Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current

Transcription

1 Journal of Oceanography, Vol. 62, pp. 457 to 471, 26 Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current MIHO ISHIZU*, YUJIRO KITADE and MASAJI MATSUYAMA Department of Ocean Science, Tokyo University of Marine Science and Technology, Konan, Minato-ku, Tokyo , Japan (Received 21 July 25; in revised form 22 January 26; accepted 3 January 26) The cold-water belt (CWB) is frequently formed off the Soya Warm Current (SWC) during summer and autumn. The detailed distribution of the flow and temperature fields observed by the R/V Sinyo-maru in the summer of 21 captured the structures of the SWC and the CWB. The temperature and density distributions showed that the vertical distribution of the CWB is associated with the upwelling formed off the SWC. Numerical experiments using a two-layer model with realistic bottom topography have been performed to understand the formation mechanism of CWB and the upwelling structure off the current. In the experiment, the sea level difference between the Japan Sea and the Okhotsk Sea, and baroclinic flow assuming the Tsushima Warm Current were given along the open boundary. The numerical model well reproduces the current system of the SWC and upwelling region off it. The upwelling region is formed at the Soya Strait first, and then it spreads on the side along the SWC as a developing current system. Analysis of the model data indicated that the geostrophic balance mainly dominates in the current system, while convergence of the bottom Ekman transport due to the SWC forms the upwelling region as the secondary circulation. In addition, the advection effect due to the SWC is found to strengthen the upwelling. Keywords: The Soya Warm Current, upwelling, two-layer model, The Okhostk Sea. 1. Introduction The Soya Warm Current (hereafter SWC) flows into the Okhotsk Sea through the Soya Strait from the Japan Sea. The SWC flows southeastward over the continental shelf along the Hokkaido coast. The SWC water has higher temperature and salinity than the Okhotsk Sea water and is the sole origin of warm saline water in the Okhotsk Sea. Yasuda (1997) and Watanabe and Wakatsuchi (1998) pointed out the important role it plays in the formation of the North Pacific Intermediate Water (NPIW). The SWC is considered to be driven by a pressure gradient, i.e., the sea level difference between the Japan Sea and the Okhotsk Sea (Aota, 1975). Long term current measurements have revealed the remarkable seasonal variation, i.e., strong in summer and weak in winter, and shown that current variations are closely correlated with the sea level difference (Aota and Kawamura, 1978; Matsuyama et al., 1999). * Corresponding author. od312@edu.s.kaiyodai.ac.jp Copyright The Oceanographic Society of Japan/TERRAPUB/Springer A drastic change of water characteristics along the northeast coast of Hokkaido appears in April and in November (Itoh and Ohshima, 2). In summer, the SWC water lies near the coast, but in winter the East Sakhalin Current water predominates, except near the sea bottom, which is occupied by the SWC water (Aota and Kawamura, 1978; Takizawa, 1982). The SWC has been considered to have the characteristics of a barotropic current (Aota, 1975; Aota et al., 1985), so it is difficult to estimate the velocity and transport of the SWC from the density distribution and thus direct measurement is necessary. Aota and Kawamura (1978), Aota and Matsuyama (1987) and Matsuyama et al. (1999) employed current measurements at mooring stations. Recently, Matsuyama et al. (21) carried out current measurement by ADCP (Acoustic Doppler Current Profiler) with temperature and salinity measurements in summer 1998 to obtain the details of velocity structures across the SWC and estimate the volume transport. The observations indicated a current width of about 35 km and a maximum velocity of about 1. ms 1 at 2 25 km off the coast. The horizontal current profile did not describe a parabola but rather a curve that shows the cur- 457

2 14 E E 145 E 45 N Fig. 1. Sea surface temperature for a northern region of Hokkaido, Japan, on 21 August 21. The image is composed of 1 day. (Processed by Tokyo University of Marine Science and Technology.) The vector indicates the current of warmer water, i.e., the Soya Warm Current. The cold water belt appears outside it. rent steeply decreasing in the direction. The vertical current profile of the SWC showed that it has a baroclinic component as well as a barotropic one. The volume transport in the region from 5 km to 3 km off the coast was found to be SV, being composed of a barotropic transport of.8.9 SV and a baroclinic transport of.3.4 SV. Two physically interesting phenomena have been found in AVHRR (Atmospheric Administration Advanced Very High Resolution Radiometer) satellite images in the SWC region from summer to autumn. One is the wavy pattern along the frontal zone, which was explained by Ohshima (199) as a result of barotropic instability. The other is the cold-water belt (hereafter CWB), see in Fig. 1, extending from the southwest coast of Sakhalin along the side of the SWC. The CWB is believed to be a transient event because of its disappearance from the satellite images at times. Danchenkov et al. (1999) suggested that even if the CWB does not appear at the sea surface, the cold water always exists under the sea surface in the region between the SWC Water (T > 7 C; S > 33.6) and the Fresh Surface Okhotsk Sea Water (hereafter FSOSW) (T < 14 C; S < 32.5) (Aota et al., 1985). From satellite images and observational data, it has been speculated that the CWB is caused by two possible mechanisms. One is the advection which carries cold water from the southwest coast of Sakhalin through the Soya strait (Nakata et al., 1996), and other is the upwelling which brings up the cold water in the subsurface of Okhotsk Sea (Danchenkov et al., 1999). These mechanisms are both plausible, but the formation mechanism of the upwelling has not been adequately explored thoroughly, so they are both still hypothetical. In this paper we try to clarify the detailed structure and the formation mechanism of the CWB along the SWC. Section 2 describes the velocity structures of the SWC and characteristics of the CWB using our observational dataset collected in 21. Section 3 describes the numerical model. Section 4 describes the formation process of the SWC and the upwelling according to the model. Section 5 examines the relation between the SWC and the upwelling, and then discusses the formation mechanism of the upwelling off the SWC. Section 6 gives a summary and conclusion. 458 M. Ishizu et al.

3 Fig. 2. Study site, bottom topography around Hokkaido, Japan and the observational sections (lines O and M) in 21. Isobaths are shown in meters. 2. Observation and Results 2.1 Observation The observation of hydrographical properties and current velocity were conducted to capture the oceanic structures of the SWC and CWB by the T/R V Shinyo- Maru, belonging to Tokyo University of Fisheries, in August 21. Figure 2 shows the observational lines. Along line O, hydrographical observations using FSI integrated CTD were conducted at 1 stations from to inshore within 5 hours. After the CTD observations, the current observation using ship-mounted ADCP (BB_ADCP 15-kHz by RD Instrument) was carried out from inshore to with the XBT casts at 1 minute intervals in time (at a distance interval corresponding to about 2.8 km). After 3 hours, the ADCP survey was also carried out along line M from inshore to with the XBT casts. The ADCP survey along each line took about 3 hours. The current data obtained by the ADCP survey along the lines were corrected by removing the ship s speed, estimated from the bottom-tracking data, following the method published by Ishii et al. (1986). In the handling of the CTD data, following Kawabe and Kawasaki (1993), after the response speed of the conductivity sensor was matched to that of the temperature, process data with 1 m interval were obtained by applying a 5 m running mean. XBT data were used below 3 m depth, taking the adjustment time of a thermistor to the sea water temperature into consideration. 2.2 Results of observation The current stick diagrams along lines O and M (Figs. 3(a) and (b)) show the existence of a strong coastal flow, i.e., the SWC, which was found from inshore to about 3 km off the coast. The current direction was mostly southeastward from the sea surface to the bottom and the maximum velocity was located at 2 25 km off the coast. The strong current is seen to be primarily due to a barotoropic component, including a weak baroclinic component, that is, the current speed decreased slightly with depth. On the other hand, in the area of the SWC, the current was very weak and had a baroclinic tendency, so that the lower velocity was stronger than the upper one. The characteristic of the current structure of the SWC observed in August 21 is very similar to that in August 1998 (Matsuyama et al., 21). The temperature sections observed by CTD and XBT along line O (Figs. 4 and 5) show the characteristics of the distributions of the water mass: 1) existence of relatively warm water corresponding to the SWC water (T > 7 C; S > 33.6) from inshore to 3 km ; 2) the FSOSW (T < 14 C; S < 32.5) above the thermocline further ; 3) the Okhotsk Intermediate Sea Water (OISW, T < 2 C; 32.8 < S < 33.4) sometimes dropping Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 459

4 (a) (b) Fig. 3. (a) Current stick diagram of line O by ADCP observation in 21. ADCP data with error velocity more than 2 cm/s are neglected. T indicates the true bearing. (b) As (a), but for line M in 21. below C just below the sharp seasonal thermocline. The contour lines of 6 1 C rise vertically from 1 m depth to the bottom, i.e., the temperature front, in the region of 2 3 km off the coast. The contour lines of 2 6 C tend to shift with depth. Thus a relatively uniform cold water mass existed near the bottom at 3 4 km off the coast in line M. The salinity front (centered at S = 33.5) tended to shift with depth along line O (Fig. 4), and was close to the temperature front (centered at 8. C). The difference from the temperature front is that the salinity front is exposed to the sea surface. The position of the surface salinity front mostly agrees with that of the split of the temperature contours (the shaded area in Fig. 4). This structure coincided with the result reported by Danchenkov et al. (1999). The relatively uniform salinity water existed near the bottom at 3 4 km off the coast, and its region corresponded with the cold water (4. 6. C) region. We found the lower temperature water (T = C) than the surrounding water, near the sea surface at about 4 km off the coast, shown in Fig. 4. This lower temperature water near the sea surface existed along lines O and M (Fig. 5), and is regarded as the region of the CWB. Comparing the cross sections of CTD (Fig. 4) with that of XBT (Fig. 5) along line O, the core of the cold water (T = C) shifted inshore after about 5 hour (left in Fig. 5, and Fig. 4), and its width extended from 1 km to 2 km. The upheaval of temperature contour lines in the subsurface layer was seen as a dome-like structure in both cross sections. The width of the CWB region decreased abruptly along line M. The XBT observations along line M (right in Fig. 5) were made about 6 hours after the XBT observations along line O (left in Fig. 5), so it is very difficult to clarify the temporal and spatial variations from the difference of the structure of the CWB between lines O and M. The density distribution (right in Fig. 4) shows almost the same structure as the temperature one, with a gradient up to the right inshore, while indicating a gradient down to the left below the pycnocline. As a result, the distribution indicates an upwelling structure with an upheaval shape that tends to shift with depth. These horizontal density gradients within 2 3 km off the coast suggest that the current has baroclinic structure with the upper velocity stronger than the lower one, and vice versa, in agreement with the result of the ADCP measurements (Fig. 3). The dense water appeared at the sea surface 2 3 km off the coast and is located in the onshore side before the region of the CWB. The same density distributions were also observed by Aota et al. (1985) and Matsuyama et al. (21), and this is considered to be one of representative features in the Hokkaido coast in summer. We can safely consider that the CWB is formed by the upwelling event, and the SWC itself is related to the upwelling event in some way, judging from the temperature and density distributions and current stick diagram. In the next section we reproduce the SWC and examine the generation mechanism of the upwelling off the SWC by the numerical experiment. 46 M. Ishizu et al.

5 Fig. 4. Cross sections of (a) temperature, (b) salinity, (c) σ t in line O in 21. Contour line intervals are.1 C,.25 and.25 kg/m 3, respectively. Shaded area indicates the sea surface low temperature, regarded as the region of the CWB. 3. Modeling 3.1 Governing equations and the condition of the model The positive x-, y- and z-axes are taken to be eastward, northward and upward, respectively. Under the f- plane, hydrostatic and the Boussinesq approximations, basic equations integrated in the vertical for the upper layer are given by U1 + f k U1 t 2 2 = gh+ η η η A U 1 γ u u, ( ) + () h 1 h h i η 1 h U1 h U 2, 2 t = ( ) and for the lower layer U2 + f k U2 t ρ1 ρ = g( h2 + η2) hη1 g( h2 + η2) hη2 ρ2 ρ A U + γ u u γ u u, () h h 2 i b η 2 h 2 4 t U, = ( ) where the subscripts 1 and 2 indicate the upper and lower layer properties, respectively, and h = i / x + j / y, i and j are the unit vectors of x and y directions, respectively. U is the vertically integrated velocity vector, u the velocity vector, u the difference of velocity vector between the upper and lower layers (u = u 1 u 2 ), η 1 and η 2 the vertical displacement of surface and interface, respectively; h is the layer thickness at the initial condition. f is the Coriolis parameter ( f = s 1, at 45 N), k the unit vector of z-direction, t the time, g the acceleration due to gravity (9.8 ms 2 ), ρ the water density, ρ the density difference between the upper and lower layers, A h the coefficient of horizontal eddy viscosity, and γ i 2 and γ b 2 the frictional coefficients at the interface and bottom, respectively. As shown in Fig. 6, the region from 141 E to 146 E and from 43 N to 46.5 N is considered for the computational domain, which is 468 km from east to west and 396 km from north to south. Based on 5 m 5 m grid data provided by Japan Oceanographic Data Center, the bathymetric data in this region were averaged into 2 km 2 km grid data. The bathymetric data around 46.5 N were read directly from the hydrographic chart (Hydrographic Department of Japan, 1936), and was interpolated linearly on the grid. For simplicity, depths deeper than 1 m were set to 1 m in the west of the Soya Strait, and small islands were omitted. Similarly depths deeper than 1 m were set to 1 m in the east of the Soya Strait, and depths shallower than 4 m were set to 4 m in all areas. The upper layer thickness was set to 2 m for the initial condition. The stratification condi- Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 461

6 Fig. 5. Cross sections of temperature in lines O (left) and M (right) in 21. Contour interval is 1 C. Shaded area indicate the sea surface low temperature, regarded as the region of the CWB. tion is ρ/ρ 2 =.3. The coefficient of horizontal eddy viscosity, A h, was 1 m 2 s 1. The interface and bottom friction coefficients, γ i 2 and γ b 2, were assumed to be as.13 and.26, respectively, except near the open boundary (the hatched area in Fig. 6). These values are usually used in numerical experiments in the coastal region (e.g., Kitade and Matsuyama, 2). In order to suppress the disturbance near the open boundary at the Okhotsk Sea side, the clamped condition (Chapman, 1985) is applied with the sponge condition along the open boundary. In these sponge regions (the hatched area in Fig. 6), the horizontal eddy viscosity A h is set to increase linearly within 4 km from the open boundary, to reach a value of ten times larger than that in the inner region. The non-slip condition was applied along the boundary at the coasts. Numerical experiments were performed by integrating Eqs. (1) (4) after transforming into finite difference forms, in which the centered difference and the leapfrog scheme were used for spatial and temporal differences, respectively. In addition, the Euler-backward scheme was applied every 2 steps to prevent numerical instability. The driving source of the SWC is believed to be the sea level difference between the Japan Sea and the Okhotsk Sea (Aota, 1975). Therefore in our numerical experiments the surface and interface displacements at the open boundary in the Okhotsk Sea was kept constant (η 1 = η 2 = by clamped condition), and on the side of the Japan Sea they were increased as in the following expression. η t / t ( m) t t η1 = η ( m) t t h2, η2 = η1, ( 5) h where η is the maximum of displacement (η =.1 m), t the set up time for the sea level (t = 1 day) and h the water depth. In addition, to consider the Tsushima Warm Current, which flows northward along the west coast of Hokkaido, the baroclinic current flowing northward along the west coast of Hokkaido was set on the south open boundary in the Japan Sea. The current structure was based on the observational data of the Tsushima Warm Current provided by the Hokkaido Fisheries Experimental Station (Hokkaido Fisheries Experimental Station, 199). The upper velocity on the south open boundary of the Japan Sea was set to.1 ms 1 in the section of 2 km from the coast and decreased exponentially in the outside of the section, in accordance with the expression below. x x u exp x 8 km u1 = λ u 8 km x 1 km ut / t for t t where u =, u = U for t t c ρ c g hh 1 2 λ =, = f ρ h + h ( 6) 462 M. Ishizu et al.

7 Fig. 6. Computational domain. A realistic coastline and simplified bottom topography were used; depths greater than 1 m and shallower than 4 m were set to 1 m and 4 m, respectively. Lines 2 7 are the selected monitoring sections. where u is the maximum northward velocity (U =.1 ms 1 ), x = is at the western boundary and x the position of the western coast of Hokkaido at 8 km from the western open boundary. The inertial period in the study is 16.9 hours, so that the displacement and current calculated by the model were processed by applying a 17-h running mean to remove the variations with inertial period. 4. Results of the Linear Model 4.1 Initial development and steady state Figure 7 shows the distributions of surface and interface displacements and the upper layer current at.5, 2, 5 and 1 days. The initial development of the current field as the adjustment process to a given boundary condition in the linear model is as follows. As the displacement in the Japan Sea rose gradually, the relatively large horizontal gradient of the sea surface displacement was formed along the west coast of Sakhalin and the northeast coast of Hokkaido at.5, 2 days. The strong current region was formed along the contour line of the sea surface displacement and the region spread over the continental shelf in the northeast coast of Hokkaido in the Okhotsk Sea. The baroclinic current given on the southern open boundary in the Japan Sea progressed northward at.5 days, and reached the Soya strait at 2 days. The positive interface displacement indicating the upwelling was formed along the west coast of Sakhalin from the north of the strait. The upwelling region spread as far as off Esashi at 1 days and distributed as the belt in the northeast coast of Hokkaido in horizontal distribution. The central part of the upwelling was located km off the coast in the northeast coast of Hokkaido. It is a peculiar property that the strong part of the current in the northeast coast of Hokkaido exists between the upwelling and the coast. Distributions of current and displacement almost reached a steady state at 4 days. Figure 8 shows the distributions of surface and interface displacements and the upper and lower layer currents at 4 days. The upwelling region spread southeastward from the strait. The upwelling region was only formed over the continental shelf and did not spread further. The amplitude of the upper layer current in the region of the strong current over the continental shelf is larger than that of the lower layer, and this direction almost corresponds. The strong current was formed between the coast and the upwelling region. In the eastern region off Abashiri, although the upper layer current spread and the velocity near Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 463

8 Fig. 7. Distributions of velocity (left) in the upper layer, surface (center) and interface displacements (right) in linear model from.5 days to 1 days. Reference vector is.1 m/s. Shaded regions indicate negative displacement. Contour intervals for surface displacement and interface displacement are 1 cm and 1 m, respectively. the coast was less than that over the continental shelf, the lower layer current concentrated near the coast and the velocities became large. The CWB is frequently found from the southwest coast of Sakhalin to off Monbetsu in the AVHRR image in summer, as shown in Fig. 1. The perpendicular distance of the CWB from the northeast coast of Hokkaido is about 3 km. So the upwelling region reproduced by the model (Fig. 8) shows a similar 464 M. Ishizu et al. distribution to the CWB. The numerical results shows that the upwelling region spreads southeastward with time. The spreading speeds, estimated from the distance between lines and the time lag, are presented in Table 1. The southeastward spreading speed of the upwelling region was.11 ms 1 between line 3 and line 5,.38 ms 1 between line 5 and line 6, and.13 ms 1 between line 6 and line 7, indicat-

9 Fig. 8. Velocity fields of the upper and lower layers, and the distributions of the surface and interface displacements in linear model at 4 days. ing that the spreading speeds in each segment become slower on the downstream side. The alongshore current of the SWC was about.1.4 ms 1 (right in Fig. 9), which is relatively faster than the spreading speeds. 4.2 Distribution of displacement and velocity on each line Figure 9 shows the cross section of the surface and interface displacements and alongshore currents in the upper and lower layers on lines 1, 2, 4, 6 and 7 at 4 days. The sea surface displacement on line 1 is almost m near the coast of Sakhalin, increasing to.1 m at the western open boundary. The interface displacement on line 1 shows a positive maximum near the coast, decreasing. The upper layer current on line 1 is entirely stronger than the lower one, and dominates within about 4 km from coast, while the lower layer currents dominate within 2 km from the coast. On line 2 in the Soya strait and lines 4, 6 and 7 off the northeast coast of Hokkaido, the surface displacement shows a positive maximum at the coast of Hokkaido, decreasing with distance from the coast. The distribution of the interface displacement on each line shows the upheaval shape, im- plying the upwelling which is formed about 25 5 km off the northeast coast of Hokkaido. The current distribution on each line shows that the current strengthens near the coast and weakens. In addition, the upper layer current is stronger than the lower one near the coast of Hokkaido, while from the upwelling region indicated by the interface displacement, the lower current is almost same as or a little stronger than the upper one. The observational results were compared with those of the numerical model. The density distribution (Fig. 4) on line O indicated the upwelling structure, i.e., the σ t contour line of the upheaval shape. The distribution of 25.σ t showed an outcrop on the sea surface, and the location of the outcrop was somewhat on the onshore side of the cold water region associated with the CWB on line O. The top of the upheaval shape of each σ t contour line shifted with depth. The distribution of the interface displacement on line 6 in the model, which corresponds to the line off Ohmu, shows the central part of the upwelling at 45 km off the coast (Fig. 9). The location of the crest represented by the interface displacement in the model is about 5 km further than that of the SST Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 465

10 Table 1. Time lag and spreading speed. Time lag indicates the time when the positive interface displacement appears on each line from the start of the numerical experiment. Spreading speed indicates the speed that the positive interface displacement spreads between lines and is estimated from the distance between lines and the time lag. Linear model Non-linear model Time lag (day) Spreading speed (m/s) Time lag (day) Spreading speed (m/s) Line Line Line Line (i.e., sea surface temperature) minimum, which is regarded as the region of the CWB (the hatched area in Fig. 4), but the distribution of the interface displacement agrees in shape with that of the contour line of 26.5σ t. The velocity distribution on line O (see Fig. 3(a)) shows the strong current region near the coast, where the upper layer current is stronger than the lower one. The velocity distribution on line 6 in the model agrees well qualitatively with those of observation. We can therefore state that the current system of the SWC in summer was qualitatively reproduced by the present two layer model. In the next section we discuss the formation mechanism of the upwelling. 5. Discussion 5.1 Formation mechanism of the upwelling In the results of the numerical model the southeastward current corresponding to the SWC was formed along the coast of Hokkaido. The upwelling structure formed off the SWC, indicated by the interface displacement in the model, agrees with that of 26.5σ t in the density distribution in line O (as shown in Fig. 4). In addition, the current distribution in the upper layer demonstrated in the model agrees well with that of the HF ocean radar observations around the Soya Strait and the Lagrangian observation with drifting buoys (Ebuchi et al., 26). In the distribution of the interface displacement of the model, the coastal upwelling along the west coast of the Sakhalin was formed, e.g., at 2 days in Fig. 7 (right), and was accompanied by the southward current. The cold water along the west coast of Sakhalin is frequently seen in summer and autumn on the AVHRR satellite images and is also captured in the temperature observation carried out by Nakata et al. (1996). The southward current along the west coast of Sakhalin has been reported by Kantakov and Shevchenko (1999). This current along the west coast of Sakhalin is called the West Sakhalin Current and the current structure is not yet clear in detail. The current field reproduced in the model is constructed from the contribution of both barotropic and baroclinic adjustments. At.5 days in Fig. 7, the contour lines of the surface displacement pass through the Soya strait and distribute from the west coast of Sakhalin to the northeast coast of Hokkaido. The distribution of the interface displacement at 2 days (Fig. 7) shows that the coastal upwelling is formed along the west coast of Sakhalin and the downwelling region is formed onshore in the northeast coast of Hokkaido. Judging from the structure and the propagating speed of the interface displacement, these coastal upwellings and downwellings are explained in terms of an internal Kelvin wave. The distribution of interface displacement after 5 days indicated the upwelling off the strong current corresponding to the SWC in the northeast coast of Hokkaido (Figs. 7 and 8). They should also be formed by the baroclinic adjustment. However, the upwelling region spreads slowly southeastward along the coast (.38 ms 1 ~.11 ms 1 ), being very much slower than that of the internal Kelvin wave. These characteristics imply that the dynamical formation mechanism of the upwelling off the SWC is different from that of the coastal upwelling formed along the west coast of Sakhalin. To understand the generation mechanism of the upwelling off the SWC, the momentum balance in the upper and lower layers was examined using 17-h running mean data. Figure 1 shows the momentum balance in the upper and lower layers. The momentum equations in the upper and lower layers are given by Eqs. (7) and (8), respectively, U1 + + ( + ) fk U1 g h1 η1 η2 hη1 Ah hu1 γ i u u t ( 2) ( 3) ( 4) ( 5) () 1, 7 = ( ) 466 M. Ishizu et al.

11 Surface and interface displacement Upper and lower velocity Line Surface Interface 1 km West coast of Sakhalin km S upper lower West coast of Sakhalin Line Interface Surface 1 km km SE upper lower.2 Line Surface 1 km Interface km SE upper lower Line Surface SE upper lower km Interface km.2 Line Surface 1 km Interface km SE upper lower Fig. 9. Distributions of the surface and interface displacements (left) and upper and lower velocities (right) on lines 1, 2, 4, 5 and 7 at 4 days. Horizontal axis indicates distance from the coast. Dashed line indicates water depth (m). U2 ρ1 ρ + fk U2 + g( h2 + η2) hη1 + g( h2 + η ) η t ( 2) ρ2 ρ2 () 1 ( 3) 2 h Ah hu2 γ i u u + γ bu 2 u 2 =, () 8 ( 4) ( 6) where (1) is the time derivative term, (2) the Coriolis term, (3) the pressure gradient term, (4) the horizontal eddy viscosity term, (5) the interface friction term, (6) the interface and bottom friction terms. Figure 1 shows that the terms of the cross-shore Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 467

12 Fig. 1. Cross-shore distributions of the term balances of momentum equations in line 4 at 4 days. The term a is alongshore component and c is cross-shore component. The terms upper and lower indicate the momentum equation in the upper and lower layers, respectively. Horizontal axis indicates distance from the coast. component are larger than those of the alongshore one in absolute value and the Coriolis and the pressure terms dominantly balance each other in both layers, indicating geostrophic equilibrium. In the alongshore component, the values in both layers are much smaller than those in the cross-shore component. However, we can see that in the momentum balance in the alongshore component of the lower layer, the frictional term is balanced with the pressure gradient one onshore and the Coriolis term is balanced with the pressure gradient one. The contribution of the frictional term onshore in the lower layer indicates the possibility that the strong current, corresponding to the SWC, drives the bottom Ekman transport as the secondary circulation and the convergence of the bottom Ekman transport in the lower layer forms the upwelling off the SWC. We examined the vorticity balance in the lower layer on the same line to investigate the formation mechanism of upwelling due to the bottom Ekman transport. The vorticity balance in the lower layer used in the present study is given by, 1 ς2 1 g f t 2 f J h g h + + ρ ( + )+ ρ U η, η 2 f J η + h, η ( ii) ρ ρ () i ( iii) f A hcurl hu curlτi+ curlτb =, f f ( iv) ( v) ( vi) ( ) where ς 2 = curlu 2, J(A, B) = ( A/ x)( B/ y) ( B/ x)( A/ y) and h U 2 = η 2 / t (ii-a). The term (i) is the time derivative term of vorticity, (ii) the stretching term. The term (iii) indicates the joint effect of the baroclincity and relief including the contribution of surface displacement because the momentum equations in the lower layer are only used for the derivation to describe the dynamics in the lower layer, thus we call this term the modified JEBAR term. The (iv) term is the curl of horizontal viscosity, (v) curlτ i, (vi) curlτ b. Figure 11 depicts the distributions of each term in the vorticity equation in the lower layer and interface displacement at 7 and 4 days. The distributions at 7 days 468 M. Ishizu et al.

13 Fig. 11. (A) Distributions of each term in vorticity equation in the lower layer on line 4 at 7 and 4 days. Definition for each line is the expression above. (B) The distribution of the interface displacement (solid line) on line 4 at 7 and 4 days and bottom topography (dashed line). In these distributions, 17-h running mean and box-type filter with 1 km width were applied to remove the inertia period variations and influence of the local topography. indicate the developing stage of upwelling and at 4 days there is a steady state. These vorticity balances are averaged by 17-h running mean and the box-type filter with 1 km width to remove the influence of the local topography on the generation of vorticity. As seen in Fig. 11(A), the vorticity balances at 7 and 4 days show the weak contribution of the time derivative term of vorticity. In the vorticity balance in the steady state (at 4 days), curlτ b and the modified JEBAR terms are dominant in the upwelling region, 2 45 km off coast, where the upwelling region develops. On the other hand, there is a weak contribution of the stretching term with a negative value in the developing stage (at 7 days). The stretching term of the negative value indicates convergence in the lower layer, meaning the interface rises, i.e., upwelling. The only term against the stretching term in the upwelling region is curlτ b, namely the term of curlτ b is considered to have a forcing effect (Cushman-Roisin, 1994). At 25 km off the coast at 7 days, the stretching term is 1 5 s 1 and curlτ b is s 1. So about 2% of the contribution of the bottom Ekman transport is considered to have an effect on the interface rise. From the above results, the schematic views with a two-layer model shown in Fig. 12 can be drawn for the formation mechanism of the upwelling off the SWC. The SWC flowing thorough the Soya strait is caught by the shallow slope on the continental shelf where it is to the east of the Soya strait, and forms the southeastward strong current near the coast in the northeast coast of Hokkaido and the weak current in the region. The bottom Ekman transport is driven as the secondary circulation in these regions. The volume transport of the bottom Ekman transport near the coast is much larger than that of the region (Fig. 12➀) and so the convergence region is formed in the lower layer off the SWC. Consequently, the upwelling is formed off the SWC (Fig. 12➁). 5.2 Advection effect due to the SWC In the last section, we discuss the advection effect on the formation mechanism of the upwelling off the Formation Mechanism of the Cold-Water Belt Formed off the Soya Warm Current 469

14 nism of the CWB in detail. In addition, in this study, other factors, e.g., the contribution of the wind, have not been considered and still remains as a possibility. Fig. 12. Schematic view of construction of upwelling. Shaded area indicates convergence. SWC. In the results of the model including the non-linear term, the distribution of the interface displacement and velocity fields are basically similar to that of the linear model. However, the developing time of upwelling in a non-linear model is faster than that in a linear model. Table 1 represents the time lag and spreading speed of the upwelling. In line 3 around the Soya strait, the appearing time of the upwelling in a non-linear model is slightly later than that in a linear model, but in the lines of the downstream side, from line 3, the upwelling region in a non-linear model spreads faster than that in a linear model. Consequently, in line 7, the upwelling formed in a non-linear model appears more than twice as fast as that in a linear one. On lines 3, 5 and 7, the maximum of the interface displacement in a non-linear model is about 2 m larger than in the linear model, and the distribution of upper and lower layer velocities in the nonlinear model is almost the same as the linear model. In line 5, the maximum interface displacements in the linear model and in the non-linear model are 2 and 4 m, respectively. These results explain that the bottom Ekman transport not only contributes to the upwelling off the SWC but also to the advection effect. However, it is difficult to explain why the upwelling region is directly advected by the SWC, because the spreading speed of the upwelling between lines (Table 1) is much slower than the southeastward velocity of.1.4 ms 1. We can state that the advection effect due to the SWC strengthened the upwelling. The CWB is considered to be formed with the formation of the upwelling structure off the SWC, as indicated by the density contour. Our numerical experiment could simulate the upwelling off the SWC, as indicated by the interface displacement. In future, we shall have to simulate the CWB itself with a three-dimensional model, including parameters of temperature and salinity, to understand the formation mecha- 6. Summary and Conclusion The CWB frequently appears from the southwest coast of Sakhalin along the side of the SWC during summer and autumn and is seen in the AVHRR satellite images. We carried out ADCP, CTD and XBT observations along the cross-shore line cross the SWC and the CWB to grasp the detailed vertical structure of the CWB. In the temperature section, the cold water region associated with the CWB was captured at about 3 km off the coast. In the density section, the upheaval shape, suggesting the upwelling, was found in the onshore side before the cold water region associated with the CWB. The cold water region associated with the CWB and the upheaval shape of the density contour were located off the SWC and were close together. It is safe to consider that the CWB was formed by the upwelling and the SWC itself is related to the upwelling event in some way, judging from the temperature and density distributions and current stick diagram. So, to examine the formation mechanism of the CWB off the SWC, we tried to simulate the current system of the SWC by a two layer model including realistic topography. From the numerical experiment, the current system of the SWC was simulated qualitatively and the upwelling region was formed from the southeast coast of Sakhalin along the side of the SWC. In the balance of momentum equations, the primary physics of the SWC region was geostrophic equilibrium in the cross-shore component, but the bottom Ekman transport as a secondary circulation was formed in the alongshore component, which then clarified that the convergence in the lower layer due to the bottom Ekman transport formed the upwelling off the SWC. The advection effect also contributed to the upwelling off the SWC. However, it is difficult to explain why the upwelling region is directly advected by the SWC, because the spreading speed of the upwelling between lines (Table 1) is much slower than the southeastward velocity of.1.4 ms 1. In our observational results, the strong current region of the SWC had a speed pf more than 1 ms 1, so the advection effect may make a substantial contribution, especially in the eastern region of the Soya strait, as seen in the results of the non-linear model. Acknowledgements The authors wish to thank Dr. J. Yoshida for his discussion and encouragement during this work. Thanks are also extended to the captain and crew of the R/V Sinyomaru for their assistance in field measurements. The observation would not have been possible without the sup- 47 M. Ishizu et al.

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