Why western boundary currents in realistic oceans are inviscid: a link between form stress and bottom pressure torques

It is shown that wind stress curl is balanced by bottom pressure torque in a zonal integral over any strip wide enough to smooth out the effect of nonlinear terms (typically about 3° of latitude). The derivation is completely general as long as the zonal wind stress is balanced by form stress at each latitude, as is known to be the case in the ocean. This implies that viscous torques are not important in western boundary currents, their place being taken by bottom pressure torques. The prediction is confirmed in the context of a global, eddy-permitting, numerical ocean model. This link between form stress and bottom pressure torques makes it easier to consider Southern Ocean dynamics and subtropical gyre dynamics in the same conceptual framework, with topographic interactions being important in both cases.