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  • View in gallery

    Annual mean (a) salinity (psu) and (b) oxygen concentration (ml L−1) superimposed with acceleration potential (m2 s−2; white solid lines) at 27.2 σθ relative to 2000 dbar calculated from the World Ocean Atlas 1998

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    Geographical distribution of (a) temperature–salinity and (b) oxygen profiles (asterisk) from World Ocean Database 1998 and (c) CTD stations (asterisk) along PR22 collected during WOCE. Light solid lines indicate 500-m isobath in (a) and (b) and 1500-m isobath in (c). The heavy solid and dashed lines in (c) show the geographic locations of vertical sections used for Figs. 10 and 13, respectively

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    Probability of AAIW defined as the percentage of salinity profiles that contain at least one salinity minimum at the density range of 27.0–27.4 σθ. Boxes A, B, and C show the geographic locations of the boxes used for Fig. 4

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    Relations of temperature vs salinity, temperature vs oxygen concentration, and salinity vs oxygen concentration in boxes A (5°– 10°S, 148°–153°E), B (0°–5°N, 130°–135°E), and C (10°–15°N, 125°–130°E) as indicated in Fig. 3. The solid lines in the left panels denote 26.8 and 27.2 σθ surfaces, respectively

  • View in gallery

    Distributions of potential density (kg m−3), temperature (°C), and salinity (psu) of AAIW as defined in Fig. 3. The heavy dashed lines denote the 0.5 probability contour

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    Distributions of (a) isopycnal depth (m), (b) temperature (°C), (c) salinity (psu), and (d) oxygen concentration (ml L−1) at 27.2 σθ

  • View in gallery

    Distributions of (a) isopycnal depth (m) and (b) oxygen concentration (ml L−1) at 27.0 σθ. Areas with oxygen concentration larger than 2.0 ml L−1 are shaded in (b)

  • View in gallery

    Oxygen distribution (ml L−1) against depth (m) at 160°E. Area with oxygen concentration less than 1.5 ml L−1 is shaded

  • View in gallery

    As in Fig. 7 except at 27.6 σθ

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    Geostrophic velocity (cm s−1) against depth (m) from Qu and Lukas (2003) superimposed on oxygen concentration (ml L−1) at sections A (12°N), B (14°N), C (16°N), D (18°N), E (20°N), and F (22°N). Positive values are northward, and the heavy dashed lines indicate the 27.2 σθ surface. The geographic locations of these sections are shown in Fig. 2c

  • View in gallery

    Geostrophic velocity (cm s−1) relative to 2000 dbar against depth (m) superimposed on oxygen concentration (ml L−1) and salinity (psu) averaged from six hydrographic sections at 18°20′N. Positive velocities are northward

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    As in Fig. 11 but for individual cruises

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    Vertical sections of (a) salinity (psu) and (b) oxygen concentration (ml L−1) against depth (m) along 20.25°N across the Luzon Strait. Areas with salinity less than 34.45 psu or oxygen less than 2.1 ml L−1 are shaded. The heavy dashed lines indicate 27.2, 27.5, 27.6, and 27.65 σθ surfaces, respectively. The geographic location of this section is shown in Fig. 2c as a heavy dashed line (M)

  • View in gallery

    Vertical profiles of annual mean salinity (psu) and oxygen concentration (ml L−1) against depth (m) in the South China Sea (SCS) and the North Pacific (NP). The geographic locations of the two stations (asterisks) are shown in Fig. 13: One (SCS) is at (20.25°N, 118.25°E), and the other (NP) at (20.25°N, 129.25°E)

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Northward Intrusion of Antarctic Intermediate Water in the Western Pacific

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  • 1 International Pacific Research Center, School of Ocean and Earth Science Technology, University of Hawaii at Manoa, Honolulu, Hawaii
  • 2 Office of Earth Science, National Aeronautics and Space Administration, Washington, D.C
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Abstract

The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertical mixing and eventually flows back to the Pacific through the Luzon Strait at depths of AAIW.

Corresponding author address: Dr. Tangdong Qu, IPRC/SOEST, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI 96822. Email: tangdong@hawaii.edu

Abstract

The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertical mixing and eventually flows back to the Pacific through the Luzon Strait at depths of AAIW.

Corresponding author address: Dr. Tangdong Qu, IPRC/SOEST, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI 96822. Email: tangdong@hawaii.edu

1. Introduction

Antarctic Intermediate Water (AAIW) is formed in the Antarctic convergence region and spreads throughout the middle and lower latitudes of the Pacific Ocean at intermediate depths by lateral mixing and general wind-driven circulation (Reid 1965, 1997; Talley 1999). It is entrained into the subtropical gyre in the eastern South Pacific as it flows eastward in the Antarctic Circumpolar Current. From there, it moves anticlockwise around the gyre and flows westward at low latitudes (∼20°S) as a tongue of low salinity and high oxygen (O2) extending well into the Coral Sea (Fig. 1). Its flow path near the western boundary of the South Pacific has been widely investigated. Among those early studies, Rochford (1960) and Reid (1965) showed evidence that AAIW extends from the Coral Sea into the western North Pacific, but the exact pathway had remained obscure until Lindstrom et al. (1987) revealed the existence of the New Guinea Coastal Undercurrent (NGCUC).

The NGCUC is important not only as a major source of the Equatorial Undercurrent water but also as the pathway of AAIW in the western South Pacific (Lindstrom et al. 1990; Tsuchiya 1991). This strong northwestward flow extends down to at least 900 m and carries AAIW from the Solomon Sea to the equator through the Vitiaz Strait. In a recent analysis of historical data, Qu and Lindstrom (2002) further showed that there is a strong water property connection between the Coral Sea and Solomon Sea, and the water masses in the NGCUC can be traced continuously in the flow path of the Great Barrier Reef Undercurrent, the North Queensland Current, and the NGCUC, giving positive evidence for the hypothesis of their South Pacific origin (Tsuchiya 1968; Lindstrom et al. 1987).

After crossing the equator in the far western Pacific, part of the AAIW flows eastward in the equatorial circulation, while the rest continues northward along the western boundary of the North Pacific (Reid 1965; Tsuchiya 1991). The primary pathway for this northward intrusion has been identified to be the Mindanao Undercurrent (MUC) centered at 700–900 m about 75–100 km offshore (Hu et al. 1991; Qu et al. 1998), through which AAIW extends to about 13°N along the Philippine coast (Masuzawa 1972; Lukas et al. 1991; Bingham and Lukas 1994; Fine et al. 1994; Wijffels et al. 1995; Qu et al. 1999).

Although the high-oxygen and low-phosphate characteristics of AAIW can be traced farther north to the midlatitudes of the western North Pacific (Reid 1965; Reid and Mantyla 1978), its direct pathway is still not understood. Qu et al. (1999) suggested that the MUC is merely a component of a recirculation associated with the Halmahera eddy that tilts northward at increasing depth. Water joining this recirculation from the south is advected northward via the MUC and bended eastward on its northern side before reaching about 13°N. This flow path of AAIW was also shown in numerical models (e.g., McCreary and Lu 2001).

Then, how can high-oxygen, low-phosphate water identical with AAIW extend to the midlatitudes of the western North Pacific (Reid 1965)? This study is intended to examine the northward intrusion of AAIW by utilizing all existing historical data combined with synoptic measurements from a repeated hydrographic section conducted by the People's Republic of China and United States joint program (PRC–US). We note that earlier studies of Reid (1965) and Reid and Mantyla (1978) were based on limited hydrographic observations. Their property distributions were most problematic near the western boundary, where zonal scale of the circulation is smallest. A great number of observations have become available since then, in particular, after the success of the Tropical Ocean and Global Atmosphere (TOGA) and World Ocean Circulation Experiment (WOCE) programs. Analyses of these data will, as shown below, provide improved property distributions in the western Pacific.

The results of this study are presented in the following sections. After a brief description of the data and methods of analysis in section 2, AAIW is first traced as a salinity minimum in section 3 and then along the isopycnal surface in section 4. The property distributions underlying and overlying AAIW are shown in section 5. The flow path of AAIW along the western boundary is examined in section 6. The possible link between the South China Sea (SCS) deep water and the Pacific intermediate waters is discussed in section 7, and results are summarized in section 8.

2. Data and methods of analysis

a. Historical data

The historical data used for this study are those CTD and bottle profiles at observed levels recorded on the CD-ROMs of World Ocean Database 1998 of the National Oceanic and Atmospheric Administration National Environmental Satellite, Data, and Information Service National Oceanographic Data Center for the region 30°S– 40°N, 110°E–180°. The procedures for quality control of these data are the same as those described by Qu and Lindstrom (2002). After all those procedures (see Qu and Lindstrom for more details), the final dataset provides 53 884 profiles of temperature/salinity and 31 890 profiles of oxygen extending to 500 m or deeper in the whole region studied (Figs. 2a,b), spanning the period from the 1920s to the late 1990s.

The temperature T, salinity S, and oxygen data from individual profiles are first interpolated onto a set of density surfaces with a 0.02 kg m−3 vertical resolution using a cubic spline and then averaged in a 0.5° × 0.5° grid, regardless of the date and time of observation. At the sea surface, the concept of averaging on isopycnal surfaces breaks down because of the outcropping of shallower isopycnals. Following Gouriou and Toole (1993), properties between the mean mixed layer depth (defined as the first depth at which the potential density is 0.1 kg m−3 greater than the surface value) and the first isopycnal surface present on all profiles are obtained by linear interpolation. The mean property fields are finally smoothed using a Gaussian filter with e-folding scales of about 150 km for temperature and salinity and about 200 km for oxygen.

Standard deviations of properties are also estimated during averaging and used to edit the mean fields (see Qu and Lindstrom 2002). In general, the spatial distribution of standard deviations within the density range of AAIW (∼27.2 σθ), though not presented, is uniform for temperature and salinity in the western Pacific except with slightly higher values near the international date line, where data coverage is relatively sparse. The typical temperature and salinity standard deviations at 27.2 σθ, based on ensembles of at least five samples at each grid cell, are 0.2°C and 0.025 psu. The standard deviation of oxygen concentration at 27.2 σθ is of the order 0.25 ml L−1. In a large part of the North Pacific, it is below 0.15 ml L−1, often reaching 0.1 ml L−1 near the western boundary. The oxygen standard deviation is relatively high in the South Pacific, where its typical value is about 0.3 ml L−1 and often exceeds 0.5 ml L−1 in a front area southeast of the Solomon Sea. In addition to the possible errors in the data, the high standard deviation southeast of the Solomon Sea also reflects influence of different sources both from the east and the south.

A measure of the uncertainty in the mean fields can be roughly afforded by the information of standard deviations described above. For example, the typical standard error in the mean oxygen field at 27.2 σθ, defined as the standard deviation divided by the square root of the number of measurements, is about 0.1 ml L−1 over the western North Pacific, and this value is even less near the Philippine coast. Because the mean fields to be presented in this study neglect the processes associated with mesoscale eddies whose decorrelation scale is at least several hundred kilometers in the region studied, these standard errors are not significant, and most of the large-scale phenomena shown in the mean fields of the present study are representative.

b. Hydrographic observations from WOCE

The hydrographic data used for this study are those from six hydrographic surveys repeated at 18°20′N (PR22) by a joint program between the People's Republic of China and the United States during the WOCE period (Fig. 2c, Table 1). The temperature–salinity–oxygen samples were extended at least to 3000 m or to within 100 m of the ocean floor. These data provide temporally consistent velocity and water property fields to examine the spread of AAIW along the Philippine coast.

3. AAIW as a salinity minimum

To trace AAIW as a salinity minimum, we introduce a quantity that is called the probability of salinity minimum and is defined as the percentage of salinity profiles that contain at least one salinity minimum at densities between 27.0 and 27.4 σθ. The salinity minimum (<34.5 psu) within this density range has a strong signal in the South Pacific and becomes less pronounced as it flows northward into the North Pacific. Intense upwelling and vertical mixing there tend to increase the salinity at its minimum to about 34.55 psu (Reid 1965; Qu et al. 1999). At about 10°N or farther northward, the influence of the North Pacific Intermediate Water (NPIW) is dominant, where a strong vertical minimum (<34.4 psu) in salinity occurs around 26.8 σθ (Talley 1993). To better resolve the weak salinity minimum associated with AAIW, we use a higher salinity range (>34.5 psu) to search for AAIW at latitudes north of 10°N. Then, we choose a variable horizontal radius to include at least 20 salinity profiles for each grid cell. Where multiple minima occur, the one of lowest salinity is used to map AAIW properties.

In the map of the probability of salinity minimum, the spreading of AAIW is clear (Fig. 3). The probability is high in the Coral and Solomon Seas, consonant with its South Pacific origin. Along the western boundary, a high-probability (>0.9) tongue is seen crossing the equator and extending to about 8°N. From there, it drops rather rapidly toward the north, falling below 0.1 at 15°N, and the low-salinity signal of AAIW is lost farther to the north.

This probability distribution is consistent with the pathway of AAIW reported by previous studies. Along the equator, Reid (1997) noted that the flow is essentially eastward at depths of AAIW and below. This equatorial pathway of AAIW is particularly evident in the distribution of oxygen (Fig. 1b), where a tongue of high oxygen is seen extending all the way from the western Pacific to the coast of Peru. Away from the equator, there are eastward flows at AAIW depths around 10° in both hemispheres (Reid 1997; Qu and Lindstrom 2002; Qu and Lukas 2003), presumably associated with the lower part of the Equatorial Countercurrents and/or the subsurface countercurrents (Tsuchiya 1975). Because of these eastward pathways, we see that AAIW is widespread in the western Pacific and high values (>0.8) of probability cover almost the entire region south of 10°N. Also interesting is a high-probability tongue extending toward the Molucca and Banda Seas, indicative of strong influence of South Pacific sources in the intermediate layer of the Indonesian Throughflow.

Water properties show considerable modification in the AAIW characteristics as it moves northward across the equator because of lateral exchange and vertical mixing with the low-salinity North Pacific waters. In the Solomon Sea (box A as indicated in Fig. 3), the South Pacific Tropical Water (SPTW) is seen as a salinity maximum (>35.5 psu) at 20° < T < 25°C and O2 ∼ 3.5 ml L−1 (Fig. 4). Below that depth, temperature and salinity decrease with depth, but oxygen concentration remains much the same until reaching AAIW at about 27.2 σθ that marks the bottom of ventilated thermocline water. As we progress northward along the western boundary, the influence of North Pacific sources becomes increasingly important. In box B (shown in Fig. 3), two minima of salinity (∼34.5 psu) are identified at intermediate depths. The shallower one, characteristic of NPIW, is near the 26.8 σθ surface and the deeper one, with AAIW properties T ∼ 5°C, S ∼ 34.5 psu, and O2 ∼ 2 ml L−1, is around the 27.2 σθ surface. The influence of South Pacific sources decreases farther northward. In box C (shown in Fig. 3), water properties in the intermediate layers are dominated by NPIW (Fig. 4), but careful examination of stations in this box indicates that a weak salinity minimum is not uncommon at densities between 27.0 and 27.4 σθ (see also Talley 1993). The salinity minimum signal of AAIW is lost at midlatitudes of the North Pacific, as indicated by small (<0.1) probability in Fig. 3.

The density of AAIW as a salinity minimum ranges between 27.15 and 27.25 σθ over the domain enclosed by the 0.5 probability contour (Fig. 5). This confirms that the 27.2 σθ surface is a good indicator of AAIW (Reid 1965). The temperature of AAIW is rather homogeneous (∼5.6°C) in the south. It rises slightly to about 6°C as it approaches the Philippine coast. Salinity is generally low (<34.5 psu) in the south and high (>34.5 psu) in the north. These changes in temperature and salinity seem to be well counterbalanced so that the density of AAIW remains rather stable (27.2 σθ) during its flow in the western Pacific.

4. AAIW along the 27.2 σθ surface

In the following, AAIW is traced along the 27.2 σθ surface. The depth of the 27.2 σθ surface ranges from about 700 m near the equator to almost 1000 m in the subtropics (Fig. 6). Property distributions along this density surface are slightly different from those shown in Fig. 5. Here, water of low temperature (<5.4°C) and low salinity (<34.45 psu) characteristic of AAIW is seen to extend toward the Solomon Sea in the south (Figs. 6b,c). Both temperature and salinity are homogenous near the equator with typical values of 6°C and 34.55 psu. In the north, water properties are largely influenced by the intrusion of cool, fresh North Pacific waters.

In the intermediate layers, the lateral gradients of temperature and salinity are weak, as compared with those in the shallower waters, and the low-salinity signal of AAIW is lost by mixing with overlying and underlying waters at the midlatitudes of the North Pacific. The nonconservative tracers have proven to provide much clearer indications of the spreading of AAIW (e.g., Reid 1965; Tsuchiya 1968). Among these nonconservative tracers oxygen is most useful. Although phosphate, nitrate, and several other nutrients are also useful, the number and spacing of high-quality data are very limited. Only the oxygen distribution is examined in the present study.

As mentioned by earlier studies (e.g., Reid 1965; Reid and Mantyla 1978), the pattern of oxygen at 27.2 σθ is dominated by the high values entering from the South Pacific and the lower values found in the North Pacific (Fig. 6d). At the Philippine coast, the oxygen of AAIW has been substantially decreased, falling below 2.0 ml L−1 at about 15°N. This high-oxygen water, however, does not appear to extend farther northward along the western boundary. Although relatively high oxygen (∼2.0 ml L−1) water does exist in the Okinawa Trough, it is connected with high-oxygen water in the SCS through the Luzon Strait. Near the coast of Luzon at 15°–18°N, there is actually a horizontal minimum of oxygen (<1.9 ml L−1), indicating that the low-oxygen water that originates in the North Pacific subpolar gyre (Reid 1965; Reid and Mantyla 1978) extends all the way to the western boundary. The disconnection of high-oxygen water from the south is apparent even in the highly smoothed World Ocean Atlas 1998 (WOA98; Fig. 1b), but has never been previously reported. In the earlier map of Reid (1965, his Fig. 25), for example, a high-oxygen tongue (>2.0 ml L−1) of South Pacific origin can be traced continuously to the Okinawa Trough, leading him to conclude that AAIW extends to the midlatitudes of the western North Pacific. Reid failed to capture the detailed structure of oxygen distribution presumably due to the lack of observations. As a consequence, his conclusion on the northward intrusion of AAIW needs to be modified.

In the SCS, there is a local maximum of oxygen (>2.2 ml L−1) along the continental slope south of China (Fig. 6d). Circulation in the SCS is believed to play an essential role in localizing this oxygen maximum (discussed in section 7). High-oxygen water from the SCS tends to spread eastward through the Luzon Strait. From there, most of it appears to join the Kuroshio and extend northward into the Okinawa Trough (Chen and Wang 1998).

5. Overlying/underlying water properties

Two isopycnals, 27.0 σθ and 27.6 σθ, are chosen to represent waters overlying and underlying AAIW. Water properties on these two isopycnals, as shown below, contain different patterns from those of AAIW, indicating that the influence of South Pacific sources becomes increasingly important with depth.

a. Oxygen at σθ = 27.0

The 27.0 σθ surface lies at about 500 m near the equator, and at midlatitudes it extends below 800 m, consistent with the anticyclonic circulation of the subtropical gyres (Fig. 7a). Water from the eastern subtropical Pacific at this isopycnal is seen as a tongue of low oxygen extending westward at 10°–20°N (Reid and Mantyla 1978). As a result of mixing during the westward flow, the oxygen of this water mass increases from <1.2 ml L−1 at the international date line to >1.8 ml L−1 near the western boundary (Fig. 7b). In contrast with the pattern shown in Fig. 6d, oxygen at this isopycnal is higher (>2.0 ml L−1) in the northern subtropical Pacific, roughly north of 20°N. The vertical distribution of oxygen shows that this isopycnal in the northern subtropical Pacific lies right below a strong vertical gradient of oxygen (Fig. 8). Overlying this isopycnal is the recently ventilated thermocline water, which raises the oxygen at 27.0 σθ well above the values formed elsewhere of the North Pacific (Fig. 7b).

High-oxygen water from the northern subtropical Pacific extends southwestward into the SCS (Fig. 7b). This might be interpreted as evidence for the intrusion of North Pacific waters in the upper layer of the Luzon Strait (e.g., Shaw 1991; Chao et al. 1996; Chen and Huang 1996; Metzger and Hurlburt 1996; Chen and Wang 1998; Qu et al. 2000; Qu 2002).

b. Oxygen at σθ = 27.6

Oxygen distribution in the North Pacific changes gradually with depth. Most strikingly, the broad oxygen minimum (<1.5 ml L−1) that originates in the North Pacific subpolar gyre stretches southward from about 27.4 σθ at 40°N to about 26.8 σθ at 5°N (Fig. 8). The shoaling of this oxygen minimum toward the equator could be due to the intrusion of South Pacific waters in the deeper layers (Qu et al. 1999). At depths around 27.4 σθ, low-oxygen water is widely spread in the northern subtropical Pacific, except at the western boundary where high-oxygen water from the South Pacific can be traced to the southern tip of the Luzon Strait (not shown). However, the eastward spread of the SCS water south of Taiwan is still evident, consistent with previous studies (e.g., Chao et al. 1996; Chen and Wang 1998; Qu 2002).

As we progress to the deeper levels, the influence of South Pacific sources becomes increasingly important. At 27.6 σθ, within the depth range of 1600–1800 m (Fig. 9a), high-oxygen water from the South Pacific appears to leak into the SCS directly through the Luzon Strait (Fig. 9b).

6. Flow path along the western boundary

The existence of AAIW at the Mindanao coast was reported in earlier studies (e.g., Masuzawa 1972; Lukas et al. 1991; Bingham and Lukas 1994; Wijffels et al. 1995; Qu et al. 1998, 1999), in which the northward-flowing Mindanao Undercurrent (MUC) seems to play a pivotal role (Hu et al. 1991; Tsuchiya 1991; Fine et al. 1994). However, no observational evidence had been published for a northward flow of AAIW farther northward than 13°N. Both climatological data and synoptic measurements are presented below to further investigate the flow path of AAIW along the Philippine coast.

a. Climatology

Figure 10 shows the mean oxygen distribution, superimposed on the mean geostrophic flow obtained from Qu and Lukas (2003), at every 2° latitude from 12° to 22°N at the western boundary. At 12°N, a northward flow associated with the MUC is present at depths around AAIW, where oxygen concentration is generally higher than 2 ml L−1. The Kuroshio starts to appear at about 14°N as a northward surface flow, overlying the southward-flowing Luzon Undercurrent (LUC: Qu et al. 1997). At 16°–18°N, the subsurface flow near the coast is dominated by the LUC with relatively low (<1.9 ml L−1) oxygen level. So, at least to the extent that the Qu and Lukas (2003) climatological data are representative, there is no northward flow of AAIW at these latitudes. At 20°–22°N, the LUC becomes unrecognizable, and the deep extension of the Kuroshio carries relatively high oxygen (>2.1 ml L−1) water into the Okinawa Trough.

b. Hydrographic sections

Oxygen concentration and geostrophic velocity averaged from six hydrographic sections (Table 1) provides further information on the flow path of AAIW at 18°20′N (Fig. 11, left panel). With their high special resolution of these data, the narrow western boundary currents are well resolved. Here, the Kuroshio is basically confined in the upper 500 m and about 200 km from the coast, while the LUC is dominant at the subsurface, with its maximum velocity exceeding 8 cm s−1 at 700–800 m. The oxygen concentration at depths around AAIW is <1.7 ml L−1 in the ocean interior, and the western extension of this low-oxygen water reflects strong influence of the eastern subtropical sources. Although oxygen concentration is somewhat higher (>1.9 ml L−1) near the coast, the flow at intermediate depths is predominantly southward, and its low-salinity characteristics suggest a supply from the northern subtropical gyre (Fig. 11, right panel).

Further inspection of the data from individual cruises shows that both oxygen concentration and geostrophic velocity vary with time (Fig. 12). The oxygen concentration around the depth of AAIW is relatively high (>1.8 ml L−1) during the repeat surveys PRC–US6 (April) and PRC–US8 (June), but falls below 1.4 ml L−1 during PRC–US9 (November). This variation corresponds well with the seasonal bifurcation of the North Equatorial Current (NEC). In a recent study of Qu and Lukas (2003), it is shown that the bifurcation of the NEC approaches its northernmost position in November/December, when the northeast monsoon prevails and, as a consequence, an anomalous amount of subtropical water with low oxygen concentration is advected southward at the Philippine coast. The situation during the southwest monsoon season (June–August) is reversed.

Geostrophic calculation from synoptic hydrographic observations is problematic over the continental slope, where geostrophic velocities vary wildly because of dynamic height noise associated with internal waves amplified by the small station spacing (Lukas et al. 1991; Wijffels et al. 1995; Qu et al. 1998). To reduce this dynamic height noise, we smooth the data from each individual cruise in the zonal direction using an e-folding scale of 0.25°. The resulting geostrophic velocities clearly demonstrate the undercurrent structure near the coast (Fig. 12). The LUC as a southward subsurface flow is present all the time during the period of observations, but with considerable variations both in strength and location. Despite these variations, relatively high oxygen water near the coast always flows southward as part of the LUC, and so there is no northward flow of AAIW near the coast of Luzon during the period of observations.

It is worthwhile to note that high-frequency, small-scale eddies neglected by the present study may also play a role in transporting water properties. In the front area around 15°N, for example, water that flows to the north usually has higher oxygen than that to the south. The overall action of these eddies would be to produce a net (northward) transport of oxygen along the western boundary, but that does not seem to be significant in the oxygen distribution shown in Fig. 6d.

7. Possible role of the SCS circulation

The results presented above suggest that AAIW does not extend farther northward than 15°N at the western boundary and that the high-oxygen water observed in the Okinawa Trough is merely a result of water exchange between the SCS and the Pacific through the Luzon Strait. However, because water denser than about 26.8 σθ is not formed in the North Pacific, this high-oxygen water in turn must come from the South Pacific. Our hypothesis is that high-oxygen water enters the SCS as part of the Pacific deep water around the sill depth (∼2000 m) of the Luzon Strait, where part of it joins the SCS circulation and exits the SCS through the Luzon Strait at AAIW depths.

The SCS is essentially a closed basin at depths of about 200 m or below, except in the Luzon Strait (Fig. 2c). The water exchange between the SCS and the Pacific through the Luzon Strait (called the Luzon Strait transport hereinafter) may have a notable impact on the SCS circulation, and vice versa (e.g., Wyrtki 1961). Recent studies suggested that the Luzon Strait transport has a sandwiched structure in the vertical direction; that is, water exits the SCS through the Luzon Strait in the intermediate layer and enters it from the Pacific in the upper and deeper layers (Chao et al. 1996; Chen and Huang 1996; Chen and Wang 1998; Qu 2002; Yuan 2002). Although no direct measurement of the Luzon Strait transport has been published, oxygen distribution presented in this study (Figs. 6, 7, and 9) clearly demonstrates that there is an outflow component of the SCS water at AAIW depths and an inflow component of the Pacific water in the upper and deeper layers.

Around the sill depth of the Luzon Strait, a transport of about 0.7 Sv (Sv ≡ 106 m3 s−1) has been observed to enter the SCS from the Pacific (Wang 1986). Since water in the deep layer of the SCS is warmer and of lower density than its Pacific counterpart, water entering the SCS from the Pacific sinks after it crosses the Luzon Strait (Wyrtki 1961) sill. To compensate for this descending movement, upwelling must occur elsewhere, and thus the renewal of the SCS water is rapid as compared with that in the Pacific (Nitani 1972; Broecker et al. 1986).

Mean property distributions across the Luzon Strait (Fig. 13) support this circulation schematic. Both salinity and oxygen contours slope upward toward the SCS above 1500–1600 m. As a result of intense vertical mixing (e.g., Yuan 2002), cross-isopycnal movement is particularly evident between 27.6 and 27.2 σθ, indicating a transfer of high-salinity, high-oxygen water toward shallower isopycnal surfaces. Below 27.6 σθ, the situation is reversed. Property contours slope downward toward the SCS but primarily follow isopycnal surfaces (Fig. 13), indicating a downward movement on the SCS side. This downward movement was first suggested by Wyrtki (1961) and has been well reproduced by numerical models (A. Ishida 2003, personal communication).

There is no Pacific Bottom Water in the SCS because of the shallowness of the Luzon Strait sill, and so no oxygen minimum is formed (Fig. 14). When compared with their counterparts in the North Pacific, the North Pacific Tropical Water and Intermediate Water in the SCS are also less pronounced. This is another indication that vertical mixing is more intense in the SCS. Oxygen in the SCS is higher than in the North Pacific at depths between about 700 and 1500 m but is lower in the shallower and deeper layers (also see Chen and Wang 1998; Qu, 2002; Yuan 2002). On the Pacific side of the Luzon Strait, the oxygen minimum lies at about 27.2 σθ (Fig. 8), roughly between 700 and 900 m (Fig. 14), and the outward spread of the SCS water around this density surface induces high oxygen during its pathway from the Luzon Strait to the Okinawa Trough.

8. Summary

It has been debated for several decades as to whether or not AAIW extends to the midlatitudes of the western North Pacific. The traditional view before this study has been that AAIW, as indicated by its high-oxygen tongues, extends all the way to the Okinawa Trough. However, until this time, no observational evidence has been published for a direct pathway at the western boundary. The northward intrusion of AAIW has been carefully examined in the present study using historical data combined with synoptic measurements from a repeated hydrographic section in the western Pacific.

AAIW as a salinity minimum is traced only to about 15°N in the western North Pacific. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, as already reported by previous studies, it is connected with high-oxygen water in the SCS through the Luzon Strait but not from the south as an extension of AAIW.

The high-oxygen water at depths of AAIW in the SCS is hypothesized to come from the South Pacific via a rather complex pathway, and the circulation in the SCS seems to play an essential role in localizing this oxygen maximum. Evidence exists to suggest that high-oxygen water first enters the SCS as part of the Pacific deep water around the sill depth of the Luzon Strait and then part of it upwells and is entrained into shallower isopycnal surfaces by intense vertical mixing over the continental slope south of China.

Active water exchange between the SCS and the Pacific occurs through the Luzon Strait. At depths of AAIW (∼27.2 σθ), oxygen concentration is high on the SCS side than on the Pacific side, and the outward spread of high-oxygen water from the SCS is likely responsible for the presence of high-oxygen water in the Okinawa Trough.

In conclusion, AAIW cannot reach the midlatitudes of the North Pacific via the western boundary, and the high-oxygen water observed in the Okinawa Trough is merely a result of water exchange between the SCS and the Pacific through the Luzon Strait.

Acknowledgments

This research was supported by National Science Foundation through Grant OCE00-95906 and by Frontier Research System for Global Change through its sponsorship of the International Pacific Research Center (IPRC). Author TQ is grateful to J. P. McCreary, H. Mitsudera, T. Jensen, T. Miyama, Y.‐Y. Kim, and H.-W. Kang for many valuable discussions and to G. Meyers, T. Yamagata, and R. Lukas for useful communication on the present topic. Thanks are also extended to L. Talley and two anonymous reviewers for useful comments and thoughtful suggestions. The hydrographic data from the repeated section were kindly provided by the State Oceanic Administration Data Center of China.

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Fig. 1.
Fig. 1.

Annual mean (a) salinity (psu) and (b) oxygen concentration (ml L−1) superimposed with acceleration potential (m2 s−2; white solid lines) at 27.2 σθ relative to 2000 dbar calculated from the World Ocean Atlas 1998

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 2.
 Fig. 2.

Geographical distribution of (a) temperature–salinity and (b) oxygen profiles (asterisk) from World Ocean Database 1998 and (c) CTD stations (asterisk) along PR22 collected during WOCE. Light solid lines indicate 500-m isobath in (a) and (b) and 1500-m isobath in (c). The heavy solid and dashed lines in (c) show the geographic locations of vertical sections used for Figs. 10 and 13, respectively

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 3.
Fig. 3.

Probability of AAIW defined as the percentage of salinity profiles that contain at least one salinity minimum at the density range of 27.0–27.4 σθ. Boxes A, B, and C show the geographic locations of the boxes used for Fig. 4

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 4.
Fig. 4.

Relations of temperature vs salinity, temperature vs oxygen concentration, and salinity vs oxygen concentration in boxes A (5°– 10°S, 148°–153°E), B (0°–5°N, 130°–135°E), and C (10°–15°N, 125°–130°E) as indicated in Fig. 3. The solid lines in the left panels denote 26.8 and 27.2 σθ surfaces, respectively

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 5.
Fig. 5.

Distributions of potential density (kg m−3), temperature (°C), and salinity (psu) of AAIW as defined in Fig. 3. The heavy dashed lines denote the 0.5 probability contour

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 6.
Fig. 6.

Distributions of (a) isopycnal depth (m), (b) temperature (°C), (c) salinity (psu), and (d) oxygen concentration (ml L−1) at 27.2 σθ

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 7.
Fig. 7.

Distributions of (a) isopycnal depth (m) and (b) oxygen concentration (ml L−1) at 27.0 σθ. Areas with oxygen concentration larger than 2.0 ml L−1 are shaded in (b)

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 8.
Fig. 8.

Oxygen distribution (ml L−1) against depth (m) at 160°E. Area with oxygen concentration less than 1.5 ml L−1 is shaded

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 9.
Fig. 9.

As in Fig. 7 except at 27.6 σθ

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 10.
Fig. 10.

Geostrophic velocity (cm s−1) against depth (m) from Qu and Lukas (2003) superimposed on oxygen concentration (ml L−1) at sections A (12°N), B (14°N), C (16°N), D (18°N), E (20°N), and F (22°N). Positive values are northward, and the heavy dashed lines indicate the 27.2 σθ surface. The geographic locations of these sections are shown in Fig. 2c

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 11.
Fig. 11.

Geostrophic velocity (cm s−1) relative to 2000 dbar against depth (m) superimposed on oxygen concentration (ml L−1) and salinity (psu) averaged from six hydrographic sections at 18°20′N. Positive velocities are northward

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 12.
Fig. 12.

As in Fig. 11 but for individual cruises

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 13.
Fig. 13.

Vertical sections of (a) salinity (psu) and (b) oxygen concentration (ml L−1) against depth (m) along 20.25°N across the Luzon Strait. Areas with salinity less than 34.45 psu or oxygen less than 2.1 ml L−1 are shaded. The heavy dashed lines indicate 27.2, 27.5, 27.6, and 27.65 σθ surfaces, respectively. The geographic location of this section is shown in Fig. 2c as a heavy dashed line (M)

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Fig. 14.
Fig. 14.

Vertical profiles of annual mean salinity (psu) and oxygen concentration (ml L−1) against depth (m) in the South China Sea (SCS) and the North Pacific (NP). The geographic locations of the two stations (asterisks) are shown in Fig. 13: One (SCS) is at (20.25°N, 118.25°E), and the other (NP) at (20.25°N, 129.25°E)

Citation: Journal of Physical Oceanography 34, 9; 10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2

Table 1.

Temporal distribution of the six surveys (03–09) at 18°20′N (PR22) conducted by the People's Republic of China and United States joint program

Table 1.
*

School of Ocean and Earth Science and Technology Contribution Number 6349, and International Pacific Research Center Contribution Number IPRC-264.

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