Drivers of near-surface ocean salinity variability in the Northeastern Pacific coastal transition zone

The coastal waters of the Northeast Pacific (NEP) region, along the coastlines of Oregon and Washington in the U.S. and British Columbia in Canada, receive substantial freshwater inflows from rivers. In this study, we assess the impact of these discharges on the seasonal and interannual variability in the near-surface salinity in the NEP coastal transition zone (CTZ), an area of the interior open ocean where the ocean dynamics are influenced by coastal processes. The assessment is based on satellite SMAP and SMOS observations, in-situ Argo profiler data, and outputs of a regional ocean circulation model. The model domain spans from southern Mexico to the Alaska Panhandle and includes freshwater inputs from more than 500 riverine sources along the Pacific coast, obtained from the Global Flood Assessment System (GloFAS). For the study period of 2008–2018, the model solution is compared to an earlier benchmark that only included discharges from major sources including the Columbia River and Salish Sea inputs. Adding the full suite of terrestrial discharges results in a more pronounced freshening of the surface waters from northern California to British Columbia coasts. In particular, it helps to improve the sea surface salinity (SSS) bias and variability in the CTZ off Vancouver Island compared to the Argo data. Satellite SSS combined with altimeter-based sea level anomaly maps reveal patterns indicative of eddy-driven transport of terrestrial waters from the shelf into the CTZ. The volume-averaged salinity term balance analysis in the 50-m near-surface layer in the CTZ domain off Vancouver Island shows that the oceanic transport contributes to freshening the layer in summer. The vertical diffusion term is relatively larger and positive in most winters, increasing salinity in the surface boundary layer. The net surface salinity flux is an influential contributor to the near-surface salinity balance.