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The Amazon River is a large source of terrigenous dissolved organic carbon (tDOC) to the Atlantic Ocean. The fate of this tDOC in the ocean remains unclear despite its importance to the global carbon cycle. Here, we used two decades of satellite ocean color to describe variability in tDOC in the Amazon River plume. Our analyses showed that tDOC distribution has a distinct seasonal pattern, reaching northwest toward the Caribbean during high discharge periods, and moving eastward entrained in the North Brazil Current retroflection during low discharge periods. Elevated tDOC content extended beyond the shelfbreak in all months of the year, suggesting that cross‐shelf carbon transport occurs year‐round. Maximum variability was found at the plume core, where seasonality accounted for 40% of the total variance, while interannual variability accounted for 15% of the variance. Our results revealed a seasonal pattern in tDOC removal over the shelf with increased consumption in May when river discharge is high. Anomalies in tDOC removal over the shelf with respect to the seasonal cycle were significantly correlated with anomalies in tDOC concentration offshore of the shelfbreak with a lag of 30–40 days, so that anomalously high inshore tDOC removal was associated with anomalously low tDOC content offshore. This suggests that variability in the offshore transport of tDOC in the Amazon River plume is modulated by interannual changes in tDOC removal over the shelf. Plain Language Summary: The Amazon River is an important source of dissolved organic carbon (DOC) of terrigenous origin to the Atlantic Ocean. It is often difficult to characterize the distribution and variability of this material in the ocean due to the lack of high resolution in situ observations spanning long periods of time. Here, we used satellite observations of terrigenous DOC (tDOC) to describe the behavior of the plume at seasonal and interannual scales. Our analyses showed that enhanced tDOC content is observed offshore of the continental margin during all months of the year, extending toward the Caribbean during high river discharge conditions and eastward during low discharge conditions associated with variability in the large‐scale circulation. Our results also revealed that tDOC removal over the shelf varies seasonally, being enhanced earlier in the year when river discharge is high. Anomalies in tDOC removal over the shelf are significantly correlated with anomalies in tDOC content offshore with a lag of 30–40 days, suggesting that the offshore transport of carbon is modulated by tDOC consumption over the shelf. Key Points: Satellite ocean color was used to describe variability in terrigenous dissolved organic carbon (tDOC) in the Amazon River plumetDOC degradation and/or flocculation over the shelf increase early in the year when river discharge also increasesInterannual variability in tDOC content off the continental margin is related to variability in tDOC removal over the shelf [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The Mississippi‐Atchafalaya River System (MARS) is an important source of freshwater, nutrients, carbon and sediments to the Gulf of Mexico. Here we use satellite observations of surface salinity and of dissolved organic carbon of terrigenous origin to describe the behavior of the MARS plume and its interaction with the Gulf circulation for the last 15 years. We show that the MARS plume is transported offshore during summer and toward the coast during fall/winter and that this seasonal variability is controlled by the interaction between river discharge and winds. In 2015 and 2016, the MARS plume was transported for 350 km from the coast due to interactions with the large‐scale circulation in the Gulf. While salinity from satellite observations allows us to track the river plume even in cloudy conditions, tDOC provides a more accurate characterization of sharp fronts and of the width of river‐influenced filaments. Key Points: Soil Moisture Active Passive (SMAP) salinity and MODIS‐Aqua combined provide improved description of Mississippi River plume variabilityMississippi River plume extends offshore during summer and retracts during fall/winterWinds and discharge are the dominant mechanisms controlling the Mississippi River plume on monthly time scale [ABSTRACT FROM AUTHOR]

The plume of Mississippi and Atchafalaya River system (MARS) is normally confined to the continental shelf and extends westward. During March 2010, while weather in North America was dominated by unusually low temperatures and winter storms in conjunction with large low-pressure systems, we observed an unusually broad extension of the MARS plume. Winds favorable for offshore transport along with high freshwater discharge contributed to a widespread MARS plume, which was statistically different from those normally occurring in spring. A strong relationship was observed between the location of this widespread plume and a reduced partial pressure of carbon dioxide () and elevated satellite-derived chlorophyll a. Furthermore, in the plume was reduced by 6.3 Pa (62 ppm) in March 2010 in contrast to a reduction of only 2.4 Pa in April 2009, thus exhibiting a greater CO2 uptake capacity in March 2010. This wide plume also covered at least twice the area and was a stronger CO2 sink by a factor of five when compared to more typical plume conditions in April 2009. Extending across the Louisiana shelf to the pelagic ocean, this widespread MARS plume encompassed regions that would normally exhibit nearly net zero air-sea CO2 exchange and resulted instead in a strong net sink for atmospheric CO2. Our findings demonstrate that regional weather-associated variations in wind forcing and river discharge may be important in influencing air-sea CO2 fluxes in marginal sea environments. [ABSTRACT FROM AUTHOR]