FY 2023

Triple-dip La Niñas in 1998-2001 and 2020-2023: Impact of mean state changes

Li, X., Z.-Z. Hu, M.J. McPhaden, C. Zhu, and Y. Liu

J. Geophys. Res., 128(17), e2023JD038843, doi: 10.1029/2023JD038843, View article online at AGU/Wiley (external link) (2023)

This study compares the evolution of atmospheric and oceanic anomalies as well as predictions for the two most recent triple-dip La Niña 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 Niña. In terms of the subsurface layer heat budget, the growth and maintenance of unusually cold SSTs during the triple-dip La Niña 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 Niña 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 Niña, 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.

Plain Language Summary. El Niño-Southern Oscillation is the strongest interannual variability on Earth and the main source of global seasonal climate predictability. Here, we examine the evolution of oceanic and atmospheric anomalies in the tropical Pacific during two triple-dip La Niñas in 1998–2001 and 2020–2023, comparing the physical processes that gave rise to them and the skill in predicting them. Our results show that the processes giving rise to these events were different and likely affected by mean state changes in the tropical Pacific. In particular, the easterly trade winds and zonal SST contrast across the basin have strengthened that played a crucial role in the growth, maintenance, and prediction of the La Niña in 2020–2023. The evolution of the La Niña in 2020–2023 was successfully predicted in real-time more than half a year in advance, which is surprising because there was little memory in oceanic heat content which is often considered a key predictor.