ABSTRACT

A process study is conducted on the evolution of boundary currents in a two-layer quasigeostrophic model on the f plane. These currents are composed of two strips of uniform potential vorticity (PV), one in each layer, and both hugging the coast. Coastal water separation (“detrainment”) through baroclinic instability and topographic perturbation is examined. It is shown that the key characteristics of the flow finite-amplitude destabilization can be explained with the help of a linear quantity—the critical amplitude A_c—that refers to the location of the line (often called critical layer) where the phase speed of the growing perturbation is equal to the unperturbed flow velocity. Notably, prediction on PV front breaking location is made possible. Different detrainment regimes (i.e., the way fragments of the boundary current are isolated and detached from the initially rectilinear core—e.g., filament formation, eddy shedding) are also identified, related to various A_c value ranges, and compared with observed oceanic events.