Annalisa Bracco
School of Earth and Atmospheric Science and Center for Nonlinear Sciences, Georgia Institute of Technology, Atlanta

Dynamics of Coherent Vortices in Forced-Dissipated, Rotating Hydrostatic Turbulence

The dynamics of coherent vortices is investigated numerically in a set of high-resolution primitive-equation model representing idealized, open-ocean conditions. The internal Rossby deformation radius is small compared to the domain size, and this results in the generation of wind-forced, surface intensified coherent anticyclones. In the upper 100 meters of the water column, the vortical motion dominates the divergent component, near-inertial waves are negligible and most statistical properties of the horizontal motion are found to be similar to those of quasigeostrophic turbulence. Major differences are a strong cyclone-anticyclone asymmetry, not present in quasigeostrophic models, and the complex structure of the vertical velocity field. Indeed, locally the motion can become strongly ageostrophic, and the vertical velocities associated to the eddies can reach values and levels of spatial complexity akin to those reported for oceanic fronts. Vertical velocities reach high instantaneous values -up to 100 m/day- and display a fine spatial structure linked to the presence of 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 distribution of vertical velocity is non-Gaussian and it is responsible for large vertical excursions of Lagrangian tracers.
In light of recent observations on the role played by mesoscale eddies in increasing nutrient supply, primary production, and the efficiency of the biological pump, the present results emphasize the complexity of submesoscale variability of wind-driven vortices and of the vertical velocity field. This latter is linked to ageostrophic motion, that cannot be captured by simpler models.
Finally, we discuss the role played by vertical resolution in the model representation of the transport properties associated with mesoscale eddies.