Surface Drifter Pair Spreading in the North Atlantic


The spreading of surface drifter pairs deployed as part of the CLIMODE project in the Gulf Stream region is resolved at hourly resolution, and quantified by relative dispersion and Finite Scale Lyapunov Exponents. At scales from 1–3 km to 300–500 km, the dispersion follows Richardson’s Law, indicating stirring by eddies comparable in scale to the pair separation distance. At larger scales the spreading becomes a random walk described by a constant diffusivity. The behavior from 1–3 km to the local deformation radius is qualitatively different from an earlier study conducted in the Gulf of Mexico, and is inconsistent with the enstrophy cascade of 2D quasigeostrophic turbulence. To test various hypotheses for this discrepancy, drifter pair spreading is examined for pairs that were not launched together, pairs deployed in the eastern subtropical North Atlantic, and CLIMODE pairs subsampled to daily temporal resolution. It is concluded that the waven umber spectrum of turbulence in this scale range in the Gulf Stream region is flatter than predicted by 2D turbulence, consistent with the predictions of surface quasigeostrophic turbulence theory. Results in the less energetic subtropical eastern Atlantic are more equivocal.