A subsurface shelf water export event at Cape Hatteras in January 2018

Lu Han1, Harvey Seim1, John Bane2 and Robert E Todd3, (1)University of North Carolina at Chapel Hill, Marine Sciences, Chapel Hill, NC, United States, (2)University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, (3)Woods Hole Oceanographic Institution, Physical Oceanography, Woods Hole, MA, United States
Cold and fresh Middle Atlantic Bight (MAB) shelf water typically flows southward and converges with warm and salty South Atlantic Bight (SAB) shelf water near Cape Hatteras, where both then leave the shelf and enter the adjacent open ocean. During the January 2018 PEACH cruise, there was no sign of shelf water export at the surface from either SST images or the shipboard thermosalinograph.Instead, a layer of shelf water with high chlorophyll and dissolved oxygen was detected at 100 m depth over the 220 m isobath within the edge of the Gulf Stream east of Hatteras, which indicated that shelf waters were exported at deeper depth. This layer was mostly MAB shelf water that had mixed above with Gulf Stream water and below with SAB shelf water. Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore waters. As a result, the exported shelf waters flowed down the continental slope and entrained into the shoreward flank of the Gulf Stream, as they were advected northeastward. As much as 0.6 Sv of shelf waters were exported in this fashion, and the cascading can be an important yet little studied subsurface pathway of shelf water export at Cape Hatteras.