Tracer Leakage from Modeled Agulhas Rings

Abstract

In a numerical, isopycnal, ocean model the mixing is investigated with the environment of two idealized Agulhas rings, one that splits, and one that remains coherent. The evolution of a passive tracer, initially contained within the rings, shows that tracer leakage is associated with the formation of filaments in the early stage of ring evolution. These filaments reach down to the thermocline. In the deepest layers leakage occurs on a larger scale. Self-advection of the rings is very irregular, and it is not possible to compute a Lagrangian boundary in order to estimate the transport of leakage from the rings. To describe the processes that govern tracer leakage, in a coordinate frame moving with the ring a kinematic separatrix is defined in the streamfunction field for the nondivergent flow. Initially, filaments arise because of the elongation of the ring, which is mainly governed by an m = 2 instability that is collaborating with differential rotation. Because of beta, the symmetry is destroyed related to the separatrix associated with a stagnation point in the flow. The filament upstream of the stagnation point grows much faster and is associated with the bulk of tracer leakage. Mixing is enhanced by time dependence of the separatrix. As a result, there are no large differences between the leakage from a coherent ring, where the m = 2 instability equilibrates, and from a splitting ring, where the m = 2 instability keeps growing, which confirms that the amount of leakage is mainly governed by the ring's initial deformation combined with unsteady self-advection of the ring and not by the splitting of the ring. The decay of tracer content in the thermocline shows that in the first months up to 40% of the ring water can be mixed with the environment. In deeper layers the decay of tracer content may reach up to 90%.