Hydrographic Structure and Variability in the Kuroshio- Oyashio Transition Area

Transcription

1 Journal of Oceanography, Vol. 59, pp. 389 to 402, 2003 Review Hydrographic Structure and Variability in the Kuroshio- Oyashio Transition Area ICHIRO YASUDA* Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan (Received 3 June 2002; in revised form 13 November 2002; accepted 13 November 2002) The hydrographic structure and variability in the Kuroshio-Oyashio Transition Area of the northwestern Pacific are briefly reviewed, focusing on the circulation, frontal structure and water-mass formation from surface to intermediate depths. This area is a key to understanding climate and ecosystem variations because signals can be detected earlier than major climate regime shifts and also because species replacement among small pelagic fishes could be related to environmental changes in this area. We need further studies of the effect of North Pacific Intermediate Water on surface currents and frontal structures and also studies on the formation and variability of water-masses in surface mixed layer. Keywords: Kuroshio, Oyashio, front, water-mass, climate, ecosystem. 1. Introduction The Kuroshio-Oyashio Transition Area east of Japan is a key to understanding climate and ecosystem variations. The Kuroshio and the Kuroshio Extension transport a large amount of heat and release it to the atmosphere; the Oyashio transports freshwater to the subtropical gyre. Decadal to interdecadal signal are centered on this area (e.g. Nitta and Yamada, 1989) and the western boundary currents that transport heat can alter climate regime (e.g. Latif and Barnett, 1994; Miller et al., 1998; Deser et al., 1999). This area is also one of the most important fishing grounds. The Pacific saury fishing grounds are known to be related to the meridional shift of the Oyashio Front (Yasuda and Watanabe, 1994; Yasuda and Kitagawa, 1996), and the population of the Japanese sardine is related to winter-spring SST in and south of the Kuroshio Extension (Yasuda et al., 1999; Noto and Yasuda, 1999, 2003). The Kuroshio-Oyashio Transition Area east of Japan is a confluence of the two western boundary currents (WBCs) of the wind-driven subtropical and subarctic gyres as well as WBCs of intermediate and deep circulation in the North Pacific. Various water masses are formed and transported into the subsurface, memorizing climate signals. In particular, horizontally and vertically homo- * address: ichiro@eps.s.u-tokyo.ac.jp Copyright The Oceanographic Society of Japan. geneous water masses are called Mode Water (e.g. Hanawa and Talley, 2001). In the Kuroshio-Oyashio Transition Area, several mode waters, such as Subtropical Mode Water (STMW: Masuzawa, 1969), Central Mode Water (CMW: Nakamura, 1996; Suga et al., 1997) and Okhotsk Sea Mode Water (OSMW: Yasuda, 1997), are formed locally. North Pacific Intermediate Water (NPIW: Sverdrup et al., 1942) is another water mass that is characterized by a salinity minimum in the subtropical gyre. It is literally a crossroad of water masses that merge through WBCs and are formed locally. There are complicated frontal structures in this area: Subarctic Front (SAF), Subarctic Boundary (SAB), Kuroshio Bifurcation Front (KBF), Kuroshio Extension Front (KEF) and Subtropical Front (STF). Long-term variations of these water masses and fronts could be related to decadal to inter-decadal climate variabilities in the North Pacific. In relation to global warming issues, we need to know how much anthropogenic CO 2 is absorbed in the North Pacific water masses, how the water masses are transported (circulated) into the subsurface and how they eventually return to surface and how long they are isolated from the atmosphere (Tsunogai et al., 1993; Ono et al., 2000). Mode water formation (deep winter mixed layer) and ventilation into intermediate depths are important processes for understanding carbon cycle. To achieve this goal, we need to know the formation processes and circulation of related water masses. 389

2 As mentioned above, studies on the Kuroshio- Oyashio Transition Area are quite important. However, previous studies are still not sufficient; we need to further understand the physical, chemical and biological processes occurring in this area. We here try to provide a current view of circulation and water masses with frontal structures from surface to intermediate depths (up to 1200 m or 27.4σ θ ) by briefly reviewing previous studies for the area N and 140 E 180, and try to address remaining issues. 2. Current Structures Near-surface current structures in the Kuroshio- Oyashio Transition Area are schematically illustrated in Fig. 1. The Kuroshio south of Japan and the Kuroshio Extension (KE) east of Japan are the western boundary currents (WBCs) of the North Pacific subtropical gyre and transport warm and saline surface waters. Subarctic WBCs are strongly modified along the pathway. The East Kamchatka Current (EKC) is a part of the cyclonic circulation of the Western Subarctic Gyre (WSAG) and flows southwestward from the Bering Sea. Near-surface water in EKC is characterized by dichothermal water formed in the winter mixed layer and mesothermal water in the intermediate depth below a strong halocline (e.g. Uda, 1963; Dodimead et al., 1963; Favorite et al., 1976; Ueno and Yasuda, 2000). A part of EKC flows into the Okhotsk Sea in the northern Kuril Straits (e.g. Ohtani, 1989) and is then considerably modified in the Okhotsk Sea along the cyclonic circulation through sea-ice formation and mixing with Okhotsk Sea waters (e.g. Kitani, 1973). In the Okhotsk Sea, the East Shakalin Current (ESC) is intensified in fall and winter (Itoh and Ohshima, 2000; Mizuta et al., 2001; Simizu and Ohshima, 2002; Ohshima et al., 2002). In the Okhotsk Sea Kuril Basin, a large anti-cyclonic circulation (Wakatsuchi and Martin, 1991; Yasuda, 1997) contains the Okhotsk Sea Mode Water (Yasuda, 1997; Transitional Water by Kitani, 1973) in the subsurface. The Okhotsk Sea water outflows to the Pacific and merges with EKC to form the Oyashio (e.g. Ohtani, 1989; Talley, 1991; Talley and Nagata, 1995; Yasuda, 1997; Kono and Kawasaki, 1997a). The Oyashio water has characteristics intermediate between EKC and Okhotsk Sea waters. The Oyashio (OY) flows southward and some part reaches KE (Yasuda et al., 1996, 2001; Shimizu et al., 2001; Okuda et al., 2001); whereas the other part recirculates northeastward along the WSAG. The subarctic surface water along the recirculation could drift southward across the subarctic front (SAF) as Ekman drift and could cover the surface in the Transition Domain between SAF and the Subarctic Boundary (SAB) (Fig. 1). In the area east of Japan, KE and OY converge. We Fig. 1. Schematic illustration of the near-surface current, front and water-mass structures in the Kuroshio-Oyashio transition area. EKC: East Kamchatka Current, WSAG: Western Subarctic Gyre, ESC: East Shakalin Current, OY: Oyashio, KE: Kuroshio Extension, TC: Tsushima Warm Current, SAF: Subarctic Front, SAB: Subarctic Boundary, KBF: Kuroshio Bifurcation Front, STF: Subtropical Front, MLF: Mixed Layer Front, STMW: Subtropical Mode Water, S-CMW: Shallow Central Mode Water, D-CMW: Dense Central Mode Water, DSW: Dense Shelf Water, OSMW: Okhotsk Sea Mode Water. The yellow circles denote warm-core rings and light green ones cold-core rings. often refer to this area between SAF and KE as the Kuroshio-Oyashio Interfrontal Zone (e.g. Kawamura et al., 1986; Yasuda et al., 1996), where several intense meso-scale eddies, such as Kuroshio warm-core rings (e.g. Kawamura et al., 1986; Yasuda et al., 1992) and coldcore rings (e.g. Kawai, 1972) distribute, especially west of 155 E. Eddy-eddy interaction, vortex merger and eddy- Kuroshio interaction are often observed (Kawamura et al., 1986; Yasuda et al., 1992); for theoretical and modeling studies, see Yasuda (1995) and Yasuda and Flierl (1995, 1997). Kuroshio warm-core rings near the east coast of Japan sometimes move northward along Honshu (e.g. Kitano, 1975; Yasuda et al., 1992) and further northeastward along Hokkaido and Kuril Islands (Yasuda et al., 2000a). This movement might be explained by a mirror-image effect (Yasuda et al., 1986; Itoh and Sugimoto, 2001) or by a pseudo-beta effect (Yasuda et al., 2000a: interaction between eddy and northeastward deep WBC). For the deep WBC, see Owens and Warren (2001). Modification of water-masses is quite large in this area, as is described in Section I. Yasuda

3 Fig. 2. (a) Year to year variations in (uppermost) 5-year running mean winter North Pacific Index (NPI), (second) Summer offshore Oyashio front latitude anomaly averaged in N (OOF in N), (third) southern end latitude anomaly of the Oyashio coastal intrusion (OY1 in N) (thick curve: annual mean anomaly; thin: anomaly in April) and (fourth) latitude anomaly of the location of the first meander crest of the Kuroshio Extension Front (KEF in N). (b) Annual variation in the southern end latitude of the Oyashio coastal intrusion with 95% confidence limit. The Tsushima Current in the Japan/East Sea flows out to the Pacific through the Tsugaru Strait as the Tsugaru Warm Current (e.g. Yasuda et al., 1988), and flows into the Okhotsk Sea through the Soya Strait as the Soya Warm Current. 3. Frontal Structure and Variability In the Kuroshio-Oyashio Transition Area, there are several fronts that separate water masses or circulation regimes. A schematic representation of these fronts is depicted in Fig. 1. We here review these frontal structures and variability. For chemical structures, like nutrient, oxygen and carbonate properties, see Ono et al. (1998). 3.1 Subarctic Front (SAF) or Oyashio Front (OYF) The Subarctic Front (SAF) is defined as the 4 C isotherm at 100 m depth (Favorite et al., 1976). SAF is a water mass front (Zhang and Hanawa, 1993); subarctic low-salinity water distributes north of SAF down to the density of 27.5σ θ (Yasuda et al., 2001). The Oyashio Front (OYF) is a western part of SAF in the vicinity of Japan (Kawai, 1972). OYF is identified by monthly variable temperatures at 100 m depth (5 8 C: Kawai, 1972) for the offshore SAF in E. For OYF between subarctic water and Tsugaru warm water that flows out from the Tsugaru Strait and along the coast of Sanriku, Takasugi and Yasuda (1993) proposed monthly variable temperature indices. Interannual variations of SAF in E were reported by Yasuda and Watanabe (1994) and Yasuda and Kitagawa (1996) in relation to the formation of fishing grounds for Pacific saury. Saury Fishing grounds tend to distribute nearshore (offshore) in years of northward (southward) SAF. Figure 2(a) shows interannual variations in the latitude anomaly of SAF (indicated by offshore Oyashio Front (OOF) in Fig. 2(a)) averaged in E based on summer 100 m depth temperature fields. OOF varies ±1 in latitude. The northward shift in the early 1970s and southward shift around the mid-1980s are remarkable; OOF in other periods were almost average. OOF fluctuates similarly to the 5-year running mean of the winter North Pacific Index (NPI: Trenberth and Hurrel, 1994) anomaly (OOF anomaly ( N) = 1.55 *NPIA (5 yr mean) 0.51: correlation coefficient R = 0.56). Interannual variations of summer-saf at 150 E and 180 in 1980s and early 1990s have been reported by Onishi (2001) and Isoda et al. (2002) with somewhat different definitions of SAF. Onishi (2001) used the 4 C isotherm at 100 m depth for SAF at 180 and identified a northward shift in the 1980s and a southward shift in the 1990s; whereas Isoda et al. (2002) used the isohaline of 33.5 psu at 26.3σ θ and reported the southward shift at 180 from 1982 to Suga et al. (2003) defines SAF as the boundary north of which dichothermal water (temperature minimum) exists and also reported SAF variations during We might need to reconsider the SAF definition, especially for the central and eastern part of the North Pacific. There have been no modeling studies that reproduce these frontal variations, and the mechanism of SAF variations thus remains unclear. Sea surface height (SSH) variations and current intensities around SAF during were reported by Qiu (2002b); we need further study of the correspondence between SAF and SSH. The Oyashio near the east coast of Honshu (OY1) is known to vary in seasonal and interannual to interdecadal time scales (e.g. Ogawa et al., 1987; Ogawa, 1988; Sekine, 1988). Mooring and repeated hydrographic observations revealed large seasonal variations in the Oyashio velocity and transport near the Hokkaido coast (Kono and Kawasaki, 1997b). As shown in the monthly southern end latitudes of the Oyashio coastal intrusions (Fig. 2(b): the 40-year monthly data from 1960 to 2001 by Tohoku National Fisheries Research Institute) based on the index temperature at 100 m depth by Kawai (1972), the Oyashio extends to 38.8 N in March and April while it retreats to 41.2 N in November. This seasonal migration is significant, as shown in the error bars of 95% confidence limit. This seasonal migration of OY1 has not been reproduced with numerical models. Hydrographic Structure and Variability in the Kuroshio-Oyashio Transition Area 391

4 OY1 undergoes large interannual variability. The April southern end latitude of OY1 has the largest variability. The thin curve in the third row of Fig. 2(a) shows the time series of April OY1 latitude anomalies from 1960 to The April OY1 anomalies correlate significantly with the annual mean OY1 anomalies (thick curves): the regression is OY1 Apr = *OY1 annual (correlation coefficient R = 0.67); the variation of April OY1 is greater than that of the annual mean OY1. Interdecadal variations are remarkable in both April and annual mean OY1 anomalies: the northward shifts continued in the 1960s, the southward shift in the 1970s and the 1980s, with a northward shift again from the mid-1990s. The ENSOrelated 2 4 year period variability is superposed on the interdecadal variation. The variation in annual mean OY1 anomaly is similar to the one in April OY1 anomaly, except that the annual mean OY1 anomaly changed sign from positive to negative around the mid-1970s. OY1 annual fluctuated similarly to NPI anomaly (NPIA) in the period from the 1960s to mid-1980s (OY1 annual ( N) = 0.197* NPIA ; R = 0.67 for 1960 to 1986); the correlation is reduced (OY1 annual ( N) = 0.145* NPIA ; R = 0.46 for 1960 to 2001) because variations in the 1990s are quite different. 3.2 Subarctic boundary (SAB) SAB is defined as a near-surface salinity front (salinity of 34 psu) south of SAF (Favorite et al., 1976). SAB and SAF cannot be distinguished in the area west of 150 E (Yasuda, 1997). SAF and SAB separate east of 150 E; the area between SAF and SAB is referred to as the Transition Domain (Favorite et al., 1976). In the Transition Domain west of the date line, a peculiar hydrographic structure is observed: the deep winter mixed layer reaches down to 26.6σ θ with temperature and salinity characteristics intermediate between subarctic and subtropical waters without an intermediate salinity minimum (Yasuda et al., 1994; Watanabe, 1997). That is, the salinity increases with depth, similar to subarctic water vertical profiles, while the water is much warmer and saltier than subarctic waters. Low-salinity water covers near-surface, probably because of the southward Ekman drift of subarctic waters across SAF. The water characteristics in the density of σ θ are similar to the waters south of SAB; that is, NPIW (Yasuda, 1997). NPIW formed around the Kuroshio Extension regions might flow into the Transition Domain (Ueno and Yasuda, 2000, 2001). The deep mixed layer is referred to as the Stability Gap by Roden (1991) and Yuan and Talley (1996). An example of SAF, SAB and Transition Domain is depicted in Fig. 3, which shows temperature, salinity and planetary potential vorticity (PV) vertical cross-sections across SAB (Sta. 1 2), Transition Domain (Sta. 2 6) and SAF (Sta. 7) (see Yasuda et al., 1994 for detail). Fig. 3. Temperature ( C), salinity and planetary potential vorticity (PV in m 1 s 1 ) vertical cross-sections across SAB (Sta. 1 2), Transition Domain (Sta. 2 6) and SAF (Sta. 7) in the cruise of Hokushin maru (Kushiro Hokkaido Fisheries Experimental Station) in July Superposed black contours denote potential density in σ θ (see Yasuda et al., 1994 for details). Dichothermal (temperature minimum) structures are observed at around 100 m depth north of SAF, below which PV is high at σ θ that is a typical structure of WSAG, whereas in the Transition Domain, low-pv is observed at σ θ, which could be a remnant of the winter mixed layer (Stability Gap) and probably a source of dense part of Central Mode Water, as will be described later. Interannual variations of SAB have been reported by Onishi (2001) and Isoda et al. (2002). Both authors reported the southward shift in at 180 and relatively north in the other years in This variation is similar to OOF. 392 I. Yasuda

5 Fig. 4. (a) March sea surface salinity (SSS in psu) and (b) March mixed layer depth (in m: colors) that is defined as the depth at which density is SSD Sea surface density (SSD) contours (CI = 0.2) are superposed in (b). 3.3 Kuroshio Bifurcation Front (KBF) The Kuroshio Bifurcation Front (KBF) is defined as the 6 8 C isotherm at 300 m depth (Mizuno and White, 1983). The Kuroshio Extension bifurcates near the Shatsky Rise located around 158 E, and the northward branch extends to about 40 N (Mizuno and White, 1983). KBF roughly corresponds to SAB east of 170 E (Zhang and Hanawa, 1993). Hurlburt and Metzger (1998) reproduced the bifurcation in their numerical model, in which eddy-induced deep flows around Shatsky Rise control the near-surface jet. For KBF, the location of bifurcation from the Kuroshio Extension is large in variability (Mizuno and White, 1983); the variability could induce large interannual variations around Shatsky Rise, as revealed by Joyce and Dunworth-Baker (2003). 3.4 Kuroshio Extension Front (KEF) The Kuroshio Extension (KE) is a strong jet of the western boundary current of the North Pacific subtropical gyre east of Boso Peninsula. There is a front along the northern edge of KE between KE and waters north of KEF through the confluence of different waters. Since KE transports huge amounts of heat and materials, variations could be quite important for variations in both climate and fisheries. The annual mean latitude anomaly of the first meander crest of the Kuroshio Extension Front (KEF) is shown in the fourth row of Fig. 2(a). KEF is based on the temperature indices at 100 m depth reported by Murakami (1993). KEF fluctuates in a bi-decadal time scale: a northward shift is remarkable in the 1950s, the end of 1970s and the end of 1990s with a southward shift from the mid-1960s to the mid-1970s and from the mid- 1980s to the mid-1990s. Hydrographic and satellite observations show that the axis of KE and its southern recirculation gyre are found to significantly change in annual and interannual interdecadal time scales. Mizuno and White (1983) described annual, interannual and shorter-term variations of KE in Qiu et al. (1991), Qiu (1995), Qiu (2000) and Qiu (2002a) reported interannual variations of KE and southern recirculation gyre using satellite altimeter data. Tatebe and Yasuda (2001) described standing and propagating waves with an annual time scale. There are many other studies; see Qiu (2002a) for a review focusing on the large-scale variability and air-sea interactions. 3.5 Subtropical Front (STF) and Mixed Layer Depth Front (MLF) The Subtropical Front (STF) in the western North Pacific is defined as a salinity and temperature front (e.g. Uda and Hasunuma, 1969; Roden, 1991). Figures 4 and 5 are based on the World Ocean Atlas 1998 (Antonov et al., 1998; Boyer et al., 1998). STF locates south of STMW, and tropical water with a salinity greater than 35 distributes south of STF (Fig. 4(a)). Recently Aoki et al. (2002) described the STF in detail and pointed out another STF at around 18 N. The distribution of mixed layer depth (MLD) in March, when MLD is generally the deepest in each year, is shown in Fig. 4(b). The deep MLD corresponds to wa- Hydrographic Structure and Variability in the Kuroshio-Oyashio Transition Area 393

6 Fig. 5. Distribution of annual mean potential thickness (colors), acceleration potential (black contours: geostrophic flow streamlines relative to 2000 dbar) and March sea-surface density (two red contours) between two isopycnals. (a) Subtropical Mode Water in the density of σθ, (b) Shallow Central Mode Water in σθ, (c) Dense Central Mode Water in σθ and (d) Upper North Pacific Intermediate Water in σ θ. Fig. 6. Isopycnal oxygen distribution (color in ml/l) and acceleration potential (black contours) at the density of 27.2σθ. ter-mass formation sites as the dense part of CMW (DCMW: σθ) around 42 N and E, the shallow part of CMW (S-CMW: σ θ) around 39 N and east of 165 E and STMW around 33 N E. 394 I. Yasuda There is an MLD front (henceforth MLF: Uda and Hasunuma, 1969) that is nearly zonally distributed at N between 130 E 180 (Fig. 1). In the area north of MLF where STMW distributes, MLD is over 200 m, while MLD is less than 75m south of the MLF. The cause and

7 maintenance mechanism of MLF has not been discussed; we need studies that include STF. 4. Subsurface Water-Mass Distribution The subsurface distributions of water masses, transport routes and formation sites can be roughly deduced from a map of potential thickness (PT), geostrophic streamlines and winter sea-surface density (Fig. 5). PT is defined as the thickness between two isopycnals divided by the normalized Coriolis parameter f(θ)/f(45 N) = sinθ/ sin(45 N) (θ is latitude). PT is the inverse of planetary potential vorticity (PV); for large scale subsurface circulation, both PT and PV are conserved dynamical properties that can be used as a tracer, while mixing can change PT and PV. For a mode water with vertically homogeneous density profiles, PT is large around its formation region. Along the subsurface circulation, PT is roughly conserved but gradually decreases by mixing, as shown in Fig. 5. In the regions surrounded by the two (upper and lower) density contours of the mode water (red contours), the area with large PT indicates the formation site of the mode water. The intersection between the density contour and streamline with relatively large PT can be regarded as subduction regions for the mode water, except for NPIW which does not outcrop in the Pacific (Fig. 5(d)). This analysis is an improved version of that of Talley (1988) using planetary potential vorticity. The ventilation or subduction rate for these water masses has been estimated (Qiu and Huang, 1995; Qu et al., 2002). The formation region of Subtropical Mode Water (STMW) in the density of σ θ is N and 140 E 180 south of the KEF (Fig. 5(a)), as Masuzawa (1969) and Suga and Hanawa (1995) pointed out. STMW mainly distributes in N and 130 E 180. For a shallow part of the Central Mode Water (S- CMW) with a density of σ θ, the formation region is in N and 170 E 160 W (Fig. 5(b)), as indicated by Nakamura (1996), Suga et al. (1997) and Yasuda and Hanawa (1997) around and south of KBF or SAB. The largest PT in the density of the mode water is located around 36 N and 165 W where intense subduction could occur. The large PT distributes along the outer flow path of STMW. A dense part of CMW (D-CMW) with a density of σ θ is formed in N and 150 E 180 in the Transition Domain between SAF and SAB (Fig. 5(c)). D-CMW flows along further outer streamlines of the subtropical gyre than S-CMW flow path. Subduction of D- CMW may take place around 175 E between N. The density ( σ θ ) of the upper part of NPIW is scarcely outcropped in the open Pacific (Reid, 1965, 1973; Talley, 1993); the source water of NPIW is formed in the Okhotsk Sea where the largest PT north of 30 N is observed, as shown in Fig. 5(d) (Yasuda, 1997; You et al., 2000; Qu et al., 2001). This large PT (thus low-pv and thick) water mass is called Okhotsk Sea Mode Water (OSMW). A relatively large PT is seen east of Japan along the flow path of NPIW. A large PT is seen in N and 170 E 180 around the Emperor Sea Mount Chain. The cause of this local PT maximum remains unclear. It is noted that relatively high PT extends northward to the subarctic region in N and W as well as in the eastern half of the Alaskan Gyre, as identified by Ueno and Yasuda (2000, 2001). The latter feeds a mesothermal structure (temperature inversion) in the North Pacific subarctic region (Ueno and Yasuda, 2000, 2001, 2002). The western route of NPIW around the dateline to the subarctic region needs to be examined further. The picture of the intermediate circulation in the density of σ θ is somewhat different from the one of upper-npiw. As pointed out by Reid (1997) and Yasuda et al. (2001) and as shown in Fig. 6, the isopycnal oxygen value in this density range is large along the western boundary regions, where it could be influenced by Antarctic Intermediate Water (AAIW) from the southern hemisphere. NPIW formation, transformation and circulation are described as follows (Fig. 1). In the density range of σ θ, the dense shelf water (DSW) is formed in the coastal polynya through sea-ice formation (brine rejection) and subsequent convection in the northwestern shelf region of the Okhotsk Sea (Kitani, 1973; Alfultis and Martin, 1987; Talley, 1991; Yasuda, 1997; Martin et al., 1998; Itoh, 2000; Yamamoto, 2001; Yamamoto et al., 2001; Yasuda et al., 2002). Since DSW entrains surface fresh water (provided from the Amur River and excessive precipitation) at its formation, DSW becomes fresher and colder than any other waters of the same density in the North Pacific. DSW formation is thus a freshening and cooling source for the intermediate North Pacific. DSW flows southward along ESC (Fukamachi et al., 2001; Mizuta et al., 2001; Yasuda et al., 2002) and to the anticyclonic circulation in the Okhotsk Sea Kuril Basin, being modified by mixing with warmer, saline waters from the Pacific and Japan Sea (Takizawa, 1982; Watanabe and Wakatsuchi, 1998; Itoh, 2000). The Okhotsk Sea Mode Water (OSMW) is thus formed, and its low-potential vorticity characteristics could be enhanced by tidal mixing around the Kuril Straits (Nakamura et al., 2000b). OSMW flows out to the Pacific through the Kuril Straits. Exchange between the Okhotsk Sea and the Pacific had been thought of as inflow from the Kruzenshterna Strait and an outflow at the Bussol Strait (e.g. Talley and Nagata, 1995). In contrast, recent observations have reported a reverse exchange pattern (Kono et al., 2001; Katsumata et al., 2001; Riser, 2001; Yasuda et al., 2002). A theoretical consideration with the upperlayer Island Rule (Katsumata and Yasuda, 2003) indicates Hydrographic Structure and Variability in the Kuroshio-Oyashio Transition Area 395

8 that either exchange pattern is possible, depending on the wind-stress field over the North Pacific and potential vorticity in the straits. A numerical model also suggests a tidal-driven mean exchange flow around islands (Nakamura et al., 2000a). OSMW mixes with EKC waters to form Oyashio water (Ohtani, 1989; Kono and Kawasaki, 1997a; Yasuda, 1997). The Oyashio water flows southward along the coasts of the Kuril Islands, Hokkaido and Honshu still has OSMW characteristics of great potential thickness (low-potential vorticity). A part of the Oyashio water does not return and is directly entrained into the subtropical gyre along the western boundary current regions (Yasuda et al., 1996, 2001, 2002; Yasuda, 1997; Kono, 1997). That is, this part of the Oyashio water crosses the subtropical/ subarctic gyre boundary and thus crosses SAF. There might be other processes across the offshore SAF into NPIW (Yasuda et al., 2002); that needs further study. A part of the Oyashio water that crosses the SAF flows further southward and reaches the Kuroshio Extension, where new NPIW is efficiently formed by mixing between the Oyashio water and warmer and saline old NPIW (Talley et al., 1995; Yasuda et al., 1996, 2001; Hiroe et al., 2002). In the lower-npiw ( σ θ ), NPIW along the Kuroshio and the Kuroshio Extension is largely influenced by Antarctic Intermediate Water (AAIW). The new NPIW is transported to both northern and southern sides of the Kuroshio Extension (Ueno and Yasuda, 2000; Yoshinari et al., 2001). The mixing processes between the Oyashio and relatively warm and saline old-npiw remain unclear, as is how the low-salinity Oyashio water spreads into the region south of the Kuroshio Extension (e.g. Okuda et al., 2001). There is a study that has tried to estimate the mixing parameter (e.g. Joyce et al., 2001). A salinity minimum structure is formed in the Kuroshio-Oyashio interfrontal zone. The density of the salinity minimum is still controversial; Yasuda (1997) emphasizes isopycnal mixing between the Kuroshio water (old NPIW) and Oyashio water that retains the PVminimum at around 26.8σ θ from OSMW; while Talley (1993), Talley et al. (1995) and Talley and Yun (2001) consider that the winter mixed layer water in the Oyashio sinks down to σ θ by densification due to cabbeling and double diffusion. Diapycnal processes and their role in the formation of NPIW need further study. The northern part of NPIW is transported to the Alaskan gyre and feeds the mesothermal structure (Ueno and Yasuda, 2000, 2001, 2002). This is the only isopycnal process that provides heat and salt to the intermediate subarctic region that can balance with the cooling and freshening in the Okhotsk Sea (Yasuda et al., 2002). Another NPIW flows along the subtropical gyre circulation (Reid, 1965; Talley, 1993; Yasuda et al., 1996; You et al., 2000). 5. Issues 5.1 Influence of NPIW on surface current and frontal structures Other than the role of NPIW in the carbon cycle, which is discussed in Subsection 5.2, there are possibilities that NPIW changes near-surface currents and frontal structures. This is because the original water of NPIW, Okhotsk Sea Mode Water (OSMW), has a low-potential vorticity signature (Yasuda, 1997). Potential vorticity is a dynamical variable can thus change dynamical structures in the area where NPIW distributes. The southward western boundary currents in intermediate and deep depths (Labrador Sea Water: e.g. Talley and McCartney, 1982) could shift the Gulf Stream southward and also induce interdecadal self-sustained oscillations between the zonally elongated recirculation gyre produced by high meso-scale eddy activity and the state of shrinking recirculation gyre, as shown in the numerical model of North Atlantic Gulf-Stream and NADW (Spall, 1996a, b). In the North Pacific, the recirculation gyre south of the Kuroshio Extension is known to adopt both the elongated and contracted states (Qiu, 2000, 2002a). We need modeling studies to explore the sensitivity of the Kuroshio/Oyashio system by changing potential vorticity in the Okhotsk Sea, exchange rate and the strength and distribution of wind-stress fields. Since NPIW renewal time is roughly 50 years, variations of NPIW might relate to the year period oscillation that dominates in the North Pacific climate (e.g. Minobe, 1997) and also in the Pacific ecosystem, as sardine, saury and anchovy populations. The mechanism by which the low-pv water is formed in the Okhotsk Sea is not especially clear. We hypothesized that the low-pv water is originally formed in the northwestern shelf region through sea-ice formation, flows southward along the Shakalin coast and then is modified by strong tidal mixing around the Kuril Straits (Kitani, 1973; Talley, 1991; Yasuda, 1997; Wong et al., 1998; Itoh, 2000; Yasuda et al., 2002). The low-pv water resides in the Okhotsk Sea Kuril Basin with a large anticyclonic circulation (Wakatsuchi and Martin, 1991; Yasuda, 1997). This anticyclonic circulation is not explained by wind-driven theory under cyclonic wind-stress curl fields. The exchange flows between the Okhotsk Sea and the Pacific is now being explored using an upperlayer Island rule that simply relates wind-stress fields over the Pacific with the exchange flow transports (Katsumata and Yasuda, 2003). We need further study on the exchange and low-pv water formation. 396 I. Yasuda

9 The frontal structures in the Kuroshio/Oyashio confluence are not well explained either. In high-resolution numerical models of the Kuroshio/Oyashio confluence (e.g. Hurlburt et al., 1996), the separation latitude of the Kuroshio Extension agrees with observations, whereas coarse-resolution models ( or coarser) generally fail to reproduce the separation; the Kuroshio Extension separates around Hokkaido. This could be explained by the theory of boundary current separation in relation to coastal topography (Marshall and Tansley, 2001) with the steep curvature along Boso Peninsula and turbulent situations (almost inviscid situation) in high-resolution models. Since SAF lies roughly along the boundary of wind-driven gyres, assuming Sverdrup balance, high-resolution models that can reproduce the separation of the Kuroshio Extension hence reproduce SAF and KEF. SAF or OYF west of 170 E lies significantly south of the gyre boundary determined from Sverdrup balance using annual mean wind-stress fields (see figure 5 in Ueno and Yasuda, 2000). The cause of the southward shift of SAF remains unclear. Every model with realistic wind forcing and realistic topography fails to reproduce this southward shift. Neither are there any models that reproduce seasonal migration of the Oyashio coastal intrusion, although interannual variations were examined using a barotropic model (Sekine, 1988). The Subarctic boundary (SAB) is the near-surface salinity front that has not yet been examined in detail. We need further studies including interannual variations of SAF and SAB; variations of these fronts might explain intense temperature anomalies over the western and central North Pacific (Nitta and Yamada, 1989; Tanimoto et al., 1993; Trenberth and Hurrel, 1994; Deser and Blackmon, 1995; Nakamura et al., 1997). 5.2 Water mass changes of the Oyashio water and NPIW A water mass regime shift of NPIW and Oyashio water has been reported. A long-term freshening trend of NPIW has been pointed out by Wong et al. (1999) for the 47 N and 24 N sections and by Joyce (2002) for the freshening trend after the mid-1970s. Kawasaki (1999) showed the water mass regime shift occurred in the 1990s in the western subarctic Pacific using summer data from 1990 to Isopycnal properties changed to warmer and saltier after 1994 in the density of σ θ, while freshening occurs in surface 26.6σ θ. Yasuda et al. (2001) noted that the potential vorticity in the Okhotsk Sea intermediate water changed higher and the density of potential vorticity vertical minimum was lowered in 1990s. Yasuda et al. (2000a) and Rogachev (2000) reported property changes in the anti-cyclonic eddies south of Bussol Strait in 1990s. Isoda et al. (2002) reported the warmer and saltier shift of NPIW after 1989 at 155 E and after 1993 at 180 in the period The long-term decrease of isopycnal oxygen and the increase of nutrient in the Oyashio water south of Hokkaido in the density of σ θ from 1970 to 1999 have been reported recently (Ono et al., 2001). Watanabe et al. (2001) showed the basin-wide oxygen decrease from the mid-1980s to the end of the 1990s at 47 N trans-pacific section and at 165 E section. They suggested that the formation rate of original NPIW in the Okhotsk Sea could be decreased. Ono et al. (2001) also reported that isopycnal oxygen fluctuated in bi-decadal time scale. The causes of the property changes in NPIW and Oyashio water mentioned above remain unclear. Since the density range of the property changes includes unoutcropped densities even in winter, except for the Okhotsk Sea, the direct impact from the atmosphere must be only from the sinking region in the Okhotsk Sea or through diapycnal/isopycnal mixing especially around the Kuril straits. Another possible cause is a change in the cross-gyre transport from subtropical to subarctic gyre that feeds the mesothermal structure in the North Pacific subarctic region (Ueno and Yasuda, 2000, 2001, 2002). We should also note that the change of mixing ratio of the Oyashio water (thus NPIW) between the relatively cold, fresh and high-oxygen Okhotsk Sea and WSAG waters might cause great property changes (Yasuda, 1997; Yasuda et al., 2002). The water-mass long-term changes could be crucial for carbon cycle and global warming issues. Net anthropogenic CO 2 flux from the Okhotsk Sea to the Pacific is estimated to be GtC/yr and a large part (0.02 GtC/yr) of it goes into the intermediate depth of the subtropical gyre and thus into NPIW (Yasuda et al., 2002; see also Andreev et al., 2001 and Ono et al., 2001). This flux explains about 15% of the inventory in the North Pacific. NPIW has the highest density of water masses formed in the North Pacific. If global warming proceeds, the density and water properties of NPIW must be most sensitively influenced. The future formation of NPIW might be terminated under warmer climate. We need longterm monitoring for water properties and transport of NPIW and related water masses. 5.3 Kuroshio Extension, Subtropical Mode Water (STMW), Mixed Layer Front (MLF) and Subtropical front (STF) Wintertime SST in the Kuroshio Extension and southern recirculation gyre (KESA), that is Subtropical Mode Water, is closely related to the annual Japanese sardine survival rate (Noto and Yasuda, 1999, 2003). The winter SST averaged in N and 145 E 180 changed in a 50-year cycle in the 20th century (Noto and Yasuda, 2001). An SST jump in the Kuroshio Extension and the Kuroshio south of Japan (KESA and KSJ in Fig. 7) occurred in 1987/ 1988, which is a few years earlier than major climate Hydrographic Structure and Variability in the Kuroshio-Oyashio Transition Area 397

10 and eastward Subtropical Counter-Current (STCC) distribute on the south side of the MLF. To understand the variability of SST, circulation and STMW, which have great impacts on climatic variations and fisheries, we need detailed observations in this area as well as modeling studies. Fig. 7. Time series in anomalies of (upper panel) Nov. Mar. PDOI (5-year running mean of Pacific Decadal Oscillation Index), (second) winter (Jan. Mar.) -NPI (5-year running mean of North Pacific Index), (third) winter-aoi (5-year running mean of Arctic Oscillation Index), (fourth) winter- SST (in C) in SAFZ (subarctic frontal zone: 160 E 180, N), (fifth) winter-sst (in C) in KESA (Kuroshio Extension and southern recirculation area: 145 E 180, N) and (sixth) winter-sst (in C) in the Kuroshio south of Japan (KSJ: E, N). jumps, suggesting that the variation in the Kuroshio Extension could induce climate regime shift (Yasuda et al., 2000b). As shown in Fig. 7, the winter Arctic Oscillation Index (Thompson and Wallace, 2000) could be related to the late-1980s SST shift (e.g. Yasunaka and Hanawa, 2002), rather than the Pacific Decadal Oscillation Index (Mantua et al., 1997) or NPI which is related to the mid- 1970s climate shift (e.g. Mantua and Hare, 2002). Variations of winter-sst in the subarctic frontal zone (SAFZ: N, 160 E 180 ) are similar to NPI and PDOI. AOI changed its sign in 1988/1989; the 1987/1988 SSTshift in KESA and KSJ is thus still one year earlier than the shift of AO. Similarly to the winter-sst, mixed layer depth (MLD) variations in KESA have a remarkable 50-year cycle, whereas an MLD jump occurred in 1985 which is further a few years earlier than the SST jump in 1987/ 1988 (Yasuda et al., 2000b). The winter-sst is closely related to the strength of the Kuroshio Extension and the southern recirculation gyre (Qiu and Kelly, 1993; Qiu, 2000; Yasuda et al., 2000b); the jumps of SST and MLD in the mid to late 1980s might be caused by the increased strength of the Kuroshio Extension (Yasuda et al., 2000b). The winter-mld regime shift in the mid-1980s occurred coherently in a wide extent in N and 140 E 160 W (Yasuda et al., 2000b). This mixed layer variability and mode water formation need further studies. In winter, a sharp mixed layer front (MLF) is formed around 30 N south of the deep mixed layer corresponding to STMW (Fig. 4(b)). The distribution, variations and formation of the MLF front have not been well examined and are not well understood. The Subtropical Front (STF) Acknowledgements The author thanks Prof. Takashige Sugimoto for giving him the opportunity to review the transition area. Thanks are extended to Dr. Tomowo Watanabe and Shin- Ichi Ito who provide updated Oyashio front data. Comments from Dr. Tokihiro Kono improved the paper. This work is partially supported by grants (KAKEN and SAGE) from Ministry of Culture, Education, Sports, Science and Technology. References Alfultis, M. A. and S. Martin (1987): Satellite passive microwave studies of the Sea of Okhotsk ice cover and its relation to oceanic processes, J. Geophys. Res., 92, Andreev, A., M. Honda, Y. Kuramoto, M. Kusakabe and A. Murata (2001): Excess CO 2 and ph excess in the intermediate water layer of the northwestern Pacific. J. Oceanogr., 57(2), Antonov, J. I., S. Levitus, T. P. Boyer, M. E. Conkright, T. O Brien and C. Stephens (1998): World Ocean Atlas 1998, Vol. 2: Temperature of the Pacific Ocean. NOAA Atlas NESDIS 28, 166 pp. Aoki, Y., T. Suga and K. Hanawa (2002): Subsurface subtropical fronts of the North Pacific as inherent boundaries in the ventilated thermocline. J. Phys. Oceanogr., 32, Boyer, T. P., S. Levitus, J. I. Antonov, M. E. Conkright, T. O Brien and C. Stephens (1998): World Ocean Atlas 1998, Vol. 5: Salinity of the Pacific Ocean. NOAA Atlas NESDIS 31, 166 pp. Deser, C. and M. L. Blackmon (1995): On the relationship between tropical and North Pacific sea surface temperature variations. J. Climate, 8, Deser, C., M. A. Alexander and M. S. Tilmin (1999): Evidence for a wind-driven intensification of the Kuroshio Current Extension from the 1970s to the 1980s. J. Climate, 12, Dodimead, A. J., F. Favorite and T. Hirano (1963): Salmon of the North Pacific Ocean-II, Review of oceanography of the Subarctic Pacific region. Bull. Int. North Pacific Comm., 13, 195 pp. Favorite, F., A. J. Dodimead and K. Nasu (1976): Oceanography of the Subarctic Pacific region, Bull. Int. North Pacific Comm., 33, Fukamachi, Y., G. Mizuta, K. I. Ohshima and M. Wakatsuchi (2001): Estimation of dense shelf water volume transport using long-term mooring data off the east coast of Sakhalin. Proc. of Int. Symp. oóêatmosphere-ocean-cryosphere Interaction in the Sea of Okhotsk and Surrounding Environment, published by Hokkaido Univ., p I. Yasuda

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