Ein Inversmodell fuer den Suedatlantik mit der Methode der finiten Elemente

Abstract

The large scale flow field of the South Atlantic and its associated heat and fresh water budget are studied with an inverse model. The model relies on traditional assumptions of mass, heat and salt conservation. A 3-dimensional velocity field which is in steady state and obeys geostrophy is derived. Using this flow field, the steady state advective-diffusive equations for temperature and salinity are solved and the corresponding density is calculated. An optimization approach is used that adjusts reference velocities such that modeled temperature and salinity are close to observations and that velocities are in geostrophic balance with the modeled density field. In order to allow for a variable spatial resolution, the finite element method is used. Its mesh is totally unstructured and the 3-dimensional elements are tetrahedra. Climatological hydrographic data, observations of sea surface height (SSH) from satellite altimetry and wind data are assimilated in the model. The advantages of the finite element method make it possible to use an easy representation of the model parameters on the tetrahedra. It is not difficult to find the adjoint form of the discrete equations. The unstructured mesh agrees well with the complex geometry of bottom topography. The model results show, that the reference velocities return the structure of the SSH data. In general the upper-level circulation corresponds to the circulation known from the literature. The volume transport through Drake Passage is constrained to be 130 Sv. At the open boundaries (Drake Passage, 30S, 20E) the mass, heat and salt transports are in agreement with the literature. This is also true for the transports of surface, intermediate, deep and bottom water.9.6 Sv of bottom water is formed in the South Atlantic.