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The Contribution of the Weddell Gyre to the Lower Limb of the Global Overturning Circulation Loïc Jullion, Alberto C

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The Contribution of the Weddell Gyre to the Lower Limb of the Global Overturning Circulation Loïc Jullion, Alberto C The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation Loïc Jullion, Alberto C. Naveira Garabato, Sheldon Bacon, Michael P. Meredith, Peter J. Brown, Sinhue Torres-Valdés, Kevin Speer, Paul Holland, Jun Dong, Dorothée Bakker, et al. To cite this version: Loïc Jullion, Alberto C. Naveira Garabato, Sheldon Bacon, Michael P. Meredith, Peter J. Brown, et al.. The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circu- lation. Journal of Geophysical Research. Oceans, Wiley-Blackwell, 2014, 119 (6), pp.3357-3377. 10.1002/2013JC009725. hal-01255764 HAL Id: hal-01255764 https://hal.archives-ouvertes.fr/hal-01255764 Submitted on 13 Jan 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/, 1 The contribution of the Weddell Gyre to the lower 2 limb of the Global Overturning Circulation Lo¨ıcJullion1,2, Alberto C. Naveira Garabato1, Sheldon Bacon3, Michael P. Meredith4,5, Pete J. Brown4,6, Sinhue Torres-Vald´es3, Kevin G. Speer2, Paul R. Holland4, Jun Dong2, Doroth´ee Bakker6, Mario Hoppema7, Brice Loose8, Hugh J. Venables4, William J. Jenkins9, Marie-Jos´eMessias10 and Eberhard Fahrbach7* 1University of Southampton, National D R A F T April 28, 2014, 3:07pm D R A F T X-2 JULLION ET AL.: THE WEDDELL GYRE IN THE GOC 3 Abstract. The horizontal and vertical circulation of the Weddell Gyre 4 is diagnosed using a box inverse model constructed with recent hydrographic 5 sections and including mobile sea ice and eddy transports. The gyre is found 6 3 −1 6 to convey 42 ± 8 Sv (1 Sv = 10 m s ) across the central Weddell Sea Oceanography Centre, Southampton, U.K. 2Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, U.S.A. 3National Oceanography Centre, Southampton, U.K. 4British Antarctic Survey, Cambridge, U.K. 5Scottish Association for Marine Science, Oban, U.K. 6University of East Anglia, Norwich, U.K. 7Alfred-Wegener-Institut, Bremerhaven, Germany 8University of Rhode Island, Narragansett, U.S.A. 9Woods Hole Oceanographic Institution, Woods Hole, U.S.A. 10University of Exeter, Exeter, U.K. D R A F T April 28, 2014, 3:07pm D R A F T JULLION ET AL.: THE WEDDELL GYRE IN THE GOC X-3 7 and to intensify to 54±15 Sv further offshore. This circulation injects 36± 8 13 TW of heat from the Antarctic Circumpolar Current to the gyre, and ex- 9 ports 51 ± 23 mSv of freshwater, including 13 ± 1 mSv as sea ice to the 10 mid-latitude Southern Ocean. The gyre’s overturning circulation has an asym- 11 metric double-cell structure, in which 13 ± 4 Sv of Circumpolar Deep Wa- 12 ter (CDW) and relatively light Antarctic Bottom Water (AABW) are trans- 13 formed into upper-ocean water masses by mid-gyre upwelling (at a rate of 14 2 ± 2 Sv) and into denser AABW by downwelling focussed at the western 15 boundary (8 ± 2 Sv). The gyre circulation exhibits a substantial through- 16 flow component, by which CDW and AABW enter the gyre from the Indian 17 sector, undergo ventilation and densification within the gyre, and are exported 18 to the South Atlantic across the gyre’s northern rim. The relatively mod- 19 est net production of AABW in the Weddell Gyre (6±2 Sv) suggests that 20 the gyre’s prominence in the closure of the lower limb of global oceanic over- 21 turning stems largely from the recycling and equatorward export of Indian- 22 sourced AABW. D R A F T April 28, 2014, 3:07pm D R A F T X-4 JULLION ET AL.: THE WEDDELL GYRE IN THE GOC 1. Introduction 23 The Southern Ocean plays a pivotal role in the global ocean circulation. The absence 24 of continental barriers in the latitude band of Drake Passage permits the existence of the 25 eastward-flowing Antarctic Circumpolar Current (ACC), which is supported geostrophi- 26 cally by sloping isopycnals and serves as a conduit for oceanic exchanges between the three 27 major ocean basins [Rintoul and Naveira Garabato, 2013]. Coupled to this intense zonal 28 flow, a meridional circulation exists in which Circumpolar Deep Water (CDW) upwells 29 along the southward-shoaling isopycnals of the ACC [Speer et al., 2000]. Whereas the 30 lighter classes of CDW reach the upper-ocean mixed layer within the ACC and are re- 31 turned northward near the surface, the denser classes of CDW are transported southward 32 and enter the system of cyclonic gyres and westward-flowing slope frontal jets encircling 33 Antarctica. There, CDW replenishes and mixes with Antarctic surface waters and water 34 masses found over the Antarctic continental shelves, ultimately resulting in the formation 35 of Antarctic Bottom Water (AABW). The production and northward export of AABW is 36 an integral component of the southern closure of the global overturning circulation (GOC, 37 Talley [2013]), gives rise to its lower cell [Lumpkin and Speer, 2007], and is an important 38 driver of deep global ocean ventilation [Orsi et al., 2002] and marine biogeochemical cy- 39 cling [Marinov et al., 2006]. 40 Traditionally, the Weddell Gyre (Figure 1) has been regarded as by far the primary 41 region of AABW formation, accounting for upwards of 60 - 70% of all AABW produc- 42 tion [Orsi et al., 1999, 2002]. Through several decades of oceanographic measurements, 43 a picture of the gyre has been built in which CDW enters the gyre’s southern limb near D R A F T April 28, 2014, 3:07pm D R A F T JULLION ET AL.: THE WEDDELL GYRE IN THE GOC X-5 ◦ 44 30 E[Orsi and Whitworth, 1993; Gouretski and Danilov, 1993; Park et al., 2001] and 45 is gradually cooled and freshened by mixing with ambient waters as it flows westward 46 near Antarctica. Further downstream, CDW interacts with dense, relatively saline wa- 47 ters cascading off the broad continental shelves of the southwestern and western Weddell 48 Sea, resulting in the production of AABW [Gill, 1973; Foster and Carmack, 1976]. The 49 regional variety of AABW is made up of two water masses: Weddell Sea Bottom Wa- 50 ter (WSBW), produced primarily near the Filchner-Ronne ice shelves, is the coldest and ◦ n −3 51 densest AABW in the Weddell Gyre (θ < −0.7 C, γ > 28.40 kg m , see Orsi et al. ◦ 52 [1999]). The warmer and lighter Weddell Sea Deep Water (WSDW; 0 >θ> −0.7 C, n −3 53 28.27 < γ < 28.40 kg m ) may be formed directly by mixing between shelf waters 54 and CDW, or indirectly by entrainment of CDW into WSBW as the shelf water plume 55 cascades down the continental slope. A distinct variety of WSDW is formed near the 56 Larsen ice shelves (LIS) in the western Weddell Sea that is lighter and fresher than deep 57 water formed further south [Fahrbach et al., 1995; Gordon et al., 2001; Huhn et al., 2008; 58 Gordon et al., 2010]. The reader may refer to Nicholls et al. [2009] for a detailed review 59 of AABW in the Weddell Sea. 60 The newly formed AABW is conveyed northeastward by the Weddell Gyre and exported 61 to the mid-latitude Southern Ocean and beyond through openings in the topographic 62 barriers bounding the gyre to the north, most conspicuously along the South Sandwich ◦ 63 Trench near 25 W[Orsi et al., 1999]. In spite of the presumed high-ranking status of the 64 Weddell Gyre in AABW formation, present estimates of AABW production in the gyre 65 are unsatisfactorily wide-ranging (Table 1). These differences represent a combination of 66 inconsistencies between different estimation techniques, AABW definitions or regional flow D R A F T April 28, 2014, 3:07pm D R A F T X-6 JULLION ET AL.: THE WEDDELL GYRE IN THE GOC 67 regimes; and temporal variability [Naveira Garabato et al., 2002a]. This large uncertainty 68 in the quantification of water mass transformation and ventilation in the Weddell Gyre 69 has historically posed a significant obstacle to determining its standing in the closure of 70 the GOC. 71 There are now several pieces of evidence that challenge this traditional view of the Wed- 72 dell Gyre. Most fundamentally, the long-held notion of the gyre as a largely hermetic bowl 73 with a few, well defined inflow and outflow pathways is inconsistent with observations. 74 Klatt et al. [2005] point out the existence of a substantial inflow of CDW into the gyre 75 along its northern rim, while Gordon et al. [2001] and Naveira Garabato et al. [2002a] 76 suggest that AABW may be exported from the gyre across a wider zonal swath than 77 previously thought, including the major topographic barrier of the South Scotia Ridge. 78 A most unexpected finding in this context relates to the observation of a prominent flow 79 of AABW from the Indian Ocean sector entering the southern Weddell Gyre across its 80 eastern rim [Meredith et al., 2000; Hoppema et al., 2001; Couldrey et al., 2013], which has 81 led to the (as yet untested) proposition that the role of the gyre in AABW formation has 82 been historically overstated [Jacobs, 2004].
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