Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region off the Boso Peninsula, Japan

Transcription

1 Journal of Oceanography, Vol. 60, pp. 487 to 503, 2004 Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region off the Boso Peninsula, Japan KOSEI KOMATSU 1 *, YUTAKA HIROE 1, ICHIRO YASUDA 2, KIYOSHI KAWASAKI 1, TERRENCE M. JOYCE 3 and FRANK BAHR 3 1 National Research Institute of Fisheries Science, Fukuura, Kanazawa, Yokohama , Japan 2 Department of Earth and Planetary Physics, University of Tokyo, Tokyo , Japan 3 Woods Hole Oceanographic Institution, Massachusetts, MA 02543, U.S.A. (Received 6 January 2003; in revised form 9 December 2003; accepted 9 December 2003) Hydrographic structure and transport of intermediate water were observed in the Kuroshio region south of Japan, focusing on the σ θ density in six cruises from May 1998 through September In the section off the Boso Peninsula where the Kuroshio exfoliates eastward, the intermediate water was clearly clustered into three groups meridionally composed of the coastal water, the Kuroshio water and the offshore water. Compared with the Kuroshio water characterized by warm, salty water transported by the Kuroshio, the coastal and offshore waters significantly degenerated due to mixing with cold, fresh waters originated from the subarctic region: the former was affected by alongshore spread of the coastal Oyashio and the latter by direct intrusion of the new North Pacific Intermediate Water (NPIW) into the southern side of the Kuroshio current axis. Particularly the offshore water showed higher apparent oxygen utilization (AOU) in layers deeper than 26.9σ θ while it showed lower AOU in layers shallower than 26.9σ θ, which indicated that colder, fresher and higher AOU water was distributed on the southeastern side of the Kuroshio in deeper layers. In May 1998, the Oyashio-Kuroshio mixing ratio was estimated to be typically 2:8 for the offshore water on the assumption of isopycnal mixing. Moreover, northeastward volume transport of the Kuroshio water was obtained from geostrophic velocity fields adjusted to lowered acoustic Doppler current profiler (LADCP) data to yield 6.1 Sv at σ θ and 11.8 Sv at σ θ. Keywords: Kuroshio, North Pacific Intermediate Water, hydrographic structure, volume transport. 1. Introduction The Kuroshio, western boundary current south of Japan in the North Pacific, often fluctuates between off the Kii Peninsula and the Izu Ridge and usually exfoliates eastward off the Boso Peninsula (Figs. 1 and 2). The downstream region of the Kuroshio is located at the western edge of the North Pacific Intermediate Water (NPIW) characterized by a salinity minimum around 26.8σ θ, which was found to cover the whole subtropical gyre (e.g., Reid, 1965; Talley, 1993). NPIW is thought to be formed in the region between the Kuroshio Extension and the Oyashio front through mixing of the warm, salty subtropical water transported within the Kuroshio and the cold, fresh * Corresponding author. kosei@affrc.go.jp Copyright The Oceanographic Society of Japan. subarctic water originated from the Okhotsk Sea transported within the Oyashio along the east coast of Japan (Talley, 1993; Yasuda et al., 1996; Yasuda, 1997). Recently Hiroe et al. (2002) estimated the overall mass budget in σ θ in the Kuroshio-Oyashio interfrontal zone by hydrographical survey and direct measurement of currents, and revealed that 20.7 Sv of NPIW was formed by 7.2 Sv of the subarctic water (5.2 Sv along the Oyashio and 2.0 Sv through the subarctic front) merged with 13.5 Sv of the subtropical water along the Kuroshio. The hydrographic structure and transport of the subtropical water along the Kuroshio, which is called the Kuroshio water (Fujimura and Nagata, 1992; Talley et al., 1995), however, are not known in detail, though its volume transport is dominant in the budget of NPIW (Hiroe et al., 2002). In the region off the Boso Peninsula, the entrance to the interfrontal zone, the hydrographic structure at intermediate depth is so complicated, because 487

2 Fig. 1. Station locations for the R/V Soyo-maru cruises with topography south of Japan. Crosses indicate stations along subtracks of TOPEX/POSEIDON, where interval of each station pair is 10 in latitude and large numerals are subtrack numbers. the region is under intermittent influences from the southward spread of alongshore Oyashio water (Yang et al., 1993a, b; Senjyu et al., 1998) and southwestward intrusion of the new NPIW on the offshore side of the Kuroshio (Yasuda et al., 1996; Okuda et al., 2001), and because the Kuroshio water collides with Izu Ridge just on the upstream side (Komatsu and Kawasaki, 2002). The hydrographic structure of intermediate water off the Boso Peninsula depends on both intensity and flow path of the Kuroshio; moreover, the velocity field of the Kuroshio has a deep structure and a barotoropic component (Hiroe et al., 2002). Therefore geostrophic calculation on the assumption of a motionless layer is inadequate and direct measurement is necessary for the accurate estimation of volume transport. It is the detailed property of hydrographic structure and accurate transport of the Kuroshio water off the Boso Peninsula that we shall discuss in this study. In particular, what is the mixing ratio between the Kuroshio and Oyashio waters in the region and what volume of the Kuroshio water is transported into the Kuroshio-Oyashio interfrontal zone? For this purpose, hydrographic structure and transport were observed in six cruises, focusing on the intermediate water in the density of σ θ along a subtrack of the TOPEX/POSEIDON (T/P) crossing the Kuroshio off the Boso Peninsula from May 1998 through September The first cruise was a part of an extensive expedition which clarified the hydrographic and velocity structure of the Oyashio water off Hokkaido (Yasuda et al., 2001) and the mass budget in the interfrontal zone (Hiroe et al., 2002), and which contains intensive observations of cross-stream velocity structures around the Kuroshio Extension (Joyce et al., 2001). Hydrographic structures were examined by a conductivity-temperature-depth meter with DO meter (CTD-DO) in all the cruises and velocity structures were observed directly by a lowered acoustic Doppler current profiler (LADCP) in four cruises. The following sections describe the data and methods (Section 2), hydrographic structures (Section 3), degeneration processes (Section 4), and volume transport of the Kuroshio water estimated from geostrophic currents adjusted to LADCP data (Section 5). 2. Data and Methods All datasets of six cruises were obtained by R/V Soyo-maru (National Research Institute of Fisheries Science) along the subtracks of T/P in the Kuroshio downstream region from May 1998 through September 2001 (Fig. 1). Hydrographic structures were observed with CTD-DO in all the cruises and velocity fields were measured directly with LADCP. Station points in each cruise are shown with the flow path of the Kuroshio during the cruise in Fig. 2 and the information of the cruises is summarized in Table 1. At every station of all the cruises, vertical profiles of temperature, salinity, dissolved oxygen (DO) were observed using a CTD-DO with Rosette sampling system basically down to 1500 db. The CTD system was the Sea- Bird 911plus with Beckmann DO sensor and all the sensors were calibrated in the laboratory once or twice a year by Sea-Bird Electronics Co. Ltd. The mean accuracies of the temperature and the pressure sensor were C and 1 db, respectively. 488 K. Komatsu et al.

3 (a) Cruise in May 1998 (d) Cruise in October (b) Cruise in July 1999 (e) Cruise in January 2001 (c) Cruise in August 1999 (f) Cruise in S eptember 2001 Fig. 2. Locations of CTD-DO stations for each cruise. Thick dashed lines denote the Kuroshio current axes during the cruises, referred from Quick Bulletin of Ocean Conditions of Japan Coast Guard. Open diamonds, crosses and solid circles are locations of stations where we observed hydrographic profiles classified as the Coastal water, the Kuroshio water and the Offshore water, respectively. In (f), the stations north and south of the Kuroshio current axis are denoted by Kii North and Kii South, respectively. The CTD-conductivity data were calibrated by water samples which were obtained from 2.4 l Niskin bottles controlled by Rossette Water Sampler (General Oceanics Co. Ltd.) and were measured with AUTOSAL (Guidline Co. Ltd.) at 7 to 10 layers every 2 to 5 stations with a repeatability within psu. The overall residual rms errors of CTD-salinity were within psu compared with sampled data. The CTD-DO data were calibrated by the water-samples using the Owens and Millard (1985) algorithm. The samples were obtained by Rossette Sampler and measured by the Winkler method with auto-oxygen titration (Hirama-Rika Co. Ltd.) at 7 to 10 layers every 1 to 5 stations with a repeatability within 0.04 ml/l. The overall residual rms errors of CTD-DO were 0.05 ml/l compared with sampled data. Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 489

4 Table 1. Summary information of the R/V Soyo-maru cruises. *Latitude degree between station pairs except for in coastal regions. The current profiles were directly measured by the 150 khz-ladcp (RD Instrument Co. Ltd.) for the cruise in May 1998 and 300 khz-ladcp for the other cruises with 1 second interval CTD and GPS-positioning data with a 2-dimensional rms of 30 m. The profiling data were processed with the Common Oceanographic Data Access System (CODAS) routines and the final data were averaged downward and upward velocities smoothed by a 20 db running-mean; the differences were quite small. The measurement errors were expected to lie within a few cm/s. In addition, the barotropic tidal velocities estimated from a global inverse model were subtracted from the LADCP velocities. These LADCP data were used to adjust geostrophic velocities which were calculated using a pair of CTD profiles relative to 1500 db; the details of the adjustment are given in Hiroe et al. (2002) and Yasuda et al. (2001). 3. Hydrographic Structure of Intermediate Water off the Boso Peninsula Kuroshio water is influenced by the southward spread of the subarctic water off the Boso Peninsula, the last corner of the western boundary (Yang et al., 1993a; Yasuda et al., 1996). The influence is very noticeable alongshore and on the southern side of the Kuroshio, and its extent depends on the strength and the flow path of both the Kuroshio and the Oyashio (Yang et al., 1993a; Senjyu et al., 1998). We here focus on the influence of the subarctic water on the Kuroshio water and describe the hydrographic structure of intermediate water off the Boso Peninsula. 3.1 Hydrographic structure from vertical section in May 1998 First we describe the hydrographic structure off the Boso Peninsula in detail, referring to vertical cross-sections obtained in May in 1998 (Fig. 3), as a representative section. The geostrophic current field is relative to 1500 db (Fig. 3(e)), and the geostrophic velocity field adjusted to LADCP is discussed in Section 5. Detailed descriptions of hydrographic structures found in other cruises are described in Komatsu et al. (2004). The isotherm of C descended steeply from the coast to 34 N, and gradually from 34 N to 31.5 N, and ascended gradually from 31.5 N to 30 N (Fig. 3(a)), similar to isohalines at depths shallower than 900 db (Fig. 3(b)). Corresponding to these distributions and isopycnals (Fig. 3(d)), the distribution of geostrophic current showed a maximum speed of over 1.2 m/s at 34.4 N and a countercurrent existed with speed over 0.3 m/s at the subsurface between 31.0 N and 30 N (Fig. 3(e)). The strong current region of the Kuroshio had a deep structure down to 900 db. Around 33.9 N, at depths of db, a weak countercurrent was recognizable south of the intense Kuroshio. The countercurrent corresponds to a subsurface anticyclonic eddy located from 34.2 N to 33.6 N at db (Figs. 3(a) and (b)). The pattern of the geostrophic current, on the whole, was analogous to that given by LADCP data, as described in Section 5. In the subsurface layer at db, a salinity maximum of over 34.8 psu with a temperature between 17.5 C and 20 C appeared in the latitude N, indicating the subtropical mode water. Just beneath the subsurface layer centered at 500 db around 31 N, there was a lens-shaped anticyclonic eddy of salty, warm, and low apparent oxygen utilization (AOU) water (Fig. 3(c)). In the upper layer of the intermediate depth of db, a band of salinity minimum extended from the coast to 30 N. The salinity minimum (salinity less than 34.2 psu) was observed between 31.8 N and 30 N, small cores of salinity less than 34.1 psu were seen at 31.6 N and at 30 N. This salinity minimum characterizes NPIW (e.g., 490 K. Komatsu et al.

5 (a) Potential temperature ( o C) (d) Potential density (kg/m 3 ) (b) Salinity (PSU) (e) Geostrophic velocity (m/s) (c) AOU (ml/l) Fig. 3. Vertical cross-sections of (a) potential temperature ( C), (b) salinity (psu), (c) AOU (ml/l), (d) potential density (kg/m 3 ) and (e) geostrophic current (m/s) obtained off the Boso Peninsula in May Geostrophic currents were calculated using a pair of CTD data relative to 1500 db without any corrections. Positive value indicates northeastward component. Contour intervals are 2.5 C for potential temperature, 0.1 psu for salinity, 0.5 ml/l for AOU, 0.5 kg/m 3 for potential density, and 0.15 m/s for geostrophic current. Arrow under the abscissa denotes the location of the Kuroshio current maximum. Reid, 1965; Talley, 1993); the salinity minimum of salinity less than 34.2 psu appeared on the offshore side of the Kuroshio indicates the spread of the new NPIW (Yasuda et al., 1996; Okuda et al., 2001). 3.2 Cross-stream structure of intermediate water on isopycnal surfaces Intermediate water, in general, is thought to be advected and modified along isopycnal surfaces. To show the water-mass structure in the cross-stream direction more clearly, cross-sections of salinity and AOU in May 1998 are illustrated with potential-density ordinates in Figs. 4(a) and (b), respectively. Salinity less than 34.2 psu extended from 31.8 N to 30 N at σ θ with two small patches of salinity less than 34.1 psu as shown in Fig. 3(b). Salinity distribution around the salinity minimum changed abruptly around 32 N. Contours of AOU in the density range of σ θ also slanted at the Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 491

6 (a) S alinity (PSU) (c) Deviation of salinity (PSU) (b) AOU (ml/l) (d) Deviation of AOU (ml/l) Fig. 4. Vertical cross-sections of (a) salinity (psu) and (b) AOU (ml/l), and isopycnal deviations of (c) salinity (psu) and (d) AOU (ml/l) off the Boso Peninsula in May 1998 with potential density (kg/m 3 ) ordinate. In (c) and (d), isopycnal averages in all stations were subtracted to emphasize the water mass difference across the section and negative areas were shaded. Contour intervals are 0.05 psu for salinity, 0.5 ml/l for AOU, psu for salinity deviation, and ml/l for AOU deviation. same latitude. In order to emphasize the cross-stream changes of water properties on isopycnal surfaces, deviations of salinity and AOU from section-averages on each isopycnal surface are shown in Figs. 4(c) and (d), respectively. Salinity north of 32.0 N was higher than south of 32.0 N in the range σ θ. On the other hand, AOU in the northern area was higher than in the southern area in the σ θ interval. Conversely, AOU in the northern area was lower than in the southern area in the range σ θ. In the σ θ interval, the meridional change of salinity and AOU is attributed to the contact with the new NPIW of lower salinity and lower AOU (higher oxygen) in the southern area (e.g., Talley, 1993, 1997; Yasuda et al., 1996). In the σ θ range, however, it is not clear why AOU in the northern area was lower than AOU in the southern area. Where does the low-aou water within the Kuroshio come from? As described by Yasuda et al. (2001), one candidate for the origin of the low-aou water might be the water influenced by the Antarctic Intermediate Water (AAIW), which transports relatively low AOU water (around 27.2σ θ ) from the Southern Hemisphere to the North Pacific (Yuan and Talley, 1992; Reid, 1997). Referring to the horizontal map of climatological DO in the Pacific on the 27.2σ θ surface shown in figure 5 of Yasuda et al. (2001), we can see that a high DO (low AOU) region is distributed along the western boundary. This indicates that high-do water might be transported by the Kuroshio along the south coast of Japan, being carried via the Kuroshio Extension into the Kuroshio-Oyashio interfrontal zone (Yasuda et al., 2001). Reid (1997) also described the possibility that AAIW might be the origin of low-aou water along the south coast Japan. Qu et al. (1999) and Kaneko et al. (2001), however, denied the possibility that AAIW can flow to the north of 12 N, based on an analysis of the circulation of the intermediate water in the Philippine sea. Moreover, degeneration caused by vertical mixing is not negligible, besides the confluence of the western boundary currents. More extensive analysis is necessary to identify the origin of low-aou water in the density range σ θ off the Boso Peninsula. 492 K. Komatsu et al.

7 (a) May 1998 (b) July 1999 (c) August 1999 (d) October 2000 (e) January 2001 (f) September 2001 Fig. 5. Meridional distributions of salinity and AOU averaged in the layers of σ θ and σ θ at each station. Shaded area indicates the region of countercurrents defined as negative (southwestward) volume transport in the layer of σ θ. Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 493

8 (a) Off the Boso P. (b) Off the Enshu-nada (c) Off the Kii P. Fig. 6. Vertical profiles of salinity (psu) and AOU (ml/l) obtained on all the stations off the Boso Peninsula (a), off the Enshunada (b) and off the Kii Peninsula (c) with potential density (kg/m 3 ) ordinate. In Fig. 8(c), profiles obtained north (south) of the Kuroshio axis are denoted by dotted (solid) lines. This cross-sectional change of water properties in May 1998 was common in the other cruises; that is, at σ θ, salinity and AOU were low on the southern side of N. On the other hand, at σ θ on the southern side, AOU was high while salinity was low. 3.3 Relation between water properties and the Kuroshio The cross-stream distributions of salinity and AOU depend to a remarkable extent on the Kuroshio flow path and countercurrents. Figure 5 illustrates averaged salinity and AOU obtained in each cruise, where the averages were taken in the layer of σ θ and σ θ at each station. In the countercurrent region on the coastal side of the Kuroshio current axis, salinity in the σ θ range was lower, particularly in the case where the Kuroshio flow path shifted south of 34 N (Figs. 5(b), (d) and (e)). On the other hand, salinity in the σ θ interval was not similar; in July 1999 (Fig. 5(b)) the salinity in the countercurrent region on the coastal side was a little higher than the salinity around the station of the current maximum. In contrast to the coastal side, on the offshore side both averages gave a lower salinity, probably due to the influence of the Oyashio (e.g., Talley, 1993, 1997; Yasuda et al., 1996; Shimizu et al., 2001). The Oyashio water merges into the Kuroshio water along the Kuroshio Extension and produces the new NPIW, a part of which bifurcates into the recirculation of the subtropical gyre (Yasuda et al., 1996). The location of the local low salinity almost coincided with that of countercurrents. We can see a correlation between countercurrents and southward intrusions of the new NPIW. However, there was a case where they did not coincide each other; for example, in May 1998 remarkable salinity minimum was observed south of 32 N while the countercurrent was distributed south of 31 N (Figs. 3(b) and (e)). In this case, the mismatch is thought to be mainly due to the effect of the anticyclonic eddy located around 31 N; the eddy contained the new NPIW and its northern edge was near 32 N. AOU was lower for the average in the σ θ interval but higher for the average in the range σ θ on the offshore side of the Kuroshio current axis, as described previously. In particular, AOU in the range σ θ indicated a negative correlation with salinity (correlation coefficient 0.75). This indicates that the water in the range σ θ is originally formed in a process of isopycnal mixing between the high-salinity and low-aou water and the low-salinity and high-aou water, as described below. 4. Modification of the Kuroshio Water South of Japan The Kuroshio water is the subtropical water transported by the Kuroshio along the south coast of Japan into the interfrontal zone (e.g., Yasuda, 1997). It is not so 494 K. Komatsu et al.

9 Fig. 7. Relation between salinity at the salinity-minimum layer and distance from the Kuroshio current axis observed on all the stations off the Kii Peninsula (a), off the Enshu-nada (b) and off the Boso Peninsula (c). Salinity at the salinity-minimum layer is calculated from the profile smoothed by 0.1σ θ running mean. In abscissa, positive (negative) value denotes northern (southern) side of the Kuroshio axis. clear, however, how much the Kuroshio water is modified during advection by the Kuroshio before it experiences the influence of southward spread of the Oyashio and the new NPIW off the Boso Peninsula. In order to clarify the degeneration process of the Kuroshio water here, horizontal mixing effects on the degeneration are analyzed by comparison of hydrographic properties off the Boso Peninsula with those obtained on other sections in the upstream region of the Kuroshio and in the interfrontal zone. 4.1 Variation of the salinity-minimum structure across/ along the Kuroshio Vertical profiles obtained at all stations on each section are shown in Fig. 6, indicating that the salinity minimum is located around 26.8σ θ (e.g., Reid, 1965; Talley, 1993) and the AOU maximum is located around 27.5σ θ. To clarify the horizontal distribution of the salinity minimum in the direction along/across the Kuroshio, salinity at the layer of the salinity minimum is plotted in Fig. 7 against distance from the Kuroshio current axis obtained at all stations in order from west to east. Off the Kii Peninsula (Fig. 7(a)), the value of the salinity minimum changes abruptly around the Kuroshio axis: salinity minima were higher than psu north of the Kuroshio axis, while they were less than psu south of it. Off the Enshu-nada (Fig. 7(b)), most of the stations north of the Kuroshio axis have salinity minima of salinity higher than psu; on the other hand, south of the Kuroshio axis the values of the salinity minima were mostly less than psu. Particularly on the stations two degrees to the southern side of the axis, which were located east of the Izu Ridge, the values were less than psu, indicating the influence of the new NPIW. Off the Boso Peninsula (Fig. 7(c)), most of the salinity minima around the Kuroshio axis were higher than psu, while those on the coastal side and on the offshore side were less than psu. As described in Subsection 3.3, salinity less than psu at the salinity minimum layer indicates the influence of the Oyashio on the coastal side and that of the new NPIW on the offshore side. In the cruise from August to September, 2001, hydrographic structures were observed along three subtracks of T/P crossing the Kuroshio (Fig. 2(f)). Vertical cross-sections of salinity obtained in this cruise are illustrated with density reference in Fig. 8. On the northern side of 32 N off the Enshu-nada, salinity minimum with a salinity less than psu occupied only limited regions compared with that observed off the Kii Peninsula, which extended from 33 N to 30 N in σ θ (Figs. 8(a) and (b)). On the other hand, south of 32 N, a salinity minimum of salinity less than psu was widely distributed, the low-salinity cores being less than psu. This salinity distribution off the Enshu-nada is due to the Kuroshio meander, where the Kuroshio crossed the Enshu-nada section line around 32 N (Fig. 2(f)). The water of salinity less than psu was distributed south of 32 N, while it did not appear on the coastal side. As is the case for the section off the Boso Peninsula (Fig. 8(c)), the location of the salinity minimum of salinity less than psu depends strongly on the Kuroshio flow path off the Enshu-nada. Moreover, two low-salinity patches appeared with salinity less than psu south of 31.2 N, which were not Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 495

10 (a) Salinity (PSU) off the Kii Peninsula (b) S alinity (PSU) off the Enshu-nada (c) Salinity (PSU) off the Boso Peninsula Fig. 8. Vertical cross-sections of salinity (psu) with potential density (kg/m 3 ) ordinate obtained (a) off the Kii Peninsula (a), off the Enshu-nada (b) and off the Boso Peninsula (c). Contour interval is 0.05 psu and thick contour line denotes psu. observed off the Kii Peninsula, as seen in Fig. 6(a). The section off the Enshu-nada north of 31.5 N is blocked by the Izu Ridge, suggesting that the low-salinity patches off the Enshu-nada was from the new NPIW formed east of the Izu Ridge and that the low-salinity water was not locally formed in the western region of the Izu Ridge. These results are consistent with those of Sekine et al. (2000), who showed that the low-salinity water with the salinity less than 34.1 psu is confined to the eastern region of the Izu Ridge. 4.2 Water property classification with vertical profiles of salinity and AOU According to the cross-stream variation of the salinity minimum value (Fig. 7), intermediate water can be divided into three groups off the Boso Peninsula: the coastal water, the Kuroshio water, and the offshore water. The locations of the stations where we observed each water-group are shown in Fig. 2. The coastal water was defined by a salinity minimum with salinity less than psu on the coastal side of the Kuroshio axis, the Kuroshio water by a salinity minimum value higher than psu, and the offshore water by a salinity minimum value less than psu on the offshore side of the Kuroshio axis. Averaged vertical profiles were calculated by ensemble mean on isopycnal surfaces from all the data classified into each group (Fig. 9(a)). Averaged AOU of the offshore water was lowest at σ θ and highest at σ θ. Between the coastal and Kuroshio waters, averaged AOU did not yield a significant gap through the density range of σ θ. Averaged salinity of the offshore water was lowest at σ θ. The salinity minimum was located around 26.8σ θ for the offshore water, which is the typical density characteristic of the NPIW; however, for the Kuroshio water it was located around 27.0σ θ, and for the coastal water around 26.9σ θ. As is the case with the section off the Boso Peninsula, vertical profiles of salinity and AOU can be divided into two groups for the section off the Enshu-nada (Fig. 9(b)). Off the Kii Peninsula, salinity at the salinity-minimum layer was higher than psu at all stations (Fig. 8(a)), as seen in Fig. 7(a). Averaged salinity and AOU obtained north and south of the Kuroshio current axis off the Kii Peninsula is shown in Fig. 9(c). In the σ θ range, the averages south (north) of the Kuroshio axis were mostly lower (higher) than those observed in the Kuroshio water downstream. During the observation period, the water of lower salinity and lower AOU was distributed just south of the Kuroshio off the Kii Peninsula, which can be partly attributed to an anticyclonic eddy with a diameter of 300 km centered at 31.8 N, E (Komatsu and Kawasaki, 2002). On the other hand, in the σ θ range, there was no significant difference between the averaged profiles obtained off the Kii Peninsula and the averaged one of the Kuroshio water off the Enshu-nada, while the Kuroshio water off the Boso Peninsula showed significantly lower salinity and higher AOU (Fig. 9(c)). This indicates that the Kuroshio water contacts the fresh, high-aou water on the eastern side of the Izu Ridge. Different from the Kuroshio water, the 496 K. Komatsu et al.

11 Fig. 9. Isopycnal averages of vertical profiles for salinity (psu) and AOU (ml/l) off the Boso Peninsula (a) and off the Enshu-nada (b) with potential density (kg/m 3 ) ordinate, which were calculated from all the data divided into each group. Horizontal bars denote standard deviations. Isopycnal averages of the Kuroshio water (c) and the offshore water (d) obtained off the Boso Peninsula and off the Enshu-nada, with averages obtained north (Kii North) and south (Kii South) of the Kuroshio axis off the Kii Peninsula. gap between the profiles off the Kii Peninsula and the averaged profile of the offshore water off the Enshu-nada is significant (Fig. 9(d)). One of the reasons for this is that the stations off the Enshu-nada where the offshore water was obtained were on the eastern side of the Izu Ridge. 4.3 Degeneration of the Kuroshio water in the interfrontal zone Horizontal distributions of isopycnal water properties were investigated from the DO-salinity diagrams shown in Fig. 10, where values were averaged in the isopycnal layers with the density of σ θ (Fig. Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 497

12 (a) S alinity-do at (b) S alinity-do at Fig. 10. DO-salinity diagram for the water at the density of (a) σ θ and (b) σ θ. Values are averaged in the layer of the density range and are composed of the Kuroshio water (Boso K), the coastal water (Boso C) and the offshore water (Boso O) off the Boso Peninsula, the Kuroshio water (Enshu-nada K) and the offshore water off the Enshu-nada (Enshu-nada O), the waters obtained north of the Kuroshio axis (Kii N) and south of the Kuroshio axis (Kii S), the waters obtained on the section lines along 36 N, 32 N, 150 E, and the Oyashio water off Hokkaido (Hokkaido). Data were obtained in this study for the waters off the Boso Peninsula, off the Enshu-nada and off the Kii Peninsula, obtained in a cruise in Jul for the water on 36 N, obtained in cruises in Jul. 1997, Oct and Jan for the water on 32 N, and obtained in a cruise in Oct for the water on 150 E and off Hokkaido. All the data were obtained by the R/V Soyo-maru with the method described in Section 2. 10(a)) and of σ θ (Fig. 10(b)) as representatives of the upper and lower layers of the intermediate depth, respectively. The waters obtained off the Boso Peninsula, off the Enshu-nada and off the Kii Peninsula were compared with the waters south of the Kuroshio Extension in the zonal section along 32 N, in the Oyashio-Kuroshio interfrontal zone in the sections along 36 N, and along 150 E and in the Oyashio off Hokkaido. All data were obtained by the R/V Soyo-maru as described in Section 2 and the data off the Boso Peninsula, off the Enshu-nada and off the Kii Peninsula were obtained in this study, the data on 32 N were obtained in the cruises in Jul. 1997, Oct and Jan. 2001; the data on 36 N were obtained in the cruise in Jul. 1999; and the data on 150 E and off Hokkaido were obtained in a cruise in Oct Figure 10 shows the linear relationship between DO and salinity: DO is almost negatively correlated with salinity on the σ θ surface. On the other hand, DO is positively correlated with salinity on the σ θ surface. This indicates that the water with intermediate characteristics are produced in the process of isopycnal mixing between the high-salinity, low-do Kuroshio wa- 498 K. Komatsu et al.

13 ter and the low-salinity, high-do Oyashio water on the σ θ surface (Talley, 1993; Yasuda et al., 1996), and between the high-salinity, high-do Kuroshio water and the low-salinity, low-do Oyashio water on the σ θ surface. On the σ θ surface in Fig. 10(a), the group of offshore water off the Boso Peninsula is located between the group of the Kuroshio water and the group of the water on 32 N, indicating that the offshore water contacts the new NPIW through the Kuroshio Extension, as described previously (Yasuda et al., 1996). On the other hand, on the σ θ surface in Fig. 10(b) the distribution of the offshore water mostly coincides with the distribution of the waters along 32 N and 150 E. These might be attributed to the anticyclonic gyre of the Kuroshio recirculation on the southern side of the Kuroshio jet as described for deeper layers denser than 27.5σ θ in Wijffels et al. (1998). The offshore water off the Boso Peninsula might be transported by the westward flow accompanied by the fresh and low DO (high AOU) water distributed around 150 E. It can be inferred that at σ θ fresh, high AOU water was fed into the eastern side of the Izu Ridge within the anticyclonic gyre of the Kuroshio recirculation in the southern side of the Kuroshio jet. Besides the supply of low AOU water within the western boundary, this might be a reason why the Kuroshio water showed lower AOU than the offshore water off the Boso peninsula. (a) LADCP velocity (m/s) (b) Adjusted geostrophic velocity (m/s) (c) Difference between (a) and (b) (m/s) 5. Volume Transport of the Kuroshio Water off the Boso Peninsula This section describes geostrophic currents adjusted by the LADCP data and temporal variation of volume transport of the coastal, Kuroshio and offshore waters off the Boso Peninsula. Moreover, from the data in May 1998, net volume transport of the Kuroshio water is estimated using the Oyashio-Kuroshio mixing ratio. Fig. 11. Vertical cross-sections of (a) LADCP velocity fields (m/s) perpendicular to each station pair, (b) geostrophic currents (m/s) adjusted to LADCP, and (c) the velocity difference (cm/s) between LADCP and adjusted geostrophic currents off the Boso Peninsula in May Positive value indicates northeastward component in (a) and (b), and contour intervals are 0.15 m/s for velocity and 10 cm/s for velocity difference. 5.1 Vertical structure of corrected geostrophic currents off the Boso Peninsula The vertical cross-sections of velocity fields off the Boso Peninsula in May 1998 are illustrated in Fig. 11: the LADCP component perpendicular to each station pair is shown in Fig. 11(a), the geostrophic currents adjusted to LADCP data in Fig. 11(b), and the difference between LADCP and adjusted geostrophic currents in Fig. 11(c). The adjustment was carried out for the reference-level velocity so as to minimize the difference between the geostrophic velocities and the station-pair averaged LADCP velocities with a least squares fit (Yasuda et al., 2001; Hiroe et al., 2002). Compared with geostrophic currents without the adjustment shown in Fig. 3(e), LADCP velocities indicate that the maximum speed of the Kuroshio was over 0.5 m/s slower around 34.4 N, and that a countercurrent adjacent to the Kuroshio current axis had a stronger, deeper structure down to 1500 db. Moreover, the countercurrent on the southern side of 31 N indicated a stronger core with a speed over 0.45 m/s for LADCP data. Adjusted geostrophic currents showed similar magnitude and distribution of the velocity fields as LADCP data, except Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 499

14 (a) Oyashio-Kuroshio mixing ratio (b) Volume transport of the Kuroshio water Fig. 12. Temporal variations of volume transport of the coastal water, the Kuroshio water, and the offshore water at σ θ (a) and at σ θ (b) off the Boso Peninsula. Volume transports were calculated from geostrophic velocity fields on the station pairs which obtained each water classified in Subsection 4.2, where the geostrophic velocities were adjusted to the LADCP data for the cruises in May 1998, Jul. 1997, Oct and Jan Fig. 13. Vertical cross-sections of (a) the Oyashio-Kuroshio mixing ratio and (b) volume transport (Sv) of the Kuroshio water at 0.1σ θ interval off the Boso Peninsula in May Volume transport was calculated by multiplying the adjusted geostrophic velocity by cross-section in 0.1σ θ interval and between each station pair, and northeastward transport is denoted by positive value. Solid circles on top abscissa denote the latitude of each station and contour intervals are 0.1 Sv for volume transport and 0.05 for mixing ratio. for the surface region around the Kuroshio current axis, and the difference between them was less than 10 cm/s in most of the section, suggesting that the velocity fields were almost in geostrophic balance. Three patches with difference over 10 cm/s were located from 34.2 N to 33.6 N at a depth of db, except for the region close to the surface or the coast. These patches correspond to the anticyclonic eddy described in Section Temporal variations of volume transport of the coastal, Kuroshio and offshore waters Figure 12 shows volume transports of the coastal water, the Kuroshio water, and the offshore water obtained on the section off the Boso Peninsula in each cruise. Volume transports in the layers of σ θ and σ θ were calculated from geostrophic velocity fields based on the station pairs which obtained each water classified in Subsection 4.2. Transport of the coastal water varied with an almost negative correlation with that of the Kuroshio water both at σ θ and at σ θ. We observed the maximum southwestward transport of the coastal water and the maximum northeastward transport of the Kuroshio water in July 1999, when the Kuroshio crossed the section around 33.5 N and turned southeastward east of 142 E (Fig. 2(b)). On the other hand, there was no significant relation between transports of the offshore and Kuroshio waters. The maximum southwestward transport of the offshore water occurred in May 1998, and the variation patterns of the offshore and coastal waters did not coincide with each other. It is clear that volume transports of these waters depend on both the Kuroshio and the Oyashio; however, the relation among them is not clear from the present analysis, one reason being that the coastal and offshore waters are not pure waters originating from 500 K. Komatsu et al.

15 Fig. 14. (a) The Kuroshio volume transports obtained at each station pair at σ θ (solid line with solid squares) and at σ θ (broken line with solid triangles), and the Kuroshio volume transports integrated from the coast at σ θ (solid line with open squares) and at σ θ (broken line with open triangles). All the transports were calculated with adjusted geostrophic velocity fields off the Boso Peninsula in May (b) Comparison of the Kuroshio volume transports at σ θ obtained from adjusted geostrophic velocity fields (solid lines) with ones obtained from non-adjusted geostrophic velocity fields (broken lines). Transports at each station pair are denoted by lines with solid squares and with solid triangles, and transports integrated from the coast are denoted by lines with open squares and with open triangles. the Oyashio, that is, they contain some Kuroshio water due to isopycnal mixing. Next, the net transport of the Kuroshio water, defined as the transport of the water influenced by the Kuroshio water, is estimated from the data in May 1998 using the Oyashio-Kuroshio mixing ratio under the assumption of isopycnal mixing between the Kuroshio and Oyashio waters. 5.3 Net transport of the Kuroshio water off the Boso Peninsula Vertical cross-sections of the Oyashio-Kuroshio mixing ratio and the volume transport of the Kuroshio water are illustrated in Fig. 13. The mixing ratios were calculated as the ratios averaged between the one based on the potential temperature and the one based on the salinity in 0.1σ θ intervals, where the reference potential temperature and salinity for the Kuroshio (Oyashio) water were obtained under the current-core region of the Kuroshio (Oyashio) off the Boso Peninsula (off Hokkaido). The details are contained in table 1 of Yasuda et al. (2001). Net volume transport of the Kuroshio water was estimated by multiplying the total volume transport by the rest of the Oyashio-Kuroshio mixing ratio, where the total volume transport was calculated by multiplying the adjusted geostrophic velocity by cross-section in 0.1σ θ and station-pair intervals. The Oyashio-Kuroshio mixing ratio was less than 0.05 in the most part northern from 32 N, but a region with the ratio over 0.1 appeared at σ θ between 33.7 N and 33 N, corresponding to the salinity-minimum patch with salinity less than psu (Fig. 4(a)). On the other hand, south of 32 N a region with the ratio over 0.1 was distributed south of 32 N, corresponding to the southward intrusion of the new NPIW with salinity less than 34.1 psu in Fig. 4(a). Moreover it should be noted that the layer denser than 27.4σ θ was covered with the region with the ratio over 0.1. In the Kuroshio region between 34.7 N and 34.3 N, a large volume transport was observed with a core over 0.4 Sv at σ θ. At the southern side of the Kuroshio region a southwestward transport appeared with a maximum over 0.6 Sv at σ θ, corresponding to the countercurrent observed in the velocity field. In the region between 32.8 N and 32.4 N a large transport with a core over 0.6 Sv was found in the deeper layer, which was due to an increase of isopycnal thickness with density because current speed was less than 0.15 m/s in this region (Fig. 11(b)). A notable transport was observed south of 32 N, where a strong, deep countercurrent was located with a maximum speed over 0.45 m/s. The net volume transports of the Kuroshio water at each station pair at σ θ and at σ θ are illustrated with their integrated values from the coast in Fig. 14(a). The northeastward Kuroshio transport along the coast of the Boso Peninsula from the coast to 31 N was 6.1 Sv at σ θ and was 11.8 Sv at σ θ. Net transport on the section from the coast to 30 N was 3.7 Sv at σ θ and 6.0 Sv at σ θ, as a result 9.7 Sv at σ θ as shown in Fig. 14(b). The transport in the recirculation gyre located between 32 N and 31 N was 4.8 Sv. Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region 501

16 The net volume transport of the Kuroshio water estimated with the adjusted geostrophic velocity data was compared with the one estimated with the non-adjusted geostrophic velocity data (Fig. 14(b)). The non-adjusted geostrophic velocity field featured relatively weak, shallow structures in countercurrent regions as shown in Fig. 3(e), which yielded less southwestward transport at 34.2 N, 30.8 N and 30.5 N. In addition, the non-adjusted geostrophic velocity caused northeastward transport to be small between 33 N and 31 N. As a result the integrated transport from the coast to 31 N was 4.2 Sv smaller than the adjusted-geostrophic transport. This study focused on the Kuroshio transport off the Boso Peninsula; the estimation of the water mass budget in the Kuroshio-Oyashio interfrontal zone is described by Hiroe et al. (2002). 6. Summary Hydrographic structures and volume transport of intermediate water were observed by CTD-DO and LADCP in the Kuroshio downstream region in six cruises from May 1998 through September The main results are summarized as follows. 1) Six cruises revealed that the spreads of the Oyashio and the new NPIW affected the water properties off the Boso Peninsula, depending on the flow path of the Kuroshio, and that a southward shift of the Kuroshio enabled the cold, fresh water to intrude more easily alongshore. 2) Intermediate water off the Boso Peninsula was clearly clustered into three groups composed of the coastal water, the Kuroshio water, and the offshore water. The coastal water and the offshore water exhibited cold, fresh properties due to influence of spread of the Oyashio water alongshore and of the new NPIW south of the Kuroshio, respectively. The offshore water, compared with the Kuroshio water, showed higher AOU in layers deeper than 26.9σ θ while it showed lower AOU in layers shallower than 26.9σ θ. 3) From salinity-do diagrams, the Kuroshio water decreased salinity and increased DO in the isopycnal mixing with the Oyashio water at σ θ ; on the other hand, it decreased both salinity and DO at σ θ. At σ θ the offshore water off the Boso Peninsula suffered the southwestward intrusions of the new NPIW through the Kuroshio Extension. 4) In the cruise in May 1998, the Oyashio-Kuroshio mixing ratio was estimated to be 20% for the offshore water on the assumption of isopycnal mixing. The net volume transport of the Kuroshio water adjusted to LADCP data was 6.1 Sv at σ θ and 11.8 Sv at σ θ. The above results are new in that we described degeneration processes of the Kuroshio water along and perpendicular to the Kuroshio jet by CTD-DO in six cruises, and we have discussed more comprehensively and quantitatively the volume transport of the Kuroshio water owing to direct measurements of velocity fields by LADCP compared with previous studies assuming a level of no motion. Acknowledgements Discussions with Drs. A. Masuda and I. Kaneko were very valuable. The authors greatly appreciate the captains and the crew members of R/V Soyo-maru for supporting the observations. This work is partially supported by the Fisheries Agency and the project of the Sub-Arctic Gyre Experiment conducted by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). This work was completed while the first author stayed at Max-Planck-Institute for Meteorology in support by Dr. G. Brasseur and MEXT. References Fujimura, M. and Y. 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