Isentropic zonal average formalism and the near‐surface circulation

The isentropic zonal average formalism is extended to include a rigorous treatment of the bottom boundary of the atmosphere. We define a ‘surface zone’, where isentropes in the latitudinal plane are interrupted by the earth's surface. The zonal average equations of motion and their time average in isentropic coordinates are rederived in the presence of the surface zone. Applying the extended formalism to a baroclinic wave model, we show that near‐surface equatorward mean flow is driven by eastward surface form drag in isentropic coordinates, which in turn is related to poleward geostrophic potential temperature flux at the surface. A potential vorticity–potential temperature picture of extratropical general circulation dynamics above and within the surface zone is presented. We highlight the importance of poleward mean flow in the upper region of the surface zone and investigate the antisymmetric distribution of mean meridional mass flow about the median potential temperature of surface air. Copyright © 2004 Royal Meteorological Society.