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DECEMBER 2012 D O W N E S E T A L . 2153 Tracing Southwest Pacific Bottom Water Using Potential Vorticity and Helium-3 STEPHANIE M. DOWNES Program in Atmospheric and Oceanic Sciences, Princeton University, New Jersey, and Research School of Earth Sciences, and ARC Centre of Excellence for Climate System Science, The Australian National University, Acton, Australian Capital Territory, Australia ROBERT M. KEY Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey ALEJANDRO H. ORSI Department of Oceanography, Texas A&M University, College Station, Texas KEVIN G. SPEER Department of Oceanography, The Florida State University, Tallahassee, Florida JAMES H. SWIFT Scripps Institution of Oceanography, La Jolla, California (Manuscript received 30 January 2012, in final form 23 July 2012) ABSTRACT This study uses potential vorticity and other tracers to identify the pathways of the densest form of Cir- cumpolar Deep Water in the South Pacific, termed ‘‘Southwest Pacific Bottom Water’’ (SPBW), along the 2 28.2 kg m 3 surface. This study focuses on the potential vorticity signals associated with three major dy- namical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low po- tential vorticity along rising isopycnal surfaces. Using d3He released from the hydrothermal vents, the in- fluence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector. 1. Introduction concluded that heat in the buoyant plumes from hy- drothermal vents is effectively diffused unless ‘‘com- The divergence of tectonic plates along midocean peting mechanisms are not overpowering.’’ Stommel’s ridges introduces a gap in the earth’s crust, forming an paper considered the circulation above the East Pacific axial valley, from which geothermal heat is released at Rise, located deep below the South Pacific subtropical a relatively high rate, by both conductive and convective gyre, where little ‘‘overpowering’’ circulation would processes (Wilson 1965; Morgan 1971). Stommel (1982) be expected. Hydrothermal vents are apparently com- mon along midocean ridges, but with a highly inter- mittent distribution; they have recently been identified Corresponding author address: Stephanie Downes, Research School of Earth Sciences, The Australian National University, on parts of the 2600-km-long Pacific–Antarctic Ridge, Canberra ACT 0200, Australia. southwest of the East Pacific Rise (Winckler et al. E-mail: [email protected] 2010). In complete contrast to the East Pacific Rise, the DOI: 10.1175/JPO-D-12-019.1 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/30/21 01:40 PM UTC 2154 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 42 Pacific–Antarctic Ridge lies within the main flow of the slightly converge and flow along the northern flank of Antarctic Circumpolar Current (ACC), presumably an the Pacific–Antarctic Ridge (Gordon et al. 1978; Gille ‘‘overpowering’’ flow. In addition, vigorous eddy activ- 1994). Along the ridge, the ACC flow between its ity and the upwelling of deep waters all coincide with the southern fronts is strong compared to other parts of the multiple hydrothermal sources along this ridge. Southern Ocean where the dominant flow is between the Hydrothermal plumes from the major ocean ridges Subantarctic and Polar Fronts (Rintoul and Sokolov have been traced both near and far afield from the vent 2001). East of the Pacific–Antarctic Ridge, the ACC sources. Veirs et al. (1999) used a stability function along fronts are squeezed through the Udintsev Fracture Zone the Juan de Fuca Ridge to identify which vent sources and the Eltanin Fracture Zone (Gordon et al. 1978; along the ridge are associated with temperature and Patterson and Whitworth III 1990). The ACC intensifies light attenuation signals. They concluded that the sta- as it converges (Gille 1994), is sharply deflected south- bility anomaly was confined to within about 50 km from eastward, and widens as it flows toward Drake Passage the vent source. Hydrothermal signals have also been (Read et al. 1995). The southern edge of the ACC in this detected at large distances from the sources on the ridge sector forms the outer Ross Gyre between 1708E and crests. For example, d3He and manganese have been 1408W (Gordon et al. 1978; Gille 1994; Jacobs et al. 2002; used to trace hydrothermal fluid originating at the East Rickard et al. 2010). Pacific Rise and flowing more than 2000 km west of the Two deep water masses have traditionally been as- ridge (Lupton and Craig 1981; Klinkhammer 1980).1 In sociated with the circulation in the Pacific sector of addition, Johnson and Talley (1997) compared stratifi- the Southern Ocean (Callahan 1972; Patterson and cation measures and temperature-salinity anomalies Whitworth 1990), namely, the oxygen-poor Upper Cir- with d3He to trace various pathways of the hydrothermal cumpolar Deep Water (UCDW) and high-salinity Lower plumes that had originated along the East Pacific Rise. Circumpolar Deep Water (LCDW). These water masses In the South Atlantic Ocean d3He indicates hydrother- are roughly considered to have Indo-Pacific (LCDW) mal plumes also extending thousands of kilometers from and Atlantic (UCDW) origins. Here we are interested in the hydrothermal sources along the southern half of the Lower Circumpolar Deep Water (LCDW), and the Mid-Atlantic Ridge (Ru¨ th et al. 2000). conditions that determine its properties. In terms of Winckler et al. (2010) identified the Pacific–Antarctic density, this layer is defined by the range gn 5 27.98 to 2 Ridge as a major hydrothermal plume source that can be 27.27 kg m 3, where gn is neutral density (Jackett and 2 traced via a d3He plume along the 28.2 kg m 3 neutral McDougall (1997); see also Fig. 3 and Table 1 in Orsi 23 density surface (s2 ’ 1037.12 kg m ) south of the ridge et al. (2002)). Some LCDW recirculates and upwells in along 1508W and across the 678S cruise transect. They the interior of the Weddell Sea, where it also may be concluded that this d3He plume, distinct from the much transformed into Antarctic Surface Water; but part of its stronger d3He source along the East Pacific Rise volume also shoals toward the continental shelf over (Lupton 1998), could be used for tracing the South Pa- a larger zonal extent, where it contributes to the pro- cific abyssal circulation. However, d3He measurements duction of both lighter and denser water masses (Orsi in the South Pacific are sparse, and thus this tracer et al. 1995). LCDW reaching the Pacific–Antarctic cannot solely be used to describe in detail the large- margins participates in the renewal of ventilated deep 2 scale circulation. Here, we expand upon the analysis and bottom waters (gn . 28.15 kg m 3) in the Ross Sea of Winckler et al. (2010) by showing that the d3He sig- (Orsi and Wiederwohl 2009) as well as of lighter (gn , 2 nature they observed coincides with a distinct potential 28.15 kg m 3) pycnocline waters (Wa˚hlin et al. 2010; vorticity signal in the deep South Pacific sector of the Jacobs and Giulivi 2010; Jenkins et al. 2010). Southern Ocean. Throughout the zonal domain of the ACC there is In the South Pacific sector of the Southern Ocean, a well-mixed portion of LCDW, which Orsi et al. (1999) strong interactions occur between the eastward-flowing identified as bottom water within the ACC (ACCbw; ACC and topography (Gnanadesikan and Hallberg density range 28.18 to 28.27), not directly supplied by 2000; Rintoul et al. 2001). The ACC is comprised of new bottom waters sinking in either the northern North fronts or jets (cf. Orsi et al. 1995; Sokolov and Rintoul Atlantic or around Antarctica. Yet, the lateral ventila- 2007). Around 1508E, the ACC strengthens (Che et al. tion of the ACCbw layer has been attributed to multiple 2011), and its path is deflected northward as the jets exports of Modified CDW near the Antarctic Slope Front (ASF) in the Atlantic, Indian, and Pacific sectors of the Southern Ocean (Whitworth et al. 1994; Orsi et al. 1 3 5 3 3 4 2 3 4 d He 100 f[( He/ He)water sample ( He/ He)atmosphere]/ 2001; Orsi and Wiederwohl 2009; Orsi 2010). Here we 3 4 ( He/ He)atmosphereg; Clarke et al. (1969). refer to water in the ACCbw density range in the South Unauthenticated | Downloaded 09/30/21 01:40 PM UTC DECEMBER 2012 D O W N E S E T A L . 2155 3 TABLE 1. Cruises of analyzed hydrographic (including He) data. Note: the CLIVAR S4P line also includes western (1728E), central (1708W), and eastern (1508W) Ross Sea sections that extend south of 678S to about 728S. Also, the P06 cruise had three legs and the chief scientists for each leg are listed. The cruise chief scientists and the Principal Investigator (PI) for 3He are also included. Line Latitude Longitude Ship Year Months Chief Scientist(s) 3He PI Pacific WOCE P06 328S 1538E–718W Knorr 1992 May–Jul H.Bryden/M.McCartney/J.Toole W.Jenkins WOCE P14 678–408S 1708E Discoverer 1996 Jan–Feb J.Bullister/G.Johnson — WOCE P15 678S–08 1708W Discoverer 1996 Feb–Mar R.Feely/M.Roberts — WOCE P16 628–218S 150.58W Knorr 1992 Oct–Nov J.Reid W.Jenkins WOCE P17 698–548S 1358W Knorr 1992 Dec–Jan J.Swift P.Schlosser WOCE P17N 348–578N 1458W Thomas Thompson 1993 May–Jun D.Musgrave J.

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