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1983
Cape Romain and the Charleston Bump: Historical and Recent Cape Romain and the Charleston Bump: Historical and Recent
Hydrographic Observations Hydrographic Observations
J.J. Singer
L. P. Atkinson
Old Dominion University
, latkinso@odu.edu
J. O. Blanton
J. A. Yoder
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Original Publication Citation Original Publication Citation
Singer, J. J., Atkinson, L. P., Blanton, J. O., & Yoder, J. A. (1983). Cape Romain and the Charleston Bump:
Historical and recent hydrographic observations.
Journal of Geophysical Research: Oceans, 88
(C8),
4685-4697. doi:10.1029/JC088iC08p04685|
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. CS, PAGES 4685-4697, MAY 30, 1983
Cape Romain and the Charleston Bump: Historical and Recent
Hydrographic Observations
J. J. SINGER
Science Applications, Inc., Raleigh, North Carolina 27606
L. P. ATKINSON, J. 0. BLANTON, AND J. A. YODER
Skidaway Institute of Oceanography, Savannah, Georgia 31406
A review and analysis of historical and new hydrographic data are presented for the Charleston
Bump region. An area of doming isotherms is identified primarily between 31 .5° and 34.5°N and the 200
and 400 m isoba~hs. The highest incidences of doming are found off Long Bay (86%), Cape Fear (38%),
and Cape ~omam (25%). Evidence suggests that low salinity shelf water collects in the doming area off
Lon~ Bay m July and that seasonal fluctuations in the depth of the main thermocline layer in this area
are hnked to Gulf Stream transport and local winds. At times there is a gradual offshore-onshore
~ovement of the Gulf Stream opposite Long Bay roughly following the 400 m isobath and at other
lime~ an a~rupt eastwru:d m~vement near 32°N. Much of the time there appears to be a direct seasonal
relali<;>~sh1p between _historical seasonal velocity fields and offshore deflection with higher (lower)
veloc1lies corresponding to greater (lesser) deflection.
INTRODUCTION
Recently, there has been a growing interest in the study of
the seaward deflection of the Gulf Stream off Charleston
South Carolina. This interest has been fueled significantly b;
the advent of satellite infrared imagery. Rao et al. [1971] and
DeRycke and Rao [1973] presented the first of this type data
for the region, noting the recurring meander and eddy
features. They suggested these were transient phenomena
' ... caused either by any one or a combination of factors
like ... strong northwest winds ... bottom topography,
and ... baroclinic instability associated with the Gulf Stream
boundary.' Legeckis [1976], using satellite imagery, again
tied the phenomenon to the bathymetry (see Figure 1).
However, the suggestion of a bathymetric influence was
perhaps first implied by Bache in Pillsbury [1891], where he
reported that observations off Charleston suggested some
effect on the Gulf Stream due to ' ... the shape of the
bottom of the sea.'
Through the years, many periodic observations have been
made of the excursions of the Gulf Stream in the area.
Bartlett [1883] and Pillsbury [1891] both noted an eastward
flow of the Gulf Stream from surface drift measurements
taken in the region. Fuglister and Worthington [1951]
showed an eastward trend of the 100 m temperature front
near 33°N, Von Arx et al. [1955] observed an east to
northeast trend in the surface frontal outcrop near 32°N, and
Pratt [1963] observed eastward flow in the bottom current.
Schroeder [1963], Fuglister and Voorhis [1965], and Pratt
[1966] all showed the offshore deflection in the 200 m
contour of the 15°C isotherm, and Ewing et al. [1966] found
that 'a major erosion channel cut(s) across the southward
extension of the Cape Fear arch at about 32°N 78°W. _ .. '
Busby [1969] reported the entrainment of a deep-sea sub-
mersible in an eddy, and Knauss [1969] noted the apparent
Copyright 1983 by the American Geophysical Union.
Paper number 3C0346.
0148-0227 /83/003C-0346$05 .00
occurrence of a large meander. Richardson et al. [1969],
showed the Gulf Stream's high velocity core well offshore of
the 200 m isobath just north of this area, and Pashinski and
Maul [1973] stated that on one occasion the Gulf Stream had
been observed to flow eastward as far as 76.5°W before
turning northward.
More recently, the quasi-permanence of the deflection of
the Gulf Stream surface thermal front has been documented
for this area from Very High Resolution Radiometer
(YHRR) and Radar Altimeter (RA) satellite data [Brooks and
Bane, 1978; Pietrafesa et al., 1978; Legeckis, 1979]. In
addition, at least two models have now been applied to the
region examining bottom topography as an explanation for
the feature [Rooney et al., 1978; Chao and Janowitz, 1979].
In studying this region, it is pointed out that satellite IR
imagery is presently incapable of year-round monitoring
because of periodic cloud cover and a negligible temperature
gradient during the summer [Maul et al., 1978; Pietrafesa et
al., 1978; Legeckis, 1979]. This is further emphasized by
Strack's [1953] observation that June-October are the worst
months for using sea surface temperature gradients as Gulf
Stream indicators. Subsequently, there are distinct advan-
tages of subsurface hydrographic data over satellite imagery.
Substantial hydrographic data sets have been collected in
the area by numerous investigators. They are discussed in
Bane [1983], Bane et al. [1980b, 1981], Vukovich and Criss•
man [1980], Mathews and Pashuk [1977], and Haze/worth
[1976]. Earlier, the Gill cruises of 1953 and 1954 also sampled
this region in considerable detail. Other investigators includ-
ing Pierce [1953] and Costin [1969] have also periodically
sampled the region.
The purpose of this paper is to review and examine these
and other historical hydrographic data and to present new
data for the 'Bump' region. It is the authors' view that this
will serve as a foundation for future work in the area. A
region of doming isotherms will be identified, and the
seasonal characteristics of the region will be discussed.
Onshore-offshore sections of temperature, salinity, nitrate,
and total chlorophyll will be presented as well as a discus-
4685
4686
SINGER ET AL.: CHARLESTON BUMP
81 W 80, H 78 77 76 75 W
36 N .---~-~-~-~--,,..,,.---,36N
35
/ 35
/·
. :
,'
34
34
33 33
32
32
31 31
30
29
{ t:
29
30
~~~-~-~-~-~~28N
80
79 78 77
76
75 W
Fig. I. Bathymetry of the South Atlantic Bight (SAB) and the
study area [from American Association of Petroleum Geologists,
1970].
sion of the characteristics of Gulf Stream deflection off Long
Bay.
METHODS
The historical data discussed in this paper are taken from
technical publications that describe the respective sampling
techniques and analytical methods. Data from the NOAA
Peirce [Haze/worth, 1976] and R. V. Dolphin [Mathews and
Pashuk, 1977] cruises were obtained from the National
Oceanographic Data Center (NODC) on magnetic tape and
were processed at Skidaway Institute of Oceanography
(SKIO).
A Grundy model 9400 conductivity-temperature-depth
(CTD) recorder, a General Oceanics model 1015 Rosette
with Niskin bottles, and an expendable bathythermograph
(XBT) were used on the authors' cruises. CTD data were
processed according to Chandler et al. [1978] and XBT data
were manually digitized. Nutrient samples were frozen in
125 ml polyethylene bottles and stored in the dark until
thawed and analyzed on shore. Standard Technicon Autoan-
alyzer methods were used [Glibert and Loder, 1977]. Dupli-
cate chlorophyll samples were filtered and frozen and stored
in the dark. Total chlorophyll was determined by the method
of Yentsch and Menzel [1963] as described by Strickland and
Parsons [1972].
DOMING REGION
Vertically, hydrographic data in the region opposite Long
and Onslow Bays (Figure l) are often characterized by
doming (an upward and downward bending) of the isopleths
in the area between the shelf break and the Gulf Stream. For
the purpose of this paper, this phenomenon (probably asso-
ciated with a cyclonic flow pattern) is defined for a cross-
shelf section as an upward and then downward sloping of the
l5°C isotherm in the upper layer (0-200 m) offshore of the
200 m isobath (Figure 2a). This definition distinguishes the
phenomenon from upwelling that may occur along the shelf
break (Figure 2b) and most frontal eddies [Lee et al., 1981]
observed south of 31.5°N. The 15°C dome of these latter
features is generally shoreward of the 200 m isobath and
much less broad. No attempt is made to distinguish between
doming associated with an apparent quasi-permanent mean-
der opposite Long Bay and that observed further down-
stream (NE) associated with larger frontal eddies that occur
with 2 to 14 day periods.
In examining the available data (Tables l, 2, and 3), it was
noted that in some instances only an upward sloping was
observed as the sampling did not extend far enough offshore
to detect the eastern extent of the dome. In such cases, the
location of the furthest sample offshore from the 200 m
isobath was considered the maximum dome (inflection or
center) position. Further, in most instances at least two
stations offshore of the 200 m isobath were necessary in
order to apply the above definition. When two such stations
were not available, a careful examination was made of the
contours at shallower depths. This eliminated some sections
that otherwise appeared applicable.
Doming Statistics
A region of maximum doming was identified between 31 .5°
and 34.5°N (Figure 3; sub-regions II, III, and IV). This
corresponds to the same region where the Gulf Stream
surface thermal front has been observed to move offshore
and then back onshore by Bane and Brooks [1979] and where
subsurface temperature data (l5°C at 200 m) reported by
Bane et al. [1980a, b, 1981], Bane and Brooks [1981], Pratt
[1966], Fuglister and Voorhis [1965], and Schroeder [1963]
all show the same trends. It is also the region over which the
400 m isobath has been observed to diverge from and return
toward the 200 m isobath.
Nearly 75% of all doming observations occurred in the
area between the 200 and 400 m isobaths. However, doming
was also observed offshore of the 500 m isobath, particularly
north of 33°N where the continental slope steepens. The
average water depth at the center dome position was 360 m
for all data and 310 m for the observations occurring between
the 200 and 400 m isobaths.
South of 31.5°N, a few doming events were observed in
region I. These were related to frontal eddies with the
exception of the event at 30.8°N where a Gulf Stream
meander was identified. The events in region IV are also
thought to be attributable to the periodic occurrence of
frontal eddies though much farther offshore. Region II leads
into the region of the quasi-permanent meander, and region
III is opposite the meander. No particular significance
should be inferred from the locations indicated as subregion-
al barriers. They are only intended as estimates.
Station
Stat,on
56 51
C
100 "'
~
:r
200 -
3
b
-----
0 10 20 30
Naut Miles
- C
100
~
' :r
200 3
Fig. 2. (a) Doming and (b) upwelling at the shelf break off Long
Bay, North Carolina [from Anderson et al., 1956a; Anderson and
Gehringer, 1957a).
SINGER ET AL.: CHARLESTON BUMP 4687
TABLE I. Published Data Sources Examined in Compiling the Region of Maximum Doming
Ship
Caryn and At-
lantis
Gill
Atlantis
Peirce (NOAA)
Eastward
Dolphin
Eastward
lselin
lselin
Advance II and
Blue Fin
Northstar
John de Wolf II
Blue Fin and
lselin
lselin
AXBT's*
AXBT's*
Endeavor
Endeavor
Gillis
Year
1953-1954
1953-1954
1955
1965-1966
1966-1967
1973
1973-1974
1976
1977
1977
1977
1977-1978
1977-1978
1978
1979
1979
1979
1979
1979
*Airborne XBT's dropped.
Relative Frequency
The relative frequency of doming off Cape Romain, Long
Bay, Cape Fear, and Onslow Bay was inferred from the Gill
cruises (two cruises in each season) run along these sections
(Figure 4; sections B, D, F, and G, respectively). The Gill
cruises constitute the largest data set of seasonal repetitions
of identical sections available. Doming occurred most fre-
quently off Long Bay (six of seven times, 86%) and to a
lesser extent off Cape Romain (two of eight times, 25%) and
Cape Fear (three of eight times, 38%). No doming observa-
tions were made on nine sections (not shown) that extended
south to 27°N, and only Onslow Bay (one of six times, 17%)
exhibited any doming on two northerly sections to Cape
Lookout. Only in August 1953 (Gill cruise 3) did all four
sections show no evidence of doming.
The higher incidence of doming off Long Bay tends to
support the view of Pietrafesa and Janowitz [1980] and
Pietrafesa [1983] that there is a cyclonic circulation pattern
persisting throughout much of the year in this area. Such a
circulation would be independent of the passage of frontal
eddies, though perhaps modified by their occurrence or
TABLE 2. Seasonal Cruises by the Authors
Ship
Pierce (Tracor)
/selin
lselin
lse/in
Pierce (Tracor)
Pierce (Tracor)
Pierce (Tracor)
lselin
Eastward*
lselin*
Pierce (Tracor)
NODC
Identification
PP-001-78
IC-004-78
IC-006-78
IC-008-78
PP-001-79
PP-002-79
PP-003-79
IC-002-79
EZ-002-80
IC-003-80
PP-001-80
Data presented in unpublished reports.
Dates
March 6-12, 1978
April 12-23, 1978
July 26-30, 1978
Nov. 9-14, 1978
March 14-19, 1979
May 28 to June 2, 1979
Aug. 22-27, 1979
Oct. 27 to Nov. 2, 1979
April 10-27, 1980
April 10-26, 1980
Sept. 3-14, 1980
*Cruise did not extend into the shelf break region north of 31 .5°N.
References for Vertical Plots of Temperature
Bumpus and Pierce [1955]
Anderson et al. [1956a, bl and Anderson and
Gehringer [1957a, b, 1958, 1959a, b, c]
Swallow and Worthington [1961]
Haze/worth [1976]
Steftinsson et al. [1971]
Mathews and Pashuk [1977]
Atkinson [1978]
Deschamps et al. [1979]
Atkinson et al. [1979]
O'Malley et al. [1978]
Vukovich and Crissman [1980]
Curtin [1979a, b, c, d]
Singer et al. [1980]
Lasley et al. [1979]
Bane et al. [1980a]
Bane and Brooks [1981]
Bane et al. [1980]
Brooks et al. [1980]
Lasley et al. [1981]
enhanced by NE winds, which would strengthen southward
flow along the western side of the dome. The lower inci-
dence of doming events further downstream (NE) from Long
Bay supports the view that they are caused there by the
occasional appearance of frontal eddies well offshore of the
200 m isobath.
SEASONAL VARIATIONS IN TEMPERATURE, SALINITY, AND
SIGMA T IN THE DOMING REGION OFF CAPE ROMAIN
Twenty-five biweekly NOAA cruises from 1965 to 1966
[Haze/worth, 1976] present an excellent opportunity to ex-
amine the seasonal characteristics of the doming region off
Cape Romain. The cruise track for this work is shown in
Figure 4, section A. This section occurs rather close to and
traverses southeastward across 32°N 79°W, the location at
which Brooks and Bane [1978], Pietrafesa et al. [1978], and
Legeckis [1979] reported repeated seaward deflection of the
surface thermal front from the 200 m isobath. Station spacing
was approximately 10 nautical miles (18.5 km). These data
reveal that the Gulf Stream was always east of station 4 and
that the lowest temperatures at 200 m depth occurred at
stations 3 or 4 of the data set in 316 and 403 m of water,
respectively (Figure 4). The lower station temperatures at
TABLE 3. NODC Ship of Opportunity XBT Data and Plots
Examined in Compiling the Region of Maximum Doming
NODC
Originator's
Identification
Identification
Ship
50203
7611
Lash Turkiye
51303 7703
Lash Turkiye
51390 7704
Mormac Argo
52342
7712
Mormac Argo
52990
7810
Lash Atlantico
52994 7811 Lash Atlantico
An attempt was made to include naval XBT data; however, these
efforts were unsuccessful as the data were not made available in a
usable format.
4688
SINGER ET AL.: CHARLESTON BUMP
8 1 W 80 79 78 77 7b 75 W
36 N~-~-~-~--~-~-~36 N
35
34 34
33
33
32
32
31 31
30 30
29
29
Fig. 3. Region of maximum doming. See text for discussion of
sub-regions I-IV.
this depth and corresponding salinities and densities are
plotted versus time in Figure 5. Also presented are the
corresponding surface and 100 m observations.
Surface Observations
In the surface layer, high temperatures corresponded to
low densities, and vice versa. There was no obvious relation
to salinity. The highest temperatures occurred in late August
and early September and the lowest in early February. The
lowest surface salinities occurred primarily between late
June and early August (during a period of extended south-
westerly winds) with a secondary low in mid-May. The
highest surface salinities were observed in December, Janu-
ary, February, and April.
The temperature and sigma t observations are readily
81 W 80
79
78
77
76
36 N .--~-~-~-~---,
35
34
31
30
29
28 N
81 W 80 79
78
·,>
,.,,
T
77 76
75 W
36 N
32
31
30
29
28 N
75 W
Fig. 4. Selected hydrographic sections A-G (see text).
~o 36
.
JFMAMJJASOND
Monlhs
OM , ...
1 OOM ·--
200M -
B
34
~~, ~.~~M~.-M~~,-,~~.-s--=-o~N,--~0--'
Months
..~~ ....... _._~_~_._~~~----........ --,,..__.
J f M A M J J A S O N 0
Monlhs
Fig. 5. Time series of bi-weekly observations of: (a) tempera-
ture, (b) corresponding salinity, and (c) corresponding sigma t at
three depths at Peirce cruise stations 3 or 4 (Figure 4, line A) in the
doming region. (The small arrows indicate no data and the large
unshaded arrow indicates the beginning of the time series in August
1%5.)
explained as responses to seasonal warming and cooling and
compare favorably in trend with Chase [1971] for Frying Pan
Shoals Light Station off Cape Fear. In addition, Chase [1969]
has shown that historically the lowest mean surface salinities
off Cape Fear occur in July.
Atkinson and Targett [1983] reported surface shelf break
(60 m isobath) salinities lower than 36.00 x 10-
3
in late June
1980 between 31° and 32°40'N, suggesting an offshore flow of
water in the region. Southwesterly winds, which occurred,
favor such observations, causing low salinity shelf water to
move offshore into the frictional drag of the Gulf Stream as
suggested by Bumpus [1955]. Here, however, the shelf water
first moves off-shelf into the doming region and then into the
frictional drag of the Gulf Stream.
Interestingly, Weber and Blanton [1980] have found that
southerly to southwesterly winds occur primarily in July in
this area. This suggests for the annual cycle that the largest
quantities of low salinity shelf water are moved off shelf en
masse in the 'Bump' region in July. If this is in fact the case,
then aerial mapping of surface salinity might define the path
of the Gulf Stream in this area in July when surface tempera-
ture observations are inappropriate.
100 m Observations
At 100 m, higher temperatures corresponded to higher
salinities and lower densities. Lower temperatures corre-