| This study deals with the non-linear cyclo-geostrophic adjustment of a circular density front in a two-layer fluid. Laboratory experiments have been performed to investigate the dynamical evolution of a fixed volume of buoyant water, initially confined within a bottomless cylinder, which is quickly released in a dense rotating fluid. This configuration corresponds to a rapid input of potential energy in a geostrophic fluid layer and reproduces some dynamical processes which occur during oceanic upwelling or stratospheric warming events. We focus our efforts on the visualisation techniques in order to have simultaneous and independent measurements of both the horizontal velocity field and the vertical density field. We thus obtained, for the first time, quantitative measurements of the potential vorticity and the flow balance after a geostrophic adjustment process. The density profile of the mean adjusted state observed in the experiment is in good agreement with the prediction of the standard adjustment theory based on lagrangian conservation of potential vorticity except in the frontal region. There, strong three-dimensional motions (plume structures, shocks and rapid transient instabilities) take place during the early stage of adjustment. These transient three-dimensional motions could dissipate up to 50% of the initial energy of the system, especially when the size of the initial density anomaly is close or larger than the deformation radius. Therefore, it significantly changes the velocity and the energy budget predicted by the standard Rossby adjustment. Both the kinetic energy of the mean adjusted state and the energy transferred to inertia-gravity waves modes are reduced by theses transient dissipative processes. During the adjustment, an important part of the initial potential energy is released to inertia-gravity waves modes. Theses modes exhibit a non-trivial structure where the kinetic energy fluctuations are concentrated in the frontal region while significant potential energy fluctuations may occur in the central region. For all the case studied, the first deviation of the density interface oscillate at the inertial frequency. Hence, the energy released to the wave modes during the adjustment is mainly concentrated at the inertial frequency. Afterwards, frequency doubling could occur for large amplitude wave. |
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