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Agulhas Leakage Into the Atlantic Estimated with Subsurface Floats

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Agulhas Leakage Into the Atlantic Estimated with Subsurface Floats This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author’s institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy ARTICLE IN PRESS Deep-Sea Research I 54 (2007) 1361–1389 www.elsevier.com/locate/dsri Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters Philip L. Richardsona,b,à aPhysical Oceanography Department, MS #21, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, USA bWoods Hole Research Center, Woods Hole, MA 02543, USA Received 1 March 2006; received in revised form 28 March 2007; accepted 5 April 2007 Available online 29 April 2007 Abstract Surface drifters and subsurface floats drifting at depths near 800 m were used to study the pathways of warm, salty Indian Ocean water leaking into the South Atlantic that is a component of the upper limb of the Atlantic meridional overturning circulation (MOC). Four drifters and 5 floats drifted from the Agulhas Current directly into the Benguela Current. Others looped for various amounts of time in Agulhas rings and cyclones, which translated westward into the Atlantic, contributing a large part of Indian Ocean leakage. Agulhas rings translated into the Benguela Current, where they slowly decayed. Some large, blob-like Agulhas rings with irregular shapes were found in the southeastern Cape Basin. Drifter trajectories suggest these rings become more circular with time, eventually evolving into the circular rings observed west of the Walvis Ridge. Agulhas cyclones, which form on the north side of the Agulhas Current south of Africa, translated southwestward (to 61E) and contributed water to the southern Cape Basin. A new discovery is a westward extension from the mean Agulhas retroflection measured by westward drifting floats near 411S out to at least 51W, with some floats as far west as 251W. The Agulhas extension appears to split the South Atlantic Current (SAC) into two branches and to transport Agulhas water westward, where it is mixed and blended with eastward-flowing water from the western Atlantic. The blended mixture flows northeastward in the northern branch of the SAC and into the Benguela Current. Agulhas leakage transport was estimated from drifters and floats to be at least 15 Sv in the upper 1000 m, which is equivalent to the transport of the upper layer MOC. It is suggested that the major component of the upper layer overturning circulation in the Atlantic is Agulhas leakage in the form of Agulhas rings. r 2007 Elsevier Ltd. All rights reserved. 1. Introduction global overturning circulation of the ocean (Gordon et al., 1992; Lutjeharms, 1996; de Ruijter et al., Interocean exchange of heat and salt around 1999; Sloyan and Rintoul, 2001). Indian Ocean South Africa is thought to be key in maintaining the leakage into the South Atlantic takes place near the Agulhas Current retroflection, where large Agulhas rings (400 km overall diameter) pinch off and à Corresponding author. Physical Oceanography Department, translate into the Atlantic. Agulhas rings and other MS #21, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, USA. Agulhas eddies and current filaments merge with Tel.: +1 508 289 2546; fax: +1 508 457 2163. South Atlantic Current (SAC) water in the Benguela 6 3 E-mail address: [email protected]. Current, forming the source of the 15  10 m /s 0967-0637/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2007.04.010 Author's personal copy ARTICLE IN PRESS 1362 P.L. Richardson / Deep-Sea Research I 54 (2007) 1361–1389 warm upper layer water that flows northward Acoustic float data in the region of interest have through the Atlantic in compensation for cold been previously discussed by Boebel et al. (2003), southward flowing North Atlantic Deep Water. Lutjeharms et al. (2003), Richardson and Garzoli Indian Ocean leakage water tends to be warmer and (2003), and Schmid et al. (2003). The present work saltier than South Atlantic water of Drake Passage builds on those studies by combining all available origin and therefore contributes to larger northward acoustic float trajectories with non-acoustic float heat and salt fluxes than does Drake Passage water displacements and surface drifter trajectories to (Gordon et al., 1992; Sloyan and Rintoul, 2001; further examine surface and subsurface velocity Schmid et al., 2000). Variations of the leakage characteristics of the flow field of the southeastern inferred from sea floor sediment cores and climate Atlantic where Indian Ocean leakage begins its variations over the last 550,000 years suggest an northward path toward the equator. The inter- enhanced leakage during present and past inter- mediate-level mean flow field in the area of southern glacials and a much reduced leakage during glacial Africa has been mapped at fairly coarse (31) intervals (Peeters et al., 2004). Variations of resolution using acoustic floats (Boebel et al., 2003), Agulhas leakage appear to play a crucial role in non-acoustic floats (Davis, 2005), and a combina- glacial terminations, the timing of interhemispheric tion of acoustic and non-acoustic floats (Nu´n˜ez- climate change, and the resulting resumption of the Riboni et al., 2005), but these maps tend to blur Atlantic meridional overturning circulation (MOC) smaller scale features (o31). The present study uses (Peeters et al., 2004). higher resolution maps (11) in order to try and In order to correctly model the leakage and resolve the smaller features of the circulation in the understand its effect on the overturning circulation vicinity of the Agulhas retroflection. and climate variations, the amounts and pathways of the present leakage need to be accurately 2. Scientific background measured and described. Mechanisms responsible for the leakage need to be understood and simulated The upper limb of the Atlantic MOC originates in realistically. However, the complexity of currents the southeastern Atlantic where, SAC water merges around southern Africa and the intense eddies in the with Indian Ocean leakage in the Benguela Current vicinity of the Agulhas retroflection make it very (Reid, 1989; Peterson and Stramma, 1991; Stramma difficult to measure the mean circulation patterns and England, 1999; de Ruijter et al., 1999). The two and interocean fluxes. At present we do not know main sources of the upper layer MOC are Agulhas accurately how much Agulhas leakage there is, what leakage and Drake Passage water that flows north- its variability is, and what percentage of leakage ward via the Falklands (Malvinas) Current and feeds into the upper limb of the MOC. This is part SAC. The Benguela Current flows northward as the of the larger problem of determining where and how eastern part of the subtropical gyre of the South North Atlantic Deep Water is transformed into the Atlantic and then westward over the mid-Atlantic upper layer water, which flows northward in the Ridge as the Benguela extension, which includes MOC. both the MOC and the return flow of the The purpose of this study is to use surface drifter subtropical gyre. Upon reaching the western and subsurface float trajectories to determine the boundary the Benguela extension divides with amounts and pathways of the Indian Ocean leakage (roughly) half flowing southward in the Brazil of surface and intermediate water (800 m) and to Current closing the gyre in the west and half, the investigate the physical mechanisms responsible for MOC, continuing northward as the North Brazil it. Leakage was investigated by studying individual Undercurrent and then North Brazil Current. trajectories that entered the Benguela Current from Accurate knowledge of the circulation patterns in the Agulhas Current, including those in Agulhas the southeastern Atlantic and of the amounts of rings and Agulhas cyclones, as well as overall water from different sources in the Benguela patterns of trajectories. Physical characteristics of Current has remained lacking because of the rings and cyclones were investigated in order to complexities of the ocean circulation and the sparse estimate their contribution to leakage. Finally, amount of suitable direct measurements. Complex- drifter and float velocities were used to map the ities include the partially interconnected South mean flow field at high resolution (11  11)inan Atlantic, Indian, and Pacific Ocean gyres, the attempt to reveal leakage pathways. formation of numerous energetic Agulhas rings Author's personal copy ARTICLE IN PRESS P.L. Richardson / Deep-Sea Research I 54 (2007) 1361–1389 1363 and cyclones in the southeastern Atlantic, and Haxby, 1990; Byrne et al., 1995; Gon˜i et al., 1997; longer term fluctuations. Good reviews of what is Schouten et al., 2000). Many rings have been known have been published by de Ruijter et al. measured at sea as described by Duncombe Rae (1999), Gordon (2001), and Lutjeharms (2006). (1991), McCartney and Woodgate-Jones (1991), They cite widely varying estimates of Agulhas Lutjeharms (1996), Arhan et al. (1999), de Ruijter leakage ranging from around 2 to 15 Sv et al. (1999), Garzoli et al. (1999), and McDonagh (1 Sv ¼ 106 m3/s) and estimates of leakage due to et al. (1999); however the sizes of rings and the Agulhas rings from around 3 to 9 Sv, based on the estimated amount of Indian Ocean water trans- formation of 6 rings per year each with a transport ported by them varies widely.
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