Oceanography | Vol.35, No.2
Counterclockwise mean circulation
(black curves in Figure 1a) in the Slope
Sea separates the Mid-Atlantic Bight
(MAB shelf extends from Cape Hatteras
to Cape Cod) from the mean path of the
separated Gulf Stream, downstream of
Cape Hatteras (Csanady and Hamilton,
1988). The Slope Sea is generally filled
with cooler and fresher waters than are
found in the Gulf Stream. The Deep
Western Boundary Current (blue-gray
path) flows equatorward through the
study area, carrying components of the
North Atlantic Deep Water at depths
between 1,000 m and 3,000 m, as part of
the deep limb of the Atlantic Meridional
Overturning Circulation (Toole et al.,
2017). It crosses underneath the Gulf
Stream off Cape Hatteras, allowing for
complicated interactions between the
deep and upper components of North
Atlantic circulation (Pickart and Smethie,
1993; Spall, 1996).
On the MAB shelf, the mean flow is
equatorward (blue arrows in Figure 1a)
from Georges Bank to Cape Hatteras,
including flow over the continental shelf
(Lentz, 2008) and in a narrow shelf-
break jet (purple in Figure 1a; Linder
and Gawarkiewicz, 1998), each of which
carries about 0.25 Sv (1 Sverdrup [Sv]
= 106 m3 s–1; Linder and Gawarkiewicz,
1998; K. Chen and He, 2015). These
waters originate as far north as the
Labrador Sea (Chapman and Beardsley,
1989). Shoreward of the Gulf Stream, in
the SAB south of Cape Hatteras, there is
thought to be mean poleward flow (light
green arrow in Figure 1a; Lee et al., 1991;
Blanton et al., 2003), particularly off the
northernmost part (Savidge and Bane,
2001; Savidge and Savidge, 2014). The
adjacent Gulf Stream strongly influences
SAB shelf circulation and hydrography,
and with limited river discharge into the
SAB, shelf water is difficult to distinguish
from Gulf Stream waters based on tem-
perature and salinity alone. However,
striking differences between Gulf Stream
and SAB biogeochemical properties
can develop through bio-mediated epi-
sodic shelf processes with timescales that
appear to align with shelf residence times
(Menzel, 1993; Savidge and Savidge,
2014). In the immediate vicinity of Cape
Hatteras, a multiyear mooring data set
from the early 1990s illustrated per-
sistent along-shelf convergence of MAB
and SAB shelf waters at Cape Hatteras
(Figure 2d) in both means and daily data
(Savidge and Bane, 2001). The MAB and
SAB salinity and temperature contrasts
form the Hatteras Front (Figure 1b),
whose along-shelf density gradient sup-
ports cross-shelf geostrophic flow within
the Front (Savidge and Austin, 2007).
These circulation features affect the
PEACH study region across a range of
spatial and temporal scales. Of particu-
lar interest to PEACH are wave-like lat-
eral meanders in the Gulf Stream’s path
that form nearly continuously and prop-
agate along the SAB, with largest ampli-
tude downstream of the Charleston
Bump (~31°N; Figure 2a). The crests and
troughs of the meanders and their associ-
ated frontal eddies move downstream at
speeds of 20–60 km day–1 with roughly
weekly period (Bane and Dewar, 1988).
Eddy decay regions off Georgia and
approaching Cape Hatteras are associated
with elongation of frontal features, off-
shore transport of momentum and heat,
and onshore transport of nutrients (Lee
et al., 1991). Measurements of currents
along the SAB shelf adjacent to the Gulf
36°N
38°N
40°N
42°N
76°W
74°W
72°W
70°W
28
22
16
10
°C
74°48'W
74°42'W
74°36'W
100
200
100
200
100
200
Depth (m)
100
200
–0.10
–0.05
0.00
0.05
0.10
Along-Shore
Velocity (m s–1)
24.5
25.0
25.5
26.0
26.5
27.0
Potential Density
(kg m–3)
33.5
34.0
34.5
35.0
35.5
Salinity
10
12
14
16
18
Potential
Temperature (°C)
74°W
5 cm s–1
75°W
76°W
77°W
35°N
36°N
37°N
FIGURE 2. (a) Sea surface temperature (SST) image offshore of the South Atlantic Bight (SAB) with
Gulf Stream meander crests (C) and troughs (T) labeled, and a white star marking the position of the
Charleston Bump. MAB = Mid-Atlantic Bight. CCR = Cold Core Rings. Image from Rutgers University
(b) SST image of the Gulf Stream and Slope Sea offshore of the Mid-Atlantic Bight shelf. Image
from Rutgers University (c) Average shelfbreak temperature, salinity, density, and along-shelf veloc-
ity during PEACH measured along the cross-shelf section in Figure 3. After Todd (2020), Figure 7
(d) Mean flow convergence near Cape Hatteras. From Savidge and Bane (2001)
30
26
22
18
14
°C
36°N
34°N
32°N
30°N
28°N
82°W
80°W
78°W
76°W
74°W