Cross-shelf exchange associated with a shelf-water streamer at the Mid-Atlantic Bight shelf edge

Significant exchanges between the Mid-Atlantic Bight (MAB) continental shelf and the neighboring open ocean can be induced by shelf water streamers, submesoscale filaments of shelf water entrained into the open ocean by Gulf Stream warm-core rings (WCRs) impinging onto the MAB continental shelf. Shelf water streamers have distinctive surface temperature and chlorophyll signals, and are thus visible from space. Satellite-measured sea surface height, temperature and chlorophyll show the evolution of a WCR over its 6-month lifespan in February-August 2019 and the persistent shelf water streamer it generated on its outskirt. In situ measurements from a two-week cruise in July 2019 were analyzed to investigate the physical, biological and biogeochemical characteristics of the shelf water streamer below the surface, and to quantify the associated cross-shelf transport of volume, heat, salt, carbon and oxygen. The analyses demonstrated that offshore transport of shelf water by the streamer, which was presumably balanced by either onshore intrusion of ring water or enhanced transport of shelf water from upstream, represented a major form of exchange between the MAB continental shelf and the open ocean. The streamer caused significant net onshore transport of heat and salt, and a significant net offshore transport of organic carbon and oxygen. Primary productivity in the streamer was higher than the surrounding slope and ring waters on the surface, which likely resulted from subsurface nutrients in the offshore-flowing shelf water being gradually consumed as the overlying water became clearer. WCR-induced shelf water streamers thus enhanced surface biological productivity in the slope sea. Plain Language Summary: Waters of the shallow Mid-Atlantic Bight continental shelf and the neighboring deep slope sea have distinctly different physical, biological and chemical properties. Mixing between them can affect the shelf ecosystem and the dispersal of coastal materials into the deep ocean. One type of cross-shelf-edge mixing process results from strong clockwise-rotating vortices – so-called warm-core rings – formed from meanders of the Gulf Stream. As a warm-core ring intrudes onto the shelf edge, it often draws shelf water offshore, forming a thin filament in the slope sea. This filament is called a shelf-water streamer and has distinctive surface temperature and chlorophyll signals that are visible from space. Warm-core rings can also push offshore water onto the shallow shelf. This study examines the evolution of a warm-core ring over its 6-month lifespan in 2019 and the shelf-water streamer the ring induced in 5 of the 6 months. Interdisciplinary measurements from a field expedition in July 2019 were examined to assess the subsurface patterns of the streamer and to quantify the induced cross-shelf fluxes of heat, salt, organic carbon and oxygen. The analysis showed that the streamer represented a major form of cross-shelf mixing and caused a substantial onshore transport of heat and salt, as well as a substantial offshore transport of organic carbon and oxygen.