Geoscience Reference
In-Depth Information
fluctuations in the Gulf Stream but do not enable us to learn much
about its evolution. The radials made between Greenland and Iceland
show evolution in the properties of deep waters, which are becoming
warmer. But there too, the data is still insufficient. In projections for
the next 100 years created with coupled models, the tendency is for a
slowing down of the meridional transport associated with
the overturning circulation, but it should be noted that the current
ocean models scarcely give this transport's magnitude, there is
therefore still some way to go.
Sensitivity studies have been carried out to test the ocean's
behavior in centennial climate projections. In particular, the rate at
which the ice melts is an important factor in the efficiency of oceanic
convection. The water issued from the melting ice is fresh and flows
at the surface, isolating the interior ocean from interactions with the
atmosphere. This thin surface layer prevents the phenomenon of
convection. Imposed on simple dynamic systems, this phenomenon
leads rapidly to a cessation of thermohaline circulation, which leads to
a cooling of northern regions. This sensitivity has been tested in the
framework of a coupled general circulation model with which two
climate change experiments have been carried out in parallel. These
two experiments were constructed from the current situation as far as
a doubling in the concentration of CO
2
, a concentration that then
remains constant for several hundred years. The only difference
between these two experiments came from the imposed melting rate of
the ice on the Greenland cap. The first experiment had no melting
rate; whereas for the second, a uniform melting rate was imposed. The
difference between these two experiments, at the end of 500 years, is
represented on Figure 2.24(b). It can be noted that the imposition of a
melting rate leads to a very significant cooling of the climate at high
latitudes, as much as 10°C in certain regions of northern Europe. The
oceanic convection zone, situated in this model to the east of
Greenland, shows a slight warming. These events indicate that the
thermohaline circulation slowed down a great deal and a more precise
diagnosis shows in fact that the cooling in this model is due to
feedbacks between clouds and atmosphere in the sea ice boundary
zone, more than to modification in the thermohaline circulation.
Moreover, when we look at the field of temperature anomalies
