Oceanic bottom boundary layers: their role in submesoscale vorticity generation, frontogenesis and dissipation

In this talk I review some recent progress towards unravelling the dynamics of oceanic bottom boundary layers over ocean topography. The talk is divided into two parts. The first part is focused on topographic vorticity generation and frontogenesis. Using idealized, submesoscale resolving simulations, we quantify precisely the role of bottom boundary layers (BBLs) in the vorticity generation process. The key finding is that a heretofore overlooked term, the bottom stress divergence torque (BSDT) is the primary source of vorticity generation on topographic slopes. BSDT is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom. The merits of BSDT over BPT in analyzing vorticity budgets becomes evident when applied to a realistic 1.5 km solution of the Western Mediterranean. This solution also reveals the role of topographic interactions in the emergence and intensification of an interior front. The second part of the talk is on BBL instabilities. Boundary currents flowing past seamounts precondition the BBL on the slopes towards developing instabilities associated with the potential vorticity reversing sign. We show that a useful characterization of these instabilities can be obtained by analyzing the sources of their turbulence kinetic energy and the structure of potential vorticity within the BBL. Both of these in turn depend on the non-dimensional seamount height $hat{h}$ (or slope Burger number) and lateral aspect ratio. For elongated seamounts , increasing $hat{h}$ has the effect of reducing the bottom stress in the downstream direction, owing to buoyancy adjustment within the BBL. This dynamical buoyancy adjustment on slopes suppresses BBL turbulence and dissipation. Interestingly however, the preconditioning  due to Ekman adjustment simultaneously renders the BBL flow susceptible to one of the types of negative potential vorticity instability - centrifugal/symmetric/gravitational or hybrid modes. We find that the equilibration and dissipation of these instabilities partially compensates for the reduced turbulent dissipation in the BBL. The most intense dissipation in our simulations is seen in the wake of circular seamounts ($\beta=1$), and is identified as arising primarily from a centrifugal mode of instability. By contrast, the hybrid modes that develop along the slope of elongated seamounts are only modestly dissipative relative to a pure centrifugal mode.