Vertical transition in transport and mixing in baroclinic flows

Numerical simulations in multilevel baroclinic turbulence in a beta-plane channel model are discussed, focusing on the transport and mixing behavior. The temperature field in the model is relaxed toward a field consistent with a broad zonal jet with vertical shear that is a Gaussian function of the cross-channel coordinate. The resulting statistical equilibrium flow includes an active baroclinic eddy field. The transport and mixing properties are analyzed by considering the fields of potential vorticity and a passive tracer (from which effective diffusivities/equivalent lengths are calculated). The upper part of the flow organizes itself in such a way that there is a transport barrier in the center of the channel, with eddy mixing regions on either side. In the lower part of the flow the eddy mixing occurs across a single broad region, with no central transport barrier. The transition between these two regimes takes place abruptly at a height z(T). A large set of simulations is used to map out the variation of z(T) as a function of external parameters including beta, the thermal relaxation rate k(T), and the (lower boundary) frictional relaxation rate k(M) (applied in the lowest model layer only). The transition height z(T) is argued to be relevant to sharp vertical transitions in transport and mixing observed in atmospheric and oceanic flows.