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 16

done ( http://energy.gov/downloads/key-events-timeline ) including the Riser Inser-
tion Tube Tool (RITT) to recover oil from the riser to a surface ship; the pumping
of heavy drilling fluids into the blowout preventer to restrict the flow of oil before
sealing it permanently with cement (referred to as “top kill”); and the installation of
a piece of equipment over the flowing well after the riser was removed to capture
hydrocarbons so that they could be collected at the sea surface (referred as to top hat
#4). The time line of these efforts with relation to the oil spill extent at the surface is
included in Fig. 1.5 .

1.4.2 Surface Ocean Circulation

Throughout the response it was critical to provide an early warning of possible threats
to remote regions from surface oil entrained in the northern extension of the LC. The
importance of identifying periods of time with northern extensions of the LC or
LCR is that they had the potential to create direct pathways between the northern
GOM and the LC and surrounding Gulf of Mexico including the West Florida Shelf
and Florida Keys. In addition, surface mesoscale dynamics also exhibited a close
relationship between the extent and shape of the surface oil during much of the
DWH event. The complex surface circulation in the GOM, characterized by the LC
and the presence of cyclonic and anticyclonic eddies (Figs. 1.1 , 1.2 and 1.4 )was
assessed by the joint analysis of numerical model outputs and satellite observations.
Hydrographic data and numerical models were used to assess the connectivity of the
LC with the LCR below the surface [ 30 , 31 ].
The front or core of the LC and rings were identified in terms of the highest
geostrophic velocity values, which correspond to the highest horizontal gradient in
SSH which for the LC is approximately 0.005m/km. Values of SSH associated with
these maximum gradients ranged from 0.020-0.050m for anticyclonic features and
from 0.00-0.020m for cyclonic features. The altimetry-derived fields of geostrophic
velocity were complemented by a limited number of available in situ observations
from hydrographic cruises that were specifically geared towards understanding the
connectivity between the LC and the LCR at depth [ 30 ]. Results regarding the sepa-
ration of a LCR from the LC, based on surface currents alone, may also differ from
those obtained from SST estimates, as the mesoscale features derived from dynamic
and temperature fields may not necessarily coincide.
On April 15, 2010, before the oil spill occurred, the LC presented its northern
limit at approximately 27 ⓦ N with some of its circulation contained in an anticyclonic
motion centred at 25.5 ⓦ N inside the LC (Fig. 1.4 a). At this time the LC northern
boundary was translating to the north at 40km/week. When the oil spill occurred

on April 20, 2010, the northern limit of the LCwas located at around 27.5 ⓦ N, approx-
imately 190km from the spill site. The LC reached its northernmost excursion of
approximately 28 ⓦ N during the first half of May, at approximately 150Km from the
oil spill site. Therefore, according to data analysed in this work, this is the closest
distance between the LC/LCR system with the oil spill site. Around mid May, the

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