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 sub polar 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 sub-polar
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:
