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  • View in gallery

    Map of cruise sections used in our analysis. Red sections denote those collected during the WOCE era (1990 to 1996), and blue dashed sections after 2005 (during the CLIVAR period). The P16, P18, and S4P lines were sampled during both the WOCE and CLIVAR periods. Magenta dots and numbers indicate GEOSECS cruise stations, and these are referred to later in the paper in Fig. 7.

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    WOCE P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) and (a) δ3He (black contours; %; asterisks denote sampling locations for δ3He), (b) vertical temperature gradient (black contours; °C dbar−1), and (c) vertical salinity gradient (black contours; CTD dbar−1). The gradients are the vertical difference in temperature or salinity with respect to pressure. We draw the reader’s attention to the southward-flowing upwelling Lower Circumpolar Deep Water, represented by the salinity maxima [>34.73; red dashed contours in (c)]. (d) CLIVAR P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) and vertical temperature gradient (black contours; °C dbar−1).

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    WOCE P16 potential temperature (°C) vs δ3He (%) for stations south of 55°S, between 800 and 3500 m. The potential temperatures associated with the potential vorticity (Q) minima and maxima signals (9 × 10−12 m−1 s−1 and 1.1 × 10−11 m−1 s−1, respectively) are denoted by the red vertical lines.

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    Potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (thin black contours; %) and 28.2 kg m−3 neutral density surface (yellow) for WOCE Pacific diagonal P16 line [between (135°W, 62°S) and (147°W, 58°S)] , the meridional lines P17, P18, P19, and the zonal line S4P. Asterisks denote sampling locations for δ3He. Note that only one δ3He sample was available for the P17 cruise, hence we have omitted δ3He contours and asterisks from this section. The 10 × 10−12 m−1 s−1 (11 × 10−12 m−1 s−1 for P19) thick black contour in each panel highlights the SPBW potential vorticity minima (along the 28.2 kg m−3 surface) and maxima. (bottom right) (Drake Passage, i.e. WOCE A21) potential vorticity (color) overlaid with δ3He (solid black contours) and the 28.0 kg m−3 (upper dashed contour) and 28.2 kg m−3 (lower dashed contour) density surfaces.

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    Schematic of the Pacific Ocean bathymetry (blue hues; m) overlaid with the fronts that encapsulate the ACC (from north to south): the SAF, PF, SACCF, and the SBDY. GEOSECS stations are indicated by magenta dots with stations shown adjacent in white boxes with magenta text. The WOCE P14, P15, P16 (P16A), diagonal P16 line (P16D), P17 (P17E), P18, P19, and S4P lines are denoted in red. The CLIVAR P16, P18 and S4P cruises are shown by blue dashed lines. The CLIVAR S4P cruise is divided into the main cruise section along 67°S (S4P), and the western, central, and eastern Ross Sea sections (RW, RC, and RE, respectively). Potential plume source locations are taken from Winckler et al. (2010). The bold part of the cruise lines denotes regions where we observe SPBW.

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    Potential vorticity (color and black contours, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) for Pacific lines P16, P18, and S4P. (top) Cruises during the WOCE period are shown; (bottom) CLIVAR cruise lines are shown.

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    GEOSECS Pacific stations (Legs 7, 8, and 9) south of 40°S for pressure greater than 1000 dbar. Shown is δ3He (%) in neutral density space (γn; kg m−3). The symbols for each station are shown in the legend, and station locations are shown in Fig. 1. Stations in the vicinity of the Pacific–Antarctic Ridge are: 282 (purple squares), 286 (green circles), and 287 (orange triangles).

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    Potential vorticity (color and black contours, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) for WOCE Pacific lines P14 and P15. Unfortunately, δ3He was not sampled along these lines, and thus we only show potential vorticity.

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    (a) Neutral density (kg m−3) vs potential vorticity (×10−11 m−1 s−1) for the region restricted by density range 27.9 < γn < 28.4 kg m−3 and between the Polar Front and Southern Boundary Front (see also Fig. 5). Shown are the WOCE Indian lines I08S, I09S, S03, and the WOCE Pacific lines P16, P18, P19, and S4P. Note that P16 (the most discussed cruise in this study) is denoted by a bold black curve. The vertical dotted line denotes the 28.2 kg m−3 surface. Note that I08S, I09S and S03 are located upstream of the Pacific–Antarctic Ridge and are indicated by small-dashed curves. (b) As in (a), but with neutral density (kg m−3) vs δ3He (%). P14 (red) and P15 (dark blue) are included in panel (a) in bold large dashed curves, but δ3He data was not available on these two cruises so they are omitted from panel (b).

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    Potential vorticity (color, ×10−12 m−1 s−1) overlaid with the 28.2 kg m−3 neutral density surface (yellow contour) for the CLIVAR line S4P. (top left) The zonal 67°S part of the line (referred to as ‘CLIVAR S4P’ in the text) with the longitude of the zonal Ross Sea sections denoted by red vertical lines. Also see Fig. 5 for section locations. The remaining panels show potential vorticity and neutral density surfaces along the Western, Central and Eastern Ross Sea sections (RW, RC and RE, respectively). The 10 × 10−12 m−1 s−1 black contour in each panel highlights the SPBW potential vorticity minima and maxima.

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    Four cruise lines that intersect prominent ridges with hydrothermal activity. Potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (black contours; %) for WOCE lines I03 (20°S), P17N (145°W), P06 (32°S), and A10 (30°S). Black dots denote sampling locations for δ3He.

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    WOCE P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (black contours; %) as shown in Fig. 2a. Asterisks denote sampling locations for δ3He. White dashed arrows denote UCDW and LCDW that flow southward from the North Pacific and upwell in the ACC region, bound by the SAF to the north, and the SBDY to the south. The SACCf and PF lie within the ACC. The low potential vorticity signature of modified bottom waters (orange dashed arrows) emphasizes that which is induced by the hydrothermal plume source at the Pacific–Antarctic Ridge. Lateral eddy mixing along the ridge (white spiral) and along the 28.2 kg m−3 surface, the ACC eastward flow, and the Ross Gyre circulation transport the low potential vorticity signature/high δ3He signal (i.e., SPBW) downstream.

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Tracing Southwest Pacific Bottom Water Using Potential Vorticity and Helium-3

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  • 1 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
  • | 2 Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey
  • | 3 Department of Oceanography, Texas A&M University, College Station, Texas
  • | 4 Department of Oceanography, The Florida State University, Tallahassee, Florida
  • | 5 Scripps Institution of Oceanography, La Jolla, California
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Abstract

This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical 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 potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence 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.

Corresponding author address: Stephanie Downes, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.E-mail: stephanie.downes@anu.edu.au

Abstract

This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical 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 potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence 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.

Corresponding author address: Stephanie Downes, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.E-mail: stephanie.downes@anu.edu.au

1. Introduction

The divergence of tectonic plates along midocean ridges introduces a gap in the earth’s crust, forming an axial valley, from which geothermal heat is released at a relatively high rate, by both conductive and convective processes (Wilson 1965; Morgan 1971). Stommel (1982) concluded that heat in the buoyant plumes from hydrothermal vents is effectively diffused unless “competing mechanisms are not overpowering.” Stommel’s paper considered the circulation above the East Pacific Rise, located deep below the South Pacific subtropical gyre, where little “overpowering” circulation would be expected. Hydrothermal vents are apparently common along midocean ridges, but with a highly intermittent distribution; they have recently been identified on parts of the 2600-km-long Pacific–Antarctic Ridge, southwest of the East Pacific Rise (Winckler et al. 2010). In complete contrast to the East Pacific Rise, the Pacific–Antarctic Ridge lies within the main flow of the Antarctic Circumpolar Current (ACC), presumably an “overpowering” flow. In addition, vigorous eddy activity and the upwelling of deep waters all coincide with the multiple hydrothermal sources along this ridge.

Hydrothermal plumes from the major ocean ridges have been traced both near and far afield from the vent sources. Veirs et al. (1999) used a stability function along the Juan de Fuca Ridge to identify which vent sources along the ridge are associated with temperature and light attenuation signals. They concluded that the stability anomaly was confined to within about 50 km from the vent source. Hydrothermal signals have also been detected at large distances from the sources on the ridge crests. For example, δ3He and manganese have been used to trace hydrothermal fluid originating at the East Pacific Rise and flowing more than 2000 km west of the ridge (Lupton and Craig 1981; Klinkhammer 1980).1 In addition, Johnson and Talley (1997) compared stratification measures and temperature-salinity anomalies with δ3He to trace various pathways of the hydrothermal plumes that had originated along the East Pacific Rise. In the South Atlantic Ocean δ3He indicates hydrothermal plumes also extending thousands of kilometers from the hydrothermal sources along the southern half of the Mid-Atlantic Ridge (Rüth et al. 2000).

Winckler et al. (2010) identified the Pacific–Antarctic Ridge as a major hydrothermal plume source that can be traced via a δ3He plume along the 28.2 kg m−3 neutral density surface (σ2 ≈ 1037.12 kg m−3) south of the ridge along 150°W and across the 67°S cruise transect. They concluded that this δ3He plume, distinct from the much stronger δ3He source along the East Pacific Rise (Lupton 1998), could be used for tracing the South Pacific abyssal circulation. However, δ3He measurements in the South Pacific are sparse, and thus this tracer cannot solely be used to describe in detail the large-scale circulation. Here, we expand upon the analysis of Winckler et al. (2010) by showing that the δ3He signature they observed coincides with a distinct potential vorticity signal in the deep South Pacific sector of the Southern Ocean.

In the South Pacific sector of the Southern Ocean, strong interactions occur between the eastward-flowing ACC and topography (Gnanadesikan and Hallberg 2000; Rintoul et al. 2001). The ACC is comprised of fronts or jets (cf. Orsi et al. 1995; Sokolov and Rintoul 2007). Around 150°E, the ACC strengthens (Che et al. 2011), and its path is deflected northward as the jets slightly converge and flow along the northern flank of the Pacific–Antarctic Ridge (Gordon et al. 1978; Gille 1994). Along the ridge, the ACC flow between its southern fronts is strong compared to other parts of the Southern Ocean where the dominant flow is between the Subantarctic and Polar Fronts (Rintoul and Sokolov 2001). East of the Pacific–Antarctic Ridge, the ACC fronts are squeezed through the Udintsev Fracture Zone and the Eltanin Fracture Zone (Gordon et al. 1978; Patterson and Whitworth III 1990). The ACC intensifies as it converges (Gille 1994), is sharply deflected southeastward, and widens as it flows toward Drake Passage (Read et al. 1995). The southern edge of the ACC in this sector forms the outer Ross Gyre between 170°E and 140°W (Gordon et al. 1978; Gille 1994; Jacobs et al. 2002; Rickard et al. 2010).

Two deep water masses have traditionally been associated with the circulation in the Pacific sector of the Southern Ocean (Callahan 1972; Patterson and Whitworth 1990), namely, the oxygen-poor Upper Circumpolar Deep Water (UCDW) and high-salinity Lower Circumpolar Deep Water (LCDW). These water masses are roughly considered to have Indo-Pacific (LCDW) and Atlantic (UCDW) origins. Here we are interested in Lower Circumpolar Deep Water (LCDW), and the conditions that determine its properties. In terms of density, this layer is defined by the range γn = 27.98 to 27.27 kg m−3, where γn is neutral density (Jackett and McDougall (1997); see also Fig. 3 and Table 1 in Orsi et al. (2002)). Some LCDW recirculates and upwells in the interior of the Weddell Sea, where it also may be transformed into Antarctic Surface Water; but part of its volume also shoals toward the continental shelf over a larger zonal extent, where it contributes to the production of both lighter and denser water masses (Orsi et al. 1995). LCDW reaching the Pacific–Antarctic margins participates in the renewal of ventilated deep and bottom waters (γn > 28.15 kg m−3) in the Ross Sea (Orsi and Wiederwohl 2009) as well as of lighter (γn < 28.15 kg m−3) pycnocline waters (Wåhlin et al. 2010; Jacobs and Giulivi 2010; Jenkins et al. 2010).

Throughout the zonal domain of the ACC there is a well-mixed portion of LCDW, which Orsi et al. (1999) identified as bottom water within the ACC (ACCbw; density range 28.18 to 28.27), not directly supplied by new bottom waters sinking in either the northern North Atlantic or around Antarctica. Yet, the lateral ventilation of the ACCbw layer has been attributed to multiple 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. 2001; Orsi and Wiederwohl 2009; Orsi 2010). Here we refer to water in the ACCbw density range in the South Pacific Ocean as “Southwest Pacific Bottom Water” (from herein, SPBW), based on the potential vorticity and δ3He signals it acquires at a middepth source located far from the Antarctic continental slope. The name “ACCbw” refers to a water type restricted to the ocean floor in the ACC region and then exported equatorward (see temperature and salinity distribution in plates 220–221 in Orsi and Whitworth 2005). Traces of SPBW signals are found downstream, both to the east at the bottom of southern Drake Passage, and to the west at deep levels of the southern Ross Gyre. The interior vertical mixing of new bottom waters from the Ross Sea needed for the regional AABW to continue equatorward over the ridges is therefore partly reflected in the SPBW characteristics.

The vertical layering of water masses along the path of the ACC and the formation of AABW in the Ross Sea have been extensively documented; however, our understanding of the interaction between these major elements of the deep stratification remains incomplete. We focus our study on the potential vorticity and other tracer signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. We trace the circulation of a water mass we term Southwest Pacific Bottom Water over roughly two decades, using results from the World Ocean Circulation Experiment (WOCE; 1991–2000) and Climate Variability and Predictability (CLIVAR; 2000 onward).

The outline of the remainder of the manuscript is as follows. We begin with a description of the cruise data and define potential vorticity used here as a tracer. We then describe the pathways of distinct potential vorticity signals in the Southwest Pacific bottom water density class from the Pacific–Antarctic Ridge to the Ross Sea and Drake Passage. We assess the influence of other major ridges and finally summarize and discuss our results.

2. Data and methods

a. WOCE and CLIVAR cruise data

This study uses hydrographic data from multiple cruises (Table 1; Fig. 1). All of the data and final cruise reports are available from the CLIVAR and Carbon Hydrographic Data Office (CCHDO; http://cchdo.ucsd.edu/). We have also analyzed 1973/74 hydrographic data from the Geochemical Ocean Sections Study (GEOSECS), also available from CCHDO. None of these data were collected during winter; however, our research focuses on deep and abyssal waters so we do not expect any seasonal bias. The main variables of interest are pressure, potential temperature, and salinity, measured using conductivity–temperature–depth (CTD) instruments with an accuracy of at least 2 m, 0.005°C, and 0.005, respectively. CTD results are routinely reported at 1-dbar resolution, but we subsampled that data every 10 dbar. Horizontal station spacing for WOCE and CLIVAR is routinely 55 km. Potential vorticity, a measure of stratification and a focus for our study, is calculated using neutral density. Neutral density is based on temperature and salinity and spatial location of the cast, and is calculated using software detailed in Jackett and McDougall (1997). The tracers, δ3He, silicate and chlorofluorocarbons were sampled using Niskin-type bottles and a Rosette and typically have accuracies of 1.5%, 2%, and 1%, respectively. Most of these cruises collected 36 bottle samples at each station; however, for these tracers only silicate was measured in all bottles. All of the WOCE and CLIVAR cruise data have been subjected to multiple levels of quality control and are believed to be of very high quality by modern standards. The GEOSECS data were high quality for the time (1970s), but are not quite as accurate or precise as the newer data.

Table 1.

Cruises of analyzed hydrographic (including 3He) data. Note: the CLIVAR S4P line also includes western (172°E), central (170°W), and eastern (150°W) Ross Sea sections that extend south of 67°S to about 72°S. 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.

Table 1.
Fig. 1.
Fig. 1.

Map of cruise sections used in our analysis. Red sections denote those collected during the WOCE era (1990 to 1996), and blue dashed sections after 2005 (during the CLIVAR period). The P16, P18, and S4P lines were sampled during both the WOCE and CLIVAR periods. Magenta dots and numbers indicate GEOSECS cruise stations, and these are referred to later in the paper in Fig. 7.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

b. Potential vorticity

Potential vorticity is a measure of stratification in the water column, and we are able to detect subtle changes in the abyssal ocean stratification with the use of hydrographic data that is high resolution in the vertical direction. Potential vorticity (Q) quantifies the ratio of the combined relative vorticity (ζ) and planetary vorticity (or Coriolis parameter; f; negative in the Southern Hemisphere) to the thickness of the water column, and is given by
e1
where ρ is the mean density, and the local vertical change in density over that in depth is given by ∂ρ/∂z. We assume that the relative vorticity is negligible for large-scale flows, and we use neutral density γn.
Hence,
e2
We assume a positive sign for potential vorticity throughout our text, figures, and equations. A small potential vorticity corresponds to a small change in density with respect to depth, and hence weak stratification (and a well mixed fluid).
In a layered framework, the potential vorticity equation can be written as
e3

The first term represents the meridional advection of planetary vorticity. The second term (h is layer thickness) is vortex stretching because of the isopycnal component of the velocity (conservative stretching). The third term represents the diapycnal (nonconservative) stretching because of diapycnal fluxes at the top (T) and bottom (B) of the layer, and the fourth term is the isopycnal diffusion of potential vorticity (where mixing κ is assumed to be constant).

Potential vorticity is not conserved near the plume source. Heat injected from the earth’s mantle and flow over the bottom can generate diapycnal fluxes that modify potential vorticity. Frictional torques experienced by the abyssal ocean will alter the local density field and break the conservation constraint for potential vorticity as well. Large-scale lateral eddy stirring κ does not directly modify density since the stirring is essentially along isopycnals, but it does smooth out potential vorticity gradients on these isopycnals, spreading tracers away from sources. These nonconservative influences may be a factor in setting the potential vorticity signals of SPBW once it moves away from the direct influence of the plume source.

3. Results

a. Identification of potential vorticity signals along the Pacific–Antarctic Ridge

Winckler et al. (2010) illustrated the existence of a unique δ3He plume south of the Pacific–Antarctic Ridge along the WOCE P16 section, as well as along the 67°S WOCE S4P section. We found that this distinct δ3He plume signal of over 10% at the ridge crest around 59°S coincides with a potential vorticity minima signal (Fig. 2a). The minimum in the potential vorticity of less than 5 × 10−12 m−1 s−1 rises above the ridge along the 28.2 kg m−3 surface (see yellow contour). Directly below the potential vorticity minima, we find a maximum in the potential vorticity of 1.1 × 10−11 m−1 s−1 that is evident south of 60°S. We note that the potential vorticity of the plume is not conserved near the ridge crest, where it is influenced by geothermal heat, eddies, and bottom friction. South of 61°S and downstream of the ridge, the plume retains a potential vorticity close to 9 × 10−12 m−1 s−1. The density range of SPBW encompasses the low potential vorticity signal along the 28.2 kg m−3 surface.

Fig. 2.
Fig. 2.

WOCE P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) and (a) δ3He (black contours; %; asterisks denote sampling locations for δ3He), (b) vertical temperature gradient (black contours; °C dbar−1), and (c) vertical salinity gradient (black contours; CTD dbar−1). The gradients are the vertical difference in temperature or salinity with respect to pressure. We draw the reader’s attention to the southward-flowing upwelling Lower Circumpolar Deep Water, represented by the salinity maxima [>34.73; red dashed contours in (c)]. (d) CLIVAR P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) and vertical temperature gradient (black contours; °C dbar−1).

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

The WOCE P16 section at 150°W lies within 30 km downstream of potential plume source regions identified by Winckler et al. (2010) (see their Fig. 3). We infer that the WOCE P16 section is close to a plume source, though we cannot determine the exact distance with the presently available measurements. We calculate the temperature anomaly in density space along the 28.2 kg m−3 surface, defined as the difference between the background temperature away from the ridge at 56°S (taken to be 0.6°C) and the temperature at the ridge crest at 59.5°S. We find that the Pacific-Antarctic Ridge crest has a 0.07°C temperature anomaly, which compares well with the 0.04°C anomaly found at the Juan de Fuca Ridge (Cannon and Pashinski 1997).

The localized minimum in the vertical temperature gradient (i.e., the local change in temperature with depth) along WOCE P16 clearly coincides with the potential vorticity minima (Fig. 2b); however, the relatively shallower minimum of the vertical salinity gradient is unrelated to the potential vorticity signals around the 28.2 kg m−3 surface (black contours in Fig. 2c). We also observed a strong correlation between the vertical temperature gradient and the potential vorticity signal for the CLIVAR P16 section (Fig. 2d). The temperature gradient is weaker in the modern data, possibly because of the small offset in location and large time difference between occupations. Along the WOCE P16 line, there is a clear δ3He maxima within the vicinity of the Pacific-Antarctic Ridge, between the mean temperature 0.4° and 0.6°C (Fig. 3).

Fig. 3.
Fig. 3.

WOCE P16 potential temperature (°C) vs δ3He (%) for stations south of 55°S, between 800 and 3500 m. The potential temperatures associated with the potential vorticity (Q) minima and maxima signals (9 × 10−12 m−1 s−1 and 1.1 × 10−11 m−1 s−1, respectively) are denoted by the red vertical lines.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Hydrothermal plumes generated close to, but upstream, of the WOCE P16 section undergo several modifications. Geothermal heat released into the water column at the Pacific–Antarctic Ridge would destabilize the water column, thus reducing the potential vorticity. However, potential vorticity can also be influenced by the enhanced flow over a rough bottom and mixing induced in the canyons on the flank of the ridge (Thurnherr et al. 2005).

To determine if additional mixing sources are at play, we can estimate the scale (H) over which the plume effects might occur (without knowledge of the exact depths of the sources we are limited to scaling estimates). We use the equation from Speer and Helfrich (1995), H ≈ 3.76(FoN−3)1/4, where 3.76 is a coefficient based on laboratory and in situ observations. The local buoyancy frequency squared, N2 = −gQ/f is based on the Coriolis parameter (f), the acceleration due to gravity (g = 9.81 m s−2), and the potential vorticity (Q) estimated in Eq. (2), and has units of s−2. The source buoyancy flux (Fo) is typically between 10−2 and 10−1 m4 s−3 (Speer and Helfrich 1995). Taking the local background potential vorticity of the vent source along the WOCE P16 section to be 5 × 10−12 m−1 s−1 and the range of Fo values, we predict a vertical scale between 300 and 530 m. Given that the plume appears to have vertical scales substantially larger, rising to the 28.2 kg m−3 surface that flows ~1 km above the ridge crest (Fig. 2), it is likely that other mixing mechanisms do occur at the ridge crest and on its flank. However, distinguishing these mixing mechanisms is beyond the scope of this study.

As it rises, the plume transports warmer waters into the surrounding fluid and entrains cooler ambient waters until it reaches a height where the background density is similar to its own (Joyce et al. 1986; Speer and Rona 1989). The plume (along with bottom mixed layer contributions) attains eventually a minimum value in potential vorticity of about 9 × 10−12 m−1 s−1 and is advected along the 28.2 kg m−3 surface both poleward into the Ross Gyre and downstream along the ACC southern fronts. We find that the layer affected by the plume flows deeper than the salty LCDW (Fig. 2c).

There is in addition a dynamical effect of the mixing and plume source near the bottom. Once the plume reaches its maximum vertical spreading height, it is transported westward via a diffusive phase speed or eastward via advection (Joyce and Speer 1987). The 28.2 neutral density surface lies within the coldest portion of LCDW, and hence within the deep meridional overturning cell (Speer et al. 2000), so when the δ3He tracer is injected from the vent, it spreads laterally in the absence of ambient currents. We estimate the phase speed of the long baroclinic Rossby waves (Joyce and Speer 1987) using the formulation:
e4
where the beta plane approximation β = ∂f/∂y ≈ 10−11 m−1 s−1 is the latitudinal variation in the Coriolis parameter, f ≈ −1.25 × 10−4 s−1. The thermal expansion coefficient, α ≈ 3 × 10−4 °C−1, and g is acceleration due to gravity. The background vertical temperature gradient is estimated as 5 × 10−4 °C m−1 from Fig. 3. The vertical wavenumber m = /H, where n = 1, 2, … is the forcing mode, and H is the previously calculated height the plume rises before equilibrating with the background buoyancy. The first three modes and an average plume rise height of 450 m give a westward phase speed of less than 2 × 10−3 cm s−1.
We estimate the eastward advection along the WOCE P16 section using the thermal wind equation:
e5
where the ρ is the ocean density, with a mean of ρ0 = 1028 kg m−3. The meridional density gradient is integrated over depth (z), giving a mean eastward flow of at least 1 cm s−1, 500 m above the ridge crest. The eastward advection, due to the ACC in this case, is one to three orders of magnitude greater than the diffusive phase speed, implying that the plume is entirely passive and swept downstream along the 28.2 kg m−3 surface. Other studies have quantified a larger zonal velocity within the ACC (e.g., Renault et al. 2011; Zhang et al. 2012), further emphasizing our conclusion that advection via the ACC dominates the plume’s pathway.

b. Tracing SPBW downstream of the Pacific–Antarctic Ridge

We trace SPBW potential vorticity signals along the path of the ACC in the WOCE cruises south and east of the Pacific–Antarctic Ridge (Fig. 4). We find that potential vorticity signals along the WOCE P16 diagonal section sampled between (135°W, 62°S) and (147°W, 58°S), as well as the southern parts of the meridional sections (P16, P17, P18, P19; Figs. 2 and 4), coincide with the δ3He tracer (~10%). The stronger δ3He signal (~14%) in the WOCE P18 and P19 sections are associated with the East Pacific Rise at a lower density (~28.0 kg m−3; Fig. 4). SPBW is carried along the ACC between the Polar Front (PF) and Southern Boundary Front (SBDY), as shown by the bold parts of the cruise lines in Fig. 5. Along the WOCE S4P section, the SPBW potential vorticity and δ3He signals are present across the entire section, as noted in the Winckler et al. (2010) study. This indicates that SPBW flows poleward of the Pacific–Antarctic Ridge, in the Ross Gyre (discussed further in section 3d) and downstream along the ACC.

Fig. 4.
Fig. 4.

Potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (thin black contours; %) and 28.2 kg m−3 neutral density surface (yellow) for WOCE Pacific diagonal P16 line [between (135°W, 62°S) and (147°W, 58°S)] , the meridional lines P17, P18, P19, and the zonal line S4P. Asterisks denote sampling locations for δ3He. Note that only one δ3He sample was available for the P17 cruise, hence we have omitted δ3He contours and asterisks from this section. The 10 × 10−12 m−1 s−1 (11 × 10−12 m−1 s−1 for P19) thick black contour in each panel highlights the SPBW potential vorticity minima (along the 28.2 kg m−3 surface) and maxima. (bottom right) (Drake Passage, i.e. WOCE A21) potential vorticity (color) overlaid with δ3He (solid black contours) and the 28.0 kg m−3 (upper dashed contour) and 28.2 kg m−3 (lower dashed contour) density surfaces.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Fig. 5.
Fig. 5.

Schematic of the Pacific Ocean bathymetry (blue hues; m) overlaid with the fronts that encapsulate the ACC (from north to south): the SAF, PF, SACCF, and the SBDY. GEOSECS stations are indicated by magenta dots with stations shown adjacent in white boxes with magenta text. The WOCE P14, P15, P16 (P16A), diagonal P16 line (P16D), P17 (P17E), P18, P19, and S4P lines are denoted in red. The CLIVAR P16, P18 and S4P cruises are shown by blue dashed lines. The CLIVAR S4P cruise is divided into the main cruise section along 67°S (S4P), and the western, central, and eastern Ross Sea sections (RW, RC, and RE, respectively). Potential plume source locations are taken from Winckler et al. (2010). The bold part of the cruise lines denotes regions where we observe SPBW.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

East of the S4P section, we have no evidence that the potential vorticity or δ3He signals associated with SPBW survive the intense mixing encountered in Drake Passage (Fig. 4; bottom right panel). The stronger δ3He signal (~17%) on the 28.0 kg m−3 surface represents the diluted signal from the East Pacific Rise hydrothermal vents that is advected into the ACC from the north in the Southeast Pacific (Lupton 1998; Well et al. 2003; Naveira Garabato et al. 2007). The observations presented in our results thus far show strong evidence of SPBW potential vorticity and δ3He signals that are advected along the path of the ACC east of the Pacific-Antarctic Ridge. In the next section we will demonstrate that the SPBW we trace in this study originates along the Pacific-Antarctic Ridge, and not upstream of the ridge.

Remarkably, across the WOCE and CLIVAR periods the magnitude and location of the potential vorticity minima and maxima in the SPBW density range remain unchanged (Fig. 6), and the 28.2 kg m−3 surface remains almost fixed in depth for the P16, P18 and S4P sections. We can also trace the δ3He signal pre-WOCE using Pacific data collected during the GEOSECS (Fig. 7). There are two δ3He signals present south of 40°S. The first is the strong δ3He signal of approximately 20% near 28.0 kg m−3 that originates along East Pacific Rise (station 322; 43°S, 130°W), and dissipates by the time it reaches stations 282 and 290. The second δ3He maxima of ~10% occurs near 28.2 kg m−3 for Stations 286 and 287 (67°S to 56°S, 170°E to 170°W), that lie around the Pacific–Antarctic Ridge. This second δ3He signal agrees in magnitude and density with the WOCE P16 (150°W) observations and other Pacific sections. The GEOSECS, WOCE and CLIVAR δ3He data presented here provide evidence of hydrothermal vent activity along the Pacific-Antarctic Ridge near the 28.2 kg m−3 surface with δ3He of ~10% since the early 1970s.

Fig. 6.
Fig. 6.

Potential vorticity (color and black contours, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) for Pacific lines P16, P18, and S4P. (top) Cruises during the WOCE period are shown; (bottom) CLIVAR cruise lines are shown.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Fig. 7.
Fig. 7.

GEOSECS Pacific stations (Legs 7, 8, and 9) south of 40°S for pressure greater than 1000 dbar. Shown is δ3He (%) in neutral density space (γn; kg m−3). The symbols for each station are shown in the legend, and station locations are shown in Fig. 1. Stations in the vicinity of the Pacific–Antarctic Ridge are: 282 (purple squares), 286 (green circles), and 287 (orange triangles).

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

c. Signals upstream of the Pacific–Antarctic Ridge

To examine the uniqueness of the SPBW potential vorticity and δ3He signals found in the Pacific basin, we analyze cruise data directly upstream of the WOCE P16 section (WOCE P14 and P15; Fig. 8) and three Indian Ocean lines upstream of the Pacific–Antarctic Ridge, namely I08S, I09SS, and S03 (Fig. 9 and Table 1). In Fig. 8 the 28.2 kg m−3 surface does not intersect the Pacific–Antarctic Ridge (~64°S), but rather flows at least 500 m above the ridge crest. We observe a maxima in the potential vorticity (1.1 × 10−11 m−1 s−1) south of the ridge crest along both the P14 (~64.5°S) and P15 (~66°S) sections. The maxima is below the 28.2 kg m−3 surface for the P14 section (similar to the maxima illustrated in Fig. 4), however, the maxima and 28.2 kg m−3 surface coincide along the P15 line. The potential vorticity minima for the P14 section is difficult to observe in Fig. 8, however, it is clearly highlighted in Fig. 9a; the P15 section minima is less well defined as data unavailable at densities greater than ~28.21 kg m−3. In addition, the GEOSECS station 282, located north of the crest near the WOCE P14 section (Fig. 5), indicates a δ3He maxima near 28.2 kg m−3 (Fig. 7). The P14 (and to a lesser extent, P15) minima in potential vorticity near the 28.2 kg m−3 surface combined with the GEOSECS station 282 δ3He data imply hydrothermal plume sources ~900 km farther upstream than the region proposed in Winckler et al. (2010) (see yellow bars in Fig. 5 for their potential plume sources).

Fig. 8.
Fig. 8.

Potential vorticity (color and black contours, ×10−12 m−1 s−1) overlaid with 28.2 kg m−3 neutral density surface (yellow) for WOCE Pacific lines P14 and P15. Unfortunately, δ3He was not sampled along these lines, and thus we only show potential vorticity.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Fig. 9.
Fig. 9.

(a) Neutral density (kg m−3) vs potential vorticity (×10−11 m−1 s−1) for the region restricted by density range 27.9 < γn < 28.4 kg m−3 and between the Polar Front and Southern Boundary Front (see also Fig. 5). Shown are the WOCE Indian lines I08S, I09S, S03, and the WOCE Pacific lines P16, P18, P19, and S4P. Note that P16 (the most discussed cruise in this study) is denoted by a bold black curve. The vertical dotted line denotes the 28.2 kg m−3 surface. Note that I08S, I09S and S03 are located upstream of the Pacific–Antarctic Ridge and are indicated by small-dashed curves. (b) As in (a), but with neutral density (kg m−3) vs δ3He (%). P14 (red) and P15 (dark blue) are included in panel (a) in bold large dashed curves, but δ3He data was not available on these two cruises so they are omitted from panel (b).

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Near 28.2 kg m−3, we also observe a strong deflection in the P16 potential vorticity curve, and less so for P18, P19 and S4P at a slightly lighter density (Fig. 9a). The upstream I08S, I09S, and S03 curves show no distinct deflection near this density. However, there is a potential vorticity minima evident at ~28.05 kg m−3 for the upstream sections that is primarily associated with interaction of the density surfaces and the regional topography. We find no clear deflections in δ3He for the sections upstream of P16 (Fig. 9b), including at 28.2 kg m−3 where we observe decreases in potential vorticity (Fig. 9a). In contrast, the Pacific sections (P16, P18, P19 and S4P) show a clear elevation in δ3He (to ~10%) at 28.2 kg m−3, followed by a sharp decline in δ3He at higher densities. The lower Indian δ3He signature (~8.5% near 28.05 kg m−3) is derived from the Central Indian Ridge (see Fig. 11) and other tropical sources.

d. The Ross Gyre and Ross Sea

Recent high-resolution model- and data-based studies have indicated large eddy mixing and topographic roughness on the southern flank of the Pacific-Antarctic Ridge where the ACC and Ross Gyre interact (Thompson 2008; Lu and Speer 2010; Nikurashin and Ferrari 2011). Eddy mixing provides a means for the SPBW potential vorticity signals to be transported poleward from the Pacific–Antarctic Ridge, and across and between the ACC fronts. Recent data from the CLIVAR S4P cruise (Fig. 10) includes both repeated stations of the WOCE S4P zonal line along 67°S, and three meridional sections in the Ross Gyre that are essentially poleward continuations of the P14, P15, and P16 lines (see Figs. 1, 5, and 8). δ3He data are not yet available for the CLIVAR S4P cruise, but we can trace the SPBW potential vorticity minima and maxima near the 28.2 kg m−3 surface along the Eastern and Central Ross Sea sections (Fig. 10).

Fig. 10.
Fig. 10.

Potential vorticity (color, ×10−12 m−1 s−1) overlaid with the 28.2 kg m−3 neutral density surface (yellow contour) for the CLIVAR line S4P. (top left) The zonal 67°S part of the line (referred to as ‘CLIVAR S4P’ in the text) with the longitude of the zonal Ross Sea sections denoted by red vertical lines. Also see Fig. 5 for section locations. The remaining panels show potential vorticity and neutral density surfaces along the Western, Central and Eastern Ross Sea sections (RW, RC and RE, respectively). The 10 × 10−12 m−1 s−1 black contour in each panel highlights the SPBW potential vorticity minima and maxima.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

In the Central Ross Sea section, the SPBW potential vorticity signal is evident until it reaches the continental shelf around 74°S, where deep waters are modified and then mix with shelf waters to form AABW (e.g., Orsi et al. 1999, 2002). The interannual variability in the properties of SPBW and Circumpolar Deep Waters and Ross Gyre strength are principal drivers in AABW production rates (Assmann and Timmermann 2005). From the Central and Eastern Ross Sea sections, we conclude that the SPBW originating at the Pacific-Antarctic Ridge flows poleward via the Ross Gyre. The modification of SPBW north of the continental shelf, and the formation of AABW on the shelf dilutes the potential vorticity signal (Western Ross Sea section; Fig. 10).

4. Discussion and conclusions

We use a combined PV and δ3He tracer analysis to identify the mechanisms modifying the properties and circulation of Southwest Pacific Bottom Water (SPBW) in the South Pacific and show the SPBW potential vorticity signature and large-scale pathways change minimally over the past two decades. This provides a physical interpretation of the pathway of hydrothermal plumes originating from the Pacific–Antarctic Ridge, as described by Winckler et al. (2010).

Hydrothermal activity can be identified using several tracers, such as manganese, silicate, and germanium (e.g., Klinkhammer 1980; Mortlock et al. 1993). We analyzed silicate data available on South Pacific WOCE cruises; however, high silica also originates from other regions surrounding the Pacific–Antarctic Ridge, in the North Pacific, and Ross Gyre. Hence, the silicate found near the ridge vent sites is thus not necessarily indicative of plumes. Cholorfluorocarbons (CFCs) and oxygen have been repeatedly used to trace deep and bottom waters in the Southern Ocean (Orsi et al. 1999, 2002). However, LCDW and SPBW are both old, well mixed water masses with a low CFC signal, and are thus difficult to separate using CFCs. In the recent CLIVAR S4P Ross Sea sections, we identified distinct nutrient signals (oxygen, nitrate, and phosphate) that were evident but irrelevant to the SPBW potential vorticity signals (figure not shown). Use of high resolution potential vorticity and helium data have provided a means of separating these deep water masses in the Pacific Ocean.

We also investigated the potential vorticity and δ3He along several other cruise lines around the global ocean and found no ridges where the two tracers were coincident (Fig. 11). The WOCE I03 line intersects the Central Indian Ridge at ~65°E. Helium from the ridge flows northeastward and in a cyclonic path around the Ninety-East Ridge (e.g., Srinivasan et al. 2009; Drijfhout and Naveira Garabato 2008). We also observe the 15% δ3He signal at 8°S on the I02 cruise line (figure not shown). Along the Juan de Fuca Ridge (40°–45°N; WOCE P17N), a δ3He signal of 24% can be observed; however, the spread of the plumes originating along this ridge are focused in their source region to within a degree or so of longitude (Veirs et al. 1999; Cannon and Pashinski 1997). In the South Pacific (WOCE P06; Fig. 11), hydrothermal plumes originating along the East Pacific Rise flow westward and counterclockwise over the ridge, then southeastward toward Drake Passage (Lupton 1998; Well et al. 2003; Naveira Garabato et al. 2007; Bianchi et al. 2010).

Fig. 11.
Fig. 11.

Four cruise lines that intersect prominent ridges with hydrothermal activity. Potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (black contours; %) for WOCE lines I03 (20°S), P17N (145°W), P06 (32°S), and A10 (30°S). Black dots denote sampling locations for δ3He.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

We observe the high δ3He signal (>24%) on both sides of the East Pacific Rise at 32°S (WOCE P06; Fig. 11). Johnson and Talley (1997) suggested that stratification and δ3He signals associated with hydrothermal plumes downstream of the East Pacific Rise are likely to coincide in the absence of vertical mixing. Along the Mid-Atlantic Ridge a distinct, but weaker, δ3He signal of ~6% has been observed along, and equatorward of, the WOCE A10 section (Fig. 11; Well et al. 2001; Rüth et al. 2000). Primordial helium (defined by anomalies relative to the background) plumes emanating from the Mid-Atlantic Ridge at 30° and 11°S spread across the western Atlantic basin (Rüth et al. 2000). In contrast, potential vorticity signals created above the ridge have been related to local mixing (Thurnherr et al. 2005). These examples illustrate the dependence of these two tracer signals on background fields as well as local processes.

After analyzing several cruise lines near other major ocean ridges, we infer that the SPBW potential vorticity coinciding with δ3He signals are likely unique to the Pacific–Antarctic Ridge. Several physical processes coincide along the Pacific–Antarctic Ridge to form the potential vorticity and δ3He signals and then transport them across the South Pacific (see Fig. 12):

  • Hydrothermal vents along most of the ridge inject significant heat into the water column, destabilizing the stratification.

  • The efficiency of eddy mixing is large and persistent along the entire Pacific Antarctic Ridge for the intermediate and deep ocean (Lu and Speer 2010).

  • The ACC overlies the vents along the Pacific–Antarctic Ridge. This sweeps them downstream, between the Polar Front and Southern Boundary. Strong westerlies driving the ACC induce a steep gradient in the density surfaces across the Pacific–Antarctic Ridge. Bottom waters originating in the Ross Sea flow equatorward to the ridge, carrying a low potential vorticity signal, and mix with lighter LCDW. The multiple hydrothermal sources along the Pacific–Antarctic Ridge (Winckler et al. 2010) provide abundant opportunity for deep waters to interact with the hydrothermal plume within the ACC along the 28.2 kg m−3 surface. Thus, different layers within the ACC can have vastly different origins.

Fig. 12.
Fig. 12.

WOCE P16 potential vorticity (color, ×10−12 m−1 s−1) overlaid with δ3He (black contours; %) as shown in Fig. 2a. Asterisks denote sampling locations for δ3He. White dashed arrows denote UCDW and LCDW that flow southward from the North Pacific and upwell in the ACC region, bound by the SAF to the north, and the SBDY to the south. The SACCf and PF lie within the ACC. The low potential vorticity signature of modified bottom waters (orange dashed arrows) emphasizes that which is induced by the hydrothermal plume source at the Pacific–Antarctic Ridge. Lateral eddy mixing along the ridge (white spiral) and along the 28.2 kg m−3 surface, the ACC eastward flow, and the Ross Gyre circulation transport the low potential vorticity signature/high δ3He signal (i.e., SPBW) downstream.

Citation: Journal of Physical Oceanography 42, 12; 10.1175/JPO-D-12-019.1

Several questions remain unanswered. From the results presented here and in Winckler et al. (2010), it is evident that more detailed surveying of the Pacific–Antarctic Ridge and surroundings are required, to determine exact vent locations, source strengths and local spreading rates. Such high-resolution surveys have been performed extensively over ridges such as the Mid-Atlantic Ridge, Juan de Fuca Ridge, and East Pacific Rise (e.g., Lupton 1998; Rüth et al. 2000, and references therein), but would be valuable in this region, since decomposing the diapycnal and along isopycnal eddy mixing, deep water upwelling, hydrothermal activity, and ACC flow requires better information about the abyssal interactions with the midocean ridge.

Acknowledgments

The authors wish to thank M. Nikurashin, J. Sarmiento, and two anonymous reviewers for useful comments and discussion. We greatly appreciate the efforts of those who collected and processed samples on the cruises mentioned in this paper. SD was supported by the Office of Science (BER), U.S. Department of Energy, Grant DE-FG02-07ER64467 and NOAA Grant NA07OAR4310096. RK was supported by National Ocean and Atmospheric Administration, Grant 08OAR4320752. KS acknowledges support from NSF OCE-0622670 and NSF OCE-0822075.

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