Loop Current eddy merger exposed by satellites during Gulf of Mexico oil spill

The clockwise flow of water that extends northward into the Gulf of Mexico known as the Loop Current, and its associated eddies, regularly produces strong currents of 2–4 knots in the northern Gulf. Cyclonic (i.e., counterclockwise rotating) eddies, migrating along its outer margin, are difficult to study due to their rapid and unpredictable growth and propagation, as well as persistent cloud cover. We have found that night-time mid-infrared satellite images obtained every 30 minutes from geostationary satellites used to quantify sea surface temperature, superimposed with daily updated gridded sea surface height data based on several satellite altimeters, allows us to track the Loop Current and cyclonic eddies more effectively than previous methods.^{1, 2}

During the Deepwater Horizon oil spill, we assessed daily changes in the motion of the Loop Current, its cyclonic eddies (called Loop Current frontal eddies), and detached anticyclonic eddies by analyzing daily-updated maps of sea surface temperatures and heights. We used ship-board acoustic Doppler current profiler current measurements and time-series positions from satellite-tracked Global Positioning System (GPS) buoys to validate our satellite data interpretations.³

Surface oil is often mapped using readily available passive radiometric optical imaging, such as ‘true-color’ images from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) sensors (see Figure 1). During the Deepwater Horizon oil spill these data were useful, but not totally reliable due to solar interference with the optical sensors. Synthetic aperture radar (SAR) sensors proved more valuable, as the backscatter signal is very sensitive to surface oil and is usable in all weather, day and night.⁴ The University of Miami Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) facility was able to provide daily near-real-time access to SAR images from several satellites. The distribution of surface oil was quantified by digitizing the oil's areal extent based on gradient analyses using both SAR and MODIS images. SAR imagery revealed a larger surface area of oil coverage than did MODIS imagery. The difference in area shown to be covered by oil was 49–100%, based on two cases of near-simultaneous image acquisition.

Figure 1. Top: Moderate Resolution Imaging Spectroradiometer (MODIS) ‘true-color’ image depicting the offshore entrainment of the Deepwater Horizon oil toward the Loop Current front on 17 May 2010. Middle: Radarsat-2 synthetic aperture radar (SAR) image also depicting the surface oil on 17 May 2010. Bottom: Geostationary Operational Environment Satellite East (GOES-East) sea surface temperature image depicting the Loop Current and the large merged cyclone north of the Loop Current on 17–19 May 2010. Superimposed on the color-coded sea surface temperatures are the oiled area, traced from the 17 May SAR image and the track of the satellite-tracked buoy from 26 April to 27 May 2010 (blue dots). White and red dots indicate the location of the submerged leaking well-head. Imagery was processed at the Louisiana State University (LSU) Earth Scan Laboratory and the University of Miami Center for Southeastern Tropical Advanced Remote Sensing (CSTARS).

Both SAR and MODIS data revealed an elongated flow of oil extending from the submerged well-head region toward the Loop Current on 17 May 2010 (see Figure 1). This feature indicated the Lagrangian flow field related to the positioning of eddies and the Loop Current. A close inspection of the combined sea surface temperature and height data provided strong evidence for the merging of three Loop Current frontal eddy cyclones during the first two weeks of May. This is the first documented case of cyclonic eddies merging along the Loop Current margin. The satellite data revealed a two-phase merging event, with two eddies along the northwest quadrant merging first and moving eastward to merge with a larger eddy on the northeast quadrant. Both phases resulted in increased circulation area and intensity.

This eddy-merging event led to the rapid offshore entrainment of oil clearly apparent in Figure 1 as a 325-km long and 10–20km wide oil slick. We estimate a surface oil coverage of 33,575km², based on SAR imagery. A satellite-tracked drifter, drogued at 45m, showed a maximum current speed of 2.25m/s as it was advected between the merged eddies and the Loop Current. SAR imagery clearly revealed an accumulation of surface oil within the merged cyclone, which contrasted with many numerical modeling simulations showing transport around the Loop Current's margin and into the Florida Straits.^{3, 5}

This research overturns our former understanding of the Loop Current eddy circulation processes.³ We showed the merger of three cyclonic eddies, which resulted in a much larger (>280×130km in area) and more intense (minimum sea surface height of −35cm) cyclonic circulation that controlled the motion and fate of much of the surface oil during our study period. Our satellite assessment of circulation events was supported by a wealth of in situ measurements collected by Horizon Marine Inc. during the event.

Advances in the numerical modeling of circulation and oil will require a better representation of these relatively small and fast-moving cyclonic eddies and their mergers along the Loop Current front. Our future research will focus on how often merging events occur and incorporation of this information into numerical models of circulation. The upcoming launch of the Geostationary Operating Environment Satellite R series in 2015 will provide significant advances in both spatial and temporal resolution, benefiting real-time surveillance and research. And future launch of the first satellite-borne wide-swath interferometric radar altimeter will dramatically improve altimetric sampling of submesoscale and mesoscale features, allowing even better assessments of spatial characteristics and growth processes of these dynamic eddies.

Funding for research and data collection was provided to Walker, Pilley, and D'Sa by NASA grant NNA07CN12A (Applied Sciences Program), and from BP. Support for Robert Leben came from Bureau of Ocean Energy Management (BOEM) contracts M08PC20043 and M10PC00112 to Science Applications International Corporation, and the NASA Ocean Surface Topography Mission Science Team Grant NNX08AR60G. The findings do not necessarily represent the opinions of BOEM. Support for Hans Graber and CSTARS came from Defense Intelligence Agency (DIA) Contract HHM402-06-D-0014.