Steady Baroclinic Flow through Ridges with Narrow Gaps

Abstract

The steady baroclinic flow in a basin containing a meridional barrier representing a midocean ridge is studied in the linear, quasigeostrophic limit of a two-layer model. Thermal damping and a simple friction provide dissipation of thickness (heat) and momentum. The ridge is pierced by two gaps in the upper layer but only a single gap in the lower layer. The flow in the model is forced by specified upwelling at the upper surface and by a specified cross-isopycnal velocity at the interface in addition to the autogenerated 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 barotropic response to the forcing, and this has profound consequences for the resulting circulation. In particular, when the baroclinic interaction of the two layers is strong, the recirculation region to the east of the ridge, previously discovered in earlier barotropic models of the circulation, grows in meridional extent so that the flow along the ridge segment may be unidirectional along the ridge. It is suggested that the theory may explain observations of such flow in the Angola Basin, which appeared previously to violate an application of Kelvin's theorem. The theory also predicts zonal jets west of the gaps in the ridge, spreading meridionally with distance from the ridge. The jets are strongly barotropic whether the external forcing is baroclinic or barotropic.