Dynamics of wind-forced coherent vortices in the open ocean

Authors: Inga Koszalka (ISAC-CNR, Torino, Italy)
Annalisa Bracco (EAS, Georgia Institute of Technology, GA, USA)
Antonello Provenzale (ISAC-CNR, Torino, Italy)

Abstract:

The dynamics and transport properties of wind forced anticyclones are investigated in a idealized model domain representative of open ocean conditions. It is shown that: (1) the horizontal circulation is in many aspects similar to that of vortex-dominated quasigeostrophic turbulence (3) locally the circulation is strongly ageostrophic, and (4) vertical motions associated with eddies might reach values and level of spatial complexity akin to those reported for oceanic fronts. Indeed, w can reach values of 100 m/day, and displays a fine 3D spatial structure linked to the presence of coherent vortices and filaments and to their interactions with the Ekman circulation. Within and around vortices and filaments upwelling and downwelling regions alternate and do not correlate with relative vorticity but result from the interplay of advection, stretching and instantaneous vorticity changes. The distributions of vertical velocity are strongly non-Gaussian and their tails account for the large vertical escursions of Lagrangian tracers.

In light of recent observations on the critical role played by mesoscale eddies by increasing nutrient supply, primary production, and efficiency of the biological pump, this work remarks the complexity of submesoscale variability of wind-driven vortices, particularly in the vertical velocity field, which is linked to ageostrophic motion and cannot be captured by low resolution (or simple) models. Our results may help understanding the extraordinarly intense and sustained plankton blooms observed in mode-water eddies near Bermuda and the submesoscale variability within (McGillicuddy et al., 2007).