Diapycnal mixing in submesoscale-permitting simulations of the deep Brazil Basin

Abstract. Modeling diapycnal mixing in the deep-ocean is challenging, particularly in regions with complex topography. Here we diagnose the relative roles of sub-inertial motions and tidal forcing focusing on the deep Brazil Basin in four simulations using a hydrostatic, high-resolution regional model (CROCO), at 1 km and 3 km horizontal resolution, in presence or absence of tides. Tracer particles are released at multiple depths to investigate the variability of modeled mixing estimates. In the model, horizontal resolution exerts the primary control on diapycnal mixing, while tidal forcing plays a secondary and resolution-dependent role. Increasing resolution significantly increases number and intensity of eddies and enhances diapycnal mixing across the water column. The comparison with the in-situ observations indicates that the simulated diffusivities near the bottom boundary layer are comparable in value to observational estimates. However, diffusivities in the stratified interior are overestimated due to bathymetric smoothing, which causes an underestimation of high-mode internal tides and allows eddy-driven motions to dominate.