Seasonal Salinification of the US Northeast Continental Shelf Cold Pool Driven by Imbalance Between Cross-Shelf Fluxes and Vertical Mixing

The US Northeast continental shelf “cold pool” comprises winter-cooled Shelf Water that is trapped below the warm surface layer during the stratified season. The regional ecosystem relies on the preservation of winter temperatures within the cold pool throughout the year. Here, we present first evidence of a significant increase in the cold pool's salt content on the US Northeast continental shelf throughout the stratified season, suggesting that shelfbreak exchange contributes strongly to the seasonal erosion of the cold pool. Cold pool salinification rates of $0.18\text{PSU/month}$ remain steady throughout the stratified season, leading to salinity differences of over $1\text{PSU}$ between April and October. A cold-pool salinity budget reveals that the observed salinification is caused by an imbalance between cross-shelf salt fluxes, which deposit salt into the cold pool at all times of year, and the strong seasonal cycle of vertical mixing. During the stratified season, vertical mixing is inhibited and no longer counteracts the cross-shelf flux, leading to net salinification of the cold pool over the summer. Along-shelf freshwater advection from upstream is only present in the fall and contributes some additional freshening to shut down the salinification trend. Seasonal variability in the position of the US Northeast shelfbreak front is too small and out of phase to contribute to the salinity increase. The strong relationship between the seasonal cycle of cold pool modification and seasonal stratification points toward the importance of the timing of spring re- and fall de-stratification on near-bottom continental shelf temperature and salinity.