Surface drifter pair spreading in the Atlantic Ocean

PIs: Rick Lumpkin

Collaborators: Shane Elipot, (National Oceanography Centre, Liverpool)

When particles such as plankton or pollutants are released in the ocean, turbulence acts to stir them and disperse them over broad areas. This process can be quantified using satellite-tracked surface drifters of the Global Drifter Program.

This study examines the spreading of surface drifter pairs deployed as part of the CLIVAR Mode Water Dynamic Experiment (CLIMODE) project in the Gulf Stream region. The spreading 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 inconsistent with the enstrophy cascade of 2-D quasigeostrophic turbulence, indicating the presence of significant energy at the submesoscale in the Gulf Stream region which flattens the wave number spectrum and dominates surface stirring at this scale range.

Results from this study were published in Lumpkin and Elipot (2010)

Figure 1. - Zonal (heavy solid curve) and meridional (heavy dashed curve) dispersion (mean squared separation) as a function of distance for 55 drifter pairs in the CLIMODE experiment. Axis on right indicates root-mean-squared separation distance. Time t is days after release. 95% confidence intervals are shown for zonal dispersion. Thin horizontal gray lines indicate the estimated noise level for separation and the squared deformation radius. Slanted black lines indicate random-walk behavoir with constant relative diffusivity indicated by the labels. Best fits to the data obeying Richardson's Law in the range 1-40 days (gray dashed line) and exponential growth in the range 7h to 1d (heavy gray curve) are superimpsed.