An Observational and Modeling Study of the Physical Processes Driving Exchanges Between the Shelf and the Deep Ocean at Cape Hatteras

Boundaries between subtropical and subpolar oceanic gyres are characterized by confluent western boundary currents in the open ocean (e.g., the Brazil/Malvinas, Kuroshio/Oyashio, and Gulf Stream/Slope Sea confluences) and convergence in the adjacent shelf and slope waters (Piola et al., 2000; Jilan and Lobanov, 1998), which leads to large net export of shelf waters to the deep ocean. However, complex, bidirectional shelf-open ocean exchange also occurs and is driven by variability in the adjacent western boundary currents, atmospheric forcing, and shelf water properties, and is influenced by local bathymetric characteristics. Exchanges between the shelf and deep ocean are central to global carbon budgets, marine ecosystem dynamics, larval and pollutant transports, and modulation of storm tracks and intensity, and thus have significant environmental, economic, and societal implications.

Recent examples of anomalous forcing along the U.S. East Coast underscore the importance of understanding the dynamics that control exchange between the deep ocean and continental shelf at the confluence of the North Atlantic gyres near Cape Hatteras (Fig. 1). Large deviations in Gulf Stream position relative to the typical meander envelope (Gawarkiewicz et al., 2012), extreme wintertime wind stress and buoyancy fluxes (Chen et al., 2014), accelerated shelf warming (Forsyth et al., 2015), and sea level rise north of Cape Hatteras (Sallenger et al., 2012; Andres et al., 2013) have been documented in recent years. All these recent trends are potential harbingers of larger shifts in atmospheric and oceanic forcing, yet their effects on shelf-open ocean exchange are unknown due to our lack of dynamical understanding under present conditions. Developing better understanding and predictive capacity

Shelf-open ocean exchange processes have remained poorly understood in part because of the technical challenge of resolving the broad range of relevant spatial and temporal scales. For example, coastal features of O(10 km) width are known to shift their alongshelf position by up to 150 km on timescales of days to weeks while boundary currents of O(100 km) width and O(1000 km) length may vary on time scales from days to years. Newer observational systems recently adopted by the community are now capable of filling parts of the space-time resolution continuum between high spatial resolution shipboard snapshots and the sparse spatial coverage of mooring data. Furthermore, advances in computing and modeling facilitate a new capability to pursue better simulations of circulation response to wide ranges of forcing conditions and to address causality in a quantitative way.

This program combines independent projects previously proposed by two groups focused on separate aspects of shelf-open ocean exchange processes at Cape Hatteras. With encouragement from program managers at NSF, our integrated team now seeks to address the range of interacting processes that likely control exchange. The goal of our “Processes driving Exchange At Cape Hatteras” (PEACH) program is to advance the fundamental understanding of the processes and dynamics underlying shelf-open ocean exchange at convergent, energetically forced coastal margins using a combined observational and modeling approach.