Geoscience Reference
In-Depth Information
icebergs very far south. These events, called Heinrich events, indicate
that oceanic circulation has without doubt experienced important
modifications and suggests that the cause of the instability is linked to
thermohaline circulation; this phenomenon of instability in thermohaline
circulation, well marked in the data from the last Glacial Maximum, is
it susceptible to being reproduced today, in the context of a climatic
warming that might destabilize the current caps?
Theoretical studies have focused on control phenomena for the
formation of deep water in particular regarding the surface exchanges
that fix the extremes of temperature and salinity which lead, or not, to
convection. The two variables, salinity and temperature, contribute to
the phenomenon of convection: the temperature is fixed by exchanges
of heat fluxes between the ocean and the atmosphere and in return it
acts strongly on the flux itself (negative feedback on the fluxes of
latent heat and sensible heat); the salinity depends on the sum of
“evaporation minus precipitation” whose value does not depend, or
depends very little, on the salinity: there is no feedback between the
fields. This differentiation in the formulation of forcing has important
consequences on the dynamic system that flows from it. The fact that
the oceanic stratification depends on two variables - one controlled,
the other not controlled - introduces a solution with chaotic character:
a single flux, “evaporation minus precipitation”
can lead to different
solutions for the oceanic convection zones. The current situation is
that of an active North Atlantic, but it could turn to an active Pacific,
with important consequences in terms of heat transport in the northern
hemisphere. The phenomenon that has been called “thermohaline
catastrophe” following the work of Stommel corresponds to a
cessation in the formation of deep water in the North Atlantic. This
phenomenon has without doubt happened in a glacial climate,
following an extension of sea ice over the formation zones of deep
water, thus intensifying the evolution of very cold conditions, but
could this occur in the near future?
The observations and projections for the future climate are
scrutinized to discover how the Gulf Stream would evolve as well the
northward heat and mass transports. The observations are still too
sparse to reconstruct a long history. Data from recent years show
