Covariation of finite‐amplitude wave activity and the zonal mean flow in the midlatitude troposphere: 1. Theory and application to the Southern Hemisphere summer

Abstract Tropospheric eddy‐mean flow interaction is formulated in terms of the vertically integrated budget of finite‐amplitude wave activity (FAWA). At each latitude the dynamics is governed by three coupled equations for the interior and surface FAWA and barotropic zonal mean zonal flow. In midlatitude austral summer, the budget reveals a largely adiabatic, antiphase covariation of FAWA and the mean flow. A marked periodicity is found for FAWA around 20–30 days, but not the mean flow, consistent with the recently discovered baroclinic annular mode. The difference in the spectra of FAWA and the mean flow arises from (i) distinct spectra of low‐level meridional eddy heat flux and the barotropic eddy momentum flux convergence and (ii) a strong thermal damping of surface wave activity: the latter makes the FAWA respond largely to the low‐level meridional eddy heat flux at low frequencies, whereas the zonal mean flow responds to the momentum flux convergence whose spectrum is broader and occupies higher frequencies.