A Simplified 3D Oceanic Model Assimilating Geostrophic Currents: Application to the POMME Experiment

Abstract

A simplified oceanic model is developed to easily perform cheap and realistic mesoscale simulations on an annual scale. This simplified three-dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis-derived geostrophic currents Ug into the momentum equation in substitution of the horizontal pressure gradient. Simplification is provided by a time sequence of Ug called guide, which is used as a low-resolution and low-frequency interpolator. This model is thus necessarily coupled to systems providing geostrophic currents?that is, ocean circulation models, analyzed/reanalyzed fields, or climatologies. In this model, the mass and currents fields are constrained to adjust to the geostrophic guide at all scales. The vertical velocity is deduced from the vorticity equation, which ensures the coherence between the vertical motion and the geostrophic structures evolution. Horizontal and vertical advection are the coupling processes that can be activated independently from each other and offer the possibility to (i) continuously derive a three-dimensional model when all processes are activated, (ii) understand how some retroaction loops are generated, and (iii) study development of structures as a function of the geostrophic environment. The model was tested during a 50-day lasting simulation over the Program Océan Multidisciplinaire Méso Echelle (POMME) experiment (northeast Atlantic Ocean, September 2000?October 2001). Optimal analyzed geostrophic currents were derived weekly during POMME from a quasigeostrophic model assimilating altimeter data. Comparison with independent in situ and satellite data indicates that this simulation is very realistic and does not drift, thanks to the prescribed geostrophic guide.