Abstract:
On December 1st, 2005, Prof. Bryden and his team reported in Nature (Bryden et
al., 2005. Slowing of the Atlantic meridional overturning circulation at 25N.
Nature 438,
655-657:
http://www.nature.com/nature/journal/v438/n7068/edsumm/e051201-11.html ) that the Atlantic meridional overturning circulation (MOC) seems to have
weakened by about 30% in the last decade or so.
The team analyzed
... measurements taken along a line at about 25N from the Bahamas
to Morocco in 2004, and compared these to measurements taken along
approximately this line in 1957, 1981, 1992 and 1998. The results suggest that
the Gulf Stream flow across 25N has remained nearly constant in this period,
but the MOC has slowed from about 20 Sv in 1957 to 14 Sv in 2004.
The evidence for the slow-down can be found both in the increase in southward
recirculation of warm water in the surface layer, and in a decrease in
southward transport of lower North Atlantic Deep Water between 3000 and 5000m
depth.
Some climate models have suggested that we could expect such a slowdown as a
result of global warming, and that this could cool northwest Europe by a few
degrees C, with the change happening relatively fast, perhaps over just a
decade. These measurements are evidence that the models could be right.
The five sections are just "snapshots" and cannot tell us for sure whether the
changes are a result of natural variability. Very little is known about how the
MOC varies seasonally or from year to year. To be sure that the observed change
is the result of a long-term trend rather than natural fluctuations we will
need to monitor flow across 25N continuously for about a decade.
In April 2004 Prof. Bryden's team deployed an array of instruments across the
Atlantic at 25 N. The array measures temperature, salinity, currents and
pressure. The first full year of measurements were collected in April 2005, and
analysis of this data is now underway.
See: http://www.noc.soton.ac.uk/rapid/sis/moc_monitor.php 
Links
