Abstract
The parameterization of the unresolved scales of motion in large-scale numerical ocean models is analyzed through a combination of Lagrangian and Eulerian observations. The limitations that coarse spatial resolution imposes on the advective-diffusive equation are tested by comparing the evolution of a passive tracer field in high and low-resolution numerical models.The estimation of the spatial distribution of the diffusivity function from the trajectories of the particles released in the eddy-resolving numerical model accurately reproduces the most important characteristics observed in the analysis of surface drifters in the South Atlantic Ocean: anisotropy of the horizontal components of the diffusivity matrix with zonal values usually being larger than meridional diffusivities; and an inhomogeneous diffusivity field, with large values in those regions where the eddy kinetic energy is larger.The use of the various diffusivity fields in the simulation of the evolution of the tracer field revealed that there is no eddy diffusivity field consistently determined, either by Lagrangian statistical analysis or from the velocity shear that can produce a tracer distribution and meridional diffusive fluxes comparable to those obtained from the eddy-resolving version of the model.A deformation-dependent diffusivity and an upper limit Lagrangian diffusion provided a better representation (using the eddy-resolving simulations as the ground truth) of the meridional tracer transport across the jet region than the constant diffusivity case. However, they also "smear out" the tracer front more than the other diffusivity alternatives.The evolution of the tracer field in the eddy-resolving experiments suggest the transfer of tracer across the region of the free jet is the result of three mechanisms. The first one is due to the subgridscale diffusivity. The second is represented by the amount of anomalous tracer trapped in the formation and shedding of rings. The third is due to the occurrence of phase shifts between the meandering streamlines and the meandering tracer front.The occurrence of these phase shifts suggests that caution should be exercised when inferring velocity fields or circulation patterns from passive tracer distributions, and when interpreting surface drifter trajectories based on sea surface temperature maps from satellites. (Abstract shortened with permission of author.)