Disentangling Mesoscale Strain and Internal Waves Using Surface Drifters

In an effort to determine the processes governing diffusivity on scales of 1-10 kilometers, both dye and GPS-tracked surface drifters were released below a shallow pycnocline and tracked for six days. This experiment was conducted twice, once in a region of weak, nearly constant mesoscale strain, $O(10^{-2} f)$ and again in a region of moderate, slowly varying strain, $O(10^{-1} f)$. Because mesoscale strain is diffusive and acts across all spatial and temporal scales, disentangling its effects from other processes is difficult in the best case, or even impossible if the processes are coupled across scales.

Here we show how a simple model for mesoscale strain can be used to remove the effects of the mesoscale strain field for both experiments. In both cases the remaining signal, including the inferred diffusivity, is consistent with linear internal waves acting on fixed-depth particles. This suggests that, to first approximation, mesoscale strain and internal waves remain uncoupled. This hypothesis is tested in a numerical model and shown to be consistent with the observations.