Particle dispersion within zonal jets in two-layer beta-plane turbulence

Abstract

Passive tracer dispersion is studied in a two-layer, quasigeostrophic, betaplane model in which persistent, steady, zonal jets are observed. Particle trajectories and statistics are examined for barriers to or mechanisms for mixing. Simulations are performed for two different values of the planetary vorticity gradient, and an Eulerian energy timescale and a lengthscale dependent on the number of jets in the flow are developed to enable comparison between the two systems. Using this new timescale and lengthscale, it is shown that zonal particle root-mean-square displacements are enhanced as b increases, and meridional rms displacements in regions away from wave-breaking activity are unaffected By an increase in the planetary vorticity gradient. Two-particle spreading also increases with b. Larger values of beta result in diminished Rossby wave-breaking on the maximum gradient of potential vorticity associated with jet cores, so that tracer dispersion across westerly jets is decreased. The dependence of tracer statistics on the scales of waves included in the advecting flow is also examined. Hyperviscosity rather than laminar or biharmonic diffusion, is used to parameterize dissipation in the model, so that enstrophv is allowed to cascade to small scales. As a result, single-and two-particle statistics do show a dependence on the number of small scale waves included, and in particular, it is shown that removal of small scales increases zonal displacements and inhibits meridional displacements. This is thought to be a result of the decreased Rossby wave-breaking that accompanies the removal of small scales from the advecting flow.