Why the most long-lived oceanic vortices are found in the subtropical westward flows

This paper examines factors contributing to the remarkable longevity of coherent vortices in the subtropical westward flows. Baroclinic vortices embedded in large-scale vertical shears generate Rossby waves which form an opposite sign eddy associated with inertial Taylor columns on the beta-plane. The combination of the vortex and lee Rossby wave can be viewed as a hetonic dipole that induces meridional drift and heat flux leading to self-amplification of vortices in baroclinically unstable flows. The analytical tractability is achieved by considering the marginally stable flow, where the beta-effect is nearly compensated by the potential vorticity gradient (PVG) associated with the meridional slope of the density interface. In the two-layer model such compensation can occur in the upper layer with the westward flow or in the lower layer with the eastward flow on top. In baroclinically unstable mean flows, vortices are shown to intensify due to the Lagrangian conservation of potential vorticity inside their cores, drifting meridionally in the layer with negative PVG and supporting baroclinic turbulence. The theory is confirmed by numerical simulations indicating that for westward flows in subtropical oceans, reduced PVG in the upper layer provides favorable conditions for eddy longevity and pathways.

Automated summary

This study investigates the factors contributing to the remarkable longevity of coherent vortices in subtropical westward flows, including the interaction of Rossby waves, vortices, and eddy waves, as well as the self-amplification mechanism in baroclinically unstable flows. The analysis suggests that under reduced PVG conditions, vortices can maintain longevity in the upper layer.