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D. M. G. C. Luz (Observatorio Astronómico de Lisboa), F. Hourdin (Laboratoire de Meteorologie Dynamique du CNRS, Paris)
Numerical modeling of Titan by Hourdin et al. (Icarus, Vol. 117, p. 358, 1995) has suggested that the mean zonal circulation at 1 mb may be barotropically unstable. Their results indicate that large scale barotropic eddies may carry angular momentum equatorward from high latitudes, thereby closing the angular momentum budget, and may play an important role in the transport and mixing of chemical species and in the maintenance of the state of superrotation of the stratosphere.
We have used a 2-dimensional, latitude-longitude model of the shallow-water equations on the sphere to investigate the dynamics of the interaction between transient eddies and the mean flow. High-resolution numerical experiments seem to show that a barotropically unstable latitudinal wind profile may be kept in steady state by thermal forcing, against the transport of angular momentum by the eddies. Furthermore, the mixing of trace species by the global circulation appears to depend strongly on latitude: tracer transport from the poles to lower latitudes is negligible, whereas there is considerable mixing by eddies up to some 60 degrees, where the unstable region lies. The possibility of this being due to polar vortex confinement will be discussed.
This work is part of a global study of the net effect of eddies on the meridional transport of angular momentum and trace species. Its main goal is to develop a parameterisation of the effect of eddies, to be used in a 2-dimensional, latitude-altitude model of Titan coupling atmospheric dynamics, aerosol physics and chemistry. Such a model will be a cost-effective alternative to coupled 3D models, which are computationally very expensive.
The author(s) of this abstract have provided an email address for comments about the abstract: dluz@oal.ul.pt