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| Enlarge ImageA global system of ocean circulation—often called the Great Ocean Conveyor—transports vast amounts of heat and salt around the planet via warmer surface currents (red) and colder deep currents (blue). It plays a central role in determining Earth's climate. (Jack Cook, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe North Atlantic and Arctic Oceans are critical components of the ocean-climate system. Warm tropical waters flow northward, releasing heat to the North Atlantic region, and eventually flow into the depths of the Arctic Ocean. Cold waters sink in the North Atlantic and flow southward to drive the Ocean Conveyor. (E. Paul Oberlander, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe Greenland-Scotland Ridge looms like a great undersea barrier, stretching from East Greenland to Iceland and the Faroe Islands, and across to Scotland. There are a few gaps in the ridge, and they act as critical checkpoints that regulate the flow of warmer, saltier waters north to the Arctic Ocean and cold, fresher waters south across the ridge into the the main body of the North Atlantic Ocean. (E. Paul Oberlander, Woods Hole Oceanographic Institution) |
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| Enlarge ImageAlmost like a subsea waterfall, cold, dense waters flow over the Greenland-Scotland Ridge and then underneath warmer, lighter waters, heading southward. (E. Paul Oberlander, Woods Hole Oceanographic Institution) |
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| Enlarge ImageIcebergs hinder scientists' abilty to study currents off the southeast coast of Greenland that are important to ocean circulation and climate, including a surface current hugging the coastline that brings cold, fresh Arctic water into the North Atlantic. (Photo by Marika Marnela, University of Hamburg) |
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| Enlarge ImageUniversity of Hamburg researchers designed "tube" moorings that make way for icebergs, then bounce back up, affording protection for instruments taking measurements near the surface. (E. Paul Oberlander, Woods Hole Oceanographic Institution) |
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| Enlarge ImageLong, protective, buoyant tubing (on the deck and snaking behind the ship) is deployed atop moorings in ice-infested seas. The tubing is less easily smashed or snagged by the keels of drifting icebergs. (Photo by Marika Marnela, University of Hamburg) |
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» The RAPID Programme
The Natural Environment Resource Council-directed programme on rapid climate change research
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By Bob Dickson and Stephen Dye
Centre for Environment, Fisheries & Aquaculture Science
Lowestoft, England
The Greenland-Scotland Ridge looms like a great undersea barrier,
stretching from East Greenland to Iceland and the Faroe Islands, and
across to Scotland. There are a few gaps in the ridge, and they act as
critical checkpoints that regulate water flowing between the Norwegian and
Greenland Seas north of the ridge and the main body of the North
Atlantic Ocean to the south.
The properties of waters in these Nordic seas and the two-way flow
across the ridge are critical components that help drive what is often
called the Great Ocean Conveyor—the global system of ocean circulation
that transports heat and salt around the planet. But only in recent
years have oceanographers deployed instruments in these remote,
violent, ice-infested subpolar waters to obtain long-term measurements.
These measurements aim to help answer several questions: Does ocean
circulation, especially in the North Atlantic, play a central role in
climate? Is greenhouse warming causing changes in the Arctic
Ocean and the subpolar Nordic Seas that are moving downstream into the
North Atlantic? Is the North Atlantic component of the Conveyor
actually slowing down?
The ocean-climate connection
The debate on the first question is not contentious. Most scientists
agree that waters in the North Atlantic become cold (and therefore
dense) enough to sink to the abyss and flow out of the ocean basin via
a deep southward current. The dense waters are replaced by salty,
tropical, surface waters that flow north, where they release heat to
the atmosphere and temper the climate of the North Atlantic region.
This conveyor-like movement of water is called the Atlantic Meridional
Overturning Circulation.
The amounts of water, heat, and salt that pass north across the
Greenland-Scotland Ridge from the Atlantic have now been directly
measured by a consortium of European and North American oceanographers
working under the Arctic/Subarctic Ocean Fluxes Programme. So have
corresponding fluxes into the Arctic Ocean by researchers from the
Institute of Marine Science and the University of Bergen in Norway and
the Alfred Wegener Institute for Polar and Marine Research in Germany.
We now know that 8.5 million cubic meters (225 million U.S. gallons) of
warm, salty Atlantic water passes north across the Greenland-Scotland
Ridge per second, carrying with it an average of about 313 million
megawatts of power and 303 million kilograms (668 million pounds) of
salt. As dense water returns south and flows over the ridge, its
salinity has decreased from about 35.25 to 34.88 salinity units, and
its temperature has dropped from 8.5°C (47°F) to 2°C (35.6°F) or less.
Not surprisingly, the ocean’s surrendering of that amount of heat to
the atmosphere has more than local climatic importance.
In a “what-if” experiment, scientists at the Hadley Centre for Climate
Prediction and Research at the U.K. Meteorological Office instructed a
computer model that simulates the general circulation of the atmosphere
and ocean to shut down the Atlantic Meriodional Overturning
Circulation. The results showed that within a decade, mean air
temperatures over most of the Northern Hemisphere cooled by several
degrees; over the northern Norwegian and Barents Seas, it cooled by
more than 15°C (27°F).
The Hadley research group “shut down” the overturning circulation by
artificially releasing a large pulse of fresh water into the North
Atlantic in their model. No model predicts this will actually happen,
but the addition of more buoyant fresh water to sensitive locales in
the North Atlantic Ocean could decrease the density of surface water
enough to curtail sinking. That could slow down the overturning
circulation and convey less tropical water and heat northward.
A wide range of opinion
The obvious follow-up question is much harder to answer: Is the
Atlantic Meridional Overturning Circulation actually slowing? Most
computer simulations of the ocean system in a climate with increasing
greenhouse-gas concentrations predict that the Atlantic overturning
circulation will weaken as the subpolar seas become fresher and warmer.
But opinions are divided on whether a slowdown is already under way
and on whether any changes we are seeing are natural or caused by human
activities such as fossil-fuel burning.
Modelers at the Geophysical Fluid Dynamics Laboratory at Princeton, for
example, suggested that aerosols from human activity have blocked solar
radiation and actually may have delayed a greenhouse-gas-induced
weakening of the overturning circulation. Modelers at Kiel University
in Germany suggested that the circulation will not weaken substantially
over the next several decades. Hadley Centre modelers reported in 2004
that although waters in the deep North Atlantic became less salty in
recent decades, the overturning circulation has increased (because an
increasing north-to-south difference in density of waters in the upper
North Atlantic).
In 2005, Harry Bryden and colleagues at the National Oceanography Centre,
Southampton, in the United Kingdom, reported that the Atlantic
Meridional Overturning Circulation had already slowed by 30 percent
since 1957 (particularly strongly since the early 1990s), after they
analysed data from five periodic survey cruises along latitude 25°N,
roughly from the Bahamas to the Canary Islands. Part of this long-term
decline, they said, resulted from a reduced southward flow of deep
waters originating from the overflow of the Greenland-Scotland Ridge.
Modeling the same transect across the Atlantic, however, Carl Wunsch and
Patrick Heimbach of the Massachusetts Institute of Technology inferred
in 2006 that the circulation of deep water had become stronger. They concluded that
the 25°N line, no matter how closely observed, is not immune to
uncertainties.
Following up on their earlier research, the Southampton researchers used
current meters to measure continuously for one year along approximately
the same line. In August 2007, they reported that the Meridional
Overturning Circulation varied widely within one year—so much so that the
previously reported 30-percent decrease over almost 50 years is
unlikely to have been significant by comparison.
Complex changes over time and space
None of these opinions—and there are others!—is controversial in the
sense that they are all based on established and accepted techniques.
The controversy exists is in the interpretation of what has been found.
The problem is that our oceanographic measurements are simply too short
or patchy to grasp unambiguously the complex changes that the Atlantic
is exhibiting over space, time, and depth. We are still developing
ideas about the causes and mechanisms of ocean circulation changes, so
these are too crudely represented in the models. Our observations
cannot yet supply many of the numbers the modelers need to reduce the
high levels of uncertainty in the present generation of climate models.
The fact remains, however, that understand it or not, our climate is
changing. In their careful reassessment of the climatic record, Timothy
Osborne and Keith Briffa of the University of East Anglia in England
concluded in 2006 that “the most significant and longest duration
feature during the last 1,200 years is the geographical extent of
warmth in the middle to late 20th century.”
Data from many researchers, collated and published in 2006 by the
International Council for the Exploration of the Seas, show clearly
enough that the Atlantic waters crossing over the Greenland-Scotland
Ridge to the Nordic Seas and Arctic Ocean are generally at their
warmest and saltiest since records began. Reflecting this trend,
Jonathan Overpeck of the University of Arizona and a broad team of
colleagues reported in 2005 that the Arctic system remains on
trajectory to a new seasonally ice-free state—“a state not witnessed
for at least a million years,” they wrote.
A valiant but disastrous effort
To improve understanding of the air-sea-ice system of subarctic seas,
we have focussed on measuring perhaps two of the most climatically
important oceanic flows on Earth—off southeast Greenland. Recognising
the significance of the region, Val Worthington, a well-known physical
oceanographer at Woods Hole Oceanographic Institution (WHOI), deployed
30 current meters in February and March of 1967 across the violent flow
through the Denmark Strait between Greenland and Iceland. There, the
density contrast of waters north and south of the Greenland-Scotland
Ridge drive cold dense water formed in the western Nordic Seas
southward over the ridge. This so-called Denmark Strait Overflow
cascades downward to fill the depths of the Irminger and Labrador Seas
and drive the lower limb of the Atlantic overturning circulation.
Worthington later wrote of his pioneering effort: “It was a disaster.
...When you put out 30 current meters and get one usable record, you
can’t crow too much.” But four decades after his heroic but
unsuccessful attempt, the Centre for Environment, Fisheries &
Aquaculture Science, with partners at University of Hamburg and the
Finnish Institute of Marine Research, have proven and fully developed
an array of instrumented moorings offshore of the town of Angmagssalik,
on the continental slope of East Greenland, to measure the
characteristics and variability of the cold, dense Denmark Strait
Overflow.
Initial findings
A decade of continuous observations from the Angmagssalik array now
reveals that cold, denser water flows over the ridge in the Denmark
Strait at a rate of about 4 Sverdrups (a Sverdrup is 1 million cubic
meters or 264,000 U.S. gallons per second). That is close to the 3.8
Sverdrups predicted in 1998 by WHOI physical oceanographer Jack
Whitehead on the basis of calculations of the hydraulic forces of
fluids forced through oceanic passages.
However, our longer observational record has moved us on a little from
thinking of the Denmark Strait Overflow as an unchanging flow solely
governed by hydraulics to one that alternately strengthens and weakens
over time. A combination of forces probably drives the overflow; among
these are the amount of dense water in the reservoir north of the
ridge; changes in local and regional winds; and effects of the large
gyre-scale ocean circulation that feeds water into and around the
region.
The array has not shown any long-term trend so far. Nor has it turned
up evidence of interrelationships, as has been supposed, between the
transport of water in the Denmark Strait and in Faroe Bank Channel, a
gap in the ridge east of Iceland. But it may simply be that we haven’t
been measuring in these places long enough to distinguish changes that
occur over decades.
In 2002, we and colleagues reported in the journal Nature that Denmark
Strait Overflow waters had become fresher remarkably rapidly and
steadily over the previous four decades. In 2005, Ruth Curry of WHOI
and Cecilie Mauritzen of the Norwegian Meteorological Institute made
the next logical step. Using Whitehead’s hydraulic equation, they
calculated how much more fresh water would have to be added to the
western parts of the Nordic seas to produce a significant slowdown of
the overturning circulation.
Not anytime soon, they found: “At the observed rate, it would take
about a century to accumulate enough fresh water ... to substantially
affect the ocean exchanges across the Greenland-Scotland Ridge, and
nearly two centuries of continuous dilution to stop them,” they wrote.
“In this context, abrupt changes in ocean circulation do not appear
imminent.” Reinforcing this conclusion is the fact that the freshening
trend for both Greenland-Scotland Ridge overflows, which we had been
observing over four decades, has slowed to a stop during the last
10 years.
In iceberg-infested waters
Over the last several years, we and our partners at the University of
Hamburg have also deployed an array of moorings across the
continental shelf, nearer the coast of southeast Greenland. These are
aimed at measuring a flow of fresh surface waters that passes south
from the Arctic Ocean to the North Atlantic under the East
Greenland ice shelf. We believe that via this route, the North Atlantic
receives its largest dose of fresh water from the Arctic, and
ocean-climate models have implicated this fresh water in slowing down
the overturning circulation.
This freshwater flux array is at a much less advanced state of
development than the Angmagssalik array, but we are improving it with the help funding from the WHOI
Ocean and Climate Change Institute. Moorings that consist of
protective, buoyant “tubes,” 40 meters (130 feet) long, bring
salinity-measuring sensors up to the base of the sea ice, in the region
where the less dense fresh water flows. The tubes protect against
strikes by drifting ice on the principle of those inflatable dolls with
weighted bottoms. In the U.K., we have been known to christen them
“Margaret Thatcher dolls,” after our former prime minister, because
when knocked down, they bounce back up.
The tube moorings have brought real advances, notably the recovery of
up to four years of continuous salinity measurements in the upper
waters of near the coast of East Greenland. However, since 2000, the
nearby Kangerdlugssuak Glacier has been calving icebergs at an accelerated
rate, creating a major new and perhaps unsurvivable hazard for our
moorings. When we attempted in August 2005 to recover the rudimentary
mooring we deployed the year before, it was gone. But we did see many
grounded icebergs in the vicinity, which may well have swept our
mooring away.
In the summer and autumn of 2006, further losses and some instrumental
failures combined with unsuitable conditions forced us to curtail our
operations for the year. We deployed one tube mooring and an acoustic
profiling current meter.
The climatic importance of measuring the water flow in this region
argues against withdrawing the array at our first reverse. But this
will never be a safe site, and only time will tell if our plans to
re-extend the array across this dangerous shelf are justified or
foolhardy.
Val Worthington would have known the feeling!
The mooring array work is supported
by the Ocean and Climate Change Institute at WHOI and other
organisations in Europe and the U.S. They include the national
programmes of Germany, Finland, and the U.K. (Department for
Environment, Food and Rural Affairs); European Union Framework Programmes, inclduing VEINS (Variability of Exchanges in the Northern Seas), ASOF (Arctic/Subarctic Ocean Fluxes-ASOF), and DAMOCLES (Developing Arctic Modeling and Observing Capabilities for
Long-term Environmental Studies); and the U.S. National Oceanic and Atmospheric Administration's Abrupt Climate Change Program (CORC-ARCHES).
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Posted: September 6, 2007 [top] |
