Forced Symmetric and Centrifugal Instability in the Bottom Boundary Layer

Flow along sloping topography generates an across-isobath Ekman transport in the bottom boundary layer (BBL). When this Ekman transport is downslope, it leads to convective instability that destroys stratification and potential vorticity, acting much like a 'downfront' wind in the surface boundary layer. Here we show, using theory and high-resolution numerical models, that this allows for persistent forced symmetric or centrifugal instability in the BBL. These submesoscale instabilities extract energy directly from the balanced background flow, enhancing turbulent dissipation in the boundary layer, and potentially altering the time-dependent bottom drag. The connection of these BBL instabilities to the stability and energetics of submesoscale topographic wakes will also be discussed, particularly in light of the very different mixing efficiencies associated with the symmetric and centrifugal modes, and the related implications for water mass transformation generated by flow encountering topography.