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

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 simulations using a hydrostatic, high-resolution regional model (CROCO) at 1, 3 and 6 km horizontal resolution, in presence or absence of tides. Tracer particles are released at multiple depths to investigate the variability of modeled mixing estimates. Horizontal resolution exerts the primary control on diapycnal mixing, while tidal forcing plays a secondary and resolution-dependent role. Increasing resolution significantly increases the number and intensity of eddies and enhances diapycnal mixing across the water column, from the bottom to at least 3000 m. The comparison with the in-situ observations indicates that the simulated diffusivities near the bottom boundary layer are comparable in value to observational estimates in the 1 and 3 km cases and too small in the 6 km run. However, diffusivities in the stratified interior are overestimated and the model-observation discrepancy grows for increasing resolution. At submesoscale-permitting resolutions, due to bathymetric smoothing, high-mode internal tides remain significantly underestimated, while submesoscale eddy-driven motions are overrepresented, leading to the biased estimate of diapycnal mixing.