Seamount-induced mixing revealed through idealized experiments and its parameterization in an Oceanic General Circulation Model

Abstract

Though hidden under hundreds if not thousands of meters of seawater, seamounts and the vertical mixing they induce are crucial components of mixing in the global ocean. To gain a deeper understanding of the influence of different seamount topographies on vertical mixing, and distributional characteristics and causes of seamount-induced vertical mixing in different grid scales, this study builds a three-dimensional idealized seamount in a regional ocean model. Results illustrate that, given a constant westward inflow, seamount-induced vertical mixing is mainly distributed on the north and south sides of the summit, the foot of the north and east sides, and downstream from the seamount. Mixing on the north and south sides of the seamount summit, and at its foot, is caused by density fronts induced by friction between ambient currents and the seamount. By contrast, mixing downstream of the seamount is due to convective instability, lee waves, and seamount wake eddies. Vertical mixing enhancement at subgrid scales mainly occurs on the southern and eastern sides of the seamount summit, at its foot, and at the seabed. Here, vertical mixing is primarily caused by friction in the bottom boundary layer. Sensitivity experiments reveal that mixing intensity is affected by seamount topographic roughness. As the seamount height and slope increases, vertical mixing at the seamount summit is enhanced, and mixing on its slope gradually shifts to the seamount base. Due to limitations of global ocean model resolutions, seamount-induced vertical mixing is often ignored. Consequently, a global background mixing coefficient (BMC) relationship is built from an Argo dataset-estimated BMC combined with seafloor topography roughness, and then applied to a global Oceanic General Circulation Model (OGCM). Results show that simulations of deep-sea water temperature and salinity improved after modifying the BMC in seamount-rich areas, with a simultaneous reduction of root-mean-square error by 17% and 12%, respectively.