Mesoscale (10-500km) and submesoscale (1-10km) flows in th12 e open ocean are characterized by the presence of strong coherent vortices which can induce intense vertical motions. There is now a wide observational evidence for the impact of vortices and of the vertical velocity field they generate on chlorophyll distribution, primary production and transport of organic matter into the deep ocean. To characterize vortex dynamics in the open ocean, here we use the Regional Ocean
Modelling System (ROMS) to study wind-forced mesoscale turbulence.

Structure of the vortex field: (a-b) Vertical component of the normalized relative vorticity /f at the surface, and at 78m depth, respectively – note the cyclonic rings that surround anticyclonic vortices. (c) Vertical velocity in m/day at 78m depth; (d) vertical section of vertical velocity (in m/day) across the line indicated in panels (a) and (b). Isolines of relative vorticity/f are superposed in white. All fields are averaged over three day (see Koszalka et al., 2007 for details)

In our integration, the internal Rossby deformation radius is small compared to the domain size, resulting in the generation of wind-forced, surface intensified coherent anticyclones. In the upper 150 meters of the water column, the vortical motion dominates over the divergent component, near-inertial waves are negligible and most statistical properties of horizontal flows (apart from the cyclone-anticyclone asymmetry) are similar to those of barotropic and quasigeostrophic turbulence. The vertical velocity field, w, however, is far more complex than we expected. 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. w is linked to ageostrophic motion and its structure cannot be captured by simpler models.
The distribution of vertical velocity is non-Gaussian and is responsible for large vertical excursions of Lagrangian tracers. In light of recent observations on the critical role played by mesoscale eddies in increasing nutrient supply, primary production, and efficiency of the biological pump, those results emphasize the complexity of submesoscale variability of wind-driven vortices in the vertical velocity field.

Future work

Our ongoing effort focuses on investigating the role of vertical vertical motion inside coherent vortices on the transport of biological tracers. We will have new results with a standard NPZD model embedded in the physical one at higher vertical resolution very soon.