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 in potential temperature coordinates are re-derived in consideration of the presence of the surface zone, and a time-average formulation is also presented. Applying the extended formalism to a baroclinic wave model, we show that near-surface equatorward mean flow is driven by eastward surface form drag, which in turn is related to poleward geostrophic potential temperature flux at the surface. 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 flux about the median potential temperature of surface air.