Investigating Turbulence regimes in the Meso- to Submesoscale-transition from Surface Drifter observations in the Benguela upwelling region

Drifter data can be used to
extract information about the dominant physical mechanisms acting at
the different scales of motion (e.g. chaotic advection, turbulence,
diffusion)
and to deduce energy spectra in specific regions.
Here we analyze position and velocity data
from surface drifters deployed in groups of triplets at the boundaries of
a filament at the upwelling front off Namibia.
Due to the underlying rich mesoscale system, the pair dispersion
and relative velocity increment statistics are very different
depending on the location of release.
The drifters of the group released at the southern boundary of the
filament (Southern Release)
are trapped in the filament
with a narrow distribution of pair separations.
The drifters in the group released closer to the upwelling system
at the northern boundary of the filament (Northern Release) follow
distinct paths within the complex surface currents
and the mean squared pair separation shows the characteristic distinct
dispersion regimes
(non-local, local (Richardson) and diffusive) of an
ocean surface mixed layer.

We contrast our findings by a Semi-Lagrangian analysis, where we
treated the drifter velocity observations as scattered
point Eulerian measurements and
examined the probability distribution of relative longitudinal
velocity, as a function of spatial separation.
For the Northern Release
we find the PDF to be positively skewed third order structure
function) for relative separations
of 10 km - 80 km, supporting the above findings of an inverse energy
cascade (Richardson scaling of pair separations).
For the Southern Release
we find the second order structure function (variance) follows a 2/3 power law
for relative separations of 1 km - 800 km.
We investigate dependence on filtering and compare our findings with
the corresponding analysis of model-trajectories from a high
resolution model of the region.
We also perform a Helmholtz decomposition, to glean more into the
dynamical origin of this scaling behavior.