A Deep Water Dispersion Experiment in the Gulf of Mexico.

The Deep Water Horizon oil spill dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment (DWDE), which consisted of the release of surface drifters and acoustically tracked RAFOS floats drifting at 300 and 1,500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated and decreases with depth: on average, diffusivity at 1,500 dbar is 5 times smaller than at the surface, suggesting that the dispersion of contaminants at depth is a significantly slower process than at the surface. This study also examines the turbulent regimes driving the dispersion, although conflicting evidences and large uncertainties do not allow definitive conclusions. At all depths, while the growth of dispersion and kurtosis with time supports the possibility of an exponential regime at very short time scales, indicating that early dispersion is nonlocal, finite size Lyapunov exponents support the hypothesis of local dispersion, suggesting that eddies of size comparable to the initial separation (6 km), may dominate the early dispersion. At longer time scales, the quadratic growth of dispersion is indicative of a ballistic regime, where a mean shear flow would be the dominating process. Examination of the along‐ and across‐bathymetry components of float velocities supports the idea that boundary currents could be the source for this shear dispersion. Plain Language Summary: The 2010 Deep Water Horizon oil spill has dramatically impacted the Gulf of Mexico's marine environment from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied over the past decades, little is known about the deep water dispersion properties of the ocean, and the fate of deep contaminants is uncertain. This paper describes the results of the Deep Water Dispersion Experiment that took place in the western Gulf of Mexico, a deep water drilling operation area. The experiment consisted in the simultaneous release of surface drifters and floats drifting at 300 and 1,500 m, to assess the variations of dispersion properties with depth for the first time. It is shown that diffusivity is weaker in depth that at the surface, so that contaminants would spread less rapidly. Key Points: Pairs of surface drifters and RAFOS floats drifting at 300 and 1,500 dbar were released simultaneously in the Gulf of Mexico between 2016 and 2018Relative diffusivity at 300 and 1,500 dbar is on average 2 and 5 times weaker than at the surface, respectivelyTopography and sheared boundary currents might play a major role in float dispersion [ABSTRACT FROM AUTHOR]