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Multiple equilibria in the thermohaline ocean circulation


The thermohaline circulation in the ocean is the result of the delicate interplay between two competing mechanisms: the
saline and the thermal buoyancy fluxes. Low surface
temperatures in the polar regions, and high temperatures in
the equatorial belt favour sinking at the poles and upwelling at the equator. This thermal forcing is opposed by the excess of evaporation over precipitation and runoff at low latitudes which induces a salt flux favouring sinking in the equatorial region and upwelling at the poles. Indeed in the North
Atlantic Ocean a direct circulation prevails, while in the
South Atlantic there is a weak indirect circulation. The net result is a single pole-to-pole cell so that the heat flux is everywhere northward, i.e. heat is transported towards the
equator in the southern hemisphere.

There is evidence from paleoceanographic data that the ocean meridional circulation was reversed during glacial times.
Moreover results of numerical simulations of the general
circulation suggest that the sensitivity of the system is
extremely acute. A variety of models, ranging from complex
coupled ocean-atmosphere simulations to simple 'box' models, has shown that the competition between thermal and saline
forcing results in multiple equilibria. Both senses of
circulation are possible as alternative stable solutions,
depending on small differences in the initial conditions.

We propose the study of 'intermediate models' where the
thermohaline circulation is viewed as the result of convection induced by large scale thermohaline forcing. Specifically, we wish to analyze two- and three- dimensional Boussiness
convection forced by prescribed surface temperature and
salinity flux boundary conditions. In these 'process' models a single mechanism is isolated and studied. The goal of these
models is not to produce realistic simulations of observed
phenomena, but to clarify the physical mechanisms that control processes such as the transition from a state with pole to
pole circulation to one with a cell in each hemisphere.

It is our belief that the study of these models is an
essential step towards the understanding of the oceanic
'conveyor-belt' and its role in the earth's climate.

Call for proposal

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31057 Toulouse

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