The Rossby-inertia-buoyancy instability in baroclinic vortices

The behavior of vortex Rossby (VR) waves undergoing inertia-buoyancy (IB) wave emission on vortices with baroclinic vertical structures is studied. We consider monotonic vortices, where the potential vorticity (PV) decays monotonically with radial distance from the vortex center, and "single-peak" vortices, where the PV reaches a single radial maximum off the vortex center. A linear anelastic model is numerically solved as an eigenvalue problem. The difference between Rossby-inertia-buoyancy (RIB) waves and traditional coupled VR wave barotropic instability is shown to be associated with the region of the vortex where the mode extracts its energy for growth. The dependence of the growth rate of these RIB modes on the radial and vertical PV gradients of the vortex is studied. New interpretations of the damping of RIB modes on baroclinic, monotonic vortices are found to be as follows: (1) RIB mode growth depends on a sensitive arrangement of radial momentum fluxes such that it is disrupted by the vertical tilting of the mode by the vortex vertical shear and (2) RIB modes have a more complicated structure than traditional barotropic instabilities such that eddy viscosity acts more strongly. It is also found that single-peak vortices with annular PV regions that are very thin favor barotropic instabilities with weak IB wave emission, but in contrast, relatively wider annular regions favor barotropic instabilities with strong IB wave emission. The modification of the vortex induced by RIB mode growth is briefly explored in a fully nonlinear model. (C) 2008 American Institute of Physics.