Mississippi River and sea surface height drive migration of surface oil slick

Millions of barrels of oil escaped into the Gulf of Mexico (GoM) after the 20 April, 2010 explosion of Deepwater Horizon (DH). Ocean circulation models were used to forecast oil slick migration in the GoM, however such models do not explicitly treat local Mississippi River (MR) hydrodynamics and its variability, or the buoyancy effects of oil. Here we identify a previously unreported effect of the MR plume: under conditions of relatively high river discharge and weak winds, an anomalously large bulge in sea surface height (SSH) can form around the Delta. Formation of a freshwater bulge acted to push the buoyant DH oil slick seaward from the Delta coastline. We explain this slick migration using a two-layer flow model driven by coupled pressure gradients, buoyancy and friction. Strong onshore winds following Hurricane Alex (late June, 2010) overwhelmed seaward SSH gradients, however pressure-driven slick migration resumed after winds declined. Results show how SSH variations can drive a first order, non-geostrophic migration of surface oil slicks. Incorporating these effects into more complex ocean models will improve forecasts of slick migration for future spills. More generally, large SSH anomalies at the MR mouth may also affect the dispersal of freshwater, nutrients and sediment associated with the MR plume.