PROJECT SUMMARY REPORT

Project 1

Project Title	Frictional and nonfrictional decay of surface frontal anticyclonic vortices in a stratified environment
Name of Group Leader	Dr. Angelo Rubino
Home Laboratory	Institut für Meereskunde der Universität Hamburg, Troplowitzstr. 7, 22529 Hamburg, Germany
E-mail address	rubino@ifm.uni-hamburg.de
Telephone	0049-40-42838-6518

1. Project objectives (no more than 10 lines)

The experiments are aimed at improving our knowledge on the dynamics of geophysical surface frontal axisymmetric anticyclonic vortices, in particular on their quasi-periodic oscillations and their energy decay due to frictional dissipation and to nonfrictional transfer via internal wave radiation as well as baroclinic instability formation. A comparison between results obtained experimentally and results obtained using a reduced-gravity frontal numerical model and a two-active-layer frontal numerical model enable us to assess the appropriateness of the reduced-gravity assumption in the description of geophysical surface frontal axisymmetric anticyclonic vortices as well as the capability of numerical models to describe the temporal and spatial evolution of surface frontal features.

2. Main achievements and difficulties encountered (no more than 20 lines)

The eddies were produced by lifting a two-meter-radius, bottomless cylinder which had been partially filled with buoyant fluid. This technique proved to represent a valid system of producing surface vortices in laboratory. In the totality of the performed experiments we were able to observe, for the first time to the best of our knowledge, inertial oscillations of surface vortices. The produced vortices showed in fact pulsations in which contractions and deepenings, expansions and shoalings alternated during an (exact) inertial period. These oscillations have been prevised theoretically and simulated numerically, but they had never been measured previously in situ or in laboratory. In the experiments carried out in a stratified environment we observed also the generation, due to the vortex pulsation, of near-inertial internal waves which propagated toward the tank periphery. These waves contributed substantially to the decay of the vortex pulsations. The produced vortices lost their coherency due to baroclinic instabilities which developed from large meanders at the eddy rim. Aspects of the dynamics observed experimentally were reproduced using a reduced-gravity nonlinear, hydrostatic, frontal numerical model. In this model a special technique for the numerical treatment of movable lateral boundaries allows for the description of eddy contractions and expansions. Good agreement was obtained for the initial stage of the vortex evolution. Its further evolution is however dominated also by the dynamics of the ambient layer and cannot be described using a reduced-gravity model. In this case, a two-active layer extension of the reduced-gravity frontal numerical model was able to reproduce the vortex long-term behavior including baroclinic instability formation. The obtained results will be presented at the General Assembly of the European Geophysical Society which will take place in Nice, France, during March 2001. A paper to be submitted to an international journal is in preparation