Tropical Pacific Quasi-Decadal Variability Suppressed by Submesoscale Eddies 

Tropical Pacific quasi-decadal (TPQD) climate variability is characterized by quasi-decadal sea surface temperature variations in the central Pacific. This low-frequency climate variability is suggested to influence extreme regional weather and substantially impact global climate patterns and associated socio-economy through teleconnections. Previous studies mostly attributed the TPQD climate variability to basin-scale air-sea coupling processes. However, due to the coarse resolution of the majority of the observations and climate models, the role of sub-basin-scale processes in modulating the TPQD climate variability is still unclear. Using a long-term high-resolution global climate model, we find that energetic small-scale motions with horizontal scales from tens to hundreds of kilometers (loosely referred to as equatorial submesoscale eddies) act as an important damping effect to retard the TPQD variability. During the positive TPQD events, compound increasing precipitation and warming SST in the equatorial Pacific intensifies the upper ocean stratification and weakens the temperature fronts along the Pacific cold tongue. This suppresses the growth of submesoscale eddies as well as their associated upward vertical heat transport by inhibiting baroclinic instability and frontogenesis; Conversely, during the negative TPQD events, the opposite is true. Using a series of coupled global climate models that participated in the Coupled Model Intercomparison Project Phase 6 with different oceanic resolutions, we show that the amplitude of the TPQD variability becomes smaller as the oceanic resolution becomes finer, providing evidence for the impacts of submesoscale eddies on damping the TPQD variability. Our study suggests that explicitly simulating equatorial submesoscale eddies is necessary for gaining a more robust understanding of low-frequency tropical climate variability.