Steady baroclinic flow through ridges with narrow gaps

The steady baroclinic flow in a basin containing a meridional barrier repre senting a midocean ridge is studied in the linear, quasigeostrophic limit o f a two-layer model. Thermal damping and a simple friction provide dissipat ion of thickness (heat) and momentum. The ridge is pierced by two gaps in t he upper layer but only a single gap in the lower layer. The flow in the mo del is forced by specified upwelling at the upper surface and by a specifie d cross-isopycnal velocity at the interface in addition to the autogenerate d cross-isopycnal velocity associated with the thermal damping. The forcing may be either broad in longitude or narrowly confined. The nature of the geometry of the model ridge mixes the baroclinic and baro tropic response to the forcing, and this has profound consequences for the resulting circulation. In particular, when the baroclinic interaction of th e two layers is strong, the recirculation region to the east of the ridge, previously discovered in earlier barotropic models of the circulation, grow s in meridional extent so that the flow along the ridge segment may be unid irectional along the ridge. It is suggested that the theory may explain obs ervations of such flow in the Angola Basin, which appeared previously to vi olate an application of Kelvin's theorem. The theory also predicts zonal jets west of the gaps in the ridge, spreadin g meridionally with distance from the ridge. The jets are strongly barotrop ic whether the external forcing is baroclinic or barotropic.