Material transport in the inner shelf of the northern Gulf of Mexico

Abstract

Material transport is investigated in the northern Gulf of Mexico with field observations obtained within the inner shelf, offshore of a beach where oil washed ashore following the Deepwater Horizon oil spill. In the fall, winter, and spring, extratropical cold air outbreaks frequently pass from the north, northwest across the nearly latitudinal coastline and experiment site, bringing winds that are stronger in the cross-shore than alongshore. Cross-shore winds drive depth-averaged along-shelf currents through an ageostrophic balance with the cross-shelf pressure gradient force and Coriolis force. Eulerian Acoustic Doppler Current Profiler and Lagrangian GPS-drifter-measured surface (< 1 m) currents differ from depth-averaged subsurface currents, and are 4–7 times larger than theoretical estimates of wind and wave-driven surface flow. Differences in surface flow are attributed to the presence of a buoyant river plume. Plume boundary fronts are sources of horizontal velocity and density gradients where drifters converge, slow, and are redirected. When the plume extended along the coast, 100% of drifters deployed offshore of the plume were barred from the beach. Plume kinematics and thickness, expected to be important for sustainment of the barrier, are examined with a one-year record of velocity observations in 10 m water depth.