Absolute Volume Transports of the Oyashio Referred to Moored Current Meter Data Crossing the OICE

Transcription

1 Journal of Oceanography, Vol. 60, pp. 397 to 409, 2004 Absolute Volume Transports of the Oyashio Referred to Moored Current Meter Data Crossing the OICE KAZUYUKI UEHARA *, SHI-ICHI ITO 2, HIDEO MIYAKE 3, ICHIRO YASUDA 4, YUGO SHIMIZU 2 and TOMOWO WATAABE 5 ational Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu-Orido, Shizuoka , Japan 2 Tohoku ational Fisheries Research Institute, Fisheries Research Agency, Shiogama, Miyagi , Japan 3 Graduate School of Fisheries Science, Hokkaido University, Hakodate, Hokkaido 04-86, Japan 4 Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan 5 ational Research Institute of Fisheries Science, Fisheries Research Agency, Fukuura, Kanagawa-ku, Yokohama, Kanagawa , Japan (Received 9 February 2003; in revised form 8 August 2003; accepted 6 September 2003) During ovember 2000 June 2002, both direct current measurements from deployment of a line of five moorings and repeated CTD observations were conducted along the Oyashio Intensive observation line off Cape Erimo (OICE). All the moorings were installed above the inshore-side slope of the Kuril-Kamchatka Trench. Before calculating the absolute volume transports, we compared vertical velocity differences of relative geostrophic velocities with those of the measured velocities. Since both the vertical velocity differences concerned with the middle three moorings were in good agreement, the flows above the continental slope are considered to be in thermal wind balance. We therefore used the current meter data of these three moorings, selected among all five moorings, to estimate the absolute volume transports of the Oyashio referred to the current meter data. As a result, we estimated that the southwestward absolute volume transports in db are m 3 /sec and the largest transport is obtained in winter, January 200. The Oyashio absolute transports in January 200, crossing the OICE between 42 and 4 5 from the surface to near the bottom above the continental slope, is estimated to be at least m 3 /sec. Keywords: The Oyashio, OICE, moored current meter data, absolute volume transport.. Introduction The Subarctic Gyre of the orth Pacific has been comprehensively described by Dodimead et al. (963), Favorite et al. (976), Ohtani (989) and agata et al. (992). According to these studies, the upper ocean circulation in the Subarctic Gyre in the orth Pacific is basically recognized as one large cyclonic circulation consisting of four sub-gyres the Western Subarctic Gyre, the Alaskan Gyre, the Okhotsk Sea Gyre, and the Bering Sea Gyre. The Alaskan Stream flowing westward along the south side of the Aleutian Islands Arc, the East Kamchatka Current flowing southwestward along the east coast of the Kamchatka Peninsula, and the Oyashio flowing southwestward along the Kuril Islands and Hokkaido * Corresponding author. kuehara@fra.affrc.go.jp Copyright The Oceanographic Society of Japan. are recognized as relatively intense currents composing the Subarctic Gyre. That is, there are two striking western boundary current systems in the Subarctic Gyre of the orth Pacific: one is the Alaskan Stream in the Alaskan Gyre and the other is the East Kamchatka Current/the Oyashio in the Western Subarctic Gyre (Fig. (a)). Ohtani (970) reported that the geostrophic transports of the Alaskan Stream, the East Kamchatka Current and the Oyashio referred to 000 m are estimated to be 8 Sv (Sv 0 6 m 3 /sec), 9 Sv and 7 Sv, respectively. Since the late 980s, larger transports have been reported when the reference level is set deeper, especially in the Alaskan Stream. For example, volume transports of the Alaskan Stream are estimated to be 2 20 Sv referred to db (Reed, 984), 28 Sv referred to 4000 db (Warren and Owens, 988), 27.5 Sv referred to 3000 db on 8-year average (Onishi and Ohtani, 999) and 38 Sv referred to 6000 db (Roden, 995). Using a vessel-mounted acoustic Doppler current profiler (ADCP), Cokelet et al. (996) 397

2 (b) (a) Fig.. (a) Schematic representation of the two striking western boundary current systems (solid arrows) in the westside (the Western Subarctic Gyre) and east-side gyre (the Alaskan Gyre) of the Subarctic Gyre in the orth Pacific. EKC, Oy and AS indicate the East Kamchatka Current, the Oyashio and the Alaskan Stream, respectively. (b) Map showing the repeated CTD observation stations (open circles) and current meter moorings (open triangles) along the Oyashio Intensive observation line off Cape Erimo (OICE). computed an ADCP-referenced geostrophic transport of 24 Sv over the upper 500 db, which is 2.2 times that of only geostrophic transports referred to 500 db. On the other hand, transports of the Oyashio or the East Kamchatka Current, which are the western boundary current system of the Western Subarctic Gyre, have been reported to be smaller than those of the Alaskan Stream. Transports of the East Kamchatka Current flowing from the Kamchatka Basin to the Pacific through the Kamchatka Strait have been reported to be 9 Sv referred to db (Ohtani, 970; Verkhunov and Tkachenko, 992). From ADCP-referenced geostrophic velocities, the transports in the upper 500 db have been computed to be 9 2 Sv (Cokelet et al., 996). The Oyashio is considered to be a continuous flow from the East Kamchatka Current. After the 970s, the Oyashio southwestward transports were estimated to be roughly 4 Sv, flowing southward along the Kuril Islands, and 9 Sv, flowing southward from Hokkaido to the Sanriku coast referred to 500 db (Kawasaki et al., 99). Actually, this is the only reliable estimate of the Oyashio, since Ohtani s (970) first report. As for the Oyashio, the water mass modification processes (e.g. Ohtani, 989; agata et al., 992; Kono, 997; Kono and Kawasaki, 997) or the flow characteristics (Stabeno and Reed, 994; Stabeno et al., 994; Uehara et al., 997; Uehara and Miyake, 999) have been mainly discussed, rather than the volume transports. There have thus been very few estimates of volume transports of the Oyashio. On the other hand, the Sverdrup volume transport computed from the wind-stress field in the orth Pacific, in the region of the Subarctic Gyre, yields 0 20 Sv on annual average (e.g. Kutsuwada, 982). From numerical models, transports of the western boundary current in the Subarctic Gyre are expected to be ~70 Sv with clear seasonal and interannual variations (e.g. Sekine, 999). Isoguchi et al. (997) calculated the standard deviations of the time-varying transport constructed from TOPEX/POSEIDO-derived sea level data. They reported that the mean value of the standard deviations of the western boundary transport is Sv. The western boundary transports of the Subarctic Gyre estimated from wind-stress fields or numerical models are far larger than in-situ measurements of the East Kamchatka Current and the Oyashio. In addition, when considering continuity from the Alaskan Stream, unbridgeable gaps are evident in the transports from the Alaskan Stream to the East Kamchatka Current and the Oyashio. To fill in the gaps, it is important to accurately estimate the volume transport of the western boundary current. It is also important to grasp the magnitude of the Subarctic Gyre for understanding of the dynamics of the Gyre. As mentioned above, the western boundary transports in the Subarctic Gyre become larger when the reference level is set deeper. To estimate western boundary volume transports accurately, therefore, we need to set a deeper reference level. Furthermore, the geostrophic current might sometimes reach the bottom, especially over the slope, where we need direct current measurements with sufficiently high spatial resolution. The use of mooring systems is very effective to resolve these problems, because directly measured velocity data yield a shallow reference velocity in the region near the coast. 398 K. Uehara et al.

3 Fig. 2. Vertical section of bottom topography along the OICE obtained from the R/V Wakataka-Maru depth sounding system during the deployment cruise in June 200 and schematic representation of mooring systems. Pluses with numbers along the top horizontal axis denote CTD stations. Bottom horizontal axis represents latitude, vertical axis depth scale in m. Moorings from the coast are named TPc2, TP-c, TP-0, TP-2 and TP-3 in order. Crosses represent current-meters which measure flows at one depth such as rotor-type current-meters (see Table ). An open square and an open triangle show an ADCP, a sediment-trap, respectively. In this paper our goal is to estimate the absolute volume transports of the Oyashio (not to be confused with Oyashio Water, which is fresh and cold) referred to moored current meter data, which have not yet been fully examined. This will also contribute to an understanding of the formation and transport processes of orth Pacific Intermediate Water (PIW), characterized by a salinity minimum in the Subtropical Gyre, because PIW is qualitatively considered to be formed by mixing the low salinity Oyashio Water and the high salinity Kuroshio Water (e.g. Talley et al., 995; Yasuda et al., 996; Yasuda, 997; Shimizu et al., 200). 2. Data As a part of the Subarctic Gyre Experiment (SAGE), we conducted direct current measurements using mooring systems and repeated CTD observations along the Oyashio Intensive observation line off Cape Erimo (OICE). The OICE was set to correspond to the TOPEX/ POSEIDO ground track 060, which runs southeastward from Cape Erimo and is mostly perpendicular to the local isobaths (Fig. (b)). Figure 2 shows the bottom topography along the OICE and the mooring array. The deepest part of the Kuril-Kamchatka Trench (Trench henceforth) on the OICE is located at 4. All moorings were installed on the inshore-side slope of the Trench. Fig. 3. Dates of CTD observations along the OICE (vertical solid lines) and periods of current measurements by the mooring systems after June 2000 (horizontal arrows). Solid arrows indicate that available moored current-meter data were obtained. Short-dashed arrows show that moored current-meter data were unavailable owing to equipment problems. Long-dashed arrows indicate that data are not used in this study because current-meter depths are deeper than possible geostrophic computations. Moorings TP-c2 and TP-c were at a water depth of about km on the relatively gentle slope. Moorings TP-0, TP- 2 and TP-3 were at greater depths of about 2, 4 and 6 km, respectively, on the steep slope. The types of current meters attached in each mooring and their nominal depths are shown in Table : each current meter contains at least pressure and temperature sensors. Installation and retrieval of all the moorings on the OICE were accomplished by the R/V Wakataka-Maru and the T/V Hokusei-Maru. Each observation period at each mooring is shown in Fig. 3. Unfortunately the current meter data shown by short-dashed arrows in Fig. 3 were not properly obtained owing to problems with the current meters. Time series of water temperature, pressure and current vectors collected at -hour intervals were smoothed by a low-pass Gaussian filter with an 8-hour half power point to remove high-frequency fluctuations such as tides and inertial oscillations. Daily-mean temperature, pressure and current vectors were then derived. ext, we decomposed the daily-mean velocity vectors into components along the OICE (43 T) and across the OICE (233 T). To compare these with relative geostrophic velocities perpendicular to the OICE, and to estimate absolute volume transports, the cross-components of the dailymean velocities and pressure were averaged over 5 days, to remove motions with smaller time scales than the ones when CTD observations in the region around five moorings are occupied. Absolute Transports of the Oyashio Crossing the OICE 399

4 Table. Current meter types attached at each mooring. RCM, 3DACM and ADCP indicate the rotor-type current meter (Aanderaa RCM5), the three-dimensional acoustic current meter (FSI) and the 300 khz acoustic Doppler current profiler (RD Instruments), respectively. During the direct current measurement periods, repeated CTD observations were performed along the OICE (Fig. 3). At stations deeper than 3000 m we lowered CTD (the Sea-Bird 9plus system) equipped with a General Oceanics rosette sampler carrying 24 iskin bottles down to 3000 m with a 5-mile horizontal resolution in latitude (see Figs. (b) and 2). At stations shallower than 3000 m CTD lowerings were made down to near the bottom with a horizontal resolution finer than 5 miles in latitude (Figs. (b) and 2). Samples drawn from the iskin bottles were analyzed for salinity to calibrate the conductivity sensor of the CTD. Using calibrated salinity data, we computed relative geostrophic velocities referred to 000 db or to near the bottom in the region shallower than 3000 db, or referred to 3000 db in the region deeper than 3000 db, from pairs of CTD stations located on both sides of each mooring. 3. Basic Hydrographic Regimes along the OICE From the repeated CTD observations we found that there are three basic regimes in the CTD vertical crosssections along the OICE, which determine flow patterns above the inshore-side slope of the Trench: First, the remarkable dome structure of isopycnals in conjunction with salinity is well recognized; second, there is a case where the dome structure is not clearly seen; and third, a warmcore ring detached from the Kuroshio Extension (WCR) lies on the OICE. For example, Figs. 4, 5 and 6 are vertical sections of potential temperature (θ), salinity (S), potential density referenced to the sea surface (σ θ ) and geostrophic velocity fields (v g ) in January 200, April 200 and August 999, respectively, showing these three typical structures. 3. Isopycnal dome structure In January 200 the dome structure of isopycnals above the Trench near 4 (Sta.29) is well developed down to the depth of 3000 db as well as the salinity section (Figs. 4(b) and (c)). Consequently, there is a strong southwestward flow above the inshore-side slope of the Trench due to the baroclinic structure of the Oyashio between Sta.34 and Sta.29. In depths shallower than about 800 m, this flow is divided into two parts, between Sta.34 and Sta.32, Sta.3 and Sta.29, respectively (Fig. 4(d)). The latter flow, between Sta.3 and Sta.29, results from the isopycnal deep dome structure having its apex at Sta.29. The former flow, between Sta.34 and Sta.32, is due to superposition of the baroclinic structure by the lowsalinity, low-temperature water near the coast and the isopycnal deep dome structure. The northeastward return flows occur on the offshore side of the Trench between Sta.29 and Sta.25 (Fig. 4(d)). In general, the well-developed isopycnal dome structure on the OICE yields the situation in which the cross section above the inshore-side slope of the Trench from Sta.35 to Sta.29 is filled in southwestward flows. 3.2 Ambiguous dome structure In April 200 the dome structure is not clearly seen (Fig. 5(c)). Therefore the southwestward flow of the Oyashio does not entirely occupy the region above the inshore-side slope, and is limited to the relatively narrow band between Sta.33 and Sta.3 (Fig. 5(d)). In addition, the northeastward return flow is also unclear. Cold Oyashio Water with temperature less than 2 C and strong salinity stratification spreads more widely than in January 200 (see Figs. 4(a) and (b)), in the area between the coast and in the waters shallower than 400 m (Figs. 5(a) and (b)). 3.3 Warm-core rings In August 999 the center of a warm-core ring (WCR) detached from the Kuroshio Extension is located at near 4, which correspond to the deepest part of the Trench (Fig. 6). In this case, the flow pattern around 4 is completely different from the case of the dome structure because of the anticyclonic circulation of the WCR (Fig. 6(d)). 400 K. Uehara et al.

5 Fig. 4. Vertical cross-sections of (a) potential temperature ( C), (b) salinity (psu), (c) potential density (kg m 3) referenced to the sea surface, and (d) relative geostrophic velocities (cm s ) referred to near bottom or 3000 db (cold and warm colors indicate southwestward and northeastward flows, respectively. Contour intervals are 5 cm s ), along the OICE in January 200. Shaded portion on the left side of each section shows the bottom topography. Fig. 5. As Fig. 4, but for April 200. Absolute Transports of the Oyashio Crossing the OICE 40

6 Fig. 6. As Fig. 4, but for August Comparison of Vertical Velocity Differences between Geostrophic Velocities and Directly Measured Velocities In the first half of the SAGE project, during , WCRs were frequently distributed on the OICE. During the mooring period after 2000 (see Fig. 3), however, WCRs were not observed on the OICE. Therefore, the mooring observations were conducted under the deep isopycnal dome structure or the ambiguous isopycnal dome structure. In this section, before estimating the absolute volume transports, we examine whether thermal wind balance is achieved over the inshore-side slope along the OICE. We omitted the measured velocity data at 3000 m of TP-2 and all data of TP-3 (long-dashed arrows in Fig. 3) because the deepest CTD cast is down to 3000 db and we therefore cannot compute geostrophic velocities deeper than 3000 m (see Fig. 2). We compared vertical shears of 5-day mean measured velocities obtained from upper and lower depths of each mooring with those of geostrophic velocities computed from a pair of CTD stations at both sides of each mooring. For TP-c2, TP-c, TP-0 and TP-2, geostrophic velocities were computed from various horizontal scales, ranging from 2 to 47 km, and the thermal wind balance of the measured flows was examined. For TP-c2, the nearest mooring from the coast, we examined thermal wind balance from five pairs of the CTD stations which have different horizontal distances between two CTD stations ( L), that is pairs of Sta.533 Sta.32 ( L 43 km; Fig. 7(a)), Sta.633 Sta.532 ( L 37 km; Fig. 7(b)), Sta.533 Sta.532 ( L 26 km; Fig. 7(c)), Sta.633 Sta.33 ( L 22 km; Fig. 7(d)) and Sta.533 Sta.33 ( L 2 km; Fig. 7(e)) (see also Fig. 2). Vertical velocity differences do not agree between the directly measured velocities and the relative geostrophic velocities, regardless of L (Fig. 7), which suggests either that the measured flows obtained from TP-c2 are not in geostrophy, or that filtering for the measured velocities might not be appropriate. The vertical measured velocity differences of TP-c, TP-0 and TP-2, on the other hand, are in good agreement with geostrophic ones. (Figs. 8, 9 and 0). For TPc, the pairs of two CTD observations used for geostrophic calculations are Sta.533 Sta.32 ( L 43 km; Fig. 8(a)), Sta.33 Sta.32 ( L 32 km; Fig. 8(b)) and Sta.33 Sta.532 ( L 5 7 km; Fig. 8(c)). We obtained the best correlation of 0.88 when using the pair of Sta.33 and Sta.32 ( L 32 km; Fig. 8(b)), although the slope of the regression is slightly greater than. The pairs of two CTD observations for TP-0 are Sta.532 Sta.3 ( L 47 km; Fig. 9(a)), Sta.32 Sta.3 ( L 32 km; Fig. 9(b)) and 402 K. Uehara et al.

7 Fig. 7. Comparisons of vertical velocity differences between measured velocities (vertical axis) and relative geostrophic velocities (horizontal axis) for mooring TP-c2. Dashed lines indicate the regression line. (a) Case where Sta.533 and Sta.32 (43 km) were used for computing the relative geostrophic velocities, (b) as (a), but for Sta.633 and Sta.532 (37 km), (c) Sta.533 and Sta.532 (26 km), (d) Sta.633 and Sta.33 (22 km), and (e) Sta.533 and Sta.33 (2 km). Sta.32 Sta.53 ( L 5 7 km; Fig. 9(c)). When using Sta.32 Sta.3 and Sta.532 Sta.3, correlations exceeding 0.8 are obtained (Figs. 9(a) and (b)). The slope of the regression for Sta.32 Sta.3 (Fig. 9(b)) is closer to than that for Sta.532 Sta.3 (Fig. 9(a)). For TP-2, we compared the measured velocity differences obtained from two depths of 500 m and 2000 m with the geostrophic velocity differences computed from the pair of two CTD stations, Sta.3 Sta.30 ( L 32 km). As a result, we obtained a correlation of 0.66 where the slope of the regression is close to (Fig. 0). When calculating geostrophic velocities in the closest distances of two CTD stations in TP-c and TP-0, vertical velocity differences are not in good agreement (Figs. 8(c) and 9(c)). Here we estimate the internal Rossby radius of deformation, R i = H/f, where is the Brunt- Väisälä frequency, H is a water depth and f is the Coriolis parameter, because the quantity R i is the horizontal scale where the internal motions adjust to geostrophy. In order to simplify the arguments, we used f = 0 4 s as a constant value on the inshore-side slope along the OICE. From the vertical profile of averaged over 5 years at 4 30, 45 E (figure 6 in Uehara and Miyake, 2000), we used = 2 0 3, and 0 3 s at water depths of, 2 and 3 km, respectively. As a result, we obtained R i = km as rough estimates. The closest distances between two CTD stations at TP-c and TP-0 are smaller than the estimates of R i, which suggests that the poor correlations of both vertical velocity differences shown in Figs. 8(c) and 9(c) could be due to ageostrophic motions. 5. Absolute Transports To estimate absolute volume transports referenced to the measured velocities, we used the measured velocities of TP-c, TP-0 and TP-2, and also used relative geostrophic velocities computed from two CTD stations of Sta.33 Sta.32 for TP-c, Sta.32 Sta.3 for TP-0 and Sta.3 Sta.30 for TP-2, respectively. The distances of these pairs of CTD stations are roughly 32 km. From now on, we call these sections TPc-section, TP0-section and TP2-section, respectively. Absolute Transports of the Oyashio Crossing the OICE 403

8 Fig. 8. As Fig. 7, but for TP-c. (a) Sta.533 and Sta.32 (43 km), (b) Sta.33 and Sta.32 (32 km), and (c) Sta.33 and Sta.532 (5 7 km). Fig. 9. As Fig. 7, but for TP-0. (a) Sta.532 and Sta.3 (47 km), (b) Sta.32 and Sta.3 (32 km), and (c) Sta.32 and Sta.53 (5 7 km). The absolute velocities V abs (z) were estimated by fitting profiles of geostrophic velocities v g (z) to the moored velocities using the method of least squares (Fig. ). That is, an optimum distance p of the parallel translation of v g (z) was obtained as follows: when V CMi denotes the measured velocity at i-th depth (z i ), the sum S, squares of the difference between V CMi and the parallel translated geostrophic velocity v g (z) + p is 2 ( CMi ( g i )) = i i= i= S = V v ( z )+ p d p 2 ( ), () Fig. 0. As Fig. 7, but for TP-2. Sta.3 and Sta.30 (32 km). where is the number of current meters, d i is V CMi v g (z i ). Using the method of least squares, the optimum p is determined to minimize S. That is, the minimization condition is 404 K. Uehara et al.

9 T abs (Sv), referred to the measured velocities as follows: v g (z) T 0 L V z dz, 6 abs ref 0 = 5 abs() () z i z i+ V CMi+ V CMi V opt (z) =v g (z)+p where L is the distance (km) between the two CTD stations at both sides of each mooring, ref (m) is a reference level of geostrophic computations. In addition, using the estimated error of the absolute velocity, maximum and minimum absolute volume transports, T max and T min can be calculated as follows: 5 ref max = abs()+ 0 ( ) ( ) T 0 L V z sε dz, 7 Fig.. Diagram of the calculation method of the absolute velocities (the solid curve; V abs (z)). The dashed curve indicates the relative geostrophic velocity (v g (z)) referred to a reference level. p is the optimal distance of parallel translations of v g (z), which is determined by the method of least squares for fitting v g (z) to the measured velocities direction (V CMi is the measured velocity at the i-th depth). S p = 2p 2 d = ( ) i 0. 2 i= We therefore obtained the optimum p as follows, p d i i= =, () 3 as the arithmetic mean of differences between the measured and the geostrophic velocities at each depth. The estimated error s ε is then: s ε = S 2 = di di i= i= 2. ( 4) The absolute velocity referenced to the measured velocities therefore becomes Vabs()= z vg()+ z di. ( 5) i= Consequently, we obtain the absolute volume transports 5 ref min = abs() 0 ( ) () T 0 L V z sε dz. 8 Figure 2 shows time series of the relative geostrophic volume transports (T ctd ) referred to 000 db and the absolute volume transports T abs at each section of TP-c, TP-0 and TP-2 with the maximum and minimum estimates, T max and T min. From April 2000 to June 2002 in the TPc-section, southwestward relative geostrophic transports ( T TPc ctd ) are observed, except in September 200 (Fig. 2(a)), which suggests that the baroclinic structure of the Oyashio is well developed in the TPc-section during this period. From July 2000 to January 200 the absolute transports in the TPc-section ( T TPc abs ) are times as large as T TPc ctd, exceeding the range of the estimated error ( Tmin TPc ). In April 200 T TPc abs completely agrees with T TPc ctd, and the range of estimated error is very small. On the other hand, T TPc abs shows the opposite direction to T TPc ctd in May 200. In the TP0-section the variation of the absolute transports TP0 T abs is in good agreement with that of the relative transports T TP0 ctd (Fig. 2(b)). T TP0 abs are.4 3. times as large as T TP0 ctd, except in June 200 when the direction of T TP0 abs is opposite to that of T TP0 ctd. In the TP2-section the direction of the absolute transports T TP2 abs is opposite to that of the relative transports T TP2 ctd in April, May 200 and April In the TP-2-section the southwestward absolute transports are. 2.6 times as large as the relative transports. In each section, the maximum southwestward absolute transports occur in January 200 (Figs. 2(a), (b) and (c)) and January 2002 (Figs. 2(b) and (c)), respectively. This is consistent with other observational results (e.g. Uehara and Miyake, 996; Uehara et al., 997) showing that the maximum southward flows occur in winter. ext we estimate the absolute transports by integrating the transports of each section. By integrating the TPc- section and the TP0-section as shown in Fig. 3(a), we Absolute Transports of the Oyashio Crossing the OICE 405

10 (a) (b) (c) Fig. 2. Solid circles with solid lines show the relative geostrophic volume transports (T ctd ) referred to 000 db. Crosses with dashed lines show the absolute volume transports (T abs ) at sections of (a) TP-c, (b) TP-0 and (c) TP-2 with their error bars of the maximum and minimum estimates. egative values indicate southwestward direction. obtained four estimates of the integral absolute transports crossing the section having a vertical range of db and a horizontal width of about 64 km between Sta.33 and Sta.3 (Table 2). The strong southwestward flows of the Oyashio are usually observed on this section (see Figs. 4(d), 5(d) and 3(a)). In January and April 200 both T TPc abs and T TP0 abs are the same direction (Figs. 2(a) and (b)), and the southwestward absolute transports of 2.8 and 3.6 Sv which are integrated from the TPc to TP0- section in the upper 000 db of the water column are.8 and.4 times as large as the southwestward relative transports, respectively (Table 2). In May and September 200, however, the absolute volume transports are only 0.95 and 0.38 times as large as the relative geostrophic volume transports, in contrast to the case in January and April 200 (Table 2). This is because the direction of T TPc abs is different from that of T TP0 abs (Figs. 2(a) and (b)). In May Fig. 3. Sectional areas used for the absolute transport calculations (shaded portions) above the inshore-side slope of the Trench and the moorings. (a) Sectional area used for the absolute transports listed in Table 2. (b) As (a), but for Table the directions of the absolute transports of the TPc, TP0 and TP2-section are not in good agreement with those of the relative geostrophic transports (Figs. 2(a), (b) and (c)). We need to examine this in detail in the future. As mentioned in the Introduction, to focus the typical density range of PIW we estimate the absolute volume transports of the intermediate layer between σ θ ( T σ abs ). The geostrophic transports in the intermediate layer between σ θ ( T σ ctd ) were derived from the integrals of the relative geostrophic volume transports 406 K. Uehara et al.

11 Table 2. Volume transports integrated between 42 (Sta.33) and 4 30 (Sta.3) on the OICE (see Fig. 3(a)). T ctd, T abs, T max and T min show the relative geostrophic volume transport referred to 000 db, the absolute volume transport derived from Eq. (6) and the maximum and minimum absolute transports from Eqs. (7) and (8), respectively. The superscript σ means volume transports of the intermediate layer between σ θ. means sum of volume transports of the TPc-section and the σ TP0-section. T add = ph σ L 0 5, where p is derived from Eq. (3), h σ is the thickness of the intermediate layer and L is the horizontal distance between two CTD stations. The absolute transports of the intermediate layer ( T abs + T add. σ T ctd σ σ ) are derived from Table 3. Absolute transports and their maximum, minimum estimates of each section referred to the near bottom in January 200. T means the integral absolute volume transports, that is T TPc + T TP0 + T TP2. Values in parentheses show absolute transports between surface and 000 db. Absolute Transports of the Oyashio Crossing the OICE 407

12 at 0.05σ θ intervals from 26.6 to 27.0σ θ. To calculate T σ abs, additional volume transports which should be added to T σ σ ctd are shown by T add = ph σ L 0 5, where p (cm/s) is derived from Eq. (3), h σ (m) is thickness of the intermediate layer ( σ θ ) and L (km) is the distance between two CTD stations. The absolute transport of the intermediate layer is then calculated as T σ abs = T σ ctd + T σ add. As a result, the southwestward absolute transports of Sv are obtained (Table 2). These values are consistent with Shimizu et al. (2003). The ratios, T σ abs / T σ ctd, show the same tendency as T abs /T ctd (the db water column). However, the smallest h σ is obtained in January 200, when the largest volume transport is obtained. Only in January 200 we can integrate the absolute transports of all the three sections shown in the shaded portion of Fig. 3(b), as the core part of the southwestward transport of the Oyashio. We then obtained the southwestward absolute transport value of 3.0 Sv in January 200 (Table 3). The maximum and minimum estimates are 33.4 and 28.6 Sv, respectively. The integral absolute transport in the upper 000 db of the water column is 20.5 Sv, this means that the transport below 000 db is 0 Sv or more. This estimate, however, is only a part of the Oyashio which flows southwestward over the inshoreside slope of the Trench (Fig. 4(d)), although it contains most of the core of the Oyashio. The sectional area used this estimate, shown in Fig. 3(b), is about 55% of the total sectional area of the inshore side slope of the Trench from to 4. Therefore, the southwestward transport above the inshore-side slope in January 200 is expected to be more than 3.0 Sv, considering that the southwestward flows cover all the sectional area of the inshore-side slope in addition. Thus, although this value might still be underestimated, this study suggests that the two remarkable western boundary currents of the Western Subarctic Gyre and the Alaskan Gyre, as shown in Fig. (a), have almost the same scale, and the possible disagreement in volume transport between the west- and east-side gyre of the Subarctic Gyre is almost resolved. 6. Summary We have estimated the absolute transports from moored current meter data deployed along the OICE combined with repeated CTD observation data. From the repeated CTD observations on the OICE, we found that there are three typical oceanographic regimes characterized by the deep isopycnal dome structure, the ambiguous isopycnal dome structure and WCRs, which determine flow patterns above the inshore-side slope of the Kuril-Kamchatka Trench. During the mooring period after 2000 (Fig. 3), WCRs were not observed on the OICE. Therefore, the mooring observations were conducted under the deep isopycnal dome structure or the ambiguous isopycnal dome structure. Before estimating the absolute volume transports, we examined the thermal wind balance of the flows above the inshore-side slope of the Trench, by comparing the measured vertical velocity differences with geostrophic ones computed from the various horizontal scales ranging from 2 to 47 km. As a result, we conclude that the measured velocities above the inshore-side slope are roughly in geostrophy, except for TP-c2 located at the nearest from the coast. On the basis of this result, we estimated each absolute transport passing through the three sections from the absolute velocities with the fitting of the relative geostrophic velocities to the measured velocities, for the horizontal scale of about 32 km where vertical velocity differences are in best agreement. The results allow us to draw the following conclusions from this study. () Four absolute volume transports in db are estimated in 200. The largest southwestward absolute transport of 2.8 Sv and the smallest of 0.5 Sv are obtained in winter January 200 and autumn September 200, respectively. (2) The southwestward absolute transport of the intermediate layer between σ θ are estimated to be Sv, which is the largest in January 200 as well. The thicknesses of the intermediate layer in April and May 200 are larger than in January 200. (3) The integral absolute transport of the Oyashio in January 200, crossing the section vertically from the surface to near the bottom and horizontally between 42 and 4 5 (about 96 km), is estimated to be at least 3.0 Sv in total, and about 0 Sv is found below 000 db. This result suggests that two western boundary current systems in the west- and east-side gyre of the Subarctic Gyre have almost the same scale. Acknowledgements This research was supported by the Subarctic Gyre Experiment project of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government. We are grateful to the members of the OICE group. We would like to acknowledge the assistance of the crews and officers of the R/V Wakataka-Maru of Tohoku ational Fisheries Research Institute and of the now-defunct T/V Hokusei-Maru of Hokkaido University, who helped in the deployments and recoveries of the current meter moorings. We also thank K. Mizuno for his valuable suggestions and comments. Thanks are extended to D. Inagake and T. Kameda for their helpful discussions. We would like to thank the assistance, support and efforts of H. Yamagishi as well as the others of the staff of ational Research Institute of Far Seas Fisheries and Tohoku ational Fisheries Research Institute without whom this work would not have been successful. All the figures were produced by GFD-DEOU Library. 408 K. Uehara et al.

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