Baroclinic and Barotropic Aspects of Wave-Mean Flow Interaction

The baroclinic and barotropic components of atmospheric dynamics are usually viewed as interlinked through the baroclinic life cycle, with baroclinic growth of eddies connected to heat fluxes, barotropic decay connected to momentum fluxes, and the two fluxes connected through the Eliassen-Palm wave activity. However, recent observational studies suggest that these two components of the dynamics are largely decoupled in their variability, with variations in the zonal mean flow associated mainly with the momentum fluxes, variations in the baroclinic wave activity associated mainly with the heat fluxes, and essentially no correlation between the two. In an attempt to understand this apparent decoupling, multiscale asymptotic methods are applied to the primitive equations to derive the baroclinic and barotropic components of the mean-flow equations, and a wave activity equation for the eddies, assuming a scale separation between the eddies and the mean flow and averaging over synoptic length and time scales. These equations predict a decoupling of the barotropic and baroclinic components of the dynamics, in the sense described above, with the baroclinic component of the zonal mean flow driven primarily through diabatic and frictional processes. The predictions of the theory are tested in a dry core model (with different setups) using different frequency bands to account for the different timescales of atmospheric variability.