Frontal Subduction of the Mid‐Atlantic Bight Shelf Water at the Onshore Edge of a Warm‐Core Ring

This work studies the subduction of the shelf water along the onshore edge of a warm-core ring that impinges on the edge of the Mid-Atlantic Bight continental shelf. The dynamical analysis is based on observations by satellites and from the Ocean Observatories Initiative Pioneer Array observatory as well as idealized numerical model simulations. They together show that frontogenesis-induced submesoscale frontal subduction with order-one Rossby and Froude numbers occurs on the onshore edge of the ring. The subduction ﬂ ow results from the onshore migration of the warm-core ring that intensi ﬁ es the density front on the interface of the ring and shelf waters. The subduction is a part of the cross-front secondary circulation trying to relax the intensifying density front. The dramatically different physical and biogeochemical properties of the ring and shelf waters provide a great opportunity to visualize the subduction phenomenon. Entrained by the ring-edge current, the subducted shelf water is subsequently transported offshore below a surface layer of ring water and alongside of the surface-visible shelf-water streamer. It explains the historical observations of isolated subsurface packets of shelf water along the ring periphery in the slope sea. Model-based estimate suggests that this type of subduction-associated subsurface cross-shelfbreak transport of the shelf water could be substantial relative to other major forms of shelfbreak water exchange. This study also proposes that outward spreading of the ring-edge front by the frontal subduction may facilitate entrainment of the shelf water by the ring-edge current and enhances the shelf-water streamer transport at the shelf edge. ring that migrates toward the shallow continental shelf. Computer simulations reproduce the observed phenomenon and reveal the underlying mechanism: the onshore migration of the warm-core ring sharpens the interface of the ring and shelf waters; the subduction ﬂ ow is a part of the cross-sectional circulation trying to relax the sharpening interface. The dramatically different physical and chemical properties of the ring and shelf waters help visualize the phenomenon. The subducted shelf water is then carried into the deep sea underneath a surface layer of warm-core ring water by the strong ring current. This offshore ﬂ ow of the shelf water represents a type of water exchange process at the shelf edge that can potentially be signi ﬁ cant but has not been well studied. It does not have a surface signature and cannot be seen by satellites. The subsurface measurements from the Pioneer Array observatory provide a great opportunity for this study. water into the frontal region. The frontal current then carries the subducted shelf water offshore underneath the ring water. Dynamics of the WCR-induced shelf-water frontal subduction at the shelf break is the same as the frontal subduction in the open ocean. However, the dramatically different physical and biogeochemical properties of the shelf and ring waters at the shelf break provide a great opportunity to visualize the subduction process.