For those of you modeling how to deal with the arctic, Here is a
report on some work that you will want to give a glance. Maybe it
will be a couple more years beforfe we see the ice-free condition.
Either way, with the Atlantic conveyor back in operation, at least we
will see a bit more carbon sequestration.
d.
Published online 29 November 2008 | Nature | doi:10.1038/news.
2008.1262
News
North Atlantic cold-water sink returns to life
Convective mixing resumes after a decade due to massive loss of Arctic
ice.
Quirin Schiermeier
North Atlantic overturning — back with a vengeance?
Nature
Scientists have found evidence that convective mixing in the North
Atlantic, a mechanism that fuels ocean circulation and affects Earth's
climate, has returned after a decade of near stagnation – thanks,
perhaps, to a dramatic loss of sea-ice in the Arctic during the summer
of 2007.
Convective mixing, or 'overturning', of ocean waters at high latitudes
helps to drive the Atlantic 'heat conveyor belt' that carries warm
water northwards and cooler deep-water back south. The phenomenon also
helps to remove carbon dioxide from the atmosphere — as cold water
sinks, it carries dissolved CO2 with it, locking it away in the depths
of the ocean for centuries.
But cold winters are one of the prerequisites for convection in the
deep-water formation regions around Greenland, such as the Labrador
and Irminger Seas, and a warming climate would be expected to slow or
stop the process.
There's been very little convection in the North Atlantic over the
past decade, prompting concerns that the impact of global warming was
already being felt.
Now two teams of scientists have independently found evidence that
overturning has resumed in the North Atlantic.
A team led by Kjetil Våge of the Woods Hole Oceanographic Institution
in Massachusetts has found that convection returned to the region last
winter (2007-2008) with a vengeance1. And Igor Yashayaev and John
Loder of the Bedford Institute of Oceanography in Nova Scotia, Canada,
separately report that the convection last winter in the Labrador Sea
was the deepest since 19942.
Temporary blip?
Våge and his colleagues suspect that weather peculiarities alone
cannot explain the unexpected return of convection.
Temperatures last winter were 5-6 °C colder in the North Atlantic than
in the previous seven years. But the location of high and low pressure
systems over the region means that weather patterns did not favour
overturning.
They suggest that a cap of cooler fresh water – massive sea-ice export
from the Arctic basin along both sides of Greenland during the
previous summer – has facilitated freezing of parts of the Labrador
and Irminger Seas. As a result, cold continental-origin air blowing
over the region had not been warmed by the relatively warm ocean when
it reached the convection areas. The temperature difference between
air and open water led to a massive transfer of heat from the ocean to
the atmosphere, thus fuelling convection.
Whether the convection is back to stay, however, is not known. "There
is a lot of natural fluctuation at play," says Detlef Quadfasel, an
oceanographer and climate scientist at the University of Hamburg in
Germany. "One nice day of ocean weather doesn't really tell you a lot
about what you might have to expect in the future."
But there is little doubt that, in the long-term, ocean convection
will decrease if northern latitudes continue to warm at the current
rate. "As the water column gets ever more stable, it will get
increasingly difficult in a warming environment to produce deep
convection in the North Atlantic," says Jürgen Fischer, an
oceanographer at the IFM-Geomar Leibniz Institute of Marine Sciences
in Kiel, Germany. "You'll probably need very exceptional conditions,
such as those of last winter, to mix the oceans."
Global consequences
Reduced convection should in theory weaken the entire Atlantic
meridional overturning circulation (MOC) — responsible for carrying
warm tropical water northwards — with far-reaching consequences for
Earth's climate. But so far at least, scientists have not observed any
significant changes to that large-scale circulation. Findings
published in 2005 that seemed to indicate a big slowing of the MOC
were later found to be in the range of natural fluctuations (see
'Ocean circulation noisy, not stalling').
One reason, says Fischer, is that the observational basis is still
thin. The Argo programme, a global array of 3,000 robots that measure
temperature, salinity and water pressure, has only last year become
fully operational, for example.
But already it's clear that the response of the Atlantic Ocean
circulation to high-latitude changes is much more complex than has
been assumed. According to Våge and his colleagues, the "myriad of
factors" that favoured the return of overturning last winter make it
"difficult to predict when deep mixing is likely to occur".
References
1. Våge, K. et al. Nature Geosci. doi:10.1038/NGEO382 (2008).
2. Yashayaev, I. & Loder, J. W. Geophys. Res. Lett. doi:
10.1029/2008GL036162 (in the press).
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