Charleston Bump - Making Waves in the Gulf Stream
If you use a World Wide Web search engine to
search for the phrase "Charleston Bump", most of the hits are likely to be
about a country-western line dance! Interspersed among the dance steps,
however, there will be occasional references to an area located off the
southeastern coast of the United States where the sea floor is elevated, east
of Charleston, South Carolina and Cape Romain. This area is what most
oceanographers and marine geologists are referring to when they talk about the
"Charleston Bump".
On April 25, 1998, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS)
instrument obtained a nearly cloud-free image of the entire U.S. East
Coast. This image shows chlorophyll concentrations in the ocean waters
off the shoreline of South Carolina. Both the flow of the Gulf Stream and the
related communities of marine phytoplankton are affected by the Charleston Bump.
The Charleston Bump is only a small feature
on bathymetric maps, but it exerts a strong influence on the Gulf Stream, one
of the ocean's most familiar currents. The Gulf Stream parallels the coast as
it flows smoothly northward from Florida and the Bahamas, with hardly a churn
or ripple visible. Sea surface temperature (SST) data from the Advanced Very
High Resolution Radiometer (AVHRR) provides almost daily views of the Gulf
Stream -- one Web site that provides images of this data is satellite imagery provided by
the Rosenstiel School of Marine and Atmospheric Science in Miami, Florida.
Even though SST images show the path of the Gulf Stream on the surface of the
ocean, it should be remembered that ocean currents are three-dimensional, and
thus they can influence, and be influenced by, the topography of the sea floor.
Warm Gulf Stream waters head virtually
straight north from the passage between West Palm Beach and Grand Bahama
Island. Just after the current begins bending to the east, following the
curve of the continental shelf, it runs directly over the Charleston Bump.
The strength of the current has scoured the sea bottom in this area, which
provides highly reflective areas in GLORIA
side-scan sonar data obtained by the United States Geological Survey. The
Charleston Bump can be seen in mosaic 18 of this data: it is the small bright
area at top center.)
As the name implies, the Charleston Bump
isn't a gigantic undersea mountain jutting abruptly from the ocean bottom. The
bathymetric map here shows that it is a fairly small feature, located with the
(*) on the map. Note the confluence of the Ashley, Cooper, and Wando Rivers
into Charleston Harbor in the SeaWiFS color image. (The arrow designating the
location of the Charleston Bump in the small color image is positioned slightly
too far south. Even with detailed bathymetry, it's not easy to locate!)
When the Gulf Stream interacts with the
Charleston Bump, the powerful current is suddenly disrupted. The direction of
the flow deflects further eastward and immense oscillations are generated,
appearing like waves that roll northward toward Cape Hatteras. At the same
time, a reverse circulation forms in the trough of the first wave, forming a
circulatory feature called the Charleston Gyre. This circulation is somewhat
like the dangerous traps that form beneath waterfalls and "holes" in the rapids
of a river. Most of the water continues to flow down the river channel, but
the reverse circulation just behind the falls can trap driftwood and unskilled
kayakers.
The Charleston Gyre was observed in ocean color imagery obtained on October
28, 1979, near the end of the first year of the Coastal Zone Color Scanner
(CZCS) mission. Dr. Charles McClain and Dr. Larry Atkinson described
observations of the Gyre that were correlated with measurements made from a
surface research ship. The scientists observed elevated phytoplankton
pigment concentrations within the Gyre, indicating that this feature induced
upwelling, bringing nutrients to the surface and augmenting phytoplankton
growth. They discussed their observations in the paper "A note on the
Charleston Gyre", published in the Journal of Geophysical Research (Volume 90,
Number C6, pages 11,857-11,861.)
SeaWiFS obtained another clear view of the region on May 15, 1998. The
true-color image on this page shows the appearance of land and near the
southeastern coast of the United States. Note the strong contrast between the
dark inshore waters and the lighter blue of the Atlantic. The Charleston Gyre
is the dark oval almost directly south of Cape Fear, the southernmost sharp
point on the Carolina Coast. Cape Lookout, southern point of the Outer Banks,
is also prominent in this image, and Charleston is approximately due west of
the Charleston Gyre. The waves that are created by the interaction of the
Charleston Bump and the Gulf Stream can also be seen downstream of the
Charleston Gyre.
At present, the Charleston Bump is receiving a lot of attention due to the
amount of fishing that takes place in the region. A moratorium has been placed
on catches of certain fish (notably swordfish), which has had an economic
impact. There is renewed interest in the oceanic circulation in the Bump
region, as the currents are responsible for fish-related effects, particularly
the transport and export of larval fish), and the upwelling in Gulf Stream
meanders is responsible for a significant fraction of the nutrients used in
primary production.
Added 01/29/2003: Islands in the
Stream: Oceanography and Fisheries of the Charleston Bump, George R.
Sedberry, Editor, is a book published by the American Fisheries Society
about the ecosystem of the Charleston Bump.
Added 11/28/2003:
"Islands in the Stream" cruise logs for the Charleston Bump study:
September 27, 2001 (excellent color bathymetric image)
September 28, 2001 (geology of the Charleston Bump)
ACKNOWLEDGMENT
SeaWiFS images produced by Norman Kuring, SeaWiFS Project, NASA GSFC & James Acker, Raytheon ITSS. Page design by Robert Simmon, Research and Professional Services. Accompanying text by James Acker, Raytheon ITSS. Bathymetric map of the Charleston Bump provided by Dr. John Bane, Department of Marine Sciences, University of North Carolina at Chapel Hill.
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