Oceanography | September 2022
INTRODUCTION
The program called Processes driving
Exchange At Cape Hatteras (PEACH)
uses observations and models to study
exchanges of seawater and its constitu-
ents between the continental shelf and the
open ocean near Cape Hatteras, North
Carolina, through a US National Science
Foundation-funded collaboration among
principal investigators at the University
North Carolina at Chapel Hill, Skidaway
Institute of Oceanography (University of
Georgia), Woods Hole Oceanographic
Institution, North Carolina State Uni-
versity, and the Coastal Studies Institute
at East Carolina University. Boundaries
between subtropical and subpolar oce-
anic gyres are characterized by confluent
western boundary currents in the open
ocean and convergence in the adjacent
shelf and slope waters. Strong forcing typ-
ical of midlatitude western ocean mar-
gins modulate the resulting net export of
shelf waters and complex, bidirectional
shelf-deep ocean exchanges. Exchanges
between the shelf and the open ocean are
central to global carbon budgets, marine
ecosystem dynamics, larval and pollut-
ant transports, and modulation of storm
tracks and intensity, and thus have signif-
icant environmental, economic, and soci-
etal implications.
Recent examples of anomalous forcing
along the US East Coast underscore the
importance of understanding the dynam-
ics that control exchange between the open
ocean and the continental shelf at the con-
fluence of the North Atlantic gyres near
Cape Hatteras (Figure 1). Large deviations
in Gulf Stream position relative to the
typical meander envelope (Gawarkiewicz
et al., 2012), extreme wintertime wind
stress and buoyancy fluxes (K. 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 doc-
umented in recent years. Such trends are
potential harbingers of larger shifts in
atmospheric and oceanic forcing, yet their
effects on shelf-open ocean exchange are
not well understood. Developing better
understanding and predictive capacity are
central goals of PEACH. Early results, dis-
cussed below, both confirm expectations
and provide surprises about the region’s
physical oceanography. Beyond the Gulf
Stream, a documentary film produced by
Kyle Lawrence-Maddux based on PEACH
cruise activities, was shown at the 2020
Woods Hole Film Festival and is available
at https://vimeo.com/279906819.
BACKGROUND
The most prominent feature of the
mean circulation in the PEACH study
area is the Gulf Stream (orange curve
in Figure 1a), the subtropical west-
ern boundary current in the Northwest
Atlantic. It carries components of both
wind-driven and overturning circula-
tion (Meinen et al., 2010; Buckley and
Marshall, 2016), so its strength is coupled
to both the deep-reaching thermohaline
circulation and the surface-intensified
flows driven by the basin-wide wind field.
Largely unconstrained by topography,
the Gulf Stream transitions from a shelf-
adjacent boundary-trapped current along
the South Atlantic Bight (SAB, which
extends from Cape Canaveral to Cape
Hatteras) to a separated jet northeast of
Cape Hatteras. The Gulf Stream’s tempo-
ral and spatial changes in offshore posi-
tion, flow orientation, and transport are
discussed more fully below.
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 ques-
tions focus on the roles that wind forcing, Gulf Stream forcing, and lateral density gra-
dients play in driving exchange. PEACH observational efforts include a variety of fixed
and mobile observing platforms, and PEACH modeling included two different resolu-
tions 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.
Gulf Stream
20 m
60 m
200 m
Cape
Hatteras
37°N
36°N
35°N
34°N
77°W
76°W
75°W
74°W
MAB Shelf Water
Diamond Shoals
Shelfbreak Jet
45°N
40°N
35°N
30°N
25°N
80°W
75°W
70°W
65°W
100 m
500 m
500 m
4000 m
4000 m
100 m
Gulf Stream
Slope Sea Gyre
Deep Western
Boundary Current
Shelfbreak Jet
Hatteras Front
MAB Shelf Water
SAB Shelf Water
After Schmitz, 1996 and
Csanady and Hamilton, 1988
MAB
SAB
30'
76°W
30'
75°W
30'
74°W
30'
30'
35°N
30'
36°N
30'
37°N
2,000
1,000
200
100
50
30
0
Depth (m)
Spray Gliders
Slocum Gliders
CPIES
ADCP+CTD
Met+ADCP+CTD
Articulating Profler
NDBC Moorings
CODAR Sites
WERA Sites
Altimeter Tracks
SAB S
helf W
ater
el
FIGURE 1. (a) Schematic depiction of circulation in the Northwest Atlantic and (b) zoom in on the
Cape Hatteras region. SAB = South Atlantic Bight. MAB = Mid-Atlantic Bight. Created by Anna
Boyette, after Schmitz (1996) and Csanady and Hamilton (1988)