The role of jets as transport barriers in the Earth's stratosphere

Theoretical results relating to Kolmogorov-Arnold-Moser (KAM) theory, particularly those dealing with the stability of degenerate one-degree-of- freedom Hamiltonian systems under a time-quasiperiodic perturbation, have led to the expectation that, independent of the background Potential Vorticity (PV) distribution, associated with zonal jets should be barriers which inhibit meridional transport. In a recently submitted paper (Beron-Vera et al., 2011) the aforementioned expectation is confirmed based on the analysis of isentropic winds in the lower stratosphere as produced by a comprehensive general circulation model. Concretely, barriers for meridional transport are found to be associated with the (eastward) austral polar night jet, for which the meridional gradient of background PV is very steep, and also for the (westward) boreal summer subtropical jet, for which the background PV is rather uniform. The identification of the meridional transport barriers is based on the computation of Finite-Time Lyapunov Exponents (FTLE), which measures the amount of stretching about fluid particle trajectories. The meridional transport barriers, which are formed by fluid particle trajectories lying on invariant tori, are identified with topologically circular locally minimizing curves or trenches of the backward-plus-forward FTLE field. Results from explicit passive tracer advection experiments and flux computations are also presented, which confirm those inferred using the FTLE diagnostic. In this paper I present results based on the analysis of analytically prescribed stratospheric model winds, which complement the results presented in Beron-Vera et al. (2011) and further allow one to better assess the scope of the FTLE diagnostic.