Influence of bottom drag on the energy cycle of the atmosphere

Using simulations with idealized general circulation models, we investigate how bottom drag influences the angular momentum budget, the energy cycle, and turbulence characteristics of the atmosphere over a wide range of imposed drag parameters.

As the drag weakens, mid-latitude jets shift poleward and become stronger. However, the jets strengthen mainly through their barotropic components, while the baroclinic components remain largely unchanged. In a statistically steady state, the available potential energy generation rate G(P) balances the baroclinic energy conversion rate C(P,K), which in turn balances the total kinetic energy dissipation D(K). For different imposed equator-to-pole surface temperature differences, G(P), C(P,K), and D(K) exhibit regime transitions as drag weakens: from nearly constant to rapid decay toward a floor given by sub-grid scale dissipation. The flow becomes more barotropic, which might explain a flattening of the global barotropic eddy kinetic energy (EKE) spectra in the simulations with weak drag. These spectra are consistent with an inverse energy cascade, but the energy transfer spectra still show dominance of eddy-mean flow interaction. As the drag weakens, both the barotropic and baroclinic components of EKE increase, which indicates that the enhanced barotropic shear does not inhibit baroclinic instability. This differs from the predictions of the “barotropic governor” effect.