Ocean Dynamics, 2011, in press.
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2010 Oil Spill: Trajectory Projections Based on Ensemble Drifter Analyses
Y.-L. Chang, L.-Y. Oey*, F.-H. Xu, H.-F. Lu and A.Fujisaki
Princeton University, Princeton, NJ, USA
*Corresponding author: lyo@princeton.edu
Abstract
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An accurate method for long-term (weeks to months) projections of oil spill
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trajectories based on multi-year ensemble analyses of simulated surface and subsurface (z =
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800 m) drifters released at the northern Gulf of Mexico spill site is demonstrated during the
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2010 oil spill. The simulation compares well with satellite images of the actual oil spill
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which show that the surface spread of oil was mainly confined to the northern shelf and slope
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of the Gulf of Mexico, with some (more limited) spreading over the north/northeastern face
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of the Loop Current, as well as northwestward toward the Louisiana-Texas shelf.
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subsurface, the ensemble projection shows drifters spreading south/southwestward, and this
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tendency agrees well with ADCP current measurements near the spill site during the months
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of May~July, which also shows southward mean currents. An additional model analysis
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during the spill period (Apr-Jul/2010) confirms the above ensemble projection. The 2010
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analysis confirms that the reason for the surface oil spread to be predominantly confined to
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the northern Gulf shelf and slope is because the 2010 wind was more southerly compared to
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climatology, and also because a cyclone existed north of the Loop Current which moreover
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was positioned to the south of the spilled site.
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At
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1. Introduction
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On 20 April, 2010, an off shore oil rig at the Macondo Prospect site operated by the
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British Petroleum (BP) in the Gulf of Mexico exploded and then sank into 1500 m of waters
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approximately 80 km southeast of Venice, Louisiana. According to a federal panel of
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scientists (the “Flow Rate Technical Group”), a total of about 4.9 million barrels of oil
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leaked into the ocean from the initial explosion to 15 Jul 2010 when the flow was stopped
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(see
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The spill threatened wildlife, ecosystem, and the livelihood of people on the Gulf Coast.
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Shortly after the spill, we used simulated drifters to make long-term (weeks ~ months)
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ensemble forecasts, with error estimates, of how and where the oil might spread using a
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unique set of high-resolution, multi-year, and data-assimilated analyses of wind, Loop
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Current
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(http://www.aos.princeton.edu/WWWPUBLIC/PROFS).
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NOAA, who used it to conduct oil-spill trajectory calculations based on the Monte-Carlo
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method. Both our and NOAA’s analyses refuted some of the “doomsday” predictions at that
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time showing extensive beaching along the Florida’s western coast, and in one example oil
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being carried out of the Gulf of Mexico and across the Atlantic Ocean.
http://www.nytimes.com/interactive/2010/05/01/us/20100501-oil-spill-tracker.html).
and
eddy-driven
currents
in
the
Gulf
of
Mexico
We made our data available to
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Circulations in the eastern Gulf of Mexico are complex, comprising of currents
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which near the coast are driven by wind and river plumes, and in the open ocean by wind,
2
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Loop Current and energetic eddies from surface to 500~800 m below the surface. In deep
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layers, smaller-scale eddies and topographic Rossby waves are dominant. For a review see
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Oey et al. 2005a (or/and DiMarco et al. 2005). For more recent studies, see Hamilton, 2007;
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Lugo-Fernández and Badan, 2007; Oey, 2008; Sturges and Kenyon, 2008; Hamilton, 2009;
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Oey et al. 2009; and Chang and Oey, 2010. There exist a wide range of temporal and spatial
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scales in the circulation processes. To arrive at meaningful statistics, our approach is to
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obtain long-term (i.e. multi-year ~ decadal) analyses of the currents combining model and
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observations, and then use these currents to produce ensemble drifter trajectories.
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2. Methodology
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The Princeton Ocean Model (POM; http://www.aos.princeton.edu/WWWPUBLIC
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/htdocs.pom/; Mellor, 2002) is used together with an efficient scheme that projects satellite
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sea-surface height anomaly (SSHA) data from Archiving, Validation and Interpretation of
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Satellites Oceanographic data (AVISO) (http://www.aviso.oceanobs.com/) on 1/3o×1/3o
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Mercator grid to the model density field (Mellor and Ezer, 1991; Ezer and Mellor, 1994; Yin
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and Oey, 2007).
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computed from a (~10 year) model run without assimilation to assimilate satellite SSHA into
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the model temperature. Sea-surface temperature (SST) is similarly assimilated using data
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from MCSST (http://gcmd.nasa.gov/ records/GCMD_NAVOCEANO_MCSST.html) but its
The projection uses SSHA and temperature-anomaly correlations pre-
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effects
are
less
than
SSHA.
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(CCMP)(http://podaac.jpl.nasa.gov/DATA_CATALOG/ ccmpinfo.html) winds and daily river
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discharges from the U.S. Geological Survey (http://waterdata.usgs.gov/nwis/rt) from 51
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United State (US) rivers (34 in the Gulf and 17 in the eastern coasts) are specified. The
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CCMP wind is a 6-hourly gridded (1/4o×1/4o) product that combines ERA-40 re-analysis
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with satellite surface winds from Seawinds on QuikSCAT, Seawinds on ADEOS-II, AMSR-E,
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TRMM TMI and SSM/I, as well as wind from ships and buoys. The model domain covers
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most of the northwestern Atlantic Ocean (NWAO) from 100oW55oW and 5oN50oN at
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approximately 5~10 km resolution in the Gulf of Mexico, and there are 25 vertical terrain-
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following (i.e. sigma) levels; this will be referred to as the NWAO model (Oey and Zhang,
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2004; Chang and Oey, 2010). An accurate fourth-order pressure-gradient scheme is used
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(Berntsen and Oey, 2010). The first sigma grid is z = 1.05 m below the surface in 1500 m
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of water. Monthly temperature and salinity climatologies from National Oceanographic Data
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Center’s
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(http://www.nodc.noaa.gov/OC5/WOA05/pr_woa05.html) are used to specify the model’s
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only open boundary conditions at 55oW.
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salinities below z = 1000 m are also relaxed to WOA climatologies with a long time scale of
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600 days; this does not impede short-period mesoscale variability. A modified Mellor and
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Yamada’s [1982] 2.5-level closure scheme that inputs breaking-wave turbulence energy at the
(NODC)
Six-hourly
World
Cross-Calibrated
Ocean
Multi-Platform
Atlas
To prevent long-term drift, temperatures and
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surface is used [Craig and Banner, 1994; Mellor, 2002]. This improves mixed layer depths
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and corrects unrealistically large speeds close to the surface sometimes found in the original
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scheme. The model has been used for research in the Gulf of Mexico where we have also
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extensively compared the results against observations both in the surface and subsurface
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(Oey and Lee, 2002; Ezer et al. 2003; Wang et al. 2003; Fan et al. 2004; Oey et al. 2005a,b,
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2006, 2007, 2008, 2009; Lin et al. 2007; Yin and Oey, 2007; Oey, 2008; Wang and Oey, 2008;
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Mellor et al. 2008).
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A high-resolution domain west of ~78oW and from 6oN~33oN is nested within the
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NWAO model at double the resolution (3~5 km horizontal grids and the same 25 sigma
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levels). The nest encompasses the northwestern Caribbean Sea, the southern portion of the
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South Atlantic Bight east of Florida, and the entire Gulf of Mexico. The nest receives
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boundary conditions from the NWAO model.
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conducted.
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conditions of wind, river discharge, Loop and eddy-driven currents pertaining to their
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respective time periods. They do not, however, cover the 2010 conditions. The CCMP wind
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tends to underestimate tropical cyclone strengths, so we supplement it with the NOAA’s
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HRD (Hurricane Research Division) analysis wind when these are available (see details in
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Yin and Oey, 2007). The 8-year ensemble then provides an estimate of the condition that
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would prevail if the present Loop Current (and eddies) are forced by winds from spring
Eight-year (2000-2007) analyses were
Both of the NWAO and nested models therefore span the wide-ranging
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through end of summer. Our goal is then to assess the accuracy of the spill probability as
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projected by ensemble spread of particles tracked using this 8-year data. Given that accurate
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forecast winds are not available beyond approximately one week, our approach is potentially
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a powerful means for long-term spill projections of 1~2 months.
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Three-hourly surface (i.e. first sigma level) currents and daily subsurface currents are
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used. Drifters are tracked horizontally at this surface level only with a semi-Lagrangian 4th-
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order Runge-Kutta scheme [e.g. Awaji et al., 1980] using currents and velocity shears from
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the model, from 21 Apr through 20 Aug (120 days) of each of the 8 analysis years. Five
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drifters per day are released, one at the accident site (88.39oW, 28.74oN), and four others at
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surrounding (i.e. neighboring) grids approximately 3 km (East-West and North-South) away.
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Since the model includes satellite data, rivers and winds, the ensemble analysis samples the
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ocean state due to Loop Current and rings, as well as winds and river discharges typical of
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spring to summer. The additional four neighboring drifters are used to estimate uncertainty
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of the release position on the model’s finite-difference grid, and also due to incomplete
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model physics, forcing, and numerical truncation errors.
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3. Results
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Winds in the Gulf of Mexico from 21 Apr through 20 Jun are predominantly from the
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east-southeast with speeds of 5~7 m s-1 typical of spring to early summer conditions [Fig.1;
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Gutierrez de Velasco and Winant, 1996; Wang et al., 1998]. However, wind directions near
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the spill site and east to the Florida Big Bend are seen to be more variable spanning a wider
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range from easterly, southerly to westerly, than locations further south and west. These
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winds produce northwestward to north/northeastward surface Ekman drift. The Louisiana,
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Mississippi, Alabama and the northwestern Florida coasts are therefore potential areas where
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drifters might end up. Ohlmann and Niiler (2005) show that very few drifters released over
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the shelf of Mississippi-Alabama-Florida can reach the Louisiana-Texas shelf west of the
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Mississippi Delta. However, easterly winds are downwelling-favorable with respect to the
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northern Gulf coast and, in combination with the Mississippi river plume, a strong westward
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coastal current can develop [Kourafalou et al. 1996]. Drifters can therefore also be advected
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to the Louisiana and Texas coasts west of the Mississippi delta, as will be seen later from
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satellite data. Moreover, since the release site is over the relatively deep region of the eastern
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Gulf of Mexico, drifter trajectories are also affected by the powerful Loop Current, rings and
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smaller eddies [Oey et al. 2005a].
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Figure 1a (inset) shows wind-rose at National Data Buoy Center (NDBC) station
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42040 just north of the spill site, computed from 21 Apr-07 Jun 2010. It indicates that winds
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during the first half of the spill were more southerly and southwesterly than climatology.
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More detailed monthly (April-July) mean wind and standard-deviation ellipse plots in fig.1b
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confirm that from April through May the winds in 2010 were more southerly and
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southwesterly than climatology. In June of 2010, winds were typical of climatology, while in
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July 2010 winds were more easterly. The consequences of these will be discussed later.
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Figure 2 (top panel) shows the visitation frequencies [Csanady, 1983; henceforth
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“VF’s”] on the 60th day (20 Jun) for a daily release from 21 Apr through 20 Jun (60 days).
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The VF is the number of times each grid square of a uniform resolution (0.05o×0.05o) grid is
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visited by the simulated drifters as a percentage of the total number of drifters released (480
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= 60drifters × 8years). The plot then shows the likelihood that each region is visited by the
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drifters. Another useful statistical information is the residence time which is a measure of
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both the frequency as well as the length of time each region has been visited; this is also
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shown in figure 2 (bottom panel). It shows large values (~30 days) near the spill site and in
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regions where drifters most often visit and/or the flow is sluggish. During the first few
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weeks, drifters preferentially spread north/northeastward in the same direction as the
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dominant direction of the wind-driven Ekman currents.
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west/northwestward towards the Louisiana-Texas shelf due to wind and plume processes, and
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south/southeastward into deeper waters caused by eddies and the Loop Current.
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drifters follow the north/northeastern face of the Loop Current and are advected to the
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southern tip of Florida, but the %VF is low. The west Florida shelf and coast are largely free
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of drifters, and so is the Texas coast. The reason in both cases is due to the predominantly
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easterly winds, which produce west/northwestward Ekman drift away from the west Florida
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Later, they also spread
Some
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coast, and convergence on the northern Gulf coasts. Also, in combination with buoyancy
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forcing from rivers [Cochrane and Kelly, 1986], the easterly winds tend to force drifters to
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beach on the Louisiana coast before they reach Texas.
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It is interesting to see how each of the five northern Gulf’s states is affected by the
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drifter-release. This is shown in Fig.3 which gives percentages of drifters that are stranded at
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the coast of each of the 5 states on 20 June, and the calculation is for five drifters released
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daily for two weeks (21 Apr-15 May). The two most-affected states are Louisiana and
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Mississippi, which receive about 20% each of the total releases. Next are Alabama and
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Florida, each about 10%, while Texas is free of stranded drifters. The northern Florida coast
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west of the “Big Bend” (near 84oW) has 2~3 times longer coastline than Alabama, and
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therefore though it is farther from the release site, it receives approximately the same number
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of stranded drifters.
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We find that the VF-distributionat z = 10 m (not shown; but see
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http://www.aos.princeton.edu/WWWPUBLIC/ PROFS/bp_spill.html) is quite different from
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that at the surface as very few drifters beach even after 60 days. This shows the importance
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of resolving the near-surface layer with a fine mesh of less than 1 meter; over the shelf (water
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depths < 100m), the present model has 4~5 grid points near the surface. The mixed layer  5
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m from the model, because of small eddy diffusivity < 10-3 m2 s-1 due to strong stratification
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caused by buoyancy inputs from rivers and surface heating, and by generally weak winds.
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The downwelling-favorable wind therefore produces at z = 10 m a weak but predominantly
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offshore flow which prevents drifters at that level from reaching the coast.
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In contrast to the surface analyses, the VF-distribution at z = 800 m (fig.4, top panel)
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shows values confined near the release site. This depth is chosen because deep currents
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below z  800 m in the Gulf of Mexico are mostly depth-independent [e.g. Oey, 2008].
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Highest VF-values near the spill site are aligned along-isobath from west-southwest to east-
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northeast. In approximately 1 month, stirring by deep currents and eddies [Oey, 2008;
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Hamilton, 2009] spread drifters southward with a preferential tilt towards the west/southwest
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from the base of DeSoto Canyon after 60 days [c.f. Wang et al. 2003]. The southward spread
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agrees well with observed ADCP (Acoustic Doppler Current Profiler) measurements at z=-
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1000m (fig.4, bottom panel), which shows also southward mean currents near the spill site
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for the months of May~July in 2010.
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To understand how currents due to winds, plumes, eddies and Loop Current affect the
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fate of drifters, we group (Fig.5) drifter tracks according to their dominant drift directions: (a)
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north/northeastward towards the coasts east of the Mississippi delta, (b) westward towards
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the western Louisiana and Texas coasts, and (c) south/southeastward towards the Straits of
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Florida. Drifters in Fig.5c are clearly dominantly driven by the Loop Current, which was
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extended northward past the 26.6oN in 21 Apr-20 Jun of 2005 and 2006. Mean winds during
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those periods were weak from the east southeast, with speeds  2 m s-1. Their effects can still
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be seen however as some drifters are forced onshore by the Ekman drift (Fig.5c). In Fig.5a,b,
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except for 2003, the Loop Current was in a retracted state south of 26.6oN, and drifter tracks
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are predominantly determined by wind-induced currents.
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southerly and sometimes exceed 5 m s-1. The corresponding Ekman currents tend to force
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drifters onshore towards the Louisiana-Mississippi-Alabama coast east of the delta.
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appears that the strong southerly bursts were sufficient to overcome the Loop Current’s
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influence even for 2003. In Fig.5b, winds had strong easterly bursts, also about 5 m s-1. The
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combination of westward winds and river plumes is effective in forcing a coastal jet that
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advects drifters westward. In 2004 and 2007, influences of deep-water eddies south of the
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release site, a cyclone in 2004 (not shown) and a Loop Current ring in 2007 (Fig.5b) can also
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be seen.
In Fig.5a, winds are more
It
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To provide an estimate of the uncertainty, the analyses are repeated using a longer
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dataset from 1993-2007 (15-year) but at a lower (1/2) model resolution. The results are
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similar and are not shown here. The 15-year dataset gives more diffused VF’s as can be
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expected from a lower-resolution model. The higher-resolution model also produces more
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north/northeastward spreading towards the Mississippi-Alabama coast, as well as westward
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spread towards Texas. Both are induced by stronger coastal currents due to the finer grid.
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South of the spill site, both datasets show similar spreading along the eastern face of the
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Loop Current.
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How accurate is the above projection? We now have estimates of the surface extent
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of oil from aerial photos and satellite imagery (not from models) by NOAA, shown in fig.6,
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and they may be compared with Figure 2. The important point about our prediction (fig.2) is
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that the spill is almost totally confined within the northeastern Gulf of Mexico, with bias over
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the northern and eastern face of the Loop Current and also towards the northern and
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northeastern coasts of the Gulf. As mentioned above, our analysis also predicts that no oil
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beached along the western shore of Florida. These predictions are in good agreement with
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the observed images shown in Fig.6. These show a predominantly north and northeastward
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spreading of oil from the spill site in agreement with the predicted larger VF’s in those
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regions. It also indicates oil slicks being preferentially advected east/southeastward along the
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north/northeastern face of the Loop Current, which also agrees with the predicted string-like
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VF-distribution in Fig.2 in the region 87-86W and 25-28N. With the exception of the
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extreme northwestern coast of Florida (west of the Big Bend), the Florida coast remained
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free of oil slicks, so were most of the coastal regions west of the Mississippi Delta, where
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Fig.2 indicates low VF-values. Our analysis shows only a low percentage of oil slicks being
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dispersed out of the Gulf by the Loop Current/Gulf Stream system. The small percentage is
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moreover outside the certainty of our estimate as measured by the grey shading of fig.2 (top).
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2010 Simulation: Comparison with Observed Satellite Data
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To further assess the accuracy of the circulation and drifter-tracking models, we
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conducted an additional analysis on the nested grid by assimilating satellite altimeter (SSHA)
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data from April-July 2010, and repeated the drifter-tracking experiment. The CCMP wind
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was not available, so instead we used the 6-hourly Global Forecast System GFS wind from
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the National Centers for Environmental Predictions (NCEP), at 0.5o×0.5o resolution. We
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then compare the results against observed digital data of oil-spill images. The data are daily
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in the so called “shapefile” format, and contain information on the edges of oil spill – from
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ftp://satepsanone.nesdis.noaa.gov/OMS/disasters/DeepwaterHorizon/composites/2010/.
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Figure 7 (top panel) shows their composite from 17 May through 30 Jul 2010, and the lower
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panel of fig.7 shows the corresponding simulated-drifter composite superposed on the model
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velocity and SSH fields averaged over the same 2.5 month period. When calculating the
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model composites, only those drifter-trajectories that fall within a radius of 200 km from the
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spill site are used as initial conditions for the subsequent 2-week composite. This is to very
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roughly account for the degrading oil with time – the spilled oil has either evaporated or been
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burned, skimmed, recovered from the wellhead or dispersed. For typical currents of about
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0.15 m s-1 during the spill period, the 200 km radius corresponds to a half-life of oil of about
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1 week which seems reasonable, and is somewhat conservative compared to the half-life of
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about 3 days due to, say, bacterial consumption (e.g. Camilli et al. 2010 and references
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quoted therein; as pointed out by the editor, this estimate does not account for the possibility
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that subsurface plume can surface; however, our model is hydrostatic and cannot correctly
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simulate the rapid surfacing of plumes. In the northern Gulf (away from the Loop Current
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and strong eddies), vertical motions in the model are typically only 1~10 m/day). Figure 7
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shows that, as in the ensemble prediction (fig.2), most of the simulated drifters spread
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north/northeastward during the spill period. During the early stage (17-31 of May; dark blue
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in fig.7), modeled drifters were confined mainly to the northern shelf with little excursion to
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the southeast as in the observed spreading. On the other hand, at later stages, for example
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during the 1-15 Jul period (yellow), model correctly shows a west/northwestward spread of
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drifters towards the Texas coast during the , which is also consistent with the wind being
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predominantly from the east and southeast in July of 2010 (fig.1b).
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south/southeastward spread, and this is again mostly confined to the east/northeastern face of
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the Loop Current. The modeled velocity and SSH fields show that the reason that the spread
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is mostly confined to the northern Gulf is because of the predominantly onshore currents
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forced by the weak southerly winds, in agreements with our previous inferences based on the
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ensemble results (fig.1). Another reason is that the Loop Current was not well-extended and
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north of the Loop Current there was a cyclone which tended to produce north and
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northwestward currents near the spill site. This is a scenario that most resembles Figs.5a,b in
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the ensemble experiment.
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There is also
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4. Conclusions
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During early stages (weeks), southerly and easterly winds typical of spring and early
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summer tend to transport drifters onshore towards the northern Gulf coasts. Later (~ 2
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months), stirrings by open-ocean eddies and currents south of the release site can entrain the
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drifters south/southeastward following the eastern face of the Loop Current towards the
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Straits of Florida. However, the small percentage of drifters that go around the tip of Florida
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and move along the eastern continental slope are outside the certainty of the model estimate.
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At subsurface, drifter trajectories are controlled by smaller-scale eddies (Oey, 2008) and
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spread south/southwestward into the Gulf’s interior.
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Winds play a major role in the surface trajectories. Winds from 21 Apr through mid-
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June during the spill were more southerly and southwesterly than climatology (Fig.1,
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compare NDBC 42040 with nearby stations). Therefore, in combination with a Loop Current
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that was predominantly confined to the southern portion of the Gulf, and in the absence also
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of strong eddies during the spill period of Apr-Jul 2010, the coastal regions east of the
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Mississippi Delta would be more affected by the spill. This was confirmed by satellite
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images.
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We have shown that the ensemble prediction is accurate when compared with the
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observed surveys of the actual spill, and is therefore a useful tool for oil-spill trajectory
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predictions in future oil spills. We have further confirmed this conclusion using the actual
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2010 analysis. We show that during the spill, the combination of more southerly winds, the
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southward shift of the Loop Current away from the spill site and the existence of a cyclone
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north of the Loop Current all contribute to the spill being confined predominantly over the
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northern Gulf of Mexico.
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ACKNOWLEDGEMENTS
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We thank the reviewers and editor (Huijie Xue) for providing many useful comments.
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Kirk Bryan initiated our interests in the ensemble analyses. We thank also Walter Johnson
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and Alexis Lugo-Fernandez for their inputs. YLC received a fellowship from the Graduate
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Student Study Abroad Program (NSC97-2917-I-003-103) of the National Science Council of
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Taiwan. The model analysis data reported herein would not have been possible without years
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of supports from BOEMRE. Computing was done at NOAA/GFDL, Princeton.
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Fig.1a. Wind-Roses at 11 indicated locations in the Gulf of Mexico: winds (vectors, m/s) and
percentage occurrence (color) in 12 indicated directions in the Gulf of Mexico, ensemble
averaged for 21 Apr-20 Jun period over 8 years from 2000-2007. Wind-rose at NDBC
42040 (location marked as a black dot) for the period 21 Apr-07 Jun 2010 is also included
for comparison with the ensemble (see inset). Contours are 200m and 2000m isobaths.
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Fig.1b. Mean wind velocities (vectors; scale in m/s shown on bottom of lower-right panel)
and standard-deviations (ellipses and color background in m/s) for the month of April, May,
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July and June (clockwise from top-left) for year 2000-2009 “wind climatology” from CCMP
dataset (black vectors and ellipses) and for year 2010 (blue vectors and ellipses) from four
NDBC buoy stations, from west to east: 42019 (-95.35,27.91), 42001 (-89.67,25.90; not
available in April 2010), 42040 (-88.21,29.21) and 42036 (-84.52,28.50). On top of each
panel, the maximum CCMP standard deviation (m/s) and lon/lat location are also shown.
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Fig.2. Top panel: visitation or “hit” frequency (color, darkest red scale = 10%; number of
times a location is visited as % of total releases = 480) for the drifter released once per day at
the central (spill) site (88.39oW, 28.74oN) from 21 Apr-20 Jun and tracked for 60 days. The
gray color indicates uncertainty spread as determined by regions visited by releases
originating from the four grid points surrounding the central release site. Lower panel:
residence time in days; larger values > 5 days are within a radius of 50~100 km near the spill
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site. Note both color and grey scales are logarithmic. Contours are 200m and 2000m
isobaths.
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Figure 3 Barchart gives the percentages by states of simulated drifters that hit the northern
Gulf coast on 20 June. The drifters were released from 21 Apr through 15 May. TX=Texas,
LA=Louisiana, MI=Mississippi, AL=Alabama and FL=Florida.
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Fig.4. Top panel: visitation frequency on 20 Jun at z = 800 m (color, darkest red scale =
10%) for releases from 21 Apr-20 Jun and tracked for the entire 60 days. Contours are 200m
and 2000m isobaths. Bottom panel: variance ellipses and the corresponding mean velocity
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vectors at z=-1000m from 3 ADCP’s near the spill site (‘+’), calculated for the period May
through July 2010. Contours are 200m, 500m and 1000m isobaths. The ADCP data is from
NDBC: http://www.ndbc.noaa.gov/maps/ADCP_WestGulf.shtml.
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(B)
(A)
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(C)
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Fig.5. Drifter tracks on 20 Jun, 15-day averaged wind vectors and Loop Current positions
defined as the mean SSH = 0.1 m contour averaged from 21 Apr-20 Jun for (a)
north/northeastward drifts in 2000, 2002 and 2003; (b) westward drifts in 2001, 2004 and
2007; and (c) south/ southeastward drifts in 2005 and 2006. Colors indicate years, and
‘other years’ in wind stick plots are gray. Drifters are continuously released at the spill site,
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once per day for 15 days from 21 Apr-5 May, and are tracked until 20 Jun. Contours are
200m and 2000m isobaths.
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Fig.6a,b Surveyed (dark shades; from aerial and satellite) and estimated (light shades; wind
and ocean current estimates combined with aerial photography and satellite imagery) extents
of oil on surface from NOAA. The dotted region is fishing ban, designated if oil were found
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in the vicinity within the last few days. The Loop Current and eddies are schematic sketches
based on satellite imagery. (a) 19 May, (b) 14 Jun, (c) 19 Jun & (d) 29 Jun, 2010. From
http://www.nytimes.com/interactive/2010/05/01/us/20100501-oil-spill-tracker.html.
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Fig.6c,d.
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Fig.7 Top panel: Satellite derived surface oil analysis composite image from 17 May to 30
Jul; the different colors denote different 2-week periods: dark blue for 17-31 May, light blue
for 1-15 Jun, green for 16-30 Jun, yellow for 1-15 Jul, and orange for 16-30 Jul. Bottom
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panel: same composite calculated from simulated drifter trajectories superposed on the 17
May-30 Jul 2010 mean velocity (vectors) and SSH (color shading in meters showing positive
SSH only). Contours show 200m and 2000m isobaths.
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