and near-term forecasts, suggesting a resurgence of easterly wind anomalies in July.

In summary, ENSO-neutral is expected to continue for the next several months. However, La Niña is favored to emerge during August-October (70% chance) and persist into Northern Hemisphere winter 2024-2025 (79% chance during November 2024-January 2025).

The latest Columbia Climate School International Research Institute (IRI) for Climate and Society ENSO predictions plume, pictured just above and published on July 19, 2024, 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. As of mid-July 2024, ENSO-neutral conditions persist in the western equatorial Pacific, and oceanic and atmospheric indicators also align with ENSO-neutral. The IRI ENSO predictions plume forecasts ENSO-neutral conditions for July-September and August-October 2024. La Niña then becomes the most probable category in September-November 2024 through December 2024-February 2025. For January-March 2025, ENSO-neutral conditions become dominant (50% chance), and remain so during February-April and March-May 2025.

Similar to the most recent, official CPC ENSO Outlook (issued on July 11, 2024), the objective IRI model-based ENSO outlook forecasts indicate ENSO-neutral conditions for July-September 2024. However, there is a notable difference between the probability numbers in the early-month CPC and mid-month IRI ENSO forecasts. The CPC ENSO forecasts predict a 70% chance of La Niña onset in August-October 2024; they also predict La Niña will persist, with increasing probabilities reaching 66-81%, during the boreal autumn and winter seasons. In contrast, the objective IRI ENSO forecasts show a late onset of La Niña conditions in the equatorial Pacific (49% chance in September-November 2024), with persistence into the fall and early winter, though with lower probabilities (48-52%). The forecasts then show a return to ENSO-neutral conditions in January-March 2025, and ENSO-neutral remains dominant for the rest of the forecast period. The low probability numbers reflect a high level of uncertainty in the forecasts, due to the relatively low skill of seasonal forecast models at this time of year. Therefore, these ENSO forecasts should be interpreted with caution.

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