Triple-Dip La Ninas in 1998-2001 and 2020-2023: Impact of Mean State Changes

This study compares the evolution of atmospheric and oceanic anomalies as well as predictions for the two most recent triple-dip La Nina events in 1998-2001 and 2020-2023. Subsurface cooling in the equatorial Pacific was stronger and more persistent during 1998-2001. In contrast, surface easterly winds were stronger during 2020-2023 as was the east-west sea surface temperature (SST) contrast along the equator. We argue that in the absence of appreciable equatorial Pacific heat discharge, persistent and strong surface trade winds and a strengthened mean zonal SST contrast across the tropical Pacific contributed to the development of the 2020-2023 triple-dip La Nina. In terms of the subsurface layer heat budget, the growth and maintenance of unusually cold SSTs during the triple-dip La Nina in 1998-2001 were mainly the result of ocean vertical entrainment and diffusion, as well as meridional advection, associated with enhanced equatorial upwelling; while for the triple-dip La Nina in 2020-2023, zonal advection was the largest contributor. The two events were mostly well predicted by multi-model averages at 1-8 months lead times. We hypothesize that mean state change with enhanced zonal SST contrast and trade winds over the last several decades altered the physical processes associated with the growth and maintenance of the most recent La Nina, affecting its predictability. Successful prediction in real-time of the 2020-2023 event more than half a year in advance was surprising because there was little memory in oceanic heat content which is often considered a key predictor.