Dynamics of multiple, migrating quasi-zonal jets in the ocean

Oleg Melnichenko (University of Hawaii, United States)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

Dynamics of multiple, migrating quasi-zonal jets in the circulation of the ocean are investigated using satellite altimetry and ocean model data. In this presentation, we focus on results from a representative case study in the eastern North Pacific. Satellite sea level anomaly (SLA) observations show that low-frequency motions in this part of the ocean are characterized by long-lived jet-like features (striations) that slowly, at a speed of about 0.3-0.5 km/day, propagate toward the equator. In their presence, the space-time distribution of mesoscale eddies is not random but rather anchored to the striations. Specifically, the distribution of eddies alternates between “troughs” of enhanced concentration of cyclonic eddies (negative SLA) and “crests” of enhanced concentration of anticyclonic eddies (positive SLA), which follow the striations in their meridional propagation.

The factors controlling such a behavior are investigated using output of the Ocean general circulation model For the Earth Simulator (OFES), which is shown to reproduce the observed features remarkably well. The diagnostics suggest that the striations’ life-cycle can be characterized by two dynamically distinct components. The first one is attributable to baroclinic instability of a large-scale, weakly sheared meridional flow in the subtropical gyre, which serves as the main energy source for the quasi-zonal striations. The second component arises from the nonlinear interactions between the zonal striations and eddies. Transient striations locally alter the mean potential vorticity distribution associated with the large-scale background flow in which they reside. This alteration is, in turn, responsible for the formation of eddies preferentially along the striations, consistently with the meridionally localized regions of enhanced or reduced baroclinicity and stabilizing or destabilizing effect of the planetary vorticity gradient and horizontal shear. When the striations move, the dynamics that generate eddies move with them, producing migrating “eddy trains”. Aligned eddies feed back onto the zonal flow, reinforcing the pattern of the striations.

Finally, the effect of the jets (striations) and eddy organization on the horizontal transport and mixing of tracers is investigated using experiments with Lagrangian particle trajectories. The experiments demonstrate that perfectly isotropic eddies can cause strongly anisotropic material transport, if their distribution is not completely random but organized into polarized “eddy trains”. The striations on which “eddy trains” reside can be very weak, i.e., “latent”, but their role in localizing eddy pathways can thus be very important for mixing distribution.

Oleg Melnichenko
University of Hawaii
United States