The overarching goal of this research is to build a rapid response capability that can be deployed in the event of an oil spill. The capability will consist of an integrated observation-prediction system to map the distribution and extent of hydrocarbons in the water column in real time and to quantify hydrocarbon removal and fate including short-term predictions of dispersion induced by the current field and transport of oil to the sea floor through scavenging by marine particles. Specific research objectives are:
The prediction system will be evaluated in retrospective assimilation experiments using data from the Deepwater Horizon spill and in forecast experiments that assimilate satellite and float data in real time. Both will demonstrate the system’s capability, and improve our understanding of physical mechanisms and their impacts on the biogeochemistry in the water column.
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).
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.
Sea surface height (SSH) and surface velocity fields simulated by NCSU’s South Atlantic Bight and Gulf of Mexico (SABGOM) circulation model on June 9, 2010. Eddy Franklin (red circle) has separated from the LC and is clearly visible in the SSH field. Inset shows where drawdown of dissolved oxygen due to DwH spill was observed at 1000-1500 m from 03-25 August, 2010 and is envisaged for the EM/APEX float deployment in the northern GoM in this project.
The APEX EM float with Optode sensor.
The observation-prediction system is a cluster of ten profiling EM/APEX floats with physical, chemical and bio-optical sensors and a data-assimilative physical-biogeochemical model for hindcast, nowcast and forecast simulations. The new state-of-the-art floats will be equipped with CTD and electromagnetic current sensors, as well as oxygen, chlorophyll and colored dissolved organic matter (CDOM) fluorescence and backscatter sensors from Teledyne-Webb Research (TWR). This novel combination of current sensors with chemical and bio-optical sensors has not been deployed before. Furthermore, in contrast to existing models, this system will be high-resolution in the region of interest (~1 km) with a focus on accurate representation of processes in the mid-water column (from the mixed layer to 2000 m) including the interaction of hydrocarbon droplets with marine particles.