Local boundaries of bottom hypoxia

Veronica Ruiz Xomchuk, Texas A&M University College Station, College Station, TX, United States and Robert D Hetland, Texas A&M University, College Station, TX, United States
Shipboard estimates of hypoxic extent in the Northern Gulf of Mexico are based on a 50 km sampling resolution and curtailing of the western portion of the survey region when hypoxia is absent in a transect. This assesment assumes low dissolved oxygen (DO) slowly advecting westward from the two river sources, thus producing a large, contiguous region of influence. In contrast, hypoxia has recently been observed west of the traditionally surveyed region, suggesting some separation of hypoxic features from the main dead zone.
Idealized numerical simulations of a buoyancy-driven flow over a shelf are used to investigate the formation of low DO features at the bottom. We hypothesize that baroclinic instabilities are a primary source of DO variability, ventilating the bottom boundary layer, and hence affecting the local time of exposure to hypoxia. Results show low DO rings form at the bottom, enclosing oxygenated cores with a length scale similar to the baroclinic instabilities generated at the surface. Changes in the bottom slope (i.e., ridges) act as trapping boundaries for hypoxic features, and slope gradients as small as 1e-3 significantly affect the local time of exposure to low DO.
In congruence, results from a 25-year realistic simulation show low DO rings and quasi-permanent boundaries in the spatial distribution of hypoxia duration over the Texas shelf, relatively far from the river influence. The identified boundaries align with large gradients in the bathymetric slope. We suggest that semi-permanent features in hypoxia duration are controlled by bathymetric trapping, while the surrounding variability is controlled by submesoscale eddies. Features found in observations, similar to those reproduced in the idealized and realistic simulations, are used to further the discussion.