June 2010 – March 2011
Since the explosion of the Deep Water Horizon Oil Drilling Rig on April 20, 2010, a large amount of oil has spilled in the Gulf of Mexico (GOM), posing unprecedented threat to the gulf coasts, as well as the southeast coast of United States. While satellite remote sensing, aircraft flights, and surfaces drifters are providing valuable information on the oil surface trajectories since the accident, little is known about how the spill is distributed at depths, and to what extent they have been transported by the subsurface currents.
At present a Loop Current Eddy has separated from the northernmost portion of the Loop Current, and stands between the Deepwater Horizon well and the Loop Current itself, so that most of the spilled oil has not been entrained into this strong current. Over time, that eddy may reattach and detach several times, and will eventually drift gradually westward in the Gulf of Mexico. When that happens, the Loop Current may eventually re-extend into the northeastern Gulf, and entrain spilled oil into the Loop Current. Any oil that is entrained will eventually pass offshore of Florida, Georgia, and the Carolinas. Yet, there is no observational means to map the oil vertical and horizontal distributions to their fullest extent.
A new concern arises with the hurricane season now starting and the oil spill in the water most likely to still be around well into the season. How does the strong mixing associated with tropical storms transversing the GOM and South Atlantic Bight bring subsurface oil up to the surface, and how the storm surges transport these newly surfaced oil up on the beach are completely unknown, and practically impossible to measure. In this regard, numerical models provide a powerful and cost-effective way to address these societally important questions. At NC State, the research group led by PI Ruoying He has been running a real-time ocean circulation nowcast/forecast model for the South Atlantic Bight and Gulf of Mexico (i.e., NC State SABGOM model) since 2008. The model is based on Regional Ocean Modeling System, having 5-km horizontal resolution and 36 vertical layers. It is nested inside the data assimilative global HYCOM. Realistic tidal and meteorological forcing, as well as major river runoff in the region are all included in the SABGOM model simulations (Figure 1). Details of model configuration and skills have been peer reviewed and published (e.g., Hyun and He, 2010).
In this oil spill event, NOAA Emergency Response Division has been using the SABGOM model fields to generate official surface oil trajectory prediction (see NOAA Deepwater Horizon website) to guide response activities of federal, state and local organizations. We are now in the process of implementing a three-dimensional oil tracer tracking and prediction for both Gulf of Mexico and South Atlantic Bight. We hereby seek NC Sea Grant Support to enhance these research activities: 1. We will implement an advection, diffusion and reaction model for the “oil tracer”, which will be simulated online within our SABGOM circulation nowcast/forecast system. The advection and diffusion processes are the same for temperature and salinity simulations. For the reaction, we will consider the “behavior” of Emulsified Oil (created by the use of dispersants), which have tiny droplets that could remain below the surface for an extended period of time. The oil tracer will simulate the motions of emulsified oil droplets. Their sinking (or rising) speeds will be parameterized based on a recent, comprehensive oil spill study by National Research Council (2005). 2. Given there is still no consensus on the total amount of oil spilled from the wellhead, we will run model sensitivity experiments starting on April 20, 2010 with various amount estimates of oil spill. Existing satellite and aircraft observations will be used to validate the simulated oil spill surface features, including expansion directions and total areas.