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The Loop Current System, involving the Loop Current and Loop Current Eddies, is the principal circulation feature in the Gulf of Mexico, which exhibits salinity gradients due to Mississippi River system freshwater discharge and large salinity variability on seasonal timescales. This research uses satellite-derived sea surface salinity from NASA's Soil Moisture Active Passive and ESA's Soil Moisture and Ocean Salinity missions with altimetric sea surface height data to observe and quantify the redistribution of low-salinity water by Loop Current System interaction. Freshwater flux in this region during summer months is modulated by Loop Current System configuration as classified by three states. An extended Loop Current transports low-salinity water southward to the Florida Straits. A Loop Current eddy near the Louisiana-Texas shelf recirculates low-salinity water within the central Gulf. During a retracted Loop Current, no interaction occurs and low-salinity water remains close to the coast in the northern Gulf.

Plain Language Summary The Gulf of Mexico receives freshwater from the Mississippi and Atchafalaya Rivers with an annual peak during spring due to snow melt over the upper part of the watershed. The fresh water forms a low-salinity water plume, which evolves based on local wind forcing as well as circulation features associated with the Loop Current System. The Loop Current brings Caribbean Sea water northward into the Gulf of Mexico and transports waters out of the Gulf through the Florida Straits where it feeds into the Gulf Stream. While inside the Gulf, the Loop Current can extend northward to the Mississippi-Alabama coast. When extended northward, the Loop Current advects low-salinity water eastward along the Mississippi/Alabama/Florida shelf, and southward along the West Florida Shelf. This low-salinity water then exits the Gulf through the Florida Straits. Additionally, the Loop Current occasionally sheds anticyclonic eddies which migrate westward to the Texas coast, and can recirculate low-salinity water in the central Gulf of Mexico. Analysis of nearly a decade of satellite salinity data shows a preference for events of freshwater flux toward the southeastern Gulf of Mexico during the late summer months with substantial interannual variability linked with the configuration of the Loop Current System.