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A multi-model study of the restratification phase in an idealized convective basin
Clément Rousset, Marie-Noëlle Houssais
LOCEAN-IPSL
(Abstract received 07/15/2007 for session X)
ABSTRACT
The representation of baroclinic instability in numerical models is much dependent upon the model physics and major differences are found when the discretization of the governing dynamical equations is performed in isopycnic or cartesian coordinate systems. This dependency is explored in the context of the restratification of an idealized convective basin using three models: a z-coordinate model (OPA), an isopycnic model including a mixed layer (MICOM), and its adiabatic version in which the mixed layer physics are removed (MICOM-ADIAB). In the latter model, the convective layer is represented by a deep adiabatic layer outcropping at the surface rather than a thick dense mixed layer. The OPA simulation shows much larger eddy kinetic energy than the other two simulations because of stronger baroclinic instability growth and persistency. This leads to the most rapid restratification among the three simulations. Heat is brought in the convective zone mainly through the export of cold water filling the core of cyclonic eddies having a strong barotropic structure. The eddy heat flux expands inward and outward from the boundary of the convective patch with the same time scale. In MICOM-ADIAB, the more baroclinic dynamics introduce a strong contrast between the surface and the dense waters. Heat is transported by anticyclonic eddies and the dense water spreading occurs at a shorter time scale than the warm surface capping. OPA generates an overestimated volume of intermediate water which does not occur in MICOM-ADIAB where instead, the volume of dense water is conserved. In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of the mixed layer, maintaining a very weak level of baroclinic instability.
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2007 LOM Workshop, Bergen, Norway, August 20-22, 2007