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

Abstract

The dynamics of current retroflection and rings shedding are not yet fully understood. In this paper, the authors develop an analytical model of the Agulhas Current retroflection dynamics using three simple laws: conservation of volume, momentum balance, and Bernoulli's principle. This study shows that, for a retroflecting current with a small Rossby number, this theoretical model is in good agreement with numerical simulations of a reducedgravity isopycnal model. Otherwise, the retroflection position becomes unstable and quickly propagates upstream, leaving a chain of eddies in its path. On the basis of these findings, the authors hypothesize that the westward protrusion of the Agulhas retroflection and the local ''zonalization'' of the Agulhas Current after it passes the Agulhas Bank are stable only for small Rossby numbers. Otherwise, the retroflection shifts toward the eastern slope of the Agulhas Bank, where its position stabilizes due to the slanted configuration of the slope. This study shows that this scenario is in good agreement with several high-resolution numerical models.

Interannual Eddy Kinetic Energy Modulations in the Agulhas Return Current

“…The Agulhas Current system plays a critical role in interocean exchange between the Indian Ocean and Atlantic Ocean and the global climate system and has been studied extensively during the past decades (Arruda et al, ; Beal et al, ; Elipot & Beal, ; Leber et al, ; Lutjeharms, ; among others). Being a warm western boundary current in the Southern Indian Ocean subtropical gyre, the Agulhas Current is primarily driven by the large‐scale wind stress curl between the Southern Hemisphere westerlies and southeast trade winds (Lutjeharms, ).…”

Where and how the East Madagascar Current retroflection originates?

“…Figure 7a and 7b present characteristics of an early retroflection at 24.5 • S. This link between high volume transports and earlier retroflections is consistent with the results of previous theoretical work in the Agulhas system (Ou & De Ruijter, 1986), suggesting that the interaction between coastline curvature and high transport of the Agulhas Current tends to favour early retroflection. This finding is reinforced by Arruda et al (2014). These authors have shown that variations in the Rossby number of deformation seem to induce inertial and momentum imbalances of the Agulhas Current and, as consequence, generate an upstream retroflection (the so-called Agulhas Current early retroflection).…”

“…de Ruijter et al (2004) and Palastanga et al (2007) added that the interactions of the westward-propagating mesoscale eddies drifted from the Indian Ocean between 20 • S and 30 • S with the mean flow of the EMC appearing to initiate the formation of a retroflection. However, Ou and De Ruijter (1986) and Arruda et al (2014) were the few studies suggesting a theoretical explanation of the retroflection in the South Indian Ocean. Ou and De Ruijter (1986) investigated the key processes responsible for the Agulhas Current retroflection.…”

“…Using one-and half-layer models, they explained the formation of a meander initiating retroflection due to the interaction between the Agulhas Current intensity and the continental margin. Arruda et al (2014) suggested that the increase of the basic eddy volume associated with variation of β-effect and the net mass flux going into the eddies explained the Agulhas Current retroflection dynamics from an analyt-…”

Where and How the East Madagascar Current Retroflection Originates?

“…de Ruijter et al (2004), andPalastanga et al (2007) added that the interactions of the westward-propagating mesoscale eddies drifted from the Indian Ocean between 20° and 30°S with the mean flow of the EMC appearing to initiate the formation of a retroflection. However, Ou and De Ruijter (1986) and Arruda et al (2014) were the few studies suggesting a theoretical explanation of the retroflection in the South Indian Ocean. Ou and De Ruijter (1986) investigated the key processes responsible for the Agulhas Current retroflection.…”