Vendredi 7 décembre - Dynamics of counter-rotating gyres forced by a mass-source and a mass-sink Education and training activities within the Joint French-South-African laboratory ICEMASA
Dec 07, 2012
from 11:00 AM to 12:00 PM
Steven Herbette (Université de Brest)
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Laboratoire de Physique des Océans, UMR6523 CNRS-IFREMER-IRD-UBO, Brest, France, LMI ICEMASA, Department of Oceanography, University of Cape Town, Rondebosch, South Africa
Titre : Dynamics of counter-rotating gyres forced by a mass-source and a mass-sinkEducation and training activities within the Joint French-South-African laboratory ICEMASA*
Résumé : The following seminar is divided into two parts. The first part (~2/3) presents our understanding of the dynamics of counter-rotating gyres on the beta-plane. The second part (~1/3) resumes our achievements and future objectives, in terms of training and education, within the ICEMASA Joint French/South African Laboratory. A more detailed content of both parts is given below.
1. Gyres are ubiquitous patterns of the Ocean Circulation, that can be found at all depths, in all oceanic basins. A simple schematic of the surface circulation shows the coexistence, in most ocean basins of polar cyclonic, sub-tropical anticyclonic and tropical cyclonic gyres. The present study seeks for the understanding of an intriguing phenomenon when using a source and a sink to force a double gyre within a rotating tank on an inclined plane. On one hand, when the anticyclonic gyre is located poleward of the cyclonic gyre, the circulation becomes unstable at moderate forcing. On the other hand, reversing the position of the gyres stabilizes the system, even at high forcing. Here, the behavior of counter rotating gyres forced by a localized mass-source and sink is studied in the framework of a shallow-water beta plane model. Understanding the stability of these simple circulations is probably an important prerequisite to rationalize the much more complex eddy and wind driven mean gyres of the ocean. A linear analytical solution is found. It consists of a dipolar gyre, composed of one central zonal jet that connects to two external zonal jets through western boundary currents. Non linear numerical experiments are performed varying the intensity of the forcing, inverting the position of the source and sink, and changing the distance between the source and the sink. Results confirm qualitative observations found in previous laboratory experiments: i) When the central zonal jet is directed westward, the system becomes unstable and water mass is exchanged between the two gyres; ii) When the central zonal jet is eastward, the system remains stable, even at high pumping/injection rates, and the two gyres remain isolated from each other. We investigate the origin and the dynamics of that instability using potential vorticity arguments. Results show that within the unstable configurations, the necessary condition for instability (PV-gradient inversion) derived by Charney and Stern (1962) is always verified inside the westward zonal jet. On the other hand, stable configurations do not complete the criteria. Although western boundary currents can in some cases fulfill the criteria, our simulations do not seem to show any instabilities originating from the latter. To discriminate whether the observed instability comes from the perturbations released by the source and the sink or whether it is linked to the intrinsic instability of the zonal central jet, a linear stability analysis of the mean state is performed. It confirms the presence of unstable modes. The growth of these modes triggers a Hopf bifurcation of the system.
2. The Joint French South-African Laboratory ICEMASA is a joint venture between IRD, CNRS, the University of Brest and the University of Cape Town. It was built on a strong “Education and Training” component that I have been in charged of strengthening and developing for the last two years. After briefly summarizing our achievements, here at the University of cape Town, in terms of integrating new course works, training master and Phd students, developing a co-badging agreement with the University of Brest, a brief presentation of two more specific ongoing research collaborations will be given. One concerns the Benguela Upwelling System and the other the dynamics of the Southern Ocean.
* : International Centre for Education, Marine and Atmospheric Sciences over Africa