Advancing models of ocean mixing in the equatorial Pacific Cold tongue using an embedded LES approach

The equatorial Pacific cold tongue is the most significant regional hotspot of ocean heat uptake and carbon outgassing on Earth, and it is the center of action of El Niño. Thus, it is critical for global Earth system models to simulate the cold tongue physics accurately. However, cold tongue bias persists in many global Earth system models, and contributes to significant difficulties simulating and predicting global climate. Hence, process studies of equatorial Pacific upwelling and mixing physics have been prioritized by the community to advance global climate prediction. Toward that priority, I conducted a process study of cold tongue mixing physics using large eddy simulations (LES) embedded in a regional ocean model. These LES allow the study of explicit ocean turbulent mixing as it responds to evolving atmospheric and oceanic forcing at timescales from minutes to months (>350 simulated days in all). Several pioneering results have been obtained, including the first explicit simulations of the modulation of cold tongue turbulence by a tropical instability wave (TIW) on and off the equator, an empirical scaling that explains ~80% of the daily variance in cold tongue mixing, and evidence that changes to the KPP parameterization can improve the simulation of cold tongue mixing as well as shear and stratification in global simulations with MOM6. These results can be used to guide both future analysis and model development efforts as well as observational process studies at sea.