Mapping of a Mid-Depth Salinity Maximum Intrusion South of New England in June 2021 and Implications for Cross-Shelf Exchange

Abstract

The Northeast U.S. continental shelf is characterized by relatively cold and fresh shelf waters of Arctic origin, whereas the adjacent Slope Sea is home to warm and salty, Gulf Stream derived waters. These contrasting watermass properties form a strong Shelfbreak Front, which can act as dynamical barrier of exchange between onshore and offshore waters. A shelfbreak exchange process which has been commonly observed in the Middle Atlantic Bight, south of New England, is the mid-depth salinity maximum intrusion, occurring predominantly in the stratified season. It is easily identified within salinity profiles, and yet there have been few hydrographic surveys that have resolved the spatial extent of these features. In order to study these features in more detail, a dedicated research cruise was directed in June 2021 toward mapping and characterization of one of these intrusions. Using a combination of shipboard CTD profiles, Autonomous Underwater Vehicle missions, and a towed microstructure profiler, a strong intrusion was mapped in three-dimensions for the first time. The intrusion penetrated 33 km onshore of the upper portion of the Shelfbreak Front, was 14 km wide in the along-shelf, and was 10–30 m thick. A warm core ring was directly offshore of the intrusion and likely contributed to the initiation of the intrusion. The intrusion contributed about 2.5% additional salt relative to the ambient salinity structure. These intrusions are relevant for the shelf salinity budget as well as the onshore transport of larval fish and are thus an important process for the shelf ecosystem.