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  • View in gallery

    STMW distribution in July (gray shading), represented by the region where the thickness of θ = 16°–19°C is >100 dbar, as calculated from the World Ocean Atlas 2009 (WOA09; Locarnini et al. 2010). Black contours indicate mean SSH, reconstructed from satellite-derived altimetry SSH anomaly data and mean dynamic topography data (see details in the text), with an interval of 10 cm. The thick line represents the 110-cm SSH contour, which is regarded as the axis of the Kuroshio and KE.

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    (a) Three typical paths of the Kuroshio south of Japan based on Kawabe (1995): nNLM, oNLM, and tLM. Shading denotes bathymetric features from the National Geophysical Data Center’s 2-minute gridded elevations/bathymetry for the world (ETOPO2v2; www.ngdc.noaa.gov/mgg/fliers/01mgg04.html) (m). Open circles show repeat hydrographic stations along 137°E and ASUKA lines, conducted by the JMA. (b) Southernmost location (°N) of the Kuroshio between 136° and 140°E, produced by the JMA; the location is determined based on temperature at a depth of 200 m and satellite-derived SST.

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    (a) Research vessel (R/V) Ryuho Maru track for JMA observations in September 1977, a tLM path year. (b) Cross section of θ with a contour interval of 1°C. Shading denotes the vertical gradient of θ [°C (100 dbar)−1].

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    Number of θ–S profiles per month in the northwestern part of the subtropical gyre (20°–36°N, 125°E–180°) (gray bars) and south of Japan (28°–35°N, 130°–140°E) (black line).

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    Mean Kuroshio/KE axes in March for the years 2005–11.

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    Composite maps of geopotential anomaly at 200 relative to 1000 dbar in March of (a) meander (2005, 2007, and 2009) and (c) non-meander (2006, 2008, 2010, and 2011) years; this is averaged for each 1° × 1° grid box using the profiles from a 3° × 3° grid box centered on the 1° × 1° grid box with a weighting function, d−2 [d is the distance (°) from the center of the 1° × 1° grid box], for observation points where d is >1°, using the method of Oka et al. (2012). The contour interval is 0.5 m2 s−2. Gray shading denotes a geopotential anomaly >14.5 m2 s−2. (b),(d) As in (a),(c), but for the MTD (>300 dbar; with a contour interval of 30 dbar). Gray shading denotes MTD > 510 dbar.

  • View in gallery

    (left) Core θ–S diagram of STMW in the northwestern part of the subtropical gyre (20°–36°N, 125°E–180°) in spring (April–June) of (a) meander years and (b) non-meander years. (right) Number of core θ.

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    (left) Distribution of STMW for meander years in: (a) spring (April–June), (b) summer (July–September), and (c) autumn (October–December). Red circles indicate warm core STMW (θ > 19°C) and blue circles represent cold core (θ < 19°C). Gray symbols are observation points where no STMW was present. Contours indicate mean SSH with an interval of 20 cm. The black rectangle delineates the SBR (specifically for 29°–32°N, 134°–138°E). (right) (d)–(f) As in (a)–(c), but for non-meander years.

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    Histogram of depths of (a) upper and (b) lower boundaries of the W-STMW in the SBR during spring of meander years (bars) (dbar) and of the C-STMW in the SBR during spring of non-meander years (line).

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    Seasonal variation of core θ of the W-STMW (closed circles) and C-STMW (open circles) detected in the SBR (°C) for (a) meander and (b) non-meander years.

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    (left) Trajectory of (a) Argo float WMO 2900366 from January to December 2005 and (b) Argo float WMO 2900956 from November 2009 to October 2010. The black closed circle represents the start point of the data used in this study. Bathymetric features are indicated by brown contours; 1000- (dashed line) and 2000-dbar depth (solid line). (right) Time–depth section of Q calculated from the Argo float profiles (10−10 m−2 s−2). Black lines represent θ with an interval of 1°C; green line is the θ = 19°C isotherm. Red line indicates the MLD.

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    As Fig. 6, but for the MLD in March (>100 dbar; with a contour interval of 20 dbar) for (a) meander and (c) non-meander years. Gray shading denotes MLD > 130 dbar. (b),(d) As in Figs. 8a and 8d, but for the ML temperature with a deep MLD >150 dbar in March.

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    (a) Temporal rate of change in OHC during winter (W m−2), defined as OHC in December of the previous year minus OHC in March, at 135°E for meander (red solid line) and non-meander (red dashed line) years; the OHC was gridded on 1° lat interval for each month, applying the identical method as for Fig. 6. (b) Time rate of change in OHC during winter at 135°E (red line), NHF in winter (January–March) at 135°E (black line), and their residual, defined as time rate of change in OHC minus NHF (bar) for meander years. (c) As in (b), but for non-meander years.

  • View in gallery

    Map of the correlation coefficient between the lat position of the Kuroshio in Fig. 2b and SST for March (from Reynolds et al. 2007). The sign of the Kuroshio position is reversed from that of the original index to represent the meander or non-meander path states more naturally; that is, the positive (negative) sign represents the meander (non-meander) path state. Black contours indicate the geopotential anomaly at 200 relative to 1000 dbar for meander years, displayed in Fig. 6a. Dotted lines show the 137°E and ASUKA lines.

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    (a) Schematic diagram of sea surface geopotential anomaly relative to 1000 dbar along the ASUKA line (m2 s−2). Symbols I and II denote the northern and southern boundaries, respectively, of the westward flow associated with the recirculation. (b) Transport-averaged temperature associated due to the westward flow associated with the recirculation (°C), obtained from repeat hydrographic data along the 137°E (solid line) and ASUKA (dashed line) lines.

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Influence of Kuroshio Path Variation South of Japan on Formation of Subtropical Mode Water

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  • 1 Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan
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Abstract

Distributions of subtropical mode water (STMW) in the northwestern part of the North Pacific Subtropical Gyre were investigated, using temperature–salinity profiles from 2005 to 2011, with particular reference to the Kuroshio meander and non-meander path states, south of Japan. In spring of meander years, warm STMW with a potential temperature of 19°–20°C (potential density anomaly of 24.6–24.9 kg m−3) was found in the Shikoku Basin, whereas cold STMW below 19°C was distributed throughout the southern region of Japan in non-meander years. The warm STMW was formed in a spatially isolated and warm winter mixed layer (ML) in the Shikoku Basin, where a local recirculation developed in association with the Kuroshio meander path; both the absence of horizontal mixing with a cold ML south of the Kuroshio Extension because of the spatially isolated ML and an increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ML warming in the Shikoku Basin. After the spring shoaling of the ML, the warm STMW was preserved under the seasonal pycnocline until midsummer at a depth of 100–250 dbar; its thickness was approximately half that of the cold STMW in the Shikoku Basin in non-meander years. The warm STMW was rapidly eroded between the late summer and the following winter.

Corresponding author address: Shusaku Sugimoto, Department of Geophysics, Graduate School of Science, Tohoku University, 6-3 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8578, Japan. E-mail: sugimoto@pol.gp.tohoku.ac.jp

Abstract

Distributions of subtropical mode water (STMW) in the northwestern part of the North Pacific Subtropical Gyre were investigated, using temperature–salinity profiles from 2005 to 2011, with particular reference to the Kuroshio meander and non-meander path states, south of Japan. In spring of meander years, warm STMW with a potential temperature of 19°–20°C (potential density anomaly of 24.6–24.9 kg m−3) was found in the Shikoku Basin, whereas cold STMW below 19°C was distributed throughout the southern region of Japan in non-meander years. The warm STMW was formed in a spatially isolated and warm winter mixed layer (ML) in the Shikoku Basin, where a local recirculation developed in association with the Kuroshio meander path; both the absence of horizontal mixing with a cold ML south of the Kuroshio Extension because of the spatially isolated ML and an increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ML warming in the Shikoku Basin. After the spring shoaling of the ML, the warm STMW was preserved under the seasonal pycnocline until midsummer at a depth of 100–250 dbar; its thickness was approximately half that of the cold STMW in the Shikoku Basin in non-meander years. The warm STMW was rapidly eroded between the late summer and the following winter.

Corresponding author address: Shusaku Sugimoto, Department of Geophysics, Graduate School of Science, Tohoku University, 6-3 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8578, Japan. E-mail: sugimoto@pol.gp.tohoku.ac.jp

1. Introduction

The North Pacific subtropical mode water (STMW), characterized by a thick thermostad layer with potential temperature θ, is formed in a deep winter mixed layer (ML) just south of the Kuroshio and the Kuroshio Extension (KE) in a broad longitudinal band between about 130°E and the international date line (Masuzawa 1969; Hanawa 1987; Hanawa and Hoshino 1988; Suga and Hanawa 1990). Numerous authors have investigated winter ML depth (MLD) to understand the STMW formation, and they have identified three major factors causing MLD variations: oceanic buoyancy loss driven by the intense East Asian winter monsoon (Suga and Hanawa 1995a; Yasuda and Hanawa 1997); the upper-ocean stratification during the preceding warm season (Qiu and Chen 2006; Iwamaru et al. 2010) as weaker (stronger) stratification results in a deeper (shallower) ML in the subsequent winter; and the main thermocline depth (MTD) in association with mesoscale eddies (Uehara et al. 2003; Oka et al. 2012) and large-scale oceanic Rossby waves (Sugimoto and Hanawa 2010), which leads to deeper (shallower) ML where the MTD is deeper (shallower). When the ML shoals in spring, the main body of STMW is capped by the seasonal pycnocline, and its properties are preserved in the subsurface. The STMW is advected westward/southwestward with the Kuroshio Countercurrent (Bingham 1992; Suga and Hanawa 1995a) and is then distributed widely throughout the western part of the North Pacific Subtropical Gyre (Fig. 1; Masuzawa 1969; Hanawa 1987; Hanawa and Talley 2001; Oka and Qiu 2012).

Fig. 1.
Fig. 1.

STMW distribution in July (gray shading), represented by the region where the thickness of θ = 16°–19°C is >100 dbar, as calculated from the World Ocean Atlas 2009 (WOA09; Locarnini et al. 2010). Black contours indicate mean SSH, reconstructed from satellite-derived altimetry SSH anomaly data and mean dynamic topography data (see details in the text), with an interval of 10 cm. The thick line represents the 110-cm SSH contour, which is regarded as the axis of the Kuroshio and KE.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

The Kuroshio takes three typical paths south of Japan (Kawabe 1995): the typical large meander (tLM) path, the offshore nonlarge meander (oNLM) path, and the nearshore nonlarge meander (nNLM) path (Fig. 2a). The Kuroshio path affects the local occurrence of STMW south of Japan; STMW is not normally found in the Shikoku Basin when the Kuroshio takes the tLM path because the associated changes in the Kuroshio Countercurrent most likely cut off the westward/southwestward advection of STMW from its major formation region east of 140°E (Bingham 1992; Suga and Hanawa 1995b,c). The above studies regarded the STMW as the thermostad layer with θ < 19°C, but Nishiyama et al. (1980, 1981) observed a warm thermostad layer with θ > 19°C in the Shikoku Basin in tLM path years (see Fig. 3). Warm STMW with θ > 19°C south of Japan can be seen in Fig. 11 of Oka (2009) in 2005, a tLM path year. This relatively warm STMW may be formed in the Shikoku Basin when the Kuroshio south of Japan takes a meander path.

Fig. 2.
Fig. 2.

(a) Three typical paths of the Kuroshio south of Japan based on Kawabe (1995): nNLM, oNLM, and tLM. Shading denotes bathymetric features from the National Geophysical Data Center’s 2-minute gridded elevations/bathymetry for the world (ETOPO2v2; www.ngdc.noaa.gov/mgg/fliers/01mgg04.html) (m). Open circles show repeat hydrographic stations along 137°E and ASUKA lines, conducted by the JMA. (b) Southernmost location (°N) of the Kuroshio between 136° and 140°E, produced by the JMA; the location is determined based on temperature at a depth of 200 m and satellite-derived SST.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 3.
Fig. 3.

(a) Research vessel (R/V) Ryuho Maru track for JMA observations in September 1977, a tLM path year. (b) Cross section of θ with a contour interval of 1°C. Shading denotes the vertical gradient of θ [°C (100 dbar)−1].

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

The purpose of the present study is to investigate the influence of the Kuroshio path on STMW formation south of Japan by using temperature–salinity profiles. The number of such profiles has increased dramatically year by year since 2000 when the international Argo project (Argo Science Team 2001) started. These data can provide a new perspective on the STMW south of Japan. The remainder of this paper is organized as follows: Section 2 outlines the datasets and processing procedures. Section 3 investigates the relationship between the STMW distribution and Kuroshio path in spring. Section 4 describes the decay of STMW from spring to autumn. Section 5 explores the winter MLD and its temperature to understand the influence of the Kuroshio path on STMW formation. Finally, section 6 provides a summary and concluding remarks.

2. Datasets and processing procedures

We use temperature–salinity profiles archived in the World Ocean Database 2009 (WOD09; Boyer et al. 2009) and at the Japan Oceanographic Data Center (JODC; www.jodc.go.jp) and profiles from Argo floats (Oka et al. 2007). To capture the STMW adequately, we only use profiles with maximum depth greater than 450 dbar. To control data quality, we first remove profiles duplicated in the different data sources. For each profile, the measured data are then compared with all values measured in the same month within a 1° × 1° box centered on the observation point, and the data are excluded if they fall outside of three standard deviations of the mean. Profiles with large temperature inversions (dT/dz < −0.1°C dbar−1) anywhere between the sea surface and 450 dbar are also removed. After quality control, temperature–salinity profiles are vertically interpolated onto a 1-dbar interval using the Akima (1970) scheme. Then, θ and the potential density anomaly σθ are calculated, and the potential vorticity Q is defined by Talley (1988) as follows:
e1
where f is the Coriolis parameter, ρ is the in situ density, and z is the vertical coordinate (positive upward). The depth derivative of ρ at depth z is calculated from the values of density at z + 30 and z − 30 dbar. We checked that results of later analyses were not sensitive to the selection of 30 dbar and found that identical results were obtained with other values, such as 50 dbar. The STMW is detected as a low-Q layer less than 2.0 × 10−10 m−1 s−1, with θ > 16°C (e.g., Oka 2009) and thickness >100 dbar. A local vertical minimum of Q is identified from each profile and then labeled as a core of the STMW. We investigate the STMW south of the Kuroshio/KE, and we select the θ = 16°C isotherm at 200 dbar as the Kuroshio/KE axis rather than the θ = 15°C isotherm that is normally regarded as a good indicator of the Kuroshio/KE axis (Kawai 1972). This is to ensure there is no risk of identifying subarctic profiles as coming from north of the Kuroshio/KE. The analysis period of this study is the 7 years from 2005, when the number of θ–S profiles south of Japan increased (Fig. 4).
Fig. 4.
Fig. 4.

Number of θ–S profiles per month in the northwestern part of the subtropical gyre (20°–36°N, 125°E–180°) (gray bars) and south of Japan (28°–35°N, 130°–140°E) (black line).

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

The MLD is defined as the depth at which σθ increases by 0.1 kg m−3 from its 10-dbar depth value (Oka 2009), and the MTD is taken to be the depth of the θ = 12°C isotherm (Uehara et al. 2003). The upper-ocean stratification in the warm season is regarded as the mean of the vertical temperature gradient calculated at every 1 dbar from the sea surface to 200 dbar (Sugimoto and Hanawa 2010). The 200-dbar depth was selected so as to cover the seasonal thermocline depth change adequately, but to exclude the influence of the MTD change.

We also use repeat hydrographic observations along 137°E and Affiliated Surveys of the Kuroshio off Cape Ashizuri (ASUKA) lines (see Fig. 2a), conducted by the Japan Meteorological Agency (JMA) in the winter season (January–March). Data along the 137°E line are available throughout the analysis period and along the ASUKA line for 2005–10. The data are gridded on 0.5° latitude intervals, applying a Gaussian filter with an e-folding scale of 55 km for temperature–salinity profiles. The reference level to obtain the geostrophic current is selected as 1000 dbar.

We use the sea surface height (SSH) reconstructed by adding the satellite-derived SSH anomaly dataset of the Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO; Ducet et al. 2000) to the mean dynamic topography (www.aviso.oceanobs.com). The SSH data have a 7-day temporal resolution and ⅓° spatial grid resolution. We define the Kuroshio/KE axis as the 110-cm SSH contour. As indicated in Fig. 1, the 110-cm SSH contour is consistently located at, or near, the maximum of the horizontal gradient of SSH. We also use sea surface temperature (SST) products based on the Advanced Very High Resolution Radiometer (AVHRR) infrared satellite data (Reynolds et al. 2007), with a spatial grid resolution of ¼°.

To investigate atmospheric forcing, the net surface heat flux (NHF; the sum of latent heat flux, sensible heat flux, net surface longwave radiation flux, and net surface shortwave radiation flux) from the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) monthly dataset (Dee et al. 2011) is used. We use the monsoon index (MOI), defined as the difference in sea level pressure between Nemuro, Japan, and Irkutsk, Russia, as an indicator of the strength of the northerly winter monsoon (Watanabe 1990).

3. STMW in the Shikoku Basin for different Kuroshio paths

The STMW is formed in the deep winter ML (e.g., Suga and Hanawa 1990). We define meander (tLM and oNLM paths) and non-meander (nNLM path) years based on the Kuroshio paths in March, which is the month with the deepest ML south of Japan (Ohno et al. 2009). If the southernmost position of the Kuroshio axis is located south (north) of 32°N, it is classified as a meander (non-meander) path (Fig. 5). In meander years (Figs. 6a,b), there are two local anticyclonic subcirculations, characterized by a deep MTD and a high geopotential anomaly, one to the west and the other to the east; the western subcirculation is located in the Shikoku Basin, and the eastern is situated between approximately 142° and 150°E. These subcirculations are known as recirculations (Veronis 1966). In contrast, in non-meander years (Figs. 6c,d), a broad continuous recirculation is found south of the Kuroshio/KE between 135° and 145°E, as noted previously (Hasunuma and Yoshida 1978).

Fig. 5.
Fig. 5.

Mean Kuroshio/KE axes in March for the years 2005–11.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 6.
Fig. 6.

Composite maps of geopotential anomaly at 200 relative to 1000 dbar in March of (a) meander (2005, 2007, and 2009) and (c) non-meander (2006, 2008, 2010, and 2011) years; this is averaged for each 1° × 1° grid box using the profiles from a 3° × 3° grid box centered on the 1° × 1° grid box with a weighting function, d−2 [d is the distance (°) from the center of the 1° × 1° grid box], for observation points where d is >1°, using the method of Oka et al. (2012). The contour interval is 0.5 m2 s−2. Gray shading denotes a geopotential anomaly >14.5 m2 s−2. (b),(d) As in (a),(c), but for the MTD (>300 dbar; with a contour interval of 30 dbar). Gray shading denotes MTD > 510 dbar.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

We investigate core properties of the STMW in spring (April–June) (Fig. 7), when the water properties at the time of formation are still preserved. We specifically focus on θ in this study because the salinity variation has small effect on determining STMW density, although salinity also has interesting year-to-year variations [see Sugimoto et al. (2013) for the salinity variations]. In meander years (Fig. 7a), the STMW is separated clearly into two types by the θ = 19°C line. The cold cores (θ < 19°C) around σθ = 25.0–25.4 kg m−3 are distributed east of 140°E (Fig. 8a), which is consistent with this being the major formation area of STMW. The warm cores (θ > 19°C), with σθ = 24.6–24.9 kg m−3, are found around the Shikoku Basin west of 140°E (Fig. 8a), where the local recirculation develops in association with the meander path (Figs. 6a,b). Intriguingly, there is little STMW over the Izu Ridge along 140°E, which is consistent with this being the geographical boundary between the warm and the cold STMW. In contrast, in non-meander years, most of the STMW is characterized by core θ < 19°C (Fig. 7b) and is distributed widely (Fig. 8d), not only in the region east of 140°E but also in the Shikoku Basin and over the Izu Ridge. It is evident that the warm STMW is found in the Shikoku Basin when the Kuroshio takes meander paths, as implied by Nishiyama et al. (1980). In the following analysis, we label the STMW with a warm core of θ > 19°C as W-STMW and that with a cold core of θ < 19°C as C-STMW for comparison.

Fig. 7.
Fig. 7.

(left) Core θ–S diagram of STMW in the northwestern part of the subtropical gyre (20°–36°N, 125°E–180°) in spring (April–June) of (a) meander years and (b) non-meander years. (right) Number of core θ.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 8.
Fig. 8.

(left) Distribution of STMW for meander years in: (a) spring (April–June), (b) summer (July–September), and (c) autumn (October–December). Red circles indicate warm core STMW (θ > 19°C) and blue circles represent cold core (θ < 19°C). Gray symbols are observation points where no STMW was present. Contours indicate mean SSH with an interval of 20 cm. The black rectangle delineates the SBR (specifically for 29°–32°N, 134°–138°E). (right) (d)–(f) As in (a)–(c), but for non-meander years.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

We explore vertical structure of the STMW in the Shikoku Basin region (SBR; specifically for 29°–32°N, 134°–138°E, displayed in Fig. 8). The upper boundary of the W-STMW in the meander years (Fig. 9a) is located around 100 dbar, which is almost identical to that of the C-STMW detected in non-meander years. In contrast, the lower boundary of the W-STMW is considerably shallower than the C-STMW; the thickness of the W-STMW is approximately half that of the C-STMW in the Shikoku Basin.

Fig. 9.
Fig. 9.

Histogram of depths of (a) upper and (b) lower boundaries of the W-STMW in the SBR during spring of meander years (bars) (dbar) and of the C-STMW in the SBR during spring of non-meander years (line).

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

4. Seasonal decay of W-STMW and its possible cause

In non-meander years, the C-STMW is present throughout the year (right panels in Fig. 8), although the cores tend to gradually cool (Fig. 10b). The W-STMW in meander years (Fig. 10a) is evident in spring, and its temperature changes little through the seasons. However, the W-STMW is less frequent in late summer, and its spatial distribution becomes progressively more restricted (Figs. 8b,c), with little remaining in late autumn. To gain a better understanding of the seasonal decay of the W-STMW, we use a quasi-Lagrangian time series from an Argo float that remained in the Shikoku Basin for about 1 year. It is difficult to use such Argo data for more than a year because Argo floats gradually move away from the Shikoku Basin, carried by strong currents such as the Kuroshio and the Kuroshio Countercurrent. We selected the best Argo float, assigned the World Meteorological Organization (WMO)-ID of 2900366 for the meander year 2005, in terms of data length and location and checked that other floats gave similar results. The float (Fig. 11a) circulated clockwise within the Shikoku Basin and captures the W-STMW formed in the winter ML. After spring, when the ML shoals, the W-STMW is distributed at depths of 100–300 dbar below the seasonal thermocline and persists until August. However, its thickness rapidly decreases after September. The W-STMW properties tend to be lost until the subsequent winter, unlike the C-STMW.

Fig. 10.
Fig. 10.

Seasonal variation of core θ of the W-STMW (closed circles) and C-STMW (open circles) detected in the SBR (°C) for (a) meander and (b) non-meander years.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 11.
Fig. 11.

(left) Trajectory of (a) Argo float WMO 2900366 from January to December 2005 and (b) Argo float WMO 2900956 from November 2009 to October 2010. The black closed circle represents the start point of the data used in this study. Bathymetric features are indicated by brown contours; 1000- (dashed line) and 2000-dbar depth (solid line). (right) Time–depth section of Q calculated from the Argo float profiles (10−10 m−2 s−2). Black lines represent θ with an interval of 1°C; green line is the θ = 19°C isotherm. Red line indicates the MLD.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

We discuss possible causes of the seasonal decay of the W-STMW. The SSH field seems that the Kuroshio path gradually shifts from the meander state to the non-meander state during meander years (Fig. 8, left). Such a transition is likely to facilitate the dissipation of the W-STMW because in autumn of meander years (Fig. 8c), the recirculation south of the Kuroshio/KE is continuous, within which the STMW patches formed at different longitudes with different temperatures tend to be stirred and finally mixed, as observed by Argo and shipboard observations (e.g., Oka et al. 2011). In other words, the W-STMW would have persisted for much longer if the meander state had persisted for more than 1 year. Qiu et al. (2006) investigated the seasonal decline of the STMW south of the KE, which is equivalent to the C-STMW in this study, using float profiles, and concluded that its erosion was induced predominantly by large diapycnal diffusivity at its upper boundary, which was caused by enhanced dissipation near the seasonal pycnocline, where the internal wave energy generated in the ML is trapped. The W-STMW should also be modified by this high diffusivity because the depth of the upper boundary of the W-STMW is identical to that of the C-STMW (Fig. 9a). In addition to the narrow vertical extent of the W-STMW, horizontal mixing processes and the high vertical diffusivity may contribute to its destruction.

There is an interesting aspect of the vertical STMW distribution in Fig. 11a; C-STMW is detectable below the W-STMW. The C-STMW must have been formed in previous years because of its distribution below the winter ML. The vertical double layer structure of the W-STMW and C-STMW is observed in other meander years (2007 and 2009) (see Fig. 10a).

Previous studies have not detected the C-STMW in the Shikoku Basin in meander years (e.g., Bingham 1992). Evidently, there was no C-STMW in 1977 when the Kuroshio took the tLM path (Fig. 3b). What causes the difference in C-STMW distribution in different meander years? The main reason that we could detect the C-STMW in the Shikoku Basin in meander years is because all the meander years in this study were the first (and only) year of the meander state (see Fig. 2b), and its properties were not therefore eroded completely, while the meander states in the 1970s and 1980s, when the C-STWM was not detected in past studies, persisted for a few years. The C-STMW properties would be lost in the Shikoku Basin if the meander states were maintained for several more years because the thickness in spring is about 100 dbar (Fig. 11a), much thinner than in non-meander years (about 250 dbar in Fig. 9). In addition to the meander state, its duration is primarily responsible for the determination of the vertical ocean structure south of Japan.

5. Influence of Kuroshio meander path on winter ML

The W-STMW is formed in the deep winter ML (Fig. 11a). We investigate the winter MLD and ML temperature, which is defined as the temperature at a depth of 10 dbar, to understand the influence of the Kuroshio path on STMW formation. In meander years (Figs. 12a,b), the deep ML with θ > 19°C is spatially isolated in the Shikoku Basin, which reflects the local recirculation associated with the meander path (Figs. 6a,b). That is, the W-STMW formation region is separated geographically from the C-STMW formation region, whose boundary lies along 140°E. The winter ML east of 140°E is colder than 19°C, except south of 30°N, although some W-STMW is distributed south of the KE in spring (Fig. 8a). This means that the W-STMW east of 140°E in spring was not formed south of the KE, but was advected from the Shikoku Basin by the Kuroshio or mesoscale processes. In contrast, in non-meander years, a broad, continuous, deep ML with θ < 19°C is present to the south of the Kuroshio/KE (Figs. 12c,d).

Fig. 12.
Fig. 12.

As Fig. 6, but for the MLD in March (>100 dbar; with a contour interval of 20 dbar) for (a) meander and (c) non-meander years. Gray shading denotes MLD > 130 dbar. (b),(d) As in Figs. 8a and 8d, but for the ML temperature with a deep MLD >150 dbar in March.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

The MLD in the Shikoku Basin is not very different between meander and non-meander years (Figs. 12a,c; Table 1) despite the somewhat deeper MTD in meander years (Figs. 6b,d; Table 1). This indicates that the background MTD is not a major influence on the winter MLD in this analysis period. We investigate the influence of two other factors on the MLD: the winter monsoon (e.g., Suga and Hanawa 1995a) and the upper-ocean stratification in the preceding warm season (July–September) (e.g., Qiu and Chen 2006). Results show a weaker monsoon and stronger stratification in the SBR in meander years compared with non-meander years (see Table 1). Both effects prevent the winter MLD from deepening in meander years. It can be pointed out that the MLD in the Shikoku Basin is determined by a combination of these three factors. Interestingly, around the Shikoku Basin, the lower boundary of the C-STMW in non-meander years is considerably deeper than that of the W-STMW in meander years (Fig. 9b), despite the similar MLD in the two periods. This is because of the C-STMW formed in the previous years; the C-STMW is distributed below the W-STMW throughout meander years (Figs. 10a, 11) and is then located under the winter ML in the following year, that is, a non-meander year (Fig. 11b), which results in the deep lower boundary of the C-STMW in non-meander years.

Table 1.

MLD in March, ML temperature in March, MTD in March (dbar), MOI in winter (January–March), upper-ocean stratification in the preceding warm season (July–September), and upward NHF in March. Each value is averaged for the SBR, except for MOI.

Table 1.

The upward NHF in the Shikoku Basin in meander years is larger than in non-meander years despite the weaker monsoon in meander years (Table 1); more active upward heat releases on the warmer SST in meander years. This implies that the positive anomalies of ML temperature in meander years are not formed through a heat exchange between the ocean and atmosphere. We attempt to quantitatively assess the influence of NHF on the upper-ocean heat content (OHC) from the sea surface to 700 dbar. A large amount of heat loss occurs in the region south of the Kuroshio (Fig. 13a), the value of which is greater in non-meander years than in meander years, as expected. The ocean heat loss in the Shikoku Basin in non-meander years is approximately explained by the NHF (Fig. 13c), but the loss is somewhat larger. This ocean cooling might be induced by horizontal mixing with the colder ML to the east because of the broad, continuous, deep ML (Fig. 12c). In contrast, in meander years (Fig. 13b), the ocean heat loss in the Shikoku Basin is considerably smaller than the NHF. We expect that the heat transport associated with the local recirculation that develops in the Shikoku Basin in meander years increases the ML temperature because the water transported by the local recirculation from the Kuroshio before reaching the KE region, where intense cooling due to the northerly winter monsoon frequently occurs in winter, experiences no remarkable heat loss. We investigate the relationship between the SST anomalies and the Kuroshio path states. The path states positively correlate with the SST anomalies in the W-STMW formation region (Fig. 14). A close look at Fig. 14 shows high positive correlations in the southeastern part of the local recirculation that develops in the Shikoku Basin in meander years, suggesting that the westward flow associated with the recirculation helps set the temperature in the Shikoku Basin. Using the two repeat hydrographic observations (ASUKA and 137°E lines) across the Shikoku Basin, we calculate the transport-averaged temperature due to the westward flow associated with the local recirculation as follows:
e2
where υ is the geostrophic velocity normal to the observation lines, z is the height (ranging between 1000 dbar and the surface), and x represents the region of the westward flow associated with the recirculation along the observation line. The westward flow region is defined, following Sugimoto et al. (2010), as the region of southern downward-tilting slope from the southern boundary of the Kuroshio (i.e., symbols I–II in Fig. 15a). It is found that the transport-averaged temperature in meander years is higher than in non-meander years (Fig. 15b). Both the absence of horizontal mixing with the colder ML south of the KE because of the spatially isolated ML and the increase in horizontal heat advection due to the westward flow associated with the local recirculation that develops in the Shikoku Basin in meander years cause ML warming and then the formation of W-STMW.
Fig. 13.
Fig. 13.

(a) Temporal rate of change in OHC during winter (W m−2), defined as OHC in December of the previous year minus OHC in March, at 135°E for meander (red solid line) and non-meander (red dashed line) years; the OHC was gridded on 1° lat interval for each month, applying the identical method as for Fig. 6. (b) Time rate of change in OHC during winter at 135°E (red line), NHF in winter (January–March) at 135°E (black line), and their residual, defined as time rate of change in OHC minus NHF (bar) for meander years. (c) As in (b), but for non-meander years.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 14.
Fig. 14.

Map of the correlation coefficient between the lat position of the Kuroshio in Fig. 2b and SST for March (from Reynolds et al. 2007). The sign of the Kuroshio position is reversed from that of the original index to represent the meander or non-meander path states more naturally; that is, the positive (negative) sign represents the meander (non-meander) path state. Black contours indicate the geopotential anomaly at 200 relative to 1000 dbar for meander years, displayed in Fig. 6a. Dotted lines show the 137°E and ASUKA lines.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

Fig. 15.
Fig. 15.

(a) Schematic diagram of sea surface geopotential anomaly relative to 1000 dbar along the ASUKA line (m2 s−2). Symbols I and II denote the northern and southern boundaries, respectively, of the westward flow associated with the recirculation. (b) Transport-averaged temperature associated due to the westward flow associated with the recirculation (°C), obtained from repeat hydrographic data along the 137°E (solid line) and ASUKA (dashed line) lines.

Citation: Journal of Physical Oceanography 44, 4; 10.1175/JPO-D-13-0114.1

6. Summary and concluding remarks

We investigated distributions of STMW in the northwestern part of the subtropical gyre using temperaturesalinity profiles for 2005–11, with particular reference to the Kuroshio meander and non-meander path states, south of Japan. In spring of meander years, the W-STMW characterized by θ > 19°C (σθ = 24.6–24.9 kg m−3) was found in the Shikoku Basin. The W-STMW was formed in the spatially isolated and warm winter ML in the Shikoku Basin, where a local recirculation developed in association with the meander path; both the absence of horizontal mixing with the colder ML south of the KE because of the spatially isolated ML and the increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ML warming in the Shikoku Basin. After spring when the ML shoals, the W-STMW was preserved below the seasonal pycnocline until midsummer, located in the depth range 100–250 dbar, with thickness approximately half that of the C-STMW (θ < 19°C, σθ = 25.0–25.4 kg m−3) formed in the Shikoku Basin in non-meander years. The W-STMW properties were lost rapidly between the late summer and the following winter. In addition to the narrow vertical extent of the W-STMW, horizontal mixing processes associated with the transition from the meander state to non-meander state during meander years and the high vertical diffusivity pointed out by Qiu et al. (2006) may have contributed to its destruction. The relative contributions among these processes should be assessed quantitatively for a better understanding of the STMW distribution in future work.

In meander years, we detected a vertical double layer structure of W-STMW and C-STMW in the Shikoku Basin; the C-STMW was formed in previous years because of its distribution below the winter ML in meander years. We concluded that the main reason that we could detect the C-STMW in the Shikoku Basin in meander years is because all the meander years in this study were the first (and only) year of the meander state, so the property was not totally lost. This contrasts with the meander states in the 1970s and 1980s when the C-STMW was not detected in past studies, which persisted for a few years. In addition to the meander states, the duration of the meander state influences the STMW formation and distribution south of Japan.

Note that the SST pattern associated with the Kuroshio path states tended to modify the heat release to the overlying atmosphere in winter, giving enhanced upward heat fluxes with warmer SST in the Shikoku Basin. Recently, Nakamura et al. (2012) reported the influence of the meander state on the trajectory of extratropical cyclones. The detailed investigations of heat storage and its dissipation within the local recirculation that develops in the Shikoku Basin in the meander state is needed for understanding the air–sea interaction in this region.

Acknowledgments

We thank the members of the Physical Oceanography Group, Tohoku University for valuable discussions. Dr. Eitarou Oka provided useful and constructive comments that improved the manuscript. The comments of Professor K. Speer and an anonymous reviewer were helpful in revising the manuscript. The first author (SS) was partly supported by the Grant-in-Aid for Young Scientists (B) (23740348) from the Japan Society for the Promotion of Science, and by the Grant-in-Aid for Scientific Research on Innovative Areas (25106702; “A ‘Hot Spot’ in the Climate System: Extra-Tropical Air–Sea Interaction under the East Asian Monsoon System”) from the Ministry of Education, Culture, Sports, Science, and Technology.

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