Variability of the Deepwater Horizon Surface Oil Spill Extent and Its Relationship to Varying Ocean Currents and Extreme Weather Conditions - Mathematical Modelling and Numerical Simulation of Oil Pollution Problems - page 4

concerns about oil becoming entrained in the Loop Current (LC) and in Loop Current
Rings (LCR). Although existing methodologies [ 11 ] and numerical model efforts in
general allow estimating the upper ocean thermal structure from satellite altimetry
observations, research cruises still supply needed critical in situ information used
to validate and assess satellite ocean current observations and to provide a suite of
subsurface data that could not have been obtained otherwise [ 30 , 31 ] to perform a
correct water mass analysis.
Trajectory models, initialized and validated by satellite observations, also played
a key role in contingency planning and determining the likelihood water and oil
particles located at or in the vicinity of the oil spill site to reach remote regions, such
as the west Florida shelf, Florida Keys, etc. [ 14 , 17 ]. Northern GOM waters have
been observed in these downstream regions, such as the Florida Straits via the LC
[ 7 , 23 , 24 ]. In addition, historical surface drifter trajectories indicated that material
particles travelling near the oil spill site had the potential to enter the North Atlantic
[ 30 ]. However, oil did not reach the western-central or southwest coast of Florida,
consistent with earlier expectations of [ 35 ], who examined satellite-tracked drifter
trajectories in this region. The flow regime of this region minimizes the cross shelf
transport in the West Florida Shelf [ 13 , 28 ]. This region, however, shows seasonal
variation [ 26 ] that can be related to the northward excursions of the LC, which extends
from 24 ⓦ Nto28 ⓦ N. Specifically, the circulation in this region would allow surface
materials in the GOM to be closer to the shoreline when the edge of the LC reaches
a maximum northern excursion, and farther away when the LC is at its southernmost
location [ 6 , 26 , 28 ].
When theDWH incident began onApril 20, 2010, the LCwas in its northern exten-
sion, reaching approximately 27.5 ⓦ N, still south of the wellhead location (88.36 ⓦ W,
28.73 ⓦ N). Numerical models initialized with in situ and satellite observations reflect-
ing these conditions were used to calculate the trajectories of synthetic Lagrangian
water particles deployed in the oil spill site to examine potential transport pathways
arising from ocean currents. A numerical methodology based on Finite Time Lya-
punov Exponents, which is an averaged measure of the separation rate of initially
nearby fluid particle trajectories, was used to identify regions that could act as bar-
riers not allowing the intrusion of waters near the west Florida coast and to estimate
potential oil pathways at the surface. Numerical model-derived particle trajectories
exhibited a number of pathways with a potential to reach areas beyond the oil spill
site, to be carried into the southern GOM, and to enter into the Florida Current and
North Atlantic Ocean at the surface (Fig. 1.1 ) and subsurface. Depending on the
decay rate assigned to these particles, some could reach remote regions, although
with a very low density or probability [ 1 ].
Real-time evaluation of surface current fields mostly derived from satellite obser-
vations became a critical component of the monitoring effort. The complexity of
these fields is enhanced by the mechanisms involved in the separation of rings from
the LC [ 33 ], which usually occur at different times at surface and subsurface. The
separation of the LCRs at the subsurface is only verifiable with in situ measurements
as satellite observations are limited to surface parameters. The spatial resolution of
the fields derived from multiple satellites became especially important for real-time