April 15, 2004
Satellites Record Weakening North
Atlantic Current
A North Atlantic Ocean circulation system
weakened considerably in the late 1990s, compared
to the 1970s and 1980s, according to a NASA
study.
Sirpa Hakkinen, lead author and researcher at
NASA’s Goddard Space Flight Center,
Greenbelt, Md. and co-author Peter Rhines, an
oceanographer at the University of Washington,
Seattle, believe slowing of this ocean current is
an indication of dramatic changes in the North
Atlantic Ocean climate. The study’s results
about the system that moves water in a
counterclockwise pattern from Ireland to Labrador
were published on the Internet by the journal
Science on the Science Express Web site at:
http://www.sciencexpress.org
or http://www.aaas.org
The current, known as the sub polar gyre, has
weakened in the past in connection with certain
phases of a large-scale atmospheric pressure
system known as the North Atlantic Oscillation
(NAO). But the NAO has switched phases twice in
the 1990s, while the subpolar gyre current has
continued to weaken. Whether the trend is part of
a natural cycle or the result of other factors
related to global warming is unknown.
“It is a signal of large climate
variability in the high latitudes,”
Hakkinen said. “If this trend continues, it
could indicate reorganization of the ocean
climate system, perhaps with changes in the whole
climate system, but we need another good five to
10 years to say something like that is
happening.” Rhines said, “The
subpolar zone of the Earth is a key site for
studying the climate. It’s like Grand
Central Station there, as many of the major ocean
water masses pass through from the Arctic and
from warmer latitudes. They are modified in this
basin. Computer models have shown the slowing and
speeding up of the subpolar gyre can influence
the entire ocean circulation system.”
Satellite data makes it possible to view the
gyre over the entire North Atlantic basin.
Measurements from deep in the ocean, using buoys,
ships and new autonomous “robot”
Seagliders, are important for validating and
extending the satellite data. Sea-surface height
satellite data came from NASA’s Seasat
(July, August 1978), U.S. Navy’s Geosat
(1985 to 1988), and the European Space
Agency’s European Remote Sensing
Satellite1/2 and NASA’s TOPEX/Poseidon
(1992 to present).
Hakkinen and Rhines were able to reference
earlier data to TOPEX/Poseidon data, and
translate the satellite sea-surface height data
to velocities of the subpolar gyre. The subpolar
gyre can take 20 years to complete its route.
Warm water runs northward through the Gulf
Stream, past Ireland, before it turns westward
near Iceland and the tip of Greenland.
The current loses heat to the atmosphere as it
moves north. Westerly winds pick up that lost
heat, creating warmer, milder European winters.
After frigid Labrador Sea winters, the water in
the current becomes cold, salty and dense,
plunges beneath the surface, and heads slowly
southward back to the equator. The cycle is
sensitive to the paths of winter storms and to
the buoyant fresh water from glacial melting and
precipitation, all of which are experiencing
great change.
While previous studies have proposed winds
resulting from the NAO have influenced the
subpolar gyre’s currents, this study found
heat exchanges from the ocean to the atmosphere
may be playing a bigger role in the weakening
current. Using Topex/Poseidon sea-surface height
data, the researchers inferred Labrador Sea water
in the core of the gyre warmed during the 1990s.
This warming reduces the contrast with water from
warmer southern latitudes, which is part of the
driving force for ocean circulation.
The joint NASA-CNES (French Space Agency)
Topex/Poseidon oceanography satellite provides
high-precision data on the height of the
world’s ocean surfaces, a key measure of
ocean circulation and heat storage in the
ocean.
NASA’s Earth Science Enterprise is
dedicated to understanding the Earth as an
integrated system and applying Earth System
Science to improve prediction of climate, weather
and natural hazards using the unique vantage
point of space. NASA, the National Oceanic and
Atmospheric Administration, and the National
Science Foundation funded the study.
For more information and images from the study
on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/
0415gyre.html
http://sealevel.jpl.nasa.gov/index.html
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Contacts:
Elvia Thompson/Etta Pagani
Headquarters, Washington
Phone: 202/358-1696/1034
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
Phone: 607/273-2561
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Dominant Pattern of Variability of the
Sea-surface Height in the 1990s
This image shows the dominant pattern of
variability of the sea-surface height in the
1990s. The slope of the sea-surface height is in
balance with ocean currents, much in the way
weather maps of pressure relate to winds. The
large blue region in the northern Atlantic
represents a slowing of the counter-clockwise,
cyclonically circulating subpolar gyre. For this
image, the researchers used TOPEX/Poseidon data,
which has been combined with ERS-1/2 data into
the NASA Pathfinder data set. The Pathfinder data
set also includes the Seasat and Geosat data
which are referenced to TOPEX/Poseidon data. The
next few years will reveal how sea surface height
is going to evolve as the altimetric time series
continues with JASON-1 observations. Credit:
Sirpa Hakkinen, NASA GSFC High-Resolution
Image
Terra MODIS Sea Surface Temperatures
for North Atlantic Ocean
This image of North Atlantic Ocean sea surface
temperatures represents an eight-day composite
from Sept 6 - Sept 13, 2001 from the Moderate
Resolution Imaging Spectroradiometer (MODIS)
instrument on Terra. The red and orange colors
represent warmer water, while the blue shades
represent cold water in the higher latitudes. The
Gulf Stream is evident along the U.S. eastern
seaboard. The color bar is in units of degrees
Celsius. Credit: Ronald Vogel, SAIC for NASA
GSFC High-Resolution
Image
Iceberg in North Atlantic
Waters
An iceberg glides southward along the east
coast of Greenland in the Irminger Sea. The tip
of this iceberg is about the size of a 2-story
house. Credit: C. A. Linder. For a high
resolution version of this image please contact
Christopher Linder at http://www.chrislinder.com
Trend Velocities in North
Atlantic
The trend of the velocities (meters per second
per decade) derived from NASA Pathfinder
altimeter data for the period May 1992 to June
2002. The colored vectors are statistically
significant. Note how the vectors trace the
following graphic of the subpolar circulation in
reverse direction, which denotes a slowing gyre.
Credit: Sirpa Hakkinen, NASA GSFC
Pathways of Transformation
Pathways associated with the transformation of
warm subtropical waters into colder subpolar and
polar waters in the northern North Atlantic.
Along the subpolar gyre pathway the red to yellow
transition indicates the cooling to Labrador Sea
Water, which flows back to the subtropical gyre
in the west as an intermediate depth current
(yellow). More information
Credit: ©Jack Cook, Woods Hole Oceanographic
Institution
Woods Hole Oceanographic Institute (WHOI) and
Oceanus Magazine helped provide graphics for this
story. For more about WHOI, please see:
http://www.whoi.edu/ and http://oceanusmag.whoi.edu/
index.html
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