A numerical investigation of the Mississippi and Atchafalaya freshwater transport, filling and flushing times on the Texas-Louisiana Shelf

A high-resolution coastal model is used to investigate the transport, filling and flushing times of the freshwater introduced from the Mississippi and Atchafalaya Rivers on the Texas-Louisiana Shelf. The model is forced with realistic forcing, and is nested within hindcasts from the HYCOM operational model. The Mississippi and Atchafalaya discharges are each tagged with dye so that they can be identified and treated separately. The seasonal patterns of freshwater transport are consistent with that expected for the prevailing seasonal winds, but with significant interannual variability. In non-summer months, the major freshwater transport is downcoast and mainly occurs in a narrow band inside of 20-m isobath. In summer, the transport decreases dramatically near the coast due to the competing effects of downcoast buoyancy driven flow and upcoast wind-driven flow. The freshwater transport is upcoast over the mid shelf, in summer, with an offshore component consistent with Ekman transport. We define the shelf domain as the region enclosed by the 100-m isobath, and the along-shore limit of the entire model domain, approximately from the Louisiana-Mississippi border to the Texas-Mexico border. Filling times, based on the river discharge, range from similar to 3 months (non-summer) to similar to 6 months (summer) for Mississippi, while for Atchafalaya from similar to 3-4 months to similar to 1 year. Flushing times, based on the fresh water flux out of the shelf domain, are more variable, ranging from several months to several years.