Mississippi River Plume Variability in the Gulf of Mexico From SMAP and MODIS-Aqua Observations

Abstract

The Mississippi-Atchafalaya River System (MARS) is an important source of freshwater and terrigenous material to the Gulf of Mexico. We used sea surface salinity (SSS) measurements from the Soil Moisture Active Passive (SMAP) satellite and terrigenous dissolved organic carbon (tDOC) from Moderate Resolution Imaging Spectroradiometer (MODIS) to describe the MARS plume seasonality and interaction with the Gulf circulation. Our analyses revealed good agreement between SMAP and in situ salinity for S > 31, and that satellite-derived tDOC can serve as a useful tracer for the river plume. A boundary detection algorithm was used to quantify the frequency of plume occurrence in the Gulf for the last 15 years. The MARS plume has its maximum offshore extension during summer, retracting to closer to shore during fall/winter. Variability at monthly time scales is controlled by river discharge and alongshore winds, which explain up to 60% of the local variance in the frequency of plume occurrence near the shelfbreak. Maximum variability in plume extension is found to the east of the Mississippi River Delta due to wind reversals that generally occur during summer. The combined use of SMAP salinity and MODIS-tDOC allowed for a detailed view of anomalous events in 2015 and 2016, when the MARS plume was transported for 350 km from the coast due to interactions with the Loop Current and with dipole eddies. While SMAP observations allow for tracking the river plume even in cloudy conditions, high-resolution MODIS data allow for a more accurate characterization of sharp fronts and of the width of river-influenced filaments.