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

Abstract

The US Northeast continental shelf “cold pool” comprises winter‐cooled Shelf Water that istrapped below the warm surface layer during the stratified season. The regional ecosystem relies on thepreservation of winter temperatures within the cold pool throughout the year. Here, we present first evidence ofa significant increase in the cold pool's salt content on the US Northeast continental shelf throughout thestratified season, suggesting that shelfbreak exchange contributes strongly to the seasonal erosion of the coldpool. Cold pool salinification rates of 0.18 PSU/month remain steady throughout the stratified season, leadingto salinity differences of over 1 PSU between April and October. A cold‐pool salinity budget reveals that theobserved salinification is caused by an imbalance between cross‐shelf salt fluxes, which deposit salt into thecold 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 coldpool over the summer. Along‐shelf freshwater advection from upstream is only present in the fall andcontributes some additional freshening to shut down the salinification trend. Seasonal variability in the positionof the US Northeast shelfbreak front is too small and out of phase to contribute to the salinity increase. Thestrong relationship between the seasonal cycle of cold pool modification and seasonal stratification pointstoward the importance of the timing of spring re‐ and fall de‐stratification on near‐bottom continental shelftemperature and salinity.