Oceanic La Nina Ends; Atmospheric La Nina Continues

Highlight: ENSO is neutral; Borderline La Nina climate remains. Strong Kelvin Wave just east of Dateline.

Fig. 1: The 12-week Nino SSTA tracker reveals ENSO has shifted into neutral phase based on the Nino34 SSTA observations.

Discussion: Oceanic ENSO is now in neutral phase based on the recent weekly Nino34 SSTA observations (Fig. 1). SSTA is formidably warm off the northwest coast of South America in the Nino12 region. Atmospheric ENSO remains in a La Nina climate which is steadily weakening (Fig. 2). The daily southern oscillation index (SOI) has shifted to negative phase the past few days due to passage of the convection phase of the Madden Julian oscillation (MJO) through the eastern equatorial Pacific. Trade winds have eased and subsurface warming in the eastern equatorial Pacific has increased (Fig. 3). However, the most important part of this week’s diagnostic report is an eastward shift of an aggressive Kelvin Wave to just east of the Dateline. The next MJO event to move through the equatorial East Pacific could push this warmth to the South America coast and trigger an El Nino onset. The latest NCEP CFS V2 Nino34 SSTA forecast indicates chance of a strong El Nino later this year (Fig. 4).  

Fig. 2: A comparison between the Nino34 SSTA and MEI for the 2020-23 La Nina episode. Oceanic La Nina ends in February while atmospheric La Nina continues.

Fig. 3: The subsurface equatorial Pacific Ocean is warming as cool water fuel to drive La Nina is diminishing rapidly. A strong Kelvin Wave has shifted east reaching the Dateline during the past 1-2 weeks.

Fig. 4: The latest Nino34 SSTA forecast from NOAA reveals risk of a strong El Nino later this year.