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.

High-wind events predominantly cause the rapid breakdown of seasonal stratification on the continental shelf. Although previous studies have shown how coastal stratification depends on local wind-forcing characteristics, the locally observed ocean forcing has not yet been linked to regional atmospheric weather patterns that determine the local wind characteristics. Establishing such a connection is a necessary first step towards examining how an altered atmospheric forcing due to climate change affects coastal ocean conditions. Here, we propose a categorization scheme for high-wind events that links atmospheric forcing patterns with changes in stratification. We apply the scheme to the Southern New England shelf utilizing observations from the Ocean Observatories Initiative Coastal Pioneer Array (2015-2022). Impactful wind forcing patterns occur predominantly during early fall, have strong downwelling-favorable winds, and are primarily of two types: i) Cyclonic storms that propagate south of the continental shelf causing strong anticyclonically rotating winds, and ii) persistent large-scale high-pressure systems over eastern Canada causing steady north-easterly winds. These patterns are associated with opposite temperature and salinity contributions to destratification, implying differences in the dominant processes driving ocean mixing. Cyclonic storms are associated with the strongest local wind energy input and drive mechanical mixing and surface cooling. In contrast, steady downwelling-favorable winds from high-pressure systems likely advect salty and less buoyant Slope Water onto the shelf. The high-wind event categorization scheme allows a transition from solely focusing on local wind forcing to considering realistic atmospheric weather patterns when investigating their impact on stratification in the coastal ocean.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.