Rapid Vertical Exchange at Fronts in the Northern Gulf of Mexico

Abstract The Texas-Louisiana Shelf in the Northern Gulf of Mexico is home to the second largest human-caused dead zone in the world. Here, the nutrient-laden, stratified waters of the Mississippi River plume condition hypoxia in a bottom layer during the summer. The plume also generates strong fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using a combination of high-resolution observations and numerical simulations, we show that the vertical exchange at these fronts can be quite rapid and can lead to the oxygenation of bottom water when the fronts are forced by the summer land-sea breeze. These winds generate strong inertial oscillations, which set up a diurnally-pulsing vertical circulation at the fronts that draws bottom waters up to the surface mixed layer. The simulations suggest that during these “breaths” the rate of oxygenation of the bottom waters is comparable to deoxygenation by the respiration of organic matter over the shelf and hence could play an important role in the evolution of the region’s dead zone.