Parcel Trajectories in Quasigeostrophic Jets: Neutral Modes

Abstract

The kinematics and dynamics of linear neutral modes propagating in a mean zonal jet are studied in an effort to understand how particle pathways within and across oceanic jets, such as the Gulf Stream, are determined. Sinuous-, varicose-, and mixed-mode waves are superposed on cusped jets that are governed by quasigeostrophic dynamics and characterized by piecewise uniform potential vorticity. Significant excursions (greater than the wave amplitude) of particle trajectories are found in the vicinity of steering lines that lie in the horizontal plane of motion, particularly when the background shear vanishes locally. Monochromatic waves produce pathways with simple periodic meandering, whereas a superposition of two waves results in chaotic parcel behavior, consistent with a number of earlier studies based on kinematic models. By identifying the steering surface, the surface on which the particle speed matches the speed of a propagating meander, as the site for preferential exchange, a mixing geometry associated with baroclinic oceanic fronts is suggested. Flows containing a continuous distribution of normal modes (a wave packet) or of nonmodal disturbances are also studied. In the former case, chaos appears to be suppressed, and in the latter case, temporary disturbance amplification can lead to meridional exchange of fluid parcels.