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.

The Northeast U.S. continental shelf is a highly productive and economically important region that has undergone substantial changes in recent years. Warming exceeds the global average and several episodes of anomalously warm, sustained temperatures, so called marine heatwaves, have had profound impacts on regional fisheries. A majority of recent research focused on the analysis of temperature, however salinity can serve as a valuable tracer as well.   With now more than a decade of remote-sensing sea surface salinity data, we shed new light onto salinity variability in the region with focus on the Mid-Atlantic Bight and assess its role for modulating stratification on the shelf using historic hydrographic data. Seasonal freshwater input via local river discharge drives decreasing salinities in spring and summer on the shelf, but also in the Slope Sea. In spring, freshwater aids the build up of stratification and a freshwater lens of about 20m thickness extends to the shelf break above the pycnocline by the beginning of summer. An observed strong salinification in the fall is linked to offshore forcing over the slope associated with the presence of Warm Core Rings.   Coherent low-frequency salinity variability is found over the slope and shelf, highlighting that shelf conditions are significantly impacted by local offshore variability and vice versa. 2015 was characterized by anomalously high salinities, associated with a northerly position of the Gulf Stream. A freshening between 2015 and 2021, is in agreement with increased river discharge. Overall, salinity serves as a valuable additional tracer of these multi-variate processes.

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 Northeast U.S. shelf (NES) is an oceanographically dynamic marine ecosystem and supports some of the most valuable demersal fisheries in the world. A reliable prediction of NES environmental variables, particularly ocean bottom temperature, could lead to a significant improvement in demersal fisheries management. However, the current generation of climate model-based seasonal-to-interannual predictions exhibit limited prediction skill in this continental shelf environment. Here we have developed a hierarchy of statistical seasonal predictions for NES bottom temperatures using an eddy-resolving ocean reanalysis dataset. A simple, damped local persistence prediction model produces significant skill for lead times up to ~6 months in the Mid-Atlantic Bight and up to ~11 months in the Gulf of Maine, although the prediction skill varies notably by season. Considering temperature from a nearby or upstream (i.e. more polewawrd) region as an additional predictor generally improves prediction skill, presumably as a result of advective processes. Large-scale atmospheric and oceanic indices, such as Gulf Stream path indices (GSIs) and the North Atlantic Oscillation index, are also tested as predictors for NES bottom temperatures. Only the GSI constructed from temperature observed at 200 m depth significantly improves the prediction skill relative to local persistence. However, the prediction skill from this GSI is not larger than that gained using models incorporating nearby or upstream shelf/slope temperatures. Based on these results, a simplified statistical model has been developed, which can be tailored to fisheries management for the NES.