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 user 2007-08-22 at 12:58:00 pm Views: 65
  • #18592

    Atlantic yields climate secrets
    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.
    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
    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
    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.