Seasonal Salinification of the US Northeast Continental Shelf Driven by an Imbalance Between Along-Shelf Advection and Cross-Shelf Eddy-Covariance Fluxes

The US Northeast continental shelf “cold pool” defines the body of winter-cooled Shelf Water that decouples from the surface layer during the stratified season. The cold pool canonically preserves fresh Shelf Water properties throughout the summer, which fulfills vital needs for the regional benthic ecosystem in the economically most productive fisheries region across the United States. However, recent warming trends significantly above the global average have put the ecosystem under environmental stress. While the cold pool’s heat content has been studied in detail, data limitations and large interannual variability in salinity have hampered an assessment of the cold pool’s salt budget. Here, we provide first evidence that the cold pool’s salt content increases significantly during the stratified season and investigate dynamical drivers of this trend, using a combination of multi-year mooring and glider observations and high-resolution regional model output. Cold pool salinification rates of 6 mPSU/day remain steady throughout the stratified season, leading to salinity differences of 1 PSU between April and October. The annual cold pool salinification is caused by an imbalance between eddy-covariance salt fluxes across the US Northeast shelfbreak front and advection of freshwater from upstream. While eddy-fluxes deposit salt onto the continental shelf at all times of year, the US Northeast shelfbreak jet is weakest during the summer, which reduces along-shelf advection. A seasonal reduction in the along-shelf salinity gradient is likely caused by processes in the Gulf of Maine/on Georges Bank. The observed interannual variability of the salinification signal is shaped by the intermittency of strong cross-shelfbreak eddy-covariance fluxes that are concentrated within 3-4 episodic events per year. Capturing the hydrographic trends in coastal water mass budgets and identifying their underlying dynamical mechanisms will lead to a better understanding of ecosystem responses and support sustainable fisheries management in a rapidly changing coastal ocean region.