The 2020-2021 prolonged La Nina evolution in the tropical Pacific

The evolution of sea surface temperature (SST) in the tropical Pacific during 2020-2021 indicates a second-year cooling in late 2021 again, following the 2020 La Nina event. Its physical explanations are still lacking, and there is a clear need to understand the underlying processes involved. Observational data and reanalysis products are used to describe the characteristics and spatiotemporal evolution of upper-ocean thermal anomalies; an intermediate coupled model (ICM) is also used to perform numerical experiments to confirm these observation-based inferences. The evolution of subsurface thermal anomalies is critically important to that of SST in the central-eastern equatorial Pacific; the effects of the former on the latter can be well represented by the temperature of subsurface waters entrained into the mixed layer (T-e), a field that reflects a subsurface forcing on SST. The SST evolution is sensitively dependent on the intensities of the local effect associated with T-e anomalies in the eastern equatorial Pacific and the remote effect associated with subsurface anomalies from the western Pacific. During early- and mid-2021, a competition was present between these local and remote effects associated with T-e anomalies. When the remote warming effect dominates the local cooling effect, the cold SST condition in the east is likely to turn into neutral and warm conditions; otherwise, it tends to continue. In addition, the negative T-e anomalies were sustained and enhanced by off-equatorial processes due to equatorial wave reflections at the eastern boundary associated with the 2020 La Nina event. The SST evolution in mid-2021 corresponded to a situation in which the warming effect associated with positive subsurface thermal anomalies from the west were not strong enough to counteract the local cooling effect associated with negative anomalies in the east. In due course, cold SST anomalies in the east developed again and the second-year cooling reoccurred in late 2021, with a turning point in June 2021. Modeling experiments support these arguments and indicate that the intensity of subsurface thermal effect on SST, as represented by T-e anomalies, needs to be adequately depicted for coupled models to capture the 2021 second-year cooling conditions in the tropical Pacific.

Automated summary

The evolution of sea surface temperature in the tropical Pacific during 2020-2021, specifically the second-year cooling phenomenon, lacks clear physical explanations. Observational data and numerical experiments were used to analyze the characteristics and evolution of upper-ocean thermal anomalies, highlighting the importance of subsurface thermal anomalies on sea surface temperature. The findings have significant implications for capturing the second-year cooling conditions in the tropical Pacific using coupled models.