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THE Official Magazine of the OCEANOGRAPHY SOCIETY OceThe OFFiciala MaganZineog OF the Oceanographyra Spocietyhy CITATION Talley, L.D. 2013. Closure of the global overturning circulation through the Indian, Pacific, and Southern Oceans: Schematics and transports.Oceanography 26(1):80–97, http://dx.doi.org/10.5670/oceanog.2013.07. DOI http://dx.doi.org/10.5670/oceanog.2013.07 COPYRIGHT This article has been published inOceanography , Volume 26, Number 1, a quarterly journal of The Oceanography Society. Copyright 2013 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. doWnloaded From http://WWW.tos.org/oceanography SPECIAL IssUE ON UPPER OCEAN PROCESSES: PETER NIILER’S ConTRIBUTIons AND InsPIRATIons Closure of the Global Overturning Circulation Through the Indian, Pacific, and Southern Oceans Schematics and Transports BY LYnnE D. TALLEY 80 Oceanography | Vol. 26, No. 1 ABSTRACT. The overturning pathways for the surface-ventilated North Atlantic Recent interest has been focused on Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively the importance of wind-driven upwell- formed Indian Deep Water (IDW) and Pacific Deep Water (PDW) are intertwined. ing of NADW to the sea surface in the The global overturning circulation (GOC) includes both large wind-driven upwelling Southern Ocean, suggesting northward in the Southern Ocean and important internal diapycnal transformation in the deep return flow directly out of the Southern Indian and Pacific Oceans. All three northern-source Deep Waters (NADW, IDW, Ocean (Toggweiler and Samuels, 1995; PDW) move southward and upwell in the Southern Ocean. AABW is produced from Gnanadesikan, 1999; Marshall and Speer, the denser, salty NADW and a portion of the lighter, low oxygen IDW/PDW that 2012). In simplest form, with no diapyc- upwells above and north of NADW. The remaining upwelled IDW/PDW stays near nal mixing in the ocean interior, recent the surface, moving into the subtropical thermoclines, and ultimately sources about GOC models importantly produce this one-third of the NADW. Another third of the NADW comes from AABW upwelling southward flow of deep waters to the in the Atlantic. The remaining third comes from AABW upwelling to the thermocline Southern Ocean where they upwell in the Indian-Pacific. Atlantic cooling associated with NADW formation (0.3 PW to the sea surface, driven by Southern north of 32°S; 1 PW = 1015 W) and Southern Ocean cooling associated with AABW Ocean wind stress curl and the geometry formation (0.4 PW south of 32°S) are balanced mostly by 0.6 PW of deep diffusive of the open Drake Passage latitude band heating in the Indian and Pacific Oceans; only 0.1 PW is gained at the surface in (e.g., Marshall and Radko, 2003; Wolfe the Southern Ocean. Thus, while an adiabatic model of NADW global overturning and Cessi, 2011). driven by winds in the Southern Ocean, with buoyancy added only at the surface in Tied to this is the well-known and the Southern Ocean, is a useful dynamical idealization, the associated heat changes important dynamical similarity of all require full participation of the diffusive Indian and Pacific Oceans, with a basin- three oceans—each transports deep averaged diffusivity on the order of the Munk value of 10–4 m2 s–1. water southward to where it rises to the surface in the Southern Ocean, and each InTRODUCTION the northern North Atlantic returned transports bottom water northward, Description of the pathways and energet- by upwelling in the Indian and Pacific regardless of a northern source of deep ics of the global overturning circulation Oceans (Gordon, 1986a; Broecker, water. The principal interocean differ- (GOC) is central to understanding the 1987), and a second cell associated with ence is that the Atlantic deep layer is interaction of different ocean basins and Antarctic Bottom Water (AABW) for- mostly sourced from the sea surface layers as well as the interplay of external mation in the south (Gordon, 1986b, and is thus marked by tracers indicating forcings. Changes in the ocean’s overturn 1991; Broecker, 1991; Schmitz, 1995). young age (high oxygen, low nutrients), on decadal to millennial time scales are Connection of the two through upwell- while the Pacific and Indian deep lay- central to variations in Earth’s climate. ing of AABW into NADW in the North ers are almost entirely sourced from For many decades, the dominant para- Atlantic has long been an accepted part upwelled bottom waters, and hence digm of the global overturning circula- of the global volume transport budget. tion described two nearly independent Modern authors connect the two Lynne D. Talley ([email protected]) cells: the popularized North Atlantic dominant overturning cells (e.g. Schmitz, is Professor, Scripps Institution of Deep Water (NADW) “great ocean con- 1995; Rahmstorf, 2002; Lumpkin and Oceanography, University of California, veyor,” with the formation of NADW in Speer, 2007; Kuhlbrodt et al., 2007). San Diego, La Jolla, CA, USA. Oceanography | March 2013 81 consistent with independent and direct estimates of deepwater diffusivities, averaging 10–4 m2 s–1 (recent work of Amy Waterhouse, Scripps Institution of Oceanography, and colleagues), which is the canonical Munk (1966) value. Thus, while physical return of the deep waters to the sea surface is almost certainly dynamically controlled by Southern Ocean winds, the properties and espe- cially heat content of the upwelled waters depend strongly on diffusion at low latitudes and the pathway of abys- sal and deep waters through the Indian and Pacific Oceans. Figure 1. Schematic of the global overturning circulation. Purple = upper ocean and thermocline. Red = Schematics of the GOC presented in denser thermocline and intermediate water. Orange = Indian Deep Water and Pacific Deep Water. Green = North Atlantic Deep Water. Blue = Antarctic Bottom Water. Gray = Bering Strait components two later sections of this paper illustrate and Mediterranean and Red Sea inflows. Updated from Talley et al. (2011), based on Schmitz (1995), the intertwined NADW, AABW, IDW, Rahmstorf (2002), and Lumpkin and Speer (2007). and PDW cells, as well as the dominant location of northward upper ocean are marked by tracers indicating much sea surface. Without diapycnal upwell- transports out of the Southern Ocean. greater age (low oxygen, high nutrients). ing at low latitudes that forms IDW and They are revisions of schematics pub- Because of recent focus on the pivotal PDW from AABW and NADW, neither lished as part of a textbook explanation role of wind-driven upwelling in the AABW nor NADW could be returned of the GOC (Talley et al., 2011) and owe Southern Ocean, the essential role of eventually to their sea surface sources, a great deal to previous work, particu- diapycnal upwelling of deep waters in particularly in terms of observed larly Gordon (1991), Schmitz (1995, the Indian and Pacific Oceans has been heat content. Air-sea heat gain in the 1996), and Lumpkin and Speer (2007). sidelined, but the overturning transports Southern Ocean, invoked in Lumpkin The GOC pathways in the global map of involved are significant (e.g., Toole and and Speer (2007) and Marshall and Speer Figure 1, based on Talley et al. (2011), Warren, 1993; Schmitz, 1995; Robbins (2012), while important for return of are similar to those of Marshall and and Toole, 1997; Ganachaud and upwelled deep waters to the subtropical Speer (2012), illustrating convergence Wunsch, 2000, 2003; Sloyan and Rintoul, thermocline, is only part of the required in thinking about the GOC. The path- 2001; Talley et al., 2003; Lumpkin and heating that must begin with warming ways are associated in the Quantifying Speer, 2007; Talley, 2008; McDonagh of bottom waters, based on heat budgets Transports and Fluxes section with et al., 2008; Macdonald et al., 2009). shown later (see Quantifying Transports quantitative transports and energy bal- The Indian/Pacific upwelling of AABW and Fluxes section), and consistent with ances from Talley (2008). into the Indian and Pacific Deep Waters the best estimates of Southern Ocean The most important aspect empha- (IDW and PDW) is an integral step in air-sea heat flux (Large and Yeager, 2009; sized here is the role in the NADW global overturning circulation (Gordon, Cerovečki et al., 2011). and AABW energy balance of the volu- 1986a,b, 1991; Schmitz, 1995, 1996; The diapycnal diffusivities that are metrically large upwelling in the Indian Speer et al., 2000; Lumpkin and Speer, diagnosed from basin-scale trans- and Pacific Oceans from abyssal to deep 2007; Talley, 2008), requiring diapyc- port budgets are not negligible (Talley waters. The deep, diapycnal warming nal (dianeutral) diffusion in the deep et al., 2003; Lumpkin and Speer, in the Indian and Pacific accomplishes Indian and Pacific Oceans, far from the 2007; Macdonald et al., 2009) and are most of the heating needed to return 82 Oceanography | Vol. 26, No. 1 NADW and AABW to the upper ocean this can cross the open latitude band of continents to the north, or Antarctica (see Quantifying Transports and Fluxes Drake Passage (roughly 57° to 61°S, sill to the south), and can therefore support section). It is also important to stress depth of about 2,000 m), while water meridional geostrophic flow. that the IDW and PDW, which are on shallower isopycnals must con- This rise of NADW across the lighter than the NADW, upwell to the nect through some process other than Antarctic Circumpolar Current (ACC) sea surface in the Southern Ocean above net meridional geostrophic transport is marked by high salinity in all three and north of the upwelled NADW (see (Gill and Bryan, 1971; Toggweiler and oceans, from about 3,000 m depth up next section).
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