ATLANTIC OCEAN YIELDS CLIMATE SECRETS

[Anonymous]

Atlantic yields climate secrets
Scientists
have painted the first detailed picture of Atlantic ocean currents
crucial to Europe’s climate.Using instruments strung out across the
Atlantic, a UK-led team shows that its circulation varies significantly
over the course of a year.Writing in the journal Science, they say it
may now be possible to detect changes related to global warming.The
Atlantic circulation brings warm water to Europe, keeping the continent
4-6C warmer than it would be otherwise.

As the water reaches the cold Arctic, it sinks, returning southwards deeper in the ocean.
Some
computer models of climate change predict this Atlantic Meridional
Overturning Circulation, of which the Gulf Stream is the best-known
component, could weaken severely or even stop completely as global
temperatures rise, a scenario taken to extremes in the Hollywood movie
The Day After Tomorrow.Last year the same UK-led team published
evidence that the circulation may have weakened by about 30% over half
a century.But that was based on historical records from just five
sampling expeditions, raising concerns that the data was not robust
enough to provide a clear-cut conclusion.

Rapid changes
The
key for scientists, then, has been to measure and understand how the
circulation varies naturally, making it much easier to pick out any
changes related to man-made global warming.This has been the goal of
the Rapid/Mocha (Rapid Climate Change/Meridional Overturning
Circulation and Heatflux Array) project; and its first results show
that the circulation varies substantially, by a factor of eight, even
during a single year.”I think this is a major step forward for our
understanding of ocean circulation,” said Stuart Cunningham from the
National Oceanography Centre (NOC) in Southampton, one of the project’s
senior scientists.”The Atlantic Ocean carries a quarter of the global
northwards heat flux, so having the information to plug into climate
models will be a major addition,” he told the BBC News website.But
measuring long-term variation is, if anything, even more important.
Man-made warming could drive the flow downwards, but so could natural
climate cycles such as the Atlantic Multidecadal Oscillation.All five
of the historical flow values documented in last year’s paper, for
example, fit within the range of variability measured here, making it
very hard to argue that these observations found a long-term trend.”We
will measure very quickly any sudden shifts,” commented NOC’s Professor
Harry Bryden.”We already think we can define changes bigger than two
Sverdrups (about 10% of the average flow; one Sverdrup (Sv) is defined
as a flow of one million cubic metres of water per second).”But the
reality is that anything we measure over 10 years even is going to be
labelled interannual variability at the moment.”

Strung out
Making
the measurements has not been a trivial matter.Early in 2004, NOC
researchers deployed 19 sets of instruments during a voyage across the
Atlantic at 26.5 degrees North, from the north-western coast of Africa
to the Bahamas.US investigators subsequently installed further moorings
on the western side of the ocean.Each set of instruments is strung out
along a cable which is tethered to the sea floor at the bottom end, and
to a float at the top.The exact instruments used vary between moorings,
but typically they measure flow, salinity, temperature and water
pressure.The instruments were left in place for just over a year, then
the team made a second cruise to recover data.This has given
researchers a real-time picture of water flows inside the ocean, from
top to bottom and side to side.But this is just part of the mechanism
transporting heat northwards from the tropics to the western shores of
Europe.At 26.5N, the Gulf Stream itself shoots along a narrow channel
between the Bahamas and the coast of Florida. The strength of this has
been measured for decades using a disused submarine telephone cable –
as sea water, an electrical conductor, flows over the cable, it induces
a voltage which is continuously measured by scientists in Miami.A third
component of the circulation is movement at the ocean’s surface driven
by winds, which can be measured nowadays by satellite.The scientists
had to combine these three datasets to calculate the average flow
northwards, and by how much it varies.

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August 22, 2007 at 12:58 PM