Geoscience Reference
In-Depth Information
seasonal to decadal scales. This variability masks the anthropic signal
as their signatures are too interlinked to allow for detecting changes
and attributing their causes. Moreover, the temperature and the
salinity are not passive tracers, but they fully participate in the oceanic
dynamics, by acting on the pressure forces. In a warming climate, with
a more significant warming at the Poles than at low latitudes, the
meridional heat transfer should slow down; it is expected that the
overturning circulation will diminish too. This tendency has been
searched for in oceanic projections produced with coupled general
ocean-atmosphere circulation models and it seems to be effectively
confirmed with time. But the observations are still far too insufficient
for a conclusion to be drawn concerning the dynamics, notably in the
North Atlantic. Repeated sections are programmed at different
latitudes to pursue this monitoring, they show warmer waters at the
bottom in the subpolar region, but do not allow conclusions on the
evolution of mass and heat transports in the North Atlantic, taking
account of the large spatial and temporal variability in the transports
along the western boundaries of the oceans. In the Mediterranean, the
climate warming should lead to a decrease in the formation of deep
waters and a modification of their properties, which seems to be
observed in this basin considered, as a critical zone of change on
account of the significant risk of general drought in its perimeter.
Today, the ocean absorbs nearly half the anthropic carbon
emissions, through solubility and by absorption in the biosphere. The
solubility depends on the surface temperature: a hot ocean will absorb
less carbon than a cold ocean. The absorption by the surface biosphere
will lead to a carbon export from surface layers into the deep ocean
through detritus and the deaths of organisms. The anthropic carbon,
buried in the depths, will supply sediments but a part will return to the
surface, in a few hundreds or thousands of years, according to the
efficacy of the global circulation; from where comes the importance of
detecting the modifications underway at the surface as well as in
subsurface, and of understanding the reasons.
One particularly studied region is the Austral Ocean. For several
decades, and in particular since the 1970s, the Southern Annular Mode
has been in a positive phase, which tends to increase the zonal wind
