February
26, 2007
A
CLIMATE-CHANGE AMPLIFYING MECHANISM
During the past
ninety thousand
years there were alternating hot and cold periods lasting several
thousand
years each which resulted in a modification of global oceanic
circulation. With
the help of paleoclimatic and paleooceanographic indicators, scientists
at
CEREGE1 have highlighted a feedback mechanism of ocean circulation on
the
climate which reinforces this heating or cooling.
This mechanism
relies on a close
link between the circulation of the North Atlantic and the tropical
hydrology
of Central America.
This study, published in
the February 22, 2007 edition of the review Nature,
should allow us to better understand and therefore better predict the
effects
of climate change on oceanic circulation.
In the past, major
and rapid climatic
variations which took place notably during the last glacial period
(Heinrich
period) disturbed ocean circulation. Climatic archives (marine and lake
sediment, polar ice, stalagmites) show the close relationship existing
between
climatic variations and oceanic circulation. Changes in oceanic
circulation in
the North Atlantic
have influence on a
planetary level by affecting, in particular, the water cycle. These
changes are
accompanied by a shift in the climatic equator which separates the
trade wind
systems of the two hemispheres: southwards during cold events and
northwards
during hot ones.
Central America, a
narrow
continental strip which separates the Atlantic
and Pacific oceans, plays a key role in this system. On the Atlantic
side
surface waters evaporate, which increases salinity. The water vapor is
transferred by the trade winds to the Pacific where it is deposited as
rain,
thus lowering salinity there. This enormous transfer of water (several
hundred
thousand cubic meters per second) maintains a contrast in salinity
between the
two oceans. The surface waters of the tropical Atlantic are then
transported,
via the Gulf Stream, towards the high latitudes where they warm the
atmosphere
before plunging into the abysses in the convection zones situated in
the seas
of Norway,
Greenland and Labrador.
The deep waters formed by this process then
flow into the world ocean, purging the North
Atlantic
of part of its excess salt.
The scientists at
CEREGE1
reconstituted the variations in surface water salinity in the area
where the
water vapor coming from the Atlantic
is
deposited. To do this they worked on the measurements taken in marine
sediments
collected in 2002 west of the Isthmus
of Panama
by the French oceanographic ship the Marion Dufresne. This study shows
that the
cold Heinrich periods correspond to increases in salinity in the east
Pacific.
This is synonymous to a decrease in the transfer of water vapor. By
comparing
their results to other studies done in the Atlantic sector and in South America, the scientists
were able to describe a
feedback mechanism which amplified the climatic disturbance. During
cold
periods the trade winds, loaded with humidity, migrated southwards.
Unable to
cross the Andes
part of the rain, which would
normally have lowered the salinity of the East Pacific, fell in the
Amazon
basin. This feedback had the effect of re-injecting rainwater into the Atlantic, thereby decreasing the
ocean’s salinity. This
water was then transported to the higher latitudes, contributing to the
weakening of deep oceanic circulation, thereby reinforcing the cooling
above
and around the North Atlantic.
Today, the fact that
global
warming could disturb the water cycle and lead to a slowing down of the
North Atlantic
circulation is a real subject of concern.
Oceanographic data from the last 50 years suggest that hydrographic
changes
(temperature and salinity) as well as a lessening of the flow of water
transported by certain surface and deep-sea marine currents have
already
occurred in the North Atlantic.
The risk of an
even greater variation of oceanic circulation by the end of this
century or the
beginning of the next needs to be taken seriously and actively studied.
##
Contact:
Monica
McCarthy
CNRS
33-196-445-191
monica.mccarthy@cnrs-dir.fr
This
text derived from:
http://www.cnrs.fr/index.html
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