Seasonal Salinification of the US Northeast Continental Shelf Cold Pool
Driven by Imbalance between Cross-Shelf Fluxes and Mixing
Abstract
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 conditions within the cold pool throughout the year. Here, we
present first evidence of a significant increase in the cold pool’s salt
content throughout the stratified season, revealed by sustained
multi-year observations from the Ocean Observatories Initiative (OOI)
Coastal Pioneer Array (2015-2022) and high-resolution realistic regional
model output. Cold pool salinification rates of
$0.17\,\mbox{PSU/month}$ remain steady
throughout the stratified season, leading to salinity differences of
over $1\,\mbox{PSU}$ between March and
October. The seasonal onshore movement of the US Northeast shelfbreak
front cannot explain the salinity increase since seasonal frontal
oscillations are too small and not in phase with the salinification
signal. Instead, 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 shuts down and no longer opposes the cross-shelf
flux, leading to net salinification of the cold pool over the summer.
Along-shelf freshwater advection from upstream contributes little and is
only present in the fall. The strong relationship between the seasonal
cycle of cold pool salinification and seasonal stratification points
toward the importance of the timing of spring re- and fall
de-stratification on near-bottom continental shelf conditions.