Agulhas Return Current
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The Agulhas Return Current (ARC) is the intense flow along the Subtropical Convergence south of Africa. It forms the connecting link between the generically similar South Atlantic Current and the South Indian Ocean Current, thus contributing to the water exchange between these two basins. It originates with the almost complete retroflection of the Agulhas Current (AC) south of Africa. As the ARC emerges from the retroflection, its velocity and volume transport (75 cm/s, 54×106 m3/s) are decreased from those of the AC (110 cm/s, 70×106 m3/s). The Indian Tropical Surface Water characteristics of the AC are also removed in the retroflection, and the central water masses are somewhat freshened via admixture with Atlantic waters. The nascent ARC almost immediately executes an equator-ward meander of variable extent over the Agulhas Plateau, and then proceeds eastward.
On average the ARC lies at a latitude of 39o30’S south of Africa, increasing slowly downstream to a latitude of 44o30’S at 60oE, except where it crosses a number of meridional ridges where northward shifts of up to 2o30’ are occasionally observed. Geostrophic speeds relative to 1500 m demonstrate a gradual eastward decrease in the velocity of the current from an average of 75 cm/s at the Agulhas retroflection to 13 cm/s at 76oE. Volume transports are similarly reduced from 54×106 m3/s in the retroflection region to 15×106 m3/s at 76oE, although the θ/S characteristics remain remarkably unchanged (θ = and S= ). The distinct ARC velocity core disappears between 66oE and 70oE, and its characteristic South Indian Subtropical Surface Water is not found east of 61oE. Given this evidence, Lutjeharms and Ansorge [2001] suggest that the ARC be called such from the Agulhas retroflection region to the Crozet Basin, with the flow to the east of the basin to be called the South Indian Ocean Current.
Lutjeharms and Ansorge [2001] comment on an interesting correspondence and provide fodder for future dissertations:
It is remarkable that the extent and location of the Agulhas Return Current, typified in this way, corresponds almost exactly with one of the largest contiguous regions of high mesoscale variability in the world ocean. All these altimeter results show a tongue of very high mesoscale turbulence starting at the Agulhas retroflection and extending eastward along the path of the Agulhas Return Current, as described here, to terminate over the Crozet Basin. The dimensions and geographical location of this band of high current variability is seen in many other data sets as well, e.g. drifting buoys and sea surface temperatures. It is therefore a notably persistent characteristic of global ocean circulation. Since it is so well correlated with the course of the Agulhas Return Current, it is most probably due to enhanced levels of meandering and eddy shedding along the Subtropical Convergence, where the Agulhas Return Current’s presence causes increased horizontal shear and instability. This needs further investigation.
See Also
- Agulhas Basin
- Agulhas Current
- Mozambique Channel
Further Reading
- Peter Saundry. 2011. Seas of the world. Topic ed. C.Michael Hogan. Ed.-in-chief Cutler J.Cleveland. Encyclopedia of Earth
- R. G. Peterson and L. Stramma. Upper-level circulation in the south atlantic. Prog. Oceanog., 26:1–73, 1991.
- J. R. E. Lutjeharms and I. J. Ansorge. The Agulhas Return Current. Journal of Marine Systems, 30: 115–138, 2001.
