Bottom boundary layer instability couples equatorward western-boundary currents to the shelf circulation

An equatorward western boundary current whose flow extends to the bottom, such as the Labrador or Malvinas currents, is expected to drive a bottom Ekman flow offshore. If there is stratification near the bottom, the advection of buoyancy in this bottom boundary layer is predicted to rapidly shutdown the circulation in the bottom boundary layer. It is thought that this shutdown eliminates bottom friction on the current and prevents the dissipation of relative vorticity by this friction, thus preventing the current from spreading across isobaths and forcing equatorward flows on the shelf.

But if the boundary current is strong enough, baroclinic instabilities of the horizontal density gradients in the bottom boundary layer disrupt this shutdown and allow the frictional dissipation of alongshore momentum by bottom friction. The bottom boundary layer friction allows the slope currents to force shelf currents via the arrested topographic wave mechanism. The bottom boundary layer friction also allows the boundary current to dissipate energy and potential vorticity, with all that implies for the gyre circulation and boundary current separation.

A scaling will be presented to estimate the reduction of alongshore bottom friction by bottom boundary layer arrest in the presence of bottom boundary layer instabilities, and this will be used to estimate the coupling between slope and shelf flows on western boundaries. This will be used to predict the strength of coupling between western-boundary and shelf flows on the western boundaries of the North and South Atlantic.