The Structure of the Eddy Diffusivity in Atmospheric Transport

The role of atmospheric circulation in the tracer transport is investigated in the idealized simulations of a primitive equation model. Different components of the atmospheric circulation are used to advect passive tracers with small-scale diffusion. It is found that, while the residual mean meridional circulation is important for the tropical ascent of tracer isopleths and interhemispheric exchange of tracers, eddy mixing is critical for the equilibrium tracer isopleth slopes in the middle and high latitudes.

The structure of isentropic mixing is diagnosed using a quasi-conservative tracer as the meridional coordinate. Compared with previous isentropic diagnostics, the authors introduce a diagnostic that allows a large latitudinal change in isentropic static stability due to the intersection of the isentropic surfaces with the ground. It is found that the upper troposphere is characterized by a diffusivity minimum at the jet's center with enhanced mixing at the jet's flanks, and the lower troposphere is dominated by stronger mixing throughout the baroclinic zone. This structure of isentropic diffusivity is consistent with the diffusivity obtained from the geostrophic component of the flow.