Mesoscale eddies are ubiquitous dynamical features, accounting for over 90% of the total kinetic energy of the ocean. However, the pathway for eddy energy dissipation has not been fully understood. Here we investigate the effect of small-scale topography on eddy dissipation in the northern South China Sea by comparing high-resolution ocean simulations with smooth and synthetically-generated rough topography. The presence of rough topography is found to (1) significantly enhance viscous dissipation and instabilities within a few hundred meters above the rough bottom, especially in the slope region, and (2) change the relative importance of energy dissipation by bottom frictional drag and interior viscosity. The role of lee wave generation in eddy energy dissipation is investigated using a Lagrangian filter method. About one-third of the enhanced viscous energy dissipation in the rough topography experiment is associated with lee wave energy dissipation, with the remaining two-thirds explained by nonwave energy dissipation, at least partly as a result of the nonpropagating form drag effect.
Recent studies show that the western boundary acts as a ‘graveyard’ for westward-propagating ocean eddies. However, how the eddy energy incident on the western boundary is dissipated remains unclear. Here we investigate the energetics of eddy-western boundary interaction using an idealised MIT ocean circulation model with a spatially variable grid resolution. Four types of model experiments are conducted: (1) single eddy cases, (2) a sea of random eddies, (3) with a smooth topography and (4) with a rough topography. We find significant dissipation of incident eddy energy at the western boundary, regardless of whether the model topography at the western boundary is smooth or rough. However, in the presence of rough topography, not only the eddy energy dissipation rate is enhanced, but more importantly, the leading process for removing eddy energy in the model switches from bottom frictional drag as in the case of smooth topography to viscous dissipation in the ocean interior above the rough topography. Further analysis shows that the enhanced eddy energy dissipation in the experiment with rough topography is associated with greater anticyclonic-ageostrophic instability (AAI), possibly as a result of lee wave generation and non-propagating form drag effect.
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