Lagrangian and tracer evolution in the vicinity of an unstable jet

The dynamics of Lagrangian particles and tracers in the vicinity of a baroclinically unstable zonal jet are investigated in a simple 2-layer model with an initially quiescent lower layer. Presence of a wave induces a particle drift which is dominated by a Stokes drift rather then the mean Eulerian velocity due to the wave. Stable and unstable waves both have zonal Stokes drift with a similar meridional structure while only unstable waves possess meridional drift which is in the direction of increasing meridional displacement. Particle dispersion in the upper layer is maximum at critical lines, where the jet and phase speeds are equal. In the lower layer dispersion is maximum where the wave amplitude is maximum. Zonal mean tracer evolution is formulated as an advection-di usion equation with an order Rossby number advection and an order one eddy-di usion which is proportional to two particle dispersion. Finite amplitude simulations of the ow reveal that small amplitude theory has predictive value beyond the range for which it is strictly valid. Mixing (as opposed to stirring) is maximum near cat's-eye-like recirculation regions at the critical lines. In the lower layer the pattern of convergence and divergence of the ow locally increases tracer gradients, resulting in stirring yet with a much slower mixing rate than in the upper layer. Meridional eddy diffusion (or particle dispersion) alone is not su cient for prediction of mixing intensity. Rotation, which we quantify by the cross correlation of meridional and zonal displacements, must also be present for mixing. These results are found to have much in common with observations of tracer elds and Lagrangian oats released near the Gulf Stream, a jet which exhibits baroclinic instability, and suggest potential use of this theory in future analysis of Gulf Stream data.