Continental freshwater discharge influences sea surface salinity variability near world’s megadeltas

Sea surface salinity (SSS) is a key parameter in the thermohaline circulation of global oceans. Near the megadeltas, inland streamflow through large catchments plays a crucial role in mediating salinity. While some regional studies have investigated how SSS is impacted through water cycle and climate components, a global scale quantification of inland streamflow contribution on SSS variability is lacking. Here, we utilized remote sensing and observation-driven datasets to quantify the statistical associations between SSS and streamflow (Qbasin) at 48 megadeltas worldwide. This study uncovers a robust negative association between Qbasin and SSS, with correlation coefficients R less than -0.6 for seasonal data found in 26 of the 48 megadeltas, and less than -0.4 for deseasoned data in 21 megadeltas. The anticorrelation relationship is more pronounced in large deltas, particularly near tropical climates and in river-influenced deltas. The study also underscores the significant roles of climate, morphological, and anthropogenic stratification in impacting the natural influence of freshwater discharge on SSS. By highlighting the interconnected impacts of alterations in terrestrial water cycle upstream and SSS, this work contributes to enhancing our understanding of global ocean and climate circulation patterns and in tackling environmental issues pertaining to marine ecosystems.