JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C03010, doi:10.1029/2009JC005585, 2010 Click Here for Full Article

On the fast decay of Agulhas rings Erik van Sebille,1,2 Peter Jan van Leeuwen,1,3 Arne Biastoch,4 and Wilhelmus P. M. de Ruijter1 Received 19 June 2009; revised 24 August 2009; accepted 14 October 2009; published 6 March 2010.

[1] The water that ends up in the detaches from the retroflection predominantly in the form of Agulhas rings and cyclones. Using numerical Lagrangian float trajectories in a high‐resolution numerical ocean model, the fate of coherent structures near the Agulhas Current retroflection is investigated. It is shown that within the Agulhas Current, upstream of the retroflection, the spatial distributions of floats ending in the Atlantic Ocean and floats ending in the Indian Ocean are to a large extent similar. This indicates that Agulhas leakage occurs mostly through the detachment of Agulhas rings. After the floats detach from the Agulhas Current, the ambient water quickly looses its relative vorticity. The Agulhas rings thus seem to decay and loose much of their water in the Cape Basin. A cluster analysis reveals that most water in the Agulhas Current is within clusters of 180 km in diameter. Halfway in the Cape Basin there is an increase in the number of larger clusters with low relative vorticity, which carry the bulk of the Agulhas leakage transport through the Cape Basin. This upward cascade with respect to the length scales of the leakage, in combination with a power law decay of the magnitude of relative vorticity, might be an indication that the decay of Agulhas rings is somewhat comparable to the decay of two‐dimensional turbulence. Citation: van Sebille, E., P. J. van Leeuwen, A. Biastoch, and W. P. M. de Ruijter (2010), On the fast decay of Agulhas rings, J. Geophys. Res., 115, C03010, doi:10.1029/2009JC005585.

1. Introduction leakage crosses the equator [Donners and Drijfhout, 2004]. A large portion recirculates in the South Atlantic Ocean [2] The entrainment of Indian Ocean water into the subtropical gyre [De Ruijter and Boudra, 1985]. This latter Atlantic Ocean is called Agulhas leakage and its source is the route is closely connected to the subtropical supergyre Agulhas Current, the western of the Indian [Speich et al., 2002], a gyre encompassing both the Indian Ocean subtropical gyre [Gordon, 1985; Lutjeharms, 2006]. Ocean and the Atlantic Ocean (and possibly the Pacific Of the volume flux carried by the Agulhas Current, however, Ocean too). Knorr and Lohmann [2003] and Peeters et al. only approximately 25% takes part in this interocean ex- [2004] have shown that the magnitude of Agulhas leakage change. Once the Agulhas Current has detached from the in the southeast Atlantic Ocean is a proxy for changes in the continental slope, potential vorticity conservation causes the Atlantic meridional overturning circulation. This considered, westward flowing current to retroflect. The bulk of the water Agulhas leakage is here defined as the flux of water carried in the Agulhas Current in then advected back into the Indian by the Agulhas Current at 32°S that does not retroflect back Ocean [De Ruijter et al., 1999]. into the Indian Ocean but ends up in the Atlantic Ocean. [3] One of the reasons why Agulhas leakage is relevant is [4] The water which constitutes Agulhas leakage was because of its role of transporting heat and salt from the suggested to be mainly trapped inside large anticyclones Indian Ocean to the Atlantic Ocean. Once advected north- called Agulhas rings [Gordon, 1986]. These rings can be ward in the Atlantic Ocean, this heat and salt can influence tracked in altimetry data to move into the Atlantic Ocean, the Atlantic meridional overturning circulation [Weijer et al., while decaying and releasing their he