Seasonal variability of the export of river discharged freshwater in the northern Gulf of Mexico

Numerical simulations of the Gulf of Mexico (GoM) using the Navy Coastal Ocean Model (NCOM) predict a seasonal signal of surface salinity throughout the northern Gulf. The model variability of salinity and currents is compared to historical hydrographic data and to transport inferred from drifting buoys. Model experiments are used to examine the roles of mesoscale eddy activity and the seasonal variability of wind forcing and river discharge in connection with the upper ocean salinity field. It is shown that the annual cycle of the local winds greatly influences the fate of the Mississippi River. Model results and drifter data show that the low salinity water is directed westward over the broad Louisiana-Texas (LATEX) shelf in the fall and winter where it remains trapped to the coast. This water is transported southward as a coastally attached current and often offshore by jets associated with eddy pairs along the western continental margin. In the spring and summer, the low salinity water of the northern Gulf spreads over deeper water to the east of the Mississippi Delta where it is influenced by the offshore circulation. Mesoscale eddies associated with the Loop Current (LC) can then entrain the low salinity water and transport it to great distances from its origin.