Allahdadi, M.N.; Jose, F., and Patin, C., 2013. Seasonal hydrodynamics along the Louisiana coast: implications for hypoxia spreading.

Summer and fall inner-shelf flow characteristics, including the vertical current structure, obtained from three WAVCIS (WAVE-Current-surge Information System) stations off the Louisiana coast were analyzed to delineate the hydrodynamic conditions that contribute to the formation of seasonal hypoxia at each station location. Two of the WAVCIS data stations used for analysis are located west of the Mississippi bird-foot delta, whereas the third is located east of the delta. Relatively small vertical gradients in the horizontal velocity current (i.e., u and v velocity components) were observed during summer (2009) for both CSI-6 and CSI-9 stations, which are located well inside the hypoxia-prone zone west of the delta. In contrast, the summertime vertical gradient of horizontal current at CSI-16 (located east of the delta) was significantly higher than that of western stations. Significant differences in the vertical gradient of flow velocities along with contrast in water column density gradient during the summer culminated in a strong stratification, which is considered the main physical requirement for the formation of hypoxia. A computed criteria based on the Richardson number also pointed to higher potential for stratification at both stations west of the delta, whereas the water column east of the delta still remained subject to vertical mixing. Furthermore, summertime current fields at CSI-6 and CSI-9 were significantly less compared with CSI-16, suggesting less reoxygenation driven by advection from surrounding waters. These conditions may exacerbate hypoxia.