Overview of the Processes Driving Exchange at Cape Hatteras Program

Abstract

The Processes driving Exchange At Cape Hatteras (PEACH) program seeks to better understand seawater exchanges between the continental shelf and the open ocean near Cape Hatteras, North Carolina. This location is where the Gulf Stream transitions from a boundary-trapped current to a free jet, and where robust along-shelf convergence brings cool, relatively fresh Middle Atlantic Bight and warm, salty South Atlantic Bight shelf waters together, forming an important and dynamic biogeographic boundary. The magnitude of this convergence implies large export of shelf water to the open ocean here. Background on the oceanography of the region provides motivation for the study and gives context for the measurements that were made. Science questions focus on the roles that wind forcing, Gulf Stream forcing, and lateral density gradients play in driving exchange. PEACH observational efforts include a variety of fixed and mobile observing platforms, and PEACH modeling included two different resolutions and data assimilation schemes. Findings to date on mean circulation, the nature of export from the southern Middle Atlantic Bight shelf, Gulf Stream variability, and position variability of the Hatteras Front are summarized, together with a look ahead to forthcoming analyses.

Ocean Circulation Near Cape Hatteras: Observations of Mean and Variability

“…Like the wind stress variations described in Seim et al (2022), current variations are typically larger in the cool season (mid-September through April) and smaller in the warm season (May-mid-September). The variance of the depth-averaged current velocities in the cool season is nearly twice that in the warm season at the moorings north of Cape Hatteras on the MAB and at the northern shelfbreak.…”

“…The Shelfbreak Jet is still present but weakened by the divergence caused by the offshore movement of the Gulf Stream. The wind‐driven equatorward flows and the Gulf Stream‐driven divergence lead to a “mustache feature” at Cape Hatteras, where the MAB water on the inner shelf flows past the cape onto the SAB shelf and the MAB water on the outer shelf turns and flows northeastward with the Gulf Stream and exports to the open ocean (cf., Seim et al., 2022).…”

“…In this study, we use observations from the PEACH field program, which employed a wide range of observing platforms over 18 months (April 2017–November 2018; Seim et al., 2022). We examine current velocities from bottom moored acoustic Doppler current profilers (ADCPs), temperature and salinity from CTDs, wind measurements from meteorological buoys, surface current velocities from High Frequency Radars, acoustic travel time from bottom‐moored current and pressure‐sensor equipped inverted echo sounders (CPIESs), and Sea Surface Temperature (SST) and Sea Surface Height (SSH) measured from satellites.…”

“…On the adjacent continental shelf, both the MAB and SAB shelf waters generally flow toward Cape Hatteras. To better understand the seawater exchange between the continental shelf and the adjacent open ocean in this region, an integrated observational and modeling program, Processes driving Exchange At Cape Hatteras (PEACH; Andres, 2021; Han et al., 2021; Seim et al., 2022) was conducted in 2017–2018.…”

The Impact of Gulf Stream Frontal Eddies on Ecology and Biogeochemistry near Cape Hatteras

“…The in-situ focus area for this study is the confluence of the warm and salty SAB, the relatively fresh and cool MAB slope sea water, the even fresher and cooler MAB shelf waters, and the saltiest and hottest Gulf Stream itself (Seim et al, 2022). The anticyclonic rotation of the subtropical gyre, of which the Gulf Stream is the western expression, converges here with the cyclonic rotation of the slope sea gyre, along with inputs from the MAB shelf water and SAB shelf water (which typically converge at the Hatteras Front).…”