Episodic winter deep chlorophyll events in the Southern Mid-Atlantic Bight revealed by autonomous glider deployments

The southern Mid-Atlantic Bight (sMAB) is likely a hotspot for the episodic export of carbon-rich shelf waters to the open ocean. Over a 2 year period, from March 2017 to May 2019, Spray gliders repeatedly occupied transects, along the slope and across the shelf, generating high-resolution chlorophyll fluorescence data in the sMAB. The resultant data resolved 2 full annual cycles of chlorophyll in a region previously undersampled, particularly in winter. The seasonal surface chlorophyll maximum was associated with the onset of stratification in early spring followed by a subsurface maximum persisting over summer. Counterintuitively, our data also revealed that depth integrated chlorophyll was highest prior to the onset of stratification, with relatively high chlorophyll concentrations observed below 100m, suggesting downward transport rather than local production. We defined “deep chlorophyll events” (DCEs) as profiles where maximum chlorophyll and/or depth integrated chlorophyll in the 100-300m range were in the 99^th percentile of the dataset. We found that these episodic DCEs were key in driving the annual peak of depth integrated chlorophyll in late winter/early spring. We find that DCEs are generally driven either by sub-mesoscale exchange processes at the Gulf Stream boundary, or larger scale winter storm mixing events. Such events have likely been missed by previous ship-based surveys. Our results strongly suggest that carbon-rich shelf waters are exported from the sMAB shelf to depth below the Gulf Stream during episodic events, potentially contributing to cross-shelf exchange of particulate organic carbon.

Presentation Preference: Oral

Primary Presenter: Francesco Lane, University of North Carolina - Chapel Hill (flane@unc.edu)

Authors:

Sophie Clayton, National Oceanographic Centre  (Sophie.Clayton@noc.ac.uk)

Robert Todd, Woods Hole Oceanographic Institute  (rtodd@whoi.edu)