Vortex Generation by Topography in Locally Unstable Baroclinic Flows

Abstract

The dynamics of a quasigeostrophic flow confined in a two-layer channel over variable topography on the beta plane is numerically investigated. The topography slopes uniformly upward in the north?south direction (in the beta sense) and is a smooth function of the zonal coordinate. The bottom slope controls the local supercriticality and is configured to destabilize the flow only in a central interval of limited zonal extent. Linearized solutions indicate that, for a wide enough channel, unstable modes exist for an arbitrary short interval of instability, confirming previous analysis on disturbances with no meridional variation. For small local maximum supercriticality, the instability is maintained by a short bottom-trapped wave localized at the downstream edge of the unstable region and oscillating in phase with the upper-layer disturbance. When nonlinearity is retained in the problem, the equilibration of the bottom-trapped wave is associated with the formation of coherent vortices. Both cyclones and anticyclones are formed continuously at the northeastern edge of the unstable interval. Through vortex stretching mechanisms, dipoles inside the interval of instability can split upon reaching the northern wall: Anticyclones move downstream along the north wall and propagate into the downstream stable region, while cyclonic structures tend to remain trapped inside the interval of instability. The authors suggest the relevance of their results to the observed eddy field of the Labrador Sea.