Thermodynamic processes-induced prolongating effects on triple La Niña events in a hybrid coupled ocean-atmosphere model

Abstract

Observed ENSO cycles show apparent asymmetry, especially manifested as frequent occurrences of multi-year La Niña events but mostly single-year El Niño events, whose causes are still not well understood. Most previous research has primarily based on observational data and reanalysis products; current coupled ocean-atmosphere models still have difficulties in realistically depicting multi-year La Niña events. A new hybrid coupled model (HCM)-based 1000-year simulation presented here can adequately depict the evolution of triple La Niña events, with sensitivity experiments further performed to explore their physical mechanisms. The results reveal that off-equatorial Pacific Meridional Mode (PMM)-like sea surface temperature (SST) anomalies can promote triple La Niñas by establishing a surface cooling in the first place that is located in the central equatorial Pacific. When the PMM-related off-equatorial cold SST anomalies weaken, the equatorial surface cooling in the western-central equatorial Pacific can be sustained by thermodynamic coupling processes (wind speed–evaporation–SST feedback). Further sensitivity experiments and diagnostic analyses are conducted to confirm that the thermodynamic processes can prolong the triple La Niña events by intensifying the effect of the off-equatorial cold SST anomalies on the equatorial surface cooling and enhancing the local ocean-atmosphere coupling. Therefore, adequately representing the PMM-like SST anomalies and the related thermodynamic processes is an important factor affecting a way for coupled models to depict multi-year La Niña events.