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Rings and eddies off the tip of Africa

Meso-scale oceanic processes are defined by their size, from a few tens to a few hundred kilometres; they play a major role in the dynamics of oceans and their function of transport of energy, heat and many dissolved substances. Eddies are one of their most common form; these structures are shed by a water mass and enclose this original water in a rotation movement while following their own path for several months.

Since the 1980s, eddies are known to transport an important amount of heat from the tropical latitudes towards the Southern ocean, particularly around the tip of Africa, where the mighty "Agulhas current" carries its warm water along the east coast of South Africa. Several kinds of eddies are found in this region, with different origins which determine the direction of rotation and the main trajectory.


Off Southern Africa, cyclonic (red) or anticyclonic (blue) eddies are formed from Agulhas current or continental slope waters, resulting in a highly turbulent dynamics north of the Antarctic Circumpolar Current.

Whereas these structures are easily identifiable by satellite data, there are few opportunities to study them directly on the field. In February 2008, the route of R/V Marion Dufresne intersected two of these eddies, very close to each other, rotating in opposite directions and apparently coupled. Together with satellite data, the many in-situ measurements made allowed a better understanding of their structure, origin and history.

The anticyclonic eddy (counter-clockwise in the Southern hemisphere) called M is particularly apparent on vertical sections of salinity and temperature. It contained a water body about 150 km wide and more than 500 m deep, warmer and saltier than the surrounding ocean. Some of its hydrologic properties are found only in subtropical latitudes, showing that it is a so-called "Agulhas ring" detached from the returning branch of Agulhas current. Even though the subtropical waters it contained progressively cooled as it moved among cooler waters, it was still releasing heat to the atmosphere and to the surrounding ocean. Its rotation speed reached 1 m/s (2 knots) and was still detectable down to 2000 m below the surface, thus making possible the interaction with submarine mountains of the Agulhas Ridge.


Vertical section (from surface to 2000 m along the route of the Marion Dufresne) of temperature (left, °C) and salinity (right, grams/litre). Notice the large difference between horizontal and vertical scales: the eddy is actually 200 to 300 times wider than it is high.

The retrospective analysis of sea surface height maps obtained through altimetry satellites allowed a reconstitution of its history. Around May 20 2007, nine months before the cruise, a 150 to 200 km-wide eddy was shed by Agulhas Current and twice splitted into smaller rings. One of them is eddy M, which followed an irregular track with periods of immobility and of faster movement, under the influence of bottom topography and hydrologic fronts. Until it disappeared from the surface, eddy M could be tracked during fifteen months. Its properties and history appear rather typical of one the eddy families originating from Agulhas Current.


Vessel track (brown line), position of eddies M and S when intersected, and complete track of eddy M (blue line) from beginning (D) to end (F).

In spite of its smaller size, cyclonic eddy named S, clearly appears on vertical sections of salinity, dissolved oxygen and other water mass properties, down to about 900 m. Once again, this signature is typically subtropical, but with a different origin then M; eddy S probably comes from oxygen-depleted water flowing south along the continental slope of South African east coasts. It was born around October 10 2007, and its life duration was about eight months. After a sinuous track, it joined eddy M and kept in close association with it (they turn in opposite directions), and this is possibly what helped it to cross the underwater mountain ridge.


Vertical section (from surface to 2000 m along the route of the Marion Dufresne) of dissolved oxygen concentration.

These two examples illustrate the capacity of eddies to transport into the subantarctic region properties of subtropical of continental-slope waters and to interfere with hydrologic structures (fronts between water masses) and ocean bottom relief.


The paper

Arhan M., Speich S., Messager C., Dencausse G., Fine R., Boye M., 2011. Anticyclonic and cyclonic eddies of subtropical origin in the subantarctic zone south of Africa. Journal of Geophysical Research, 116, C11004, doi:10.1029/2011JC007140.
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The authors

This work was conducted by scientists from IUEM (Laboratoire de physique des océans and Laboratoire des sciences de l'environnement marin, Lemar) and from the University of Miami.


The journal

Journal of Geophysical Research is the flagship journal of the American Geophysical Union. Over its 115 years of continual publication, it has adapted to meet the needs of multidisciplinary science.It now has seven disciplinary sections, one of which is dedicated to oceans. JGR-Oceans embraces the application of physics, chemistry, biology, and geology to the study of the oceans and their interaction with other components of the Earth system.



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