Modeling the initial, fast Sea-Surface Height decay of Agulhas ring "Astrid"

The early Sea-Surface Height (SSH) decay of an Agulhas ring is studied using a circular symmetric, equivalent barotropic idealization of ring Astrid, which was measured during the first MARE-cruise. Observations indicate that the SSH of Agulhas rings most rapidly decays just after shedding. It is found that the observed initial fast decay of ring Astrid can be recovered by a numerical model and that a mixed baroclinic/barotropic instability accounts for most of the observed decay of SSH. In addition, a series of numerical experiments is presented in which the effects on the decay of ring strength, barotropic component, diameter, radial profile, β, and cooling were investigated. In all cases discussed, rings are linearly unstable to an m = 2 mode. The evolution of the first 10 days is well predicted by a linear stability analysis. Further growth of the m = 2 mode leads in most cases to split-up of the ring. The SSH decay of the rings is associated with a conversion from available potential energy of the parent ring to kinetic energy of nearly barotropic higher modes. Most of the energy release is associated with the m = 2 mode. Also, the parent ring features an energy conversion from its barotropic to its baroclinic components. A strong barotropic component associated with a corotating ring is essential for SSH decay to occur. Counterrotating rings may feature SSH increase by energy conversion from the baroclinic to the barotropic component. For corotating rings SSH decay becomes weaker when the instability develops less vigorous. The simulation of ring Astrid shows that tracer loss from the core scales well with the decay of SSH. In the thermocline the associated mixing of fluid occurs preferably at the extremes of the elongating ring. At the deepest levels mixing is associated with dispersion through Rossby-wave radiation. © 2002 Elsevier Science Ltd. All rights reserved.