Mixing across unsteady jets

Observations from satellite altimetry and output from high-resolution ocean models indicate that the Southern Ocean is characterized by an intricate web of narrow, meandering, filamentary jets. These jets undergo spontaneous formation, merger and splitting events, and rapid latitude shifts over periods of weeks to months. Numerical simulations using a doubly-periodic, forced-dissipative, two-layer quasi-geostrophic model in the presence of simple sinusoidal topography have shown that similar jet variability occurs when there is a disagreement between the externally-set topographic lengthscale and the internally-set jet spacing, also equivalent to the Rhines scale in this two-layer model. We focus on two particular behaviors observed in these simulations: (i) persistent jet formation and merger events caused by the migration of zonal jets across large-scale potential vorticity contours and (ii) quasi-periodic oscillations between topographically-steered jets and purely zonal jets. This study considers how these behaviors influence the large-scale meridional transport of passive tracers and Lagrangian particles. We show that in the case with migrating zonal jets, particles may experience meridional excursions much larger than the jet spacing, without crossing strong potential vorticity gradients. In the case where jets oscillate between zonal and topographically-steered structures, transport is dominated by intermittent events that are spatially localized. The patterns of transport barriers in upper and lower layers are also found to be different, even though the jets have a strong barotropic component.