Baroclinic Equilibration and the Maintenance of the Momentum Balance.Part II: 3D Results

Abstract

In this two-paper series the equilibration of a baroclinic jet is described, emphasizing the redistribution of momentum and the effects that this redistribution has on the steering level and the potential vorticity (PV) structure of the basic state. In Part I of this series the equilibration of the 2D problem is discussed, using as a model the barotropic point jet. In that problem, the zonal mean momentum is redistributed along the single direction: there is a westerly acceleration at the jet vertex and compensating easterly acceleration in the interior. Part II of this series describes the equilibration of the 3D problem. In this problem, momentum is not simply redistributed vertically, but there is also a horizontal redistribution of momentum. It is shown that this makes the dynamics very different from the 2D simulations described in Part I, and in particular that the geometrical constraints introduced there no longer apply. The results of this work give insight into why 3D effects are important to the baroclinic equilibration, and point to friction as a possible mechanism limiting thermal homogenization at the surface in the extratropical troposphere.