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, 111 (35), 12693-8

Submesoscale Dispersion in the Vicinity of the Deepwater Horizon Spill

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Submesoscale Dispersion in the Vicinity of the Deepwater Horizon Spill

Andrew C Poje et al. Proc Natl Acad Sci U S A.

Abstract

Reliable forecasts for the dispersion of oceanic contamination are important for coastal ecosystems, society, and the economy as evidenced by the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 and the Fukushima nuclear plant incident in the Pacific Ocean in 2011. Accurate prediction of pollutant pathways and concentrations at the ocean surface requires understanding ocean dynamics over a broad range of spatial scales. Fundamental questions concerning the structure of the velocity field at the submesoscales (100 m to tens of kilometers, hours to days) remain unresolved due to a lack of synoptic measurements at these scales. Using high-frequency position data provided by the near-simultaneous release of hundreds of accurately tracked surface drifters, we study the structure of submesoscale surface velocity fluctuations in the Northern Gulf of Mexico. Observed two-point statistics confirm the accuracy of classic turbulence scaling laws at 200-m to 50-km scales and clearly indicate that dispersion at the submesoscales is local, driven predominantly by energetic submesoscale fluctuations. The results demonstrate the feasibility and utility of deploying large clusters of drifting instruments to provide synoptic observations of spatial variability of the ocean surface velocity field. Our findings allow quantification of the submesoscale-driven dispersion missing in current operational circulation models and satellite altimeter-derived velocity fields.

Keywords: Lagrangian transport; geophysical turbulence; ocean dispersion; pollutant patterns.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Multiscale flows near the DwH and DeSoto Canyon region. (A) Synthetic aperture radar (SAR) image of the DwH oil slick taken on May 17, 2010. The red diamond marks the location of the DwH wellhead, and the Inset shows the geographic location. (B) Drifter launch patterns: The actual pattern obtained (red circles) for S1 at the launch time of the last drifter compared with the targeted template (black circles). The Inset shows a single node of the multiscale launch pattern. (C) Chlorophyll-a concentration (indicative of phytoplankton suspended in the upper ocean flows) derived from the moderate resolution imaging spectroradiometer sensor aboard the Aqua satellite on July 12, 2012. The similarity of this image to A indicates that the GLAD experiment sampled flow conditions similar to those during the spill. (D) The time evolution of the number of drifter pairs at given separation distances for the S2 release (pair numbers on log scale).
Fig. 2.
Fig. 2.
GLAD trajectories. Trajectories for S1 and T1 (Upper) and S2 (Lower) with initial and day-21 positions marked by symbols. Trajectories are color coded based on total residence time, τ, in the canyon: red triangles for τ < 7 d, gold circles for 7 < τ < 14 d, green circles for 14 < τ < 21 d, and blue squares for τ > 21 d. The zonal line at 28.1°N marks the latitude used as boundary for residence time estimates inside the canyon.
Fig. 3.
Fig. 3.
Sensitivity to launch positions. Initial launch locations and ship-track sea-surface salinity maps for S1 (Upper) and S2 (Lower) launches. Initial conditions are color coded based on total residence time in the canyon. Refer to Fig. 2 legend for the description of color coding. The colored tracks and the color bar indicate sea surface salinity measured along ship track.
Fig. 4.
Fig. 4.
Dispersion ellipses. Trajectories and dispersion ellipses for S1 (blue) and T1 (yellow). Launch S2 has been separated into two groups: drifters initialized in MRO water with residence times in the canyon longer than 7 d (red) and those with residence times in the canyon less than 7 d (green).
Fig. 5.
Fig. 5.
Dispersion diagrams from GLAD in comparison with NCOM and AVISO. Time dependence of the relative dispersion, D(t), for four different initial separation distances for the S1 (Top) and S2 (Middle) launches. For comparison, data from identical launches advected using geostrophic velocities produced by AVISO altimeter data are shown in red. (Bottom) The scale-dependent pair separation rate as function of separation distance for the three launches (S1, S2, and T1) shown in solid symbols with corresponding model results from a 3-km resolution NCOM simulation for S1 and S2 shown in open symbols. The slope indicates the Richardson regime, Δv/rr−2/3.
Fig. 6.
Fig. 6.
Scale-dependent relative diffusivities. Lower left axes: Tracer-based diffusivity estimates based on fitting ellipses. The solid red, green, and blue symbols show Okubo (36) estimates of Ka(3σ) = σ2/4t. Corresponding estimates from the S1 drifter data are shown by solid black squares. Upper right axes: Scale-dependent mixing length diffusivities, KL(r) = rΔv(r), observed in S1 launch plotted with uncertainty estimates in solid black lines and filled black circles. Richardson–Obukhov scaling law, KL(r) ∼ r4/3 is indicated.

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