OC4331-Mesoscale Oceanography
Final Project Summary

Topic Area

LCDR Jerel R. Johnson, USN

The Agulhas Rings are formed from the interaction of three major currents. The Agulhas Current, a western boundary current, the South Atlantic Current, and the Benguela Current. In the Agulhas Retroflection, the Agulhas Current (the boundary current flowing southwest next to the southeast African coast) runs into the eastward flow of the Antarctic Circumpolar Current (ACC) and the northeastward flow of the Benguela Current. This collision of currents causes the Agulhas flow to bend almost completely back on itself, producing a turbulent zone of mixing. Rings formed in the Agulhas Retroflection have been observed spinning into the southern Atlantic and migrating all the way to the South American coast. This process helps transport warmer and more saline Indian Ocean waters into the southern Atlantic.

The dominant mode of variability of the Agulhas Current is in the form of natal pulses These are large solitary meanders containing a cold-core cyclone on the inshore side of the. Natal pulses occur about 6 times per year and propagate downstream at approximately 10 km/day. The passage of nearly all natal pulses is followed by the spawning of an Agulhas Ring.

The most important aspects of Agulhas rings include:

• Agulhas Rings are as deep as 1200.
• Agulhas retroflection region not only spawns large (200-320 km) anti-cyclonic Agulhas Rings, but also smaller (120 km) cyclones.
• Depending on their translational speed and size, the rings trapping depth probably reaches as deep as 1100 m, thus including the AAIW layer.
• Between 2 and 9 rings are formed per year.
• From the retroflection region they drift northwestward into the south Atlantic, traveling at a speed of approximately 5-8 km per day.

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Figure 1. Temperature and salinity cross-section of an Agulhas Ring.

Figure 2. Agulhus current and Natal pulses which spawn Agulhas Rings.

Lutieharms, J. R. E, b, 1981: Features of the southern Agulhas Current circulation from satellite remote sensing. S. Afr. J Sci., 77, 231-236, 1981 S. African J. Sci.., Vol. 77, 231-236.

Van Leeuwan, P. J., W. P. M. De Ruijter and J. R. E. Lutjeharms, 2000: Natal pulses and the formation of Agulhas rings. J. Geophys Res.., Vol. 105, 6425-6436.

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