A North Atlantic deep-water eddy in the Agulhas Current system

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Abstract

One hundred and twelve stations of CTDO2 and LADCP were collected in the Agulhas Current system as part of the Agulhas Undercurrent experiment (AUCE) in March 2003. Along an offshore section, at approximately 35.6°S and 27.3°E to the northwest of the tip of the Agulhas Plateau, an unusual feature was revealed between 2200 and 3500 m depth, imbedded in the northward moving NADW layer. An anomalously high salinity of 34.83, 0.03 saltier than the surrounding water, was observed. Maximums in the potential temperature and oxygen were also found, with isotherms dropping by about 250 m over 50 km and a doming of the oxygen layers. From the convex lens structure of the neutral surfaces, we conclude that we sampled an anticyclonic eddy of NADW. Since the LADCP data reveal deep velocities up to 20 cm s−1, yet no anticyclonic circulation, whereas the geostrophic velocity referenced to the bottom shows a weak anticyclonic circulation, we inferred that we sampled the outer edge of the eddy and not its core. From an analysis of the water properties within the eddy and a comparison with known properties in the SE Atlantic Ocean and SW Indian Ocean, we conclude that the eddy was formed in the Agulhas Retroflection region. We speculate that the eddy was the result of an instability in the NADW slope current, which flows from the SE Atlantic around the Agulhas Bank. A deeply penetrating Agulhas Ring spun up the deep waters, pinching off an eddy, which later detached from the slope current and was carried southward. Once offshore, it coupled with the surface Agulhas Return Current, whose meandering path advected the eddy northeastward and ejected it over the Agulhas Plateau.

Introduction

It has been shown that the exchange of water masses between the Atlantic and Indian Oceans is a key component of the global thermohaline circulation. As the Agulhas Current separates from the Agulhas Bank at the southern tip of Africa, it retroflects and periodically sheds large eddies with diameters as large 500 km, known as Agulhas Rings. The surface and intermediate inter-ocean exchanges are dominated by the migration of Agulhas Rings from the Agulhas Retroflection region into the Cape Basin of the eastern South Atlantic (Boebel et al., 2003, Boebel et al., 2003). The majority of the Agulhas Current retroflects and flows east back into the Indian Ocean as a meandering jet known as the Agulhas Return Current (ARC) (Boebel et al., 2003, Boebel et al., 2003). Although Agulhas Rings have been observed to reach up to 4000 m depth (McCartney et al., 1991; van Veldhoven, 2005), they are identified as surface features. Once the Agulhas Rings drift into the Atlantic Ocean, they significantly alter its heat and salt budgets (de Ruijter et al., 1999; Weijer et al., 2002).

Most eddies observed in the ocean are surface-intensified, although sub-surface eddies, such as the Mediterranean Water eddies (Meddies), which are formed at depths between 500 and 1500 m (Richardson et al., 2000), are also familiar. However, observations of deep-water eddies are rare. Lately some evidence of deep eddies has been found in the Brazil Basin (Weatherly et al., 2002; Dengler et al., 2004). In particular, Dengler et al. (2004) observed that the Deep Western Boundary Current (DWBC), which flows southward along the continental slope of South America carrying North Atlantic Deep Water (NADW), breaks into eddies at about 8°S, which continue to advect south along the western boundary. They speculate that the eddies are generated during periods of strong meridional overturning circulation (MOC), while during periods of weak MOC, the DWBC continues as a more laminar flow south of 8°S. Model results suggest that baroclinic and barotropic instabilities are the responsible mechanism for the eddy generation.

In this study, we report the observation of an unusual, deep feature within the Agulhas Current system, sampled during the Agulhas Undercurrent Experiment (AUCE). We identify the feature as an anticyclonic eddy containing NADW from the SE Atlantic. Contrary to the Brazil basin, there is no DWBC in the Southwestern Indian Ocean, probably owing to the rapidly shoaling topography towards the north. Instead, there is an Agulhas Undercurrent, whose core is positioned against the continental slope at around 1200 m depth, within the intermediate water layers and inshore of the Agulhas Current (Beal and Bryden, 1997). Therefore, it was unexpected to find a NADW eddy in the absence of an energetic DWBC.

In earlier studies of Reid (1989) and Toole and Warren (1993) it had been proposed that the deep boundary current separated from the continental slope near 20°S to continue eastward in the Agulhas return flow. Arhan et al.(2003) and van Aken et al.(2004) showed evidence of a NADW slope current along the African continental slope. Specifically, van Aken et al.(2004) followed the deep salinity maximum associated with the NADW core in a narrow band along the African continental slope from the SE Atlantic near 30°S, around the tip of the Agulhas Bank to the Mozambique Channel east of Africa and estimated that around 2 Sv of upper NADW flows across the sill in the Mozambique Channel into the Somali Basin. However, most of the flow continues eastward at about 45°S. These previous findings are in agreement with Arhan et al.(2003) who concluded that once in the Indian Ocean basin most of the slope current is entrained in the deep return flow of the Agulhas Current or ARC, leaving only 2–3 Sv to continue north. The eastward path of the ARC is not a straightforward trajectory, but instead consists of a series of meanders (Boebel et al., 2003, Boebel et al., 2003). This meandering path of the ARC has maintained the same meandering pattern of troughs and crests for the past 15 years, in particular the first meander trough that wraps around the Agulhas Plateau which is considered permanent.

In this paper, we present the characteristics of this rare deep eddy and compare them with previous datasets (Section 2). We subsequently seek information on where and how such an eddy could have been formed along with its most likely advection path (Section 3).

Section snippets

Data and analysis

The data collected in February–March 2003, was part of the AUCE, which took place off the east coast of South Africa, between 26° and 37°S with the objective of improving our current knowledge of the Indian Ocean's western boundary current system. The survey region consisted of 112 stations distributed among four high resolution cross-stream sections closed by an offshore section (see Fig. 1) encompassing the Agulhas Current system. Full depth hydrographic data were collected with a

Discussion

Based on the data available, we have inferred that the region of the Naddie formation is the Agulhas Retroflection region known for its strong eddy activity (Stammer and Wunsch, 1999) associated with very high kinetic energy (Schmitz, 1996; Biastoch and Krauss, 1999; Stammer and Wunsch, 1999). Boebel et al., 2003, Boebel et al., 2003 have shown that this so-called “Cape Cauldron” region (part of the southern Cape Basin) is a zone of turbulent stirring and mixing because of high eddy kinetic

Conclusion

During the AUCE survey in February–March of 2003 off the east coast of South Africa, a NADW eddy (Naddie) was found in the northward moving NADW layer between the depths of 2200 and 3200 m at 35.6°S, 27.3°E, just northeast of the Agulhas Plateau. The Naddie has highly anomalous water properties with a high salinity of 34.83, 0.03 saltier than the surrounding water, a positive temperature anomaly of 0.2°C and a doming of the oxygen maximum. The density field revealed a convex lens structure

Acknowledgments

The authors would like to thank Hendrik M. van Aken for kindly supplying the WOCE clusters data, Michel Arhan for his figure and Bill Johns, Mohamed Iskandarani, Eric Chassignet, Hartmut Peters, and Chris Duncombe Rae for fruitful discussions; the help of Jean Carpenter with some of the figures is greatly appreciated. We also thank the reviewers and the editor for their comments and suggestions. TGDC and LMB have been funded by NSF grant number OCE-0244769 and RL by NOAA's Office of Global

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