Robust and leaky transport barriers in unstable baroclinic flows

Unstable baroclinic jets undergoing life cycles of wave growth and nonlinear equilibration are investigated numerically in a quasigeostrophic two-layer model. The focus is on understanding the properties of the upper layer transport barrier that emerges in the ensuing turbulent flow. The transport barriers in the simulated flows are representative of observed atmospheric and oceanic transport barriers, for example, at the extratropical tropopause, or at the Atlantic Gulf stream or the Antarctic circumpolar current. The simulations reveal that, depending on the value of the (inverse) criticality parameter beta associated with the initial jet, the developing transport barriers either remain almost entirely impermeable to transport or leak by allowing vortices to be shed across them. A dynamical theory to predict the final flow, based on minimization of potential energy subject to relevant kinematic and dynamical constraints, is extended to make predictions about the transport barrier behavior. The theory is able to predict the value of beta for which the barrier begins to leak and provides accurate estimates for the ensuing potential vorticity exchange across the barrier, which is primarily due to the vortex shedding. (c) 2008 American Institute of Physics.