At The Surface, La Nina Strengthened Last Week. However, Subsurface Cool Fuel Diminishing.

Climate Impact Company Weekly ENSO Climate Diagnostics

Issued: Monday December 15, 2025

Highlight: At the surface, La Nina strengthened last week. However, subsurface cool fuel to sustain La Nina is steadily diminishing.

Fig. 1-3: The 12-week monitoring of the Nino SSTA regions and subsurface temperature anomaly schemes across the equatorial Pacific Ocean.

Discussion:During the past week, the Nino34 and Nino3 sea surface temperature anomaly (SSTA) regions across the east-central and east equatorial Pacific Ocean cooled to the second strongest level of the weak late 2025 La Nina episode (Fig. 1). The central equatorial Pacific and off the northwest coast of South America SSTA are near normal. Of interest is the appearance of a Kelvin Wave shifting east of the Dateline during early December (Fig. 2). Subsurface waters of the upper 150 meters of the equatorial Pacific near the Dateline and eastward to the northwest coast of South America remain cooler than normal although less cool than 2-3 weeks ago. Consequently, the upper ocean heat east of the Dateline has warmed to near normal (Fig. 3). The Kelvin Wave requires close monitoring as an eastward shift across the equatorial Pacific signals ending to La Nina with potential for El Nino to follow. According to NCEP CFS V2, in 2026, El Nino remains in the forecast with onset occurring during meteorological summer 2026 (Fig. 4).

Fig. 4: The Nino34 SSTA forecast utilizing the CFS V2 model maintains El Nino risk by JUN/JUL/AUG 2026.