New insights into seasonal to interannual salinity variability on the Northeast U.S. continental shelf and slope

The Northeast U.S. continental shelf is a highly productive and economically important region that has experienced robust changes in upper-ocean properties in recent decades. Warming rates exceed the global and North Atlantic average and in particular several episodes of anomalously warm temperatures, so called marine heatwaves, have had devastating impacts on regional fisheries over the past decade. There are also indicators of a salinification of the region, which might be linked to large-scale changes in the North Atlantic circulation as well as changes in regional processes, such as the number of Warm Core Rings shedding of the Gulf Stream, driving an increased salinity flux into the continental slope and shelf region. With now more than a decade of remote-sensing sea surface salinity data, we revisit seasonal to interannual salinity variability and assess the role of salinity for modulating stratification on the continental shelf. We provide important regional context for the interpretation of data from the OOI Coastal Pioneer array, a local shelf-break observatory. We find that the local seasonal cycle is an interplay of seasonal freshwater input via local river discharge, driving decreasing salinities in spring and summer not just on the shelf but also in the Slope Sea. An observed salinification in the fall is likely linked to offshore forcing over the slope associated with the presence of Warm Core Rings. A coherent low-frequency salinity variability is found over the slope and shelf region in the Mid-Atlantic Bight (MAB) and Gulf of Maine, highlighting that shelf conditions in particular in the MAB are not solely dominated by upstream shelf conditions but are significantly impacted by local offshore variability. Furthermore, we synthesise hydrographic data from the NOAA ECOsystem MONitoring (ECOMON) program to construct mean cross-shelf sections along the MAB to investigate the relative contributions of thermal and haline components to the seasonal stratification. Overall, salinity serves as a valuable tracer, in addition to temperature, of these multi-variate processes and with now more than a decade of satellite surface salinity can shed new light on the spatio-temporal variability on the Northeast U.S. continental shelf.