In December 2025, La Niña was reflected in the continuation of below-average sea surface temperatures (SSTs) across the east-central and eastern equatorial Pacific Ocean [Fig. 1]. The latest weekly Niño-3.4 index value was -0.5°C, with the Niño-3 and Niño-1+2 indices remaining cooler at -0.8°C and -0.7°C, respectively [Fig. 2]. The equatorial subsurface temperature index (average from 180°-100°W) became slightly positive [Fig. 3], reflecting the expansion of above-average temperatures from the western to the east-central Pacific at depth [Fig. 4]. Atmospheric anomalies across the tropical Pacific Ocean remained consistent with La Niña. For most of the month, easterly wind anomalies were present over the central equatorial Pacific, and upper-level westerly wind anomalies continued across the equatorial Pacific. Enhanced convection persisted over Indonesia and suppressed convection strengthened near the Date Line [Fig. 5]. The equatorial Southern Oscillation index was positive. Collectively, the coupled ocean-atmosphere system remains consistent with La Niña.

The IRI multi-model predictions indicate ENSO-neutral will emerge during January-March (JFM) 2026 [Fig. 6]. In conjunction with the North American Multi-Model Ensemble, the team favors ENSO-neutral to develop during JFM 2026. Even after equatorial Pacific SSTs transition to ENSO-neutral, La Niña may still have some lingering influence through the early Northern Hemisphere spring 2026 (e.g., CPC's seasonal outlooks). For longer forecast horizons, there are growing chances of El Niño, though there remains uncertainty given the lower accuracy of model forecasts through the spring. In summary, La Niña persists, followed by a 75% chance of a transition to ENSO-neutral during January-March 2026. ENSO-neutral is likely through at least Northern Hemisphere late spring 2026 [Fig. 7].