North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects

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1 Journal of Oceanography, Vol. 60, pp. 385 to 395, 2004 Review North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects ICHIRO YASUDA* Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo Bunkyo-ku, Tokyo , Japan (Received 19 February 2003; in revised form 8 October 2003; accepted 14 October 2003) We survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE (SubArctic Gyre Experiment), including important results obtained from related projects. Intensive observations have provided the transport distributions relating to NPIW and revealed the existence of the cross-wind-driven gyre Oyashio water transport that flows directly from the subarctic to subtropical gyres through the western boundary current as well as the diffusive contribution across the subarctic front. The anthropogenic CO 2 transport into NPIW has been estimated. The northern part of NPIW in the Transition Domain east of Japan is transported to the Gulf of Alaska, feeding the mesothermal (intermediate temperature maximum) structure in the North Pacific subarctic region where deep convection is restricted by the strong halocline maintained by the warm and salty water transport originating from NPIW. This heat and salt transport is mostly balanced by the cooling and freshening in the formation of dense shelf water accompanied by sea-ice formation and convection in the Okhotsk Sea. Intensive observational and modeling studies have substantially altered our view of the intermediate-depth circulation in the North Pacific. NPIW circulations are related to diapycnal-meridional overturning, generated around the Okhotsk Sea due to tide-induced diapycnal mixing and dense shelf water formation accompanied by sea-ice formation in the Okhotsk Sea. This overturning circulation may possibly explain the direct cross-gyre transport through the Oyashio along the western boundary from the subarctic to subtropical gyres. Keywords: North Pacific Intermediate Water, Kuroshio, Oyashio, Okhotsk Sea, water mass, anthropogenic CO Introduction North Pacific Intermediate Water (henceforth NPIW) is a water mass characterized by a salinity minimum centered at 26.8σ θ that is widely distributed in the North Pacific subtropical gyre (e.g. Sverdrup et al., 1942; Reid, 1965). The low-salinity signature undoubtedly comes from the subarctic area where precipitation exceeds evaporation, and the low-salinity surface water might sink or diffuse to intermediate depths. We here define NPIW as the water including the salinity minimum layer in the density range of σ θ that is influenced by lowsalinity subarctic waters. NPIW is the densest and deepest water-mass ventilated in the North Pacific, and could thus be important for anthropogenic CO 2 absorption and long-term climate variability. * address: ichiro@eps.s.u-tokyo.ac.jp Copyright The Oceanographic Society of Japan. NPIW is different from other intermediate water masses characterized by a salinity minimum, such as Labrador Sea Water (LSW) and Antarctic Intermediate Water (AAIW), both of which have an apparent pycnostad (potential vorticity minimum), while no pycnostad is obvious in NPIW (Hasunuma, 1978; Talley, 1993). This leads to an explanation for the salinity minimum formation in NPIW that does not include overturning circulation accompanied by diapycnal transports and cross-winddriven gyre transport. Reid (1965) hypothesized that the low-salinity signature diffuses vertically into intermediate depths along the cyclonic subarctic circulation and then diffuses horizontally into the subtropical gyre across the trans-pacific gyre boundary due to isopycnal mixing. Talley (1991) indicated that vertical diffusion occurs primarily in the Okhotsk Sea. Horizontal mixing mainly occurs in the Kuroshio-Oyashio confluence region just east of Japan (Talley, 1993; Talley et al., 1995; Yasuda et 385

2 al., 1996). On the other hand, Yasuda (1997) and Yasuda et al. (1996) reported the existence of low potential vorticity (PV) water and a PV minimum centered at 26.8σ θ in the Kuril Basin of the Okhotsk Sea, along the Oyashio water east of southern Kuril Islands, Hokkaido and Honshu, and along the Kuroshio Extension just after the confluence with the Kuroshio Extension. They hence suggested that NPIW could be formed not only by isopycnal diffusion (Reid, 1965; Talley, 1993) but also by meridional overturning circulation with diapycnal and cross-gyre transports. This explanation was originally advanced by Wüst (1930). The hypotheses mentioned above were based on the water properties and were rather qualitative. In the SubArctic Gyre Experiment (SAGE) project, intensive hydrographic observations including moored current meters and shipboard/lowered acoustic Doppler current profilers (ADCP/LADCP) were made during in the Okhotsk Sea, the western subarctic gyre and the area east of Japan, in order to find quantitative arguements for the formation of NPIW. Numerical and inverse modeling were also performed to understand the intermediate-depth circulation and the formation of NPIW. For the Okhotsk Sea, another project, Okhotsk Sea studies on sea-ice and its role in the climate system represented by Prof. M. Wakatsuchi at Hokkaido University, was conducted during (henceforth OSP). We here briefly survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE, including important results from related projects. This review concentrates on the studies performed within the SAGE in this special issue of Journal of Oceanography as well as publications that have already been published and manuscripts that have been submitted to other journals, and important historical previous studies. The reader is invited to consult Yasuda (2003) for a general review of NPIW and other water masses in the northwestern Pacific. 2. Water Mass Formation in the Okhotsk Sea and Exchange with the Pacific The major cooling and freshening source for intermediate waters in the North Pacific subarctic region and NPIW is a water mass in the Okhotsk Sea (see Fig. 1). In the density range of σ θ that is not outcropped in the open North Pacific, the Okhotsk Sea water is the coldest and freshest in the North Pacific, because the dense shelf water (DSW) sinks down to σ θ due to brine rejection during sea-ice formation, entraining surface low-salinity water (Kitani, 1973; Alfultis and Martin, 1987; Talley, 1991; Martin et al., 1998). The cold, low-salinity water mass in the Kuril Basin is called Okhotsk Sea Mode Water (OSMW) and has a pycnostad (PV minimum) at around 26.8σ θ, suggesting that OSMW contributes to the formation of NPIW (Yasuda et al., 1996; Yasuda, 1997; Kono and Kawaski, 1997a; Watanabe and Wakatsuchi, 1998; You et al., 2000). Hence the OSMW formation, its exchange with the Pacific and the formation of the Oyashio water were major challenges in SAGE. In order to tackle these issues, hydrographic observations in the areas around Kuril Islands and Kuril Straits every summer and mooring observation at Bussol Strait were performed in SAGE in corporation with OSP. The observations performed at the Bussol Strait, Kruzenshtern Strait, Okhotsk Sea Kuril Basin and the area south of Kuril Islands are changing our view of the exchange between the Okhotsk Sea and the Pacific concerning such matters as the inflow at Kruzenshtern and outflow at Bussol Strait. The exchange in the Bussol Strait (Fig. 1(b)) was studied in the joint research with OSP. Three moorings with current meters and CTD (conductivity, temperature and depth) were deployed at the deepest valley and shelf slope in the northern Bussol Strait and one at the southern Bussol Strait. The Bussol Strait has a sill near the center. Kono et al. (2001) and Riser (2001) reported that water in the northern strait at 300 m depth roughly corresponding to NPIW density generally flows into the Okhotsk Sea, while the current in the southern strait is generally an outflow and of magnitude stronger than in the northern strait. That is, the exchange flow at 300 m in the Bussol Strait is bi-directional. Katsumata et al. (2003) performed repeated LADCP/CTD observation covering the whole Bussol Strait with high spatial resolution in September They revealed this bi-directional exchange flow, discovering that the net transport in the Bussol Strait shallower than 27.2σ θ is 5 Sv (Sv = 10 6 m 3 /s) out of the Okhotsk Sea, after removing tidal components. The exchange in the Kruzenshtern Strait was measured in August 1999 by Katsumata et al. (2001) with LADCP/CTD. The transport in the density range shallower than 27.2σ θ was out of Okhotsk Sea and about 2 Sv, which is opposite to the previous view that inflow occurs at the Kruzenshtern Strait and outflow at Bussol Strait. Yasuda et al. (2002) confirmed this outflow transport from observation south of the Kuril Islands. Using the summer hydrographic surveys in the Okhotsk Sea and the area south of Kuril Islands, Kono and Kawasaki (1997a) argued that the Oyashio water is formed by mixing between the Okhotsk Sea water (OSW) and Western Subarctic Gyre water (WSAG). Itoh (2000) and Itoh et al. (2003) produced the climatological property distributions using quality controls on isopycnal surfaces and estimated the transport of Forerunner Soya water (winter dense water from the Japan Sea), dense water production rate and Okhotsk Sea outflow rate. Yasuda et al. 386 I. Yasuda

3 (a) NPIW circulation diagram Okhotsk Kamchatka Pen. 2 1 ESC EKC Sakhalin 12 WSAG 3.5 SAF Hokkaido OY Sea To: Alaska Gyre Japan Sea 5 6 KBF SAB DSW OSMW 12 KE Along STG MLF (c) Possible physical explanation Dispycnal Upwelling Around Kuril Straits STF DSW (b) SAGE OBS OSMW A-line OICE-line JMA-line Fig. 1. (a) Schematic illustration of water mass distribution, transformation and transport related to North Pacific Intermediate Water (NPIW). The transport values are in Sv (=10 6 m 3 /s) in the density range between σ θ. EKC: East Kamchatka Current, WSAG: Western Subarctic Gyre, ESC: East Sakhalin Current, OY: Oyashio, KE: Kuroshio Extension, DSW: Dense Shelf Water, OSMW: Okhotsk Sea Mode Water. Broken curves represent the Subarctic Front (SAF), Subarctic Boundary (SAB), Kuroshio Bifurcation Front (KBF), and Mixed Layer Front (MLF) Subtropical Front (STF) from north to south. (b) Location of SAGE Bussol Strait mooring and repeat hydrographic observation. A (off Akkeshi) line, OICE (Oyashio Intensive observation off Cape Erimo), and JMA (Japan Meteorological Agency). (c) Possible explanation for the cross-winddriven gyre transport through the western boundary current (WBC) of the Oyashio (modified from Tatebe and Yasuda (2003)). (2002) estimated the Oyashio transport (11 12 Sv in σ θ ) from hydrographic and direct current measurement with LADCP/CTD south of Kuril Islands in summer 1999, and inferred an OSMW outflow transport to the Pacific (3.4 Sv), DSW formation rate (1.1 Sv), cooling rate (16.8 TW) and freshwater input (0.007 Sv) in the Okhotsk Sea. These cooling and freshening rates for the Okhotsk Sea intermediate water are comparable with the transport of warm and saline water from the area east of Japan to the Gulf of Alaska, feeding the mesothernmal structure in the subarctic North Pacific (Ueno and Yasuda, 2000, 2003), suggesting that the major balance of heat and freshwater in the intermediate subarctic North Pacific is between the cooling and freshening in the Okhotsk Sea and warming and being saline due to the transport to the Gulf of Alaska. North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects 387

4 Further remarkable progress is the recognition of the importance of tidally induced vertical mixing around the Kuril Islands. Non-hydrostatic numerical modeling efforts in SAGE showed the importance of strong tidal mixing around the Kuril Straits for OSMW and NPIW formations. Nakamura et al. (2000a) pointed out that subinertial strong K1 tide induces a mean circulation through numerous straits around Kuril Islands chain due to tidal rectification. Nakamura et al. (2000b) and Nakamura and Awaji (2003) estimated that the tide-induced strong diapycnal turbulent mixing around the Kuril Straits is over 1000 cm 2 /s. Yamamoto et al. (2001, 2002) and Yamamoto (2001) demonstrated that the water mass formation in the Okhotsk Sea is related to the diapycnal mixing as well as dense water formation due to sea-ice formation in the northwestern shelf region, based on oxygen isotope and CFC observations. They suggested that the intermediate water in the Kuril Basin of the Okhotsk Sea has components of deep water denser than 27.4σ θ as well as of surface cold, low-salinity water. Meso scale variability in the western subarctic gyre was examined by Yasuda et al. (2000) through the analysis of satellite altimetry and hydrographic data, particularly focusing on the cold and fresh core anticyclonic eddies that are often observed south of Bussol Strait. They showed that the Bussol eddy is formed by two processes: one is the local formation due to the outflow of Okhotsk Sea low-pv water (OSMW) and the other is the northeastward propagation of Kuroshio warm-core rings that are strongly modified with the entrainment of lowsalinity and low-pv Okhotsk Sea water. 3. Transport of the Oyashio South of Hokkaido and Cross-Gyre Transport from Subarctic to Subtropical Gyres along the Nearshore Oyashio The estimation of the Oyashio transport south of Hokkaido and of their contribution to NPIW formation are one of the major components of SAGE and were performed using four repeated hydrographic sections (Fig. 1(b)): the A (Akkeshi)-line off Akkeshi, the OICE (Oyashio Intensive observation line off Cape Erimo)-line off Cape Erimo, the 144 E-line and the 41.5 N-line. The A-line has been occupied almost bi-monthly and maintained by the Hokkaido National Fisheries Research Institute since Since 1997, when SAGE started, two to four moorings with current meters were deployed near the Hokkaido coast along the A-line. The structures and variations in water properties and currents were summarized by Kono and Kawasaki (1997b, c). Generally, in winter and spring, the southwestward Oyashio has deep current structures while in summer and fall the Oyashio becomes weaker and shallower. Kono and Kawasaki (1997b) demonstrated that the southwestward Oyashio volume transport referenced to moored current velocities varied corresponding to Sverdrup transport variations integrated from Hakkaido to 170 E during Kusaka et al. (2003) estimated the seasonal variation of the southwestward Oyashio volume transport integrated from the surface to 3000 m that takes a value of 21 Sv in winter, 18 Sv in spring, 16 Sv in summer and 10 Sv in fall, based on the average for 10 years. The OICE-line is located along the TOPEX/ POSEIDON satellite altimetry track and has been observed mainly by Tohoku National Fisheries Research Institute with other fisheries research institutes and the Japan Meteorological Agency. Since 1997, two to five moorings have been deployed and hydrographic observations were repeated about 40 times. The results of the observation are summarized in Ito et al. (2004) in terms of the relation between the sea-surface height differences and current/transport of the southwestward Oyashio. Uehara et al. (2004) estimated the southwestward Oyashio transport referenced to mooring current meter data; they reported 31 Sv of the southwestward Oyashio transport in January 2000 in m. The discovery and estimation of cross-gyre transports from subarctic to subtropical gyres along the western boundary is one of the remarkable results from SAGE. Yasuda et al. (2001) estimated that the Oyashio transport contributes to the NPIW formation based on the LADCP/ CTD data obtained in June 1998 along the A-line. The Oyashio transport integrated from the coast to just north of the subarctic (Oyashio) Front (Favorite et al., 1976) is not completely compensated, and generally the southwestward transport is larger than the northeastward one (Yasuda, 1997; Kono, 1997). The excess southwestward transport could intrude across the Oyashio Front, meaning that the Oyashio water crosses the subarctic/subtropical gyre boundary and contributes to the formation of NPIW. 5 Sv of the Oyashio water, including 2.5 Sv of Okhotsk Sea water in the density of σ θ, is reported to participate in the NPIW formation (Yasuda et al., 2001). Ito et al. (2003, personal communication) also evaluated the Oyashio cross-gyre transport south of Hokkaido in the density of σ θ that may be contributed to the NPIW formation to be 4 7 Sv as an annual average across the A-line, OICE-line and PH-line, based on the geostrophic calculation relative to 2000 dbar. The crossgyre transports are relatively large in winter-spring (5 7 Sv) and weaker in summer-fall (2 6 Sv) in the OICEline. Masujima et al. (2003) reported that the cross-gyre transports near Hokkaido were relatively large in spring (5 10 Sv) and small in summer-fall (0 3 Sv) based on directly measured LADCP currents, corresponding to the seasonal variation of the southwestward Oyashio transport (Kono and Kawasaki, 1997b; Kusaka et al., 2003). 388 I. Yasuda

5 4. NPIW Formation and Transport East of Japan and Transport to the Alaskan Gyre A large part of low-salinity Oyashio water that flows southward along the east coast of Hokkaido does not return to the subarctic region but is entrained into the subtropical gyre to form NPIW, as mentioned in the previous section. As reported by Yasuda et al. (1996) and Okuda et al. (2001), part of the Oyashio water is entrained into the Kuroshio Extension and a salinity minimum (thus NPIW) formation occurs there. However, the detection of actual processes and locations of NPIW formation (how and where NPIW is formed) and the estimate of its volume transport and transport route (how much and where NPIW goes) were still not fully studied before SAGE. In SAGE, intensive observations were conducted in the area east of Japan from the Kuroshio Extension regions to the subarctic frontal zone. The Japan Meteorological Agency (JMA) performed repeat hydrographic observations along 144 E, 147 E, 152 E and 165 E. Fisheries research institutes conducted several cruises in the Kuroshio-Oyashio interfrontal zone. High resolution surveys with a Moving Vessel Profiler (MVP), which is a new towed CTD system, were performed around the Kuroshio Extension region. The Japan Meteorological Research Institute deployed a total of 21 neutral buoyant floats, targeting at the NPIW density around 26.7σ θ, and also developed isopycnal-tracking profiling floats that trace the isopycnal density of 26.7σ θ to observe the actual modification process of NPIW. Inverse modeling using historical hydrographic data was also conducted to understand the basin-wide transport route of NPIW and relating water masses. Shimizu et al. (2001) examined the modification of Oyashio water and salinity minimum distribution along the Oyashio southward intrusion near the east coast of Japan, and reported that the Oyashio is more prominent in the intermediate layer than near the surface. Shimizu et al. (2003) revealed the transport distribution of the intermediate Oyashio and Kuroshio waters around the Kuroshio Extension regions using shipboard ADCP and CTD data obtained in spring of 1992 and Hiroe et al. (2002) also reported the transport distribution of NPIW using LADCP/CTD data in spring 1998, finding that 5 Sv of the Oyashio water flowed southward along Hokkaido and Honshu (cf. Yasuda et al., 2001) and another 2 Sv of the Oyashio water component in waters that had been already mixed with Kuroshio waters flowed southward across 39 N around 155 E, merging and mixing with 13 Sv of Kuroshio relatively high-salinity water to flow eastward across 158 E along the Kuroshio Extension. The intermediate water south of Japan and off Boso Peninsula has been studied in detail by Kaneko et al. (2001) and Komatsu et al. (2004). Kaneko et al. (2001) applied a box inverse method to derive the geostrophic transports in intermediate and deep waters south of Japan using WOCE data and reported 8 Sv of NPIW in the density range of 26.5σ θ 36.7σ 2 (σ is a density based on the reference pressure of 2000 dbar) is transported along the Kuroshio. Komatsu et al. (2004) reported that water containing relatively high oxygen concentration flows along the intermediate water in the density range of σ θ and that this could be influenced by AAIW which was also indicated by Reid (1997) and Yasuda et al. (2001). This suggests that the lower part of NPIW in the density range of σ θ is not simply recirculated but there is an input from AAIW through western boundary currents. Meridional transports of NPIW across 37 N in the mixed water region between the Kuroshio and Oyashio fronts was examined by Yoshinari et al. (2001, 2003) using LADCP/CTD data obtained in July NPIW formed near the Kuroshio Extension was transported northward west of 155 E around Shatsky Rise while a southward transport containing additional Oyashio components was observed in E, suggesting that the isopycnal mixing in the offshore subarctic front produces new NPIW around the Subarctic Front (SAF). Based on repeated hydrographic observations by JMA and geostrophic calculation relative to 2000 dbar, Miyao and Ishikawa (2003) revealed the mean transport distributions of Oyashio and Kuroshio waters across 144 E, 152 E and 165 E in the density of σ θ. Oyashio water components flows eastward: 5.0 Sv along the Kuroshio Extension and 3.5 Sv along the SAF, which coincides with the subarctic boundary (henceforth SAB) west of 150 E (Yasuda, 1997) across 144 E, 7.3 Sv along the Kuroshio Extension and 5.0 Sv along the SAB/SAF across 152 E and 9.2 Sv along the Kuroshio Extension and 6.8 Sv along SAB and 7.9 Sv along Subarctic Front across 165 E. The Kuroshio components are 11 Sv along the Kuroshio Extension across 144 E, 152 E and 165 E and 3 Sv along SAB and 2 Sv along SAF across 165 E. The total eastward transport of the Oyashio water component is 12 Sv across 152 E and 20 Sv across 165 E; these are much greater than the cross-gyre transport near Hokkaido (5 Sv: Yasuda et al., 2001; Shimizu et al., 2003; Ito et al., 2004) and along the Kuroshio Extension (7 Sv: Hiroe et al., 2002; Masujima et al., 2003), and previous estimates of 3 5 Sv published by Talley (1997). The excess eastward Oyashio transport of around 5 Sv seen along SAB across 152 E can be explained by cross-offshore SAF transport. Masujima et al. (2003) estimated the cross-offshore SAF Oyashio transport of 6 Sv west of 150 E from LADCP/CTD data in spring 2001 and fall 2000 cruises. Yasuda et al. (2002) also pointed out that 5 7 Sv of the Oyashio water crossed the offshore- SAF between E in summer These esti- North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects 389

6 mates of the cross-offshore-saf transport west of 152 E are consistent with the excess eastward Oyashio transport along SAB relative to 2000 dbar (Miyao and Ishikawa, 2003), indicating that the cross-offshore SAF transport of Oyashio water flows along SAB around 152 E. The cross-offshore SAF Oyashio transport is not composed of pure Oyashio water but is already mixed with Kuroshio waters, suggesting that the mixed water is formed near SAF through isopycnal mixing due to meso scale eddies (Masujima et al., 2003). The further increase of 8 Sv along SAF at 165 E (Miyao and Ishikawa, 2003) and 10 Sv from SAB to SAF at 162 E (Masujima et al., 2003) could also be explained by the mixed water formed along SAF between 152 E and 165 E. These results indicate the importance of NPIW formation due to the isopycnal mixing along offshore-saf in addition to the cross-gyre transport along the nearshore Oyashio. A subsurface profiling float experiment was designed to elucidate the NPIW flow field using floats of ALACE, P-ALACE and isopycnal P-ALACE, newly developed in SAGE, to track the isopycnal surface of 26.7σ θ (Iwao et al., 2003). A total of 21 subsurface floats revealed the averaged flow field around 26.7σ θ and showed relatively strong eastward currents along the Kuroshio Extension, SAB and SAF. Shimizu et al. (2004) reported the behavior of each isopycnal-tracking floats showing the low-pv Oyashio waters were actually entrained into the Kuroshio Extension. Formation of NPIW and salinity minimum around the Kuroshio Extension is found to be enhanced by strong isopycnal mixing due to baroclinically unstable frontal waves through the coupling of surface and intermediate anomalous vorticity fields from the analysis of MVP highresolution hydrographic and ADCP current data (Kouketsu et al., 2004). This might explain the rapid modification of NPIW along the Kuroshio Extension as high-salinity old NPIW and low-salinity Oyashio water mostly mixing and disappearing before 150 E. This frontal wave with a wavelength of about 100 km also explains the locally formed, remarkable salinity minimum that is frequently observed along the Kuroshio Extension. Estimates of mixing around the Kuroshio Extension and NPIW formation are also discussed by Joyce et al. (2001). Cabbeling and double diffusion processes in NPIW formation are examined by Inoue et al. (2003), showing that the density increase due to cabbeling through isopycnal mixing between the Oyashio and Kuroshio waters is 0.05 kg/m 3 and that due to salt finger is 0.02 kg/s that totally explains the density increase of 0.08 kg/s that is smaller than Talley and Yun (2001) s estimate. Part of NPIW formed in the Kuroshio-Oyashio interfrontal zone is found to be transported to the subarctic region through the Gulf of Alaska and to maintain the mesothermal water (temperature maximum) widely distributed in the North Pacific subarctic region (Ueno and Yasuda, 2000, 2001, 2003). The transport emanates from the Transition Domain between SAF and SAB. The northward transports of volume, heat and salt across 46 N between 158 E and 130 W are Sv, TW and kg/s respectively, as found from inverse analyses (Ueno and Yasuda, 2003). NPIW is a heat and salt source for the North Pacific subarctic region where deep convection is severely restricted due to the presence of a strong halocline maintained by the salt transport originating from NPIW. The modification of the intermediate water in the North Pacific subarctic region is examined by Miura et al. (2002, 2003), for the Bering Sea and by Ueno and Yasuda (2003) for the Ridge Domain in N and 150 E 120 W. The warm, saline water transported from the Transition Domain is not much modified in the density range greater than 26.7σ θ in the Gulf of Alaska and along the Alaskan Stream (Ueno and Yasuda, 2003). In the Bering Sea, winter cooling and excess precipitation create dichothermal water and erode the mesothermal water from above, but the influence is up to the density value of 26.8σ θ (Miura et al., 2002). The most intensive modification occurs in the Okhotsk Sea where dense water formation due to brine rejection and associated convection and strong diapycnal mixing due to tide occurs as mentioned in Section Climatic Significance: Anthropogenic CO 2 Transport into NPIW and Water-Mass Variability Ono et al. (2000) estimated anthropogenic-co 2 (ex- CO 2 ) in the Oyashio near Hokkaido using TCO 2 and CFC data and reported that the ex-co 2 transport through NPIW is Gt/yr, based on the estimates for the cross- SAF Oyashio transport near the east coast of Hokkaido, assuming a level of no motion. Yasuda et al. (2002) estimated the net ex-co 2 flux from the Okhotsk Sea to the North Pacific a GtC/yr in σ θ using the estimate of exchange flow transports and the ex-co 2 concentration data provided by Andreev et al. (2001). Ono et al. (2003) revised ex-co 2 estimates, finding GtC/yr into NPIW on the basis of the new estimates of cross subarctic front Oyashio transport near the Hokkaido coast as well as the cross-offshore subarctic front reported by Masujima et al. (2003), with anthropogenic CO 2 concentration for Okhotsk Sea Water, that is a improved version of the values given by Sabine et al. (2002). The concentration in the density of σ θ is larger than previously estimated and the transport value becomes greater. This new estimate explains 35% of the total inventory of ex-co 2 in the temperate North Pacific. Kawasaki (1999) and Kawasaki and Kusaka (2003) reported that a regime shift of subarctic water-masses occurred in the mid-1990s from surveys during the 1990s 390 I. Yasuda

7 in the western subarctic region: in the western subarctic Pacific the intermediate water in the density range of σ θ changed to become warmer and saltier while the surface water changed to become colder and fresher. This kind of shift was observed in the mesothermal water (Uda, 1963) in the East Kamchatka Current and WSAG in particular, and also in the Oyashio and Okhotsk Sea Kuril Basin. A water mass regime shift of NPIW and Oyashio water is currently being reported. A long-term freshening trend of NPIW was pointed out by Wong et al. (1999) for 47 N and 24 N section, and by Joyce and Dunworth- Baker (2003) for the freshening trend after the mid-1970s. Kawasaki (1999) showed that the water mass regime shift occurred in the 1990s in the western subarctic Pacific using data from every summer between 1990 and Isopycnal properties changed to warmer and saltier after 1994 in the density range of σ θ, while freshening occurs in the density from the surface to 26.6σ θ. Yasuda et al. (2001) noted that the potential vorticity in the Okhotsk Sea intermediate water changed to become higher and the density of potential vorticity vertical minimum decreased in the 1990s. Yasuda et al. (2000) and Rogachev (2000) reported property changes in the anticyclonic 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 concentrations in the Oyashio water south of Hokkaido in the density range of σ θ from 1970 to 1999 were reported recently (Ono et al., 2001). Watanabe et al. (2001) reported a basin-wide oxygen decrease from the mid-1980s to the end of the 1990s at the 47 N trans-pacific section and at the 165 E section. They suggested that the formation rate of original NPIW in the Okhotsk Sea could be decreasing. Ono et al. (2001) also reported the 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. Ono et al. (2001) and Watanabe et al. (2001) speculated that a change of vertical mixing is one possible candidate. Another possible cause is a change in the cross-gyre transport from the subtropical to the subarctic gyre that feeds the mesothermal structure in the North Pacific subarctic region (Ueno and Yasuda, 2000, 2001, 2003). 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 large property changes (Yasuda, 1997; Yasuda et al., 2002). An interdecadal change in NPIW is being demonstrated in the area south of Japan across the 137 E section maintained by JMA. Nakano et al. (2003 personal communication) reported that the sectional area with salinity less than 34.2 psu across 137 E underwent interdecadal variations: in the 1970s and the early 1980s the area was relatively small while in the late-1980s and the 1990s the area became larger. This change is coherent with the North Pacific Index with about a 10-year time lag. 6. Modeling of NPIW Ishizaki and Ishikawa (2004) reproduced realistic distribution of the Oyashio and NPIW using 1/4 (longitude) 1/6 (latitude) and 50-level eddy-permitting OGCM with the temperature and salinity restored to observational values in the whole water column in the Okhotsk Sea and Bering Sea in the conditions under the NCEP/NCAR wind-stress field and the modification of tracer advection scheme, which enhances isopycnal mixing due to meso scale eddies near the subarctic front. Nakamura et al. (2000a, b) and Nakamura and Awaji (2003) demonstrated the importance of tidally-induced exchange flow and vertical mixing around the Kuril Straits, as already mentioned in Section 2. The OGCM experiments with the sea-ice formation process in the Okhotsk Sea and the enhanced diapycnal diffusivity of 200 cm 2 /s around the Kuril straits reproduce realistic OSMW and NPIW formations (Nakamura et al., 2004). The strong diapycnal mixing around the Kuril straits enhances the overturning circulation in the Okhotsk Sea with diapycnal upwelling near the Kuril straits and diapycnal sinking of Dense Shelf Water (DSW) in the northwestern shelf region due to the increase of sea-surface salinity. The overturning circulation occurs between the surface and intermediate layers through 26.7σ θ surface in the density less than 27.4σ θ. The diapycnal mixing around the Kuril straits also induces meridional overturning circulation in the North Pacific: 3 4 Sv of overturning circulation is induced between the intermediate and deep layers across 27.2σ θ surface. Tatebe (2003) and Tatebe and Yasuda (2003) examined the impact of Okhotsk Sea diapycnal upwelling on the Oyashio southward intrusion, cross-gyre transport and circulation in the Kuroshio/Oyashio confluence region using a three-layer model with the restoration of intermediate layer thickness in the Okhotsk Sea to the observed value by giving diapycnal transports. About 3 Sv of diapycnal upwelling from deep to the intermediate layer yields a realistic Oyashio southward extension as well as realistic cross-gyre transport feeding NPIW. These results North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects 391

8 can be simply interpreted in terms of the application of Stommel and Arons (1960) theory to surface-intermediate layer instead of the abyssal layer. The diapycnal upwelling transport in the whole subarctic region feeds the southward cross-gyre transport as a western boundary current in the surface and intermediate layers; this explains the southward shift of the Oyashio extension and direct cross-gyre transport from subarctic to subtropical gyres, as reported from the observations (Fig. 1(c)). 7. Summary and Remaining Issues As mentioned above, intensive observation and modeling performed in SAGE led to great progress in NPIW studies. As a summary of the observations, a schematic diagram of NPIW formation, transformation and circulation with transport values in the density between 26.7 and 27.4σ θ is given in Fig. 1(a). In the density range of σ θ, about 1 Sv of 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. The DSW flows southward along the East Sakhalin Current (ESC) and to the anticyclonic circulation in the Okhotsk Sea Kuril Basin (Ohshima et al., 2002), being modified by mixing with warmer, saline waters from the Pacific (2 Sv) and Japan Sea. Okhotsk Sea Mode Water (OSMW) is thus formed, and its low-potential vorticity and low-salinity characteristics could be enhanced by tidal mixing around the Kuril straits. More than 3.5 Sv of OSMW flows out to the Pacific through the Kuril straits. OSMW mixes with EKC (7 9 Sv) waters to form Sv of the Oyashio water. The Oyashio water flows southward along the coasts of the Kuril Islands, Hokkaido and Honshu still has OSMW characteristics of large potential thickness (lowpotential vorticity). Part (5 Sv) of the Oyashio water does not return and is directly entrained into the subtropical gyre along the western boundary current regions. That is, this part of the Oyashio water crosses the subtropical/ subarctic gyre boundary and thus crosses SAF. This part of the cross-saf Oyashio water flows further southward and reaches the Kuroshio Extension, where new NPIW is formed by mixing with 12 Sv of warmer and saline old NPIW transported along the Kuroshio. Old NPIW in the density range of σ θ is influenced by Antarctic Intermediate Water, and its isopycnal oxygen content is higher than that of the Oyashio water. The new NPIW along the Kuroshio Extension is transported to both northern and southern sides of the Kuroshio Extension. Mixing processes between the Oyashio and relatively warm and saline old-npiw seems to be enhanced by frontal eddies, cabbeling and double diffusion. Another 6 Sv of the Oyashio water also crosses the offshore SAF/SAB west of 150 E into NPIW, probably due to isopycnal mixing around SAF and flows eastward in the Mixed Water Region north of the Kuroshio Extension. The eastward transport between SAB and SAF (Transition Domain) increases, probably due to isopycnal mixing around the SAF. 3 Sv of the northern part of NPIW and in the Transition Domain is transported to the Alaskan Gyre and feeds the mesothermal structure (temperature maximum) widely distributed in the North Pacific subarctic region. This transport with isopycnal diffusion provides heat and salt to the intermediate subarctic region which can balance with the cooling and freshening in the Okhotsk Sea. The rest of NPIW flows along the subtropical gyre circulation. Numerical studies suggest that overturning circulations in the Okhotsk Sea as well as in the North Pacific occur corresponding to tide-induced diapycnal upwelling around the Kuril Islands and dense shelf water formation in the northwestern shelf region of the Okhotsk Sea. The overturning in the Okhotsk Sea forms Okhotsk Sea Mode Water that is the origin of NPIW. The overturning in the North Pacific that is caused by diapycnal upwelling transport (3 4 Sv) through tidal mixing around the Kuril Islands enhances the cross-wind-driven gyre Oyashio transport along the western boundary current and forms NPIW around the Kuroshio Extension. Isopycnal mixing across SAF could be also important to form the northern part of NPIW. Recent studies in SAGE and related projects have greatly improved our knowledge of NPIW. 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