Scientists Uncover Diversion of Gulf
Stream Path in Late 2011; Warmer Waters Flowed to Shelfbreak South of New
England
From ScienceDaily.com
At a meeting with New
England commercial fishermen last December, physical oceanographers Glen Gawarkiewicz
and Al Plueddemann from the Woods Hole Oceanographic Institution (WHOI) were
alerted by three fishermen about unusually high surface water temperatures and
strong currents on the outer continental shelf south of New England.
"I promised them I
would look into why that was happening," Gawarkiewicz says.
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The figures show the maximum sea surface
temperatures
for the periods (a) October 12–21, 2011, and (b) December 1–15,
2011.
The coastline and shelfbreak are indicated by the
thin black contours.
The dashed black lines in (a) and (b)
denote the climatological mean location of
the Gulf Stream North Wall
in October and December. In (a), the blue line
denotes the path
of a surface drifter released off Cape Fear, NC, on
October
12, 2011, that was entrained in the Gulf Stream and
reached Georges Bank 8 days
later. The drifter's speed (blue)
and course (red) are shown as functions of
latitude in (c). In (a–b),
the blue star indicates the location of the OOI test
mooring,
and the magenta squares denote the locations of the
eMOLT
observations. (Credit: Image courtesy Robert Todd,
Woods Hole Oceanographic
Institution))
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The result of his
investigation was a discovery that the Gulf Stream diverged well to the north
of its normal path beginning in late October 2011, causing the
warmer-than-usual ocean temperatures along the New England continental shelf.
The researchers'
findings, "Direct interaction between the Gulf Stream and the shelfbreak
south of New England," were published in the August 2012 issue of the
journal Scientific Reports.
To begin to unravel the
mystery, Gawarkiewicz and his colleagues assembled data from a variety of
sources and recreated a record of the Gulf Stream path during the fall of 2011.
First, they tapped into data collected by a program called eMOLT, a non-profit
collaboration of fishing industry, research, academic and government entities,
run by James Manning of National Oceanic and Atmospheric Administration's
Northeast Fisheries Science Center. For more than a decade the program has
recorded near-bottom ocean temperatures by distributing temperature probes to
lobstermen.
Manning and scientists
from WHOI, including Robert Todd and Magdalena Andres, analyzed a time series
of temperatures from two eMOLT sites, OC01 and TA51, which were located over
the outer continental shelf near the shelfbreak, and identified two events when
temperatures suddenly increased by 6.2 and 6.7°C, respectively, to highs of
more than 18°C.
"These are very
dramatic events for the outer continental shelf, at least 2°C warmer than we've
seen since 2001," says Gawarkiewicz. "Near-bottom temperatures of
18°C on the outer shelf are extremely high for late autumn." The maximum
recorded temperature in December 2011 was the warmest bottom temperature
recorded in 6 years of records at the OC01 site.
In typical years, the
warm Gulf Stream waters only indirectly influence ocean currents and
temperatures near the continental shelfbreak south of New England when eddies,
called warm core rings, pinch off from the Gulf Stream and drift toward the
outer continental shelf. Such rings normally drift past a site after a few
weeks, and therefore cause only limited warming of the water on the outer
shelf.
Gawarkiewicz and his
colleagues collected additional data on water temperature and salinity from December
4, 2011 through January 4, 2012, from instruments on temporary test moorings
placed 12 km south of the shelfbreak by the Ocean Observatories Initiative
(OOI). The researchers compared those salinity measurements to historical data,
and discovered that high salinity levels -- consistent with the salinity of
waters carried by the Gulf Stream -- coincided with the warming periods.
The extent and duration
of the two 2011 warming events combined with the high salinity observed by the
researchers suggested the cause was not a transient warm core ring, but the
Gulf Stream itself that carried warm, salty water to the outer shelf.
To solidify that
finding, Gawarkiewicz received serendipitous help from students in the Marine
Advanced Technology Education (MATE) program at Cape Fear Community College in
Wilmington, NC, who had deployed a surface drifter during the period coinciding
with the two warming events. Drifters use satellites to transmit their
positions roughly every six hours, key information for the WHOI scientists, who
analyzed the drifter tracks and speeds.
"Drifters around
the edges of warm core rings drift toward the continental shelf at about 1
knot," Gawarkiewicz says. "But we saw the drifter cut across the
slope towards the shelf at about 2.5 knots. It only took it eight days to
travel from Cape Fear, North Carolina, to a point 40 miles south of Georges
Bank, a total distance of 580 miles."
The periods of high
speeds for the drifters coincided with the records for high temperatures on the
outer shelf, which told the scientists that the core of the Gulf Stream had
diverted to 39.9°N at 68°W -- 125 miles north of its mean position, further
north than had ever been recorded by satellite altimeters at this particular
longitude.
The temporary shift in Gulf
Stream path observed last fall potentially has significant longer-term
implications. Studies have shown that temperature increases of 2°C have caused
major shifts in silver hake populations, for example, and in spring 2012,
migratory bluefish and striped bass were observed off the coast of Cape Cod
much earlier than in previous years. But, the scientists say, more research is
needed to determine just how the Gulf Stream's behavior in 2011 affected the
continental shelf ecosystem and marine organisms.
It is unclear what might
have caused this shift in the Gulf Stream path. It occurred shortly after
Hurricanes Irene and Katia drenched the east coast with rain, and this might
have impacted the Gulf Stream separation from the continental shelf near Cape
Hatteras.
Another possibility is that a cold core ring, an eddy south of the
Gulf Stream core, might have deflected the Gulf Stream. Further research will
be necessary to determine exactly how and why this occurred, which will be
helpful in the long term in predicting Gulf Stream motions.
In the meantime,
Gawarkiewicz and his colleagues will be keeping an eye on what the Gulf Stream
does this fall, with the hope of someday being able to predict such a shift.
"We're checking in from time to time to monitor it. We'll be talking to
the fishermen, and academics, and keeping an eye on things," he says.
Fishermen David Spencer,
Fred Mattera, and Norbert Stamps first alerted the researchers to the anomaly.
Profile data were made available by the OOI, which is funded by the National
Science Foundation and managed by the Consortium for Ocean Leadership. Tim Shaw
and David Calhoun at Cape Fear Community College provided drifter data. WHOI
scientists on this project were supported by the NSF, the Cooperative Institute
for the North Atlantic Region, the Penzance Endowed Fund in Support of
Assistant Scientists, and the Postdoctoral Scholar Program at WHOI.
Story Source:
The above story is
reprinted from materials provided byWoods Hole
Oceanographic Institution.
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Journal Reference:
1. Glen G. Gawarkiewicz, Robert E. Todd, Albert J.
Plueddemann, Magdalena Andres, James P. Manning.Direct interaction between
the Gulf Stream and the shelfbreak south of New England. Scientific
Reports, 2012; 2 DOI: 10.1038/srep00553