Atmospheric weather patterns and their contributions to the fall stratification breakdown on the Southern New England shelf

High-wind events predominantly cause the rapid breakdown of seasonal stratification on mid-latitude continental shelfs. It is well established that downwelling-favorable wind forcing, i.e., wind vectors with the coastline to their right (on the northern hemisphere), leads to enhanced coastal destratification. A categorization scheme for high-wind events has identified the two atmospheric weather patterns that locally cause such favorable wind conditions on the Southern New England shelf and have the largest contribution to the annual breakdown of stratification in the region. These patterns are i) cyclonic storms that propagate south of the continental shelf and cause strong anticyclonically rotating winds, and ii) persistent large-scale high-pressure systems over eastern Canada causing steady north-easterly winds. Despite both patterns generally producing downwelling-favorable winds on the shelf, the two patterns differ in their wind direction steadiness and tend to produce opposite temperature and salinity contributions to destratification, implying differences in the dominant processes driving ocean mixing. We hypothesize that local mechanical mixing and surface cooling dominate for cyclonic storms due to their strong wind energy input and shear production. In contrast, the weaker but steady downwelling-favorable winds from high-pressure systems can lead to an enhanced cross-shelf Ekman cell that advects salty and less buoyant Slope Water onto the continental shelf. To assess which process dominates for the different impactful high-wind event patterns, we apply a simplified two-dimensional mixed-layer model framework that incorporates horizontal buoyancy gradients across the shelfbreak front. The model allows to determine the stratification change caused by one-dimensional surface forcing (wind stress and surface buoyancy flux) and Ekman-driven advection individually. Observations from moorings and glider transects across the shelfbreak, provided by the Ocean Observatories Initiative Coastal Pioneer Array (2015-2022) at the Southern New England shelfbreak, allow a comparison to investigate the importance of along-shelf processes for predicting shelf stratification changes on synoptic to intra-seasonal timescales.