Why Are There Agulhas Rings?

Abstract

The recently proposed analytical theory of Nof and Pichevin describing the intimate relationship between retroflecting currents and the production of rings is examined numerically and applied to the Agulhas Current. Using a reduced-gravity 1½-layer primitive equation model of the Bleck and Boudra type the authors show that, as the theory suggests, the generation of rings from a retroflecting current is inevitable. The generation of rings is not due to an instability associated with the breakdown of a known steady solution but rather is due to the zonal momentum flux (i.e., flow force) of the Agulhas jet that curves back on itself. Numerical experiments demonstrate that, to compensate for this flow force, several rings are produced each year. Since the slowly drifting rings need to balance the entire flow force of the retroflecting jet, their length scale is considerably larger than the Rossby radius; that is, their scale is greater than that of their classical counterparts produced by instability. Recent observations suggest a correlation between the so-called ?Natal Pulse? and the production of Agulhas rings. As a by-product of the more general retroflection experiments, the pulse issue is also examined numerically using two different representations for the pulses. The first is a meander pulse (i.e., the pulse is similar to a meander) and the second is a transport pulse. It is shown that, in this model, there is no obvious relationship between the presence of Natal pulses and the production of rings.