Agulhas cyclones

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It has previously been recognised that cyclonic motion on the landward border of the Agulhas Current is inherent in meanders such as Natal Pulses and shear edge eddies. To date no direct measurements have been made to study such cyclonic features. We present observations on the characteristics and the behaviour of the cyclones that are generated at the edge of the Agulhas Current using altimetry, RAFOS floats on two surfaces, hydrographic data as well as thermal infrared satellite images. It is shown that cyclones associated with Natal Pulses as well as with shear edge features move downstream, are shed into the South Atlantic, and often are instrumental in triggering the occlusion of an Agulhas ring. The surface thermal expression as well as the sea-surface height of Agulhas cyclones are shown to be very closely tied to their Lagrangian movement at depth. The speed of downstream translation of cyclones lies between 10 km d-1 for a Natal Pulse and 17 km d-1 for a shear edge eddy. The tangential speeds are up to 100 cm s-1 on the 26.8σθ surface and up to 80 cm s-1 on the 27.2σθ surface; the rotational period lies between 5 and 7 d (mesoscale pulses) and is around 2-3 d for sub-mesoscale pulses. Float pressure records indicate across-stream vertical displacements of more than 500 m. Once shed south of Africa Agulhas cyclones seem to disintegrate rapidly in the Cape Basin or at the subtropical convergence. © 2002 Elsevier Science Ltd. All rights reserved.

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Deep-Sea Research Part II: Topical Studies in Oceanography