The mean meridional circulation: A new potential-vorticity, potential-temperature perspective

The dynamical equations governing atmospheric flow are transformed into a system of coordinates that consists of potential vorticity as meridional coordinate and potential temperature as vertical coordinate. Within this system of coordinates the atmosphere is divided into undulating tubes bounded by isentropic and constant potential vorticity surfaces, and the air moves through the tubes without penetrating through the walls, in adiabatic and frictionless conditions. A model that uses this system of coordinates has the advantage of incorporating as built-in the dry convective adjustment and baroclinic adjustment processes, which prevent the folding of isentropic and potential vorticity surfaces but simulate the effects of interaction of waves with the flow.; When applied to the study of the mean meridional circulation this system of coordinates reveals a residual circulation driven by the Lagrangian time rate of change of potential vorticity and diabatic heating, and allows to interpret the momentum exchange that affects the zonal mean flow as the zonal component of the pressure forces exerted by the eddies on a thin zonal tube bounded by surfaces of constant potential vorticity as lateral sides and undulating bottom and top isentropes.