Generation and Growth Mechanism of the Natal Pulse

Abstract

The Natal pulses, solitary cyclonic meanders in the Agulhas Current, are reproduced in an ocean general circulation model. The model covers the region around the Agulhas Current with a grid fine enough to reproduce major eddies. The features of the reproduced Natal pulses are consistent with observational evidences in the following respects: they are generated at the Natal Bight when anticyclonic eddies come, move downstream along the Agulhas Current at speeds about 20 km day−1, and grow in its horizontal size as they move. The present simulation shows that the generation and growth of the Natal pulse occurs because of the interaction between the mean flow of the Agulhas Current and an anticyclonic eddy. A supplemental simulation, where the topography of the Natal Bight is modified, indicates that the topography of the Natal Bight does not cause the generation of the Natal pulses, contrary to a previous suggestion.

A New Model of Current Retroflection Applied to the Westward Protrusion of the Agulhas Current

“…According to Biastoch et al (2009), the retroflection occurs at 178E during shedding and 238E (i.e., east from the Agulhas Bank) afterward. Similar scenarios can be seen in the Backeberg et al (2009) and Tsugawa and Hasumi (2010) simulations.…”

Decoupling of the Agulhas Leakage from the Agulhas Current

“…Chassignet and Boudra (1988) show that ring formation occurs through mixed barotropic-baroclinic instability. Tsugawa and Hasumi (2010) suggest that barotropic instability may drive increased Agulhas variability, through Natal pulses. We note a similar upstream response here, at higher inertia.…”

South African Climates (Agulhas LGM Density Profile)

“…Instabilities of the current are caused by the migration of eddies and meanders that interact with the bathymetry and morphology of the continental margin. An example is solitary meanders that form in the Natal Bight, the so-called Natal Pulses ( Figure F5) (e.g., Lutjeharms and Roberts, 1988;Beal and Bryden, 1999;de Ruijter et al, 1999b;van Leeuwen et al, 2000;Bryden et al, 2005;Biastoch et al, 2008;Tsugawa and Hasumi, 2010;Rouault and Penven, 2011). At the tip of Africa, the current veers briefly to the west, driven by its own inertia, and then retroflects back to the east into the Indian Ocean as the ARC (Lutjeharms and Van Ballegooyen, 1988).…”

“…At the tip of Africa, the current veers briefly to the west, driven by its own inertia, and then retroflects back to the east into the Indian Ocean as the ARC (Lutjeharms and Van Ballegooyen, 1988). Mozambique Channel and EMC eddies that propagate downstream with the Agulhas Current as strongly barotropic solitary meanders (Bryden et al, 2005;Biastoch et al, 2009a;Tsugawa and Hasumi, 2010) destabilize the retroflection. Four to six anticyclonic eddies (Agulhas rings) form per year with a radius of up to 400 km; they spin off the retroflection area carrying between 2 and 20 Sv of thermocline water to the South Atlantic Ocean (de Ruijter et al, 1999a;Richardson, 2007).…”