Scientific Background

The explosion of the Deepwater Horizon (DwH) drilling rig on 20 April 2010 underscores the clear need for rapid response capabilities to aid in understanding and predicting the movement of subsurface oil and dispersants in a highly complex oceanic regime. One of the major concerns during the DwH spill was the relative position of the Loop Current (LC) and its energetic warm core eddy field relative to the Macondo well site in the northern Gulf of Mexico (GoM). This energetic current field (surface velocities exceeding 1 m s^-1) is deep and can transport hydrocarbons at depth out of the GoM, through the Straits of Florida (SoF) , and along the eastern seaboard. The need to simultaneously and rapidly map oceanic flows and subsurface hydrocarbon distribution during and subsequent to an oil spill became abundantly clear.

Hydrocarbons in water do not act like conservative tracers, but undergo physical and biogeochemical changes including dissolution of gases and enhanced mixing of emulsified fluid forms compared to larger subsurface oil droplets; estimates from the DwH spill indicate that > 50% of the leaked hydrocarbon mixture entered deep plumes as dissolved gases and emulsified oil (i.e., small droplets) (1). An incomplete understanding of oil changes and of turbulent mixing introduces the need for sub-grid scale parameterizations of these processes. Improving these parameterizations requires defining empirical coefficients that depend on the type and age of oil, as well as environmental factors such as temperatures, salinities, and currents (2).

Research Goals

Key questions are the size of oil particles and the hydrocarbon fraction for each size, and how turbulent mixing and biogeochemical changes throughout the water column impact these oil characteristics as ocean currents and buoyant forcing transport oil components. Most turbulent mixing in the ocean interior takes place at near-inertial frequencies, and it is associated with near-inertial currents as part of a geostrophic adjustment process in mesoscale ocean features. Mesoscale background currents in the GoM, such as the LC and associated warm core eddies and smaller-scale frontal cyclones, are energetic and modulate turbulent mixing throughout the water column (3, 4). Thus, directly measuring these hydrodynamic processes (including current and shear) and assessing their impact on biogeochemical distributions and processes to depths of 1500 m is central to improving process representation in coupled ocean biogeochemical models. We propose to build a new profiling float capable of simultaneously measuring hydrodynamic and biogeochemical fields and providing profile data at 2 to 4 day intervals in near real time for ingestion into a data-assimilative model for both forecasting and hindcasting the level and fate of hydrocarbons.