The role of boundary layer processes in baroclinic eddy equilibration in a simple atmosphere-slab ocean coupled model

To understand the role of baroclinic eddies in atmospheric circulation, several theories have been proposed. However, these theories either fail to work in the boundary layer or simply neglect the influence of boundary layer processes. Our previous study found that, under fixed SST boundary condition, the boundary layer thermal diffusion, along with the surface heat flux, is primarily responsible for limiting PV homogenization by baroclinic eddies in the boundary layer, which also provides an explanation for why the baroclinic adjustment theory does not work well there. The role of different boundary layer processes in eddy equilibration in a simple atmosphere-slab ocean coupled model is investigated in this study. For each boundary layer process, there exist at least two different ways that they can influence the eddy behavior. First, the direct effect of boundary layer processes is a damping of eddy energies. On the other hand, during eddy equilibration, boundary layer processes also influence the zonal-mean flow, which can further affect the eddy activity. We find that for surface friction, the direct effect is dominant. Surface friction largely damps the eddy mixing near the surface; thus, stronger surface friction results in stronger meridional temperature gradient of the coupled system. However, for surface heat flux, the indirect effect is dominant, which acts to reduce the underlying surface temperature gradient. Under the same global averaged SST, stronger surface heat flux, results in weaker SST gradient, as well as weaker eddy activity in the atmosphere.