Boundary-layer-mediated vorticity generation in currents over sloping bathymetry

Abstract

Current–topography interactions in the ocean give rise to eddies spanning a wide range of spatial and temporal scales. The latest modeling efforts indicate that coastal and underwater topography are important generation sites for submesoscale coherent vortices (SCVs), characterized by horizontal scales of O (0:1-10) km. Using idealized, sub-mesoscale and bottom boundary layer (BBL)-resolving simulations and adopting an integrated vorticity balance formu-lation, we quantify precisely the role of BBLs in the vorticity generation process. In particular, we show that vorticity generation on topographic slopes is attributable primarily to the torque exerted by the vertical divergence of stress at the bottom. We refer to this as the bottom stress divergence torque (BSDT). BSDT is a fundamentally nonconservative torque that appears as a source term in the integrated vorticity budget and is to be distinguished from the more familiar bottom stress curl (BSC). It is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom and is in fact shown to be the dominant component of BPT in solutions with a well-resolved BBL. This suggests an interpretation of BPT as the sum of a viscous, vorticity-generating component (BSDT) and an inviscid, ‘‘flow-turning’’ component. Companion simulations without bottom drag illustrate that although vorticity generation can still occur through the inviscid mechanisms of vortex stretching and tilting, the wake eddies tend to have weaker circulation, be substantially less energetic, and have smaller spatial scales.