Abstract:

Flow interacting with bathymetry has been posited to be important for dissipation and mixing in the global ocean. Despite this, there are large uncertainties regarding mixing in these environments, particularly as it pertains to the role of submesoscale structures in the dynamics and energetics. We focus on centrifugal–symmetric instabilities (CSIs), which is a class of submesoscale phenomena that has been shown to be particularly effective at removing energy from the balanced flow, and study their connections with flow-bathymetry encounters using Large-Eddy simulations. Consistent with hypotheses from previous studies, but resolved here for the first time up to small scales, we show direct evidence of submesoscale CSIs in the wake leading to a forward energy cascade. We also find that CSIs dominated by horizontal shear production tend to have a higher mixing efficiency than ones driven by vertical shear production. Moreover, the kinetic energy (KE) dissipation rate, buoyancy mixing rate, and eddy diffusivity of bathymetry-generated turbulence organize as linear functions of the bulk Rossby and Froude numbers across all simulations, despite very different dynamical regimes. The slope Burger number (Rossby over Froude number) was found to be particularly useful as it can organize aspects of both the dynamics and energetics. This suggests a different effect of submesoscales on flow energetics in the bottom from what has been observed in the upper ocean, where CSI turbulence seems to follow a different scaling from their non-CSI counterparts. Finally, comparison of KE dissipation rates with previous works suggests an underestimation of dissipation rates by regional models of up to an order of magnitude, with potential implications for global energy budgets.