Investigating vertical dependence of turbulence regimes and lateral mixing in the mixed layer from drifter observations in the South Atlantic

The energy transfers in the meso- to submesoscale regime in the ocean yield both up-scale and down-scale components from a complex pattern of flow structures which impact scale-dependent ocean turbulence and mixing that is not yet correctly parameterised in climate models.
The Walvis Ridge region in the South Atlantic is characterized by strong tidal beams and lies in the path of the Agulhas eddies and hence also features large mesoscale energy with associated submesoscale fronts and filaments.
Here, we quantify lateral mixing in the mixed layer using surface drifter observations from two observational campaigns with a unique deployment of two drifter types at two different depth levels simultaneously, one at the very surface and one at 15m depth. We quantify the contribution of the different motions that show up in the drifter trajectories at various time and space scales ranging from hours to months and 100m to 1000s of km and how they influence the applicability of the eddy-diffusion model.                                    

We find that large scale mean flow removal plays a critical role in achieving convergence in the components of the diffusivity tensor and in the major axis component after diagonalization. Writing the diffusivities as the product of time scales and kinetic energy, the significant anisotropy in the diffusivity tensor is mainly explained by the anisotropy in the Lagrangian integral time scales, while the major axis component of the velocity variance tensor is comparable to the minor axis component. The details of this anisotropy depend on scale. Motions on scales smaller than the Rossby Radius contribute significantly to the diffusivities. We discuss how the results relate to what kind of energy cascade exists at which scale.