Layers of anomalous chemical composition and potential vorticity (PV) are commonly formed as a consequence of irreversible Rossby wave breaking near the tropopause. Here we consider the dynamics of such layers, and investigate their stability properties, in a non-hydrostatic, Boussinesq model, both analytically and numerically. The key idea is that dynamical instabilities of these PV layers may determine the mixing timescales of the anomalous chemicals they contain, with implications for tropospheric and lower stratospheric chemical budgets.

For simplicity, we consider an infinite layer of constant Ertel's PV, with anomalous PV dQ, and oriented arbitrarily in space; the problem is simplified to two dimensions by defining rotating coordinates Z perpendicular to the layer, and X and Y along the layer parallel to and perpendicular to the slope respectively. The evolution in Y and Z alone is then considered.

It is shown that for dQ larger than a critical value the balanced state associated with the PV layer becomes Kelvin-Helmholtz unstable for certain angles of orientation. Furthermore, it is shown that in a weak shear flow the PV layer can be tilted from a stable to an unstable regime, providing an example of the spontaneous emission of gravity waves from a (slightly perturbed) initially balanced state.

It is argued that the critical value of dQ may often be attained during stratosphere to troposphere exchange events. Kelvin-Helmholtz instability may therefore act as one systematic mechanism for the rapid mixing of the chemical contents of stratospheric intrusions into the troposphere.