Processes controlling mid-water column oxygen minima over the Texas-Louisiana shelf
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abstract
2015. American Geophysical Union. All Rights Reserved. We investigate distributions of dissolved oxygen over the Texas-Louisiana shelf using spatially highly resolved observations in combination with a regional circulation model with simple oxygen dynamics. The observations were collected using a towed, undulating CTD during the Mechanisms Controlling Hypoxia (MCH) program. Mid-water oxygen minimum layers (dissolved oxygen lower than 3.2 mL L-1) were detected in many transects. These oxygen minimum layers are connected with the bottom boundary layer and follow the pycnocline seaward as a tongue of low oxygen into the mid-water column. T-S diagrams highlighting the low oxygen minima in both observations and simulations imply direct connections between low-oxygen bottom water and the oxygen minimum layer. The dynamics of these oxygen minimum layers in the mid-water column are examined using a three-dimensional hydrodynamic model, based on the Regional Ocean Modeling System (ROMS). Convergence within the bottom boundary layer relative to density surfaces is calculated, results show that there is a convergence in the bottom boundary layer at the location where the pycnocline intersects the bottom. Buoyancy advection forced by bottom Ekman transport creates this convergent flow, and the corresponding low-oxygen intrusion. Similar intrusions of near-bottom water into the pycnocline are observed in other regions. The presence of hypoxia within the bottom boundary layer in the northern Gulf of Mexico creates a unique situation in which these intrusions are also associated with low dissolved oxygen. Key Points: Mid-water oxygen minima are studied using both observation and simulation The buoyancy-driven convergence is essential in forming the oxygen minima The mid-water oxygen minima is actually a bottom low-oxygen water intrusion
