Fig. 1: The NCEP CFS V2 ENSO phase forecast for 2021 indicates an intensifying La Nina.
Discussion: The updated NCEP CFS V2 indicates La Nina continues with moderate intensity through April and intensifies to a “strong” La Nina the second half of the year (Fig. 1). The NCEP CFS V2 forecast is in opposition to most other forecast models which favor weakening La Nina in 2021. The ENSO forecast for 2021 is clearly uncertain. The widespread anomalous warmth of the Pacific Ocean surface away from the tropics implies ENSO events (whether El Nino or La Nina) have a tendency to be suppressed and of short duration. However, subsurface cool waters in the eastern equatorial Pacific remain in good supply and if trade winds persist at stronger than normal intensity the La Nina episode could strengthen. The most dramatic implication by this low confidence forecast that La Nina intensifies later in 2021 is another very active North Atlantic tropical cyclone season.
Currently, Nino SSTA regions indicate La Nina remains in moderate intensity (Fig. 2). Waters off the northwest coast of South America have cooled. Southern oscillation index (SOI) is very positive and indicates the atmosphere is in full-throttle La Nina response to the cool waters in the eastern equatorial Pacific Ocean.
Fig. 2: The 12-week trend of the Nino SSTA regions indicate moderate-strength La Nina continues.
There is risk of some weakening of La Nina the next 2-3 weeks if strengthening of the Madden Julian oscillation (MJO) occurs across the equatorial Pacific Ocean as forecast by most models.