Effect of instabilities in the buoyancy-driven flow on the bottom oxygen: Applications to the Louisiana Shelf

Abstract

A combination of in situ sampling and numerical modeling was used to investigate the effects of mesoscale (<50 km) circulation patterns and stratification on the evolution of hypoxia on the Louisiana Shelf. Temperature, salinity, and dissolved oxygen concentrations records reveal the presence of an alongshelf meander, which is manifested vertically and horizontally as a wave-like distribution of the properties in the water column. The observations suggest the meander is a ubiquitous characteristic of the shelf with alongshore spatial scale approximately 50 km and less, which is consistent with the locations of sandy shoals along the coast and the local deformation radius. Twelve numerical experiments using an idealized three-dimensional shelf circulation model were performed to evaluate the relative importance of the variable bottom topography and freshwater forcing on the development, evolution, and scales of the dynamic instabilities. The inclusion of the shoals into the bottom topography showed the development of the dynamic instabilities as the flow passed over the shoals and downstream. Introduction of fresh water onto the shelf resulted in greater salinity differences, and, as a consequence in the formation of the dynamically unstable salinity fronts along the plume edge. The combination of the freshwater forcing and shoaling topography produced competing and complex interactions. Six numerical experiments were analyzed in order to investigate the effect of dynamic instabilities on spatial and temporal patterns of dissolved oxygen concentrations along the shelf. Although a linear relationship between Brunt-Väisälä frequency and dissolved oxygen deficit was expected, a nonlinear loop-like relationship was discovered that reflects the response of biochemical properties to the alongshelf variability of the density field. Comparison of the numerical modeling runs to observations of density and dissolved oxygen concentrations on the Louisiana Shelf reinforces the importance of physical processes such as topographic steering and/or freshwater forcing on the alongshore distribution of physical and biochemical properties. It suggests that the time scales of respiration (~3 days) and buoyancy transfer processes (~5-7 days), associated with the physical processes that are responsible for water column stability and ventilation, are similar to the time scales associated with the benthic respiration rates.