Seasonal Variations of Water Properties and the Baroclinic Flow Pattern in Toyama Bay under the Influence

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1 Journal of Oceanography, Vol. 61, pp. 943 to 952, 2005 Seasonal Variations of Water Properties and the Baroclinic Flow Pattern in Toyama Bay under the Influence of the Tsushima Warm Current SATOSHI NAKADA 1 *, YUTAKA ISODA 2 and ISAMU UCHIYAMA 3 1 Safety and Environment Analysis Unit, Japan NUS Co., Ltd., Loop-X Bldg., 9-15 Kaigan 3, Minato-ku, Tokyo , Japan 2 Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido , Japan 3 Toyama Prefectural Fisheries Research, Namerikawa, Toyama , Japan (Received 17 September 2004; in revised form 22 March 2005; accepted 22 March 2005) The seasonal variations of water properties and the baroclinic flow pattern in the upper layer of Toyama Bay, where the shelf breaks in the passway of the eastward coastal branch of the Tsushima Warm Current, have been examined using temperature and salinity data from 26 local stations collected in the 32 years from 1963 through The results show that the flow pattern around the bay, as inferred from the distributions of the geopotential anomaly at 300 dbar and saline core water, changes remarkably from summer to autumn. There are two obvious inflows into Toyama Bay in a year. One is the surface inflow of less saline water from east of the Noto Peninsula as the coastal-trapped density-driven flow of the coastal branch during the transition from May to July. In September, this inflow is abruptly weakened by a transient northwestward reversal flow in the intermediate layer around 100 m depth. This reversal flow is accompanied by the temporary shallowness of the pycnoclines inside the bay. At that time, another inflow with more saline water of the year occurs in the intermediate layer. From November until January, this reversal flow disappears and a southeastward passing through-flow gradually intensifies across the bay mouth, accompanied by deepening of the pycnoclines inside the bay. According to our interannual analysis over the 32-year study period, this reversal flow has been a stable seasonal phenomenon, except for only 4 years, in which a local warm region or warm eddy developed just north of the Noto Peninsula. Keywords: Seasonal variations, Toyama Bay, coastal branch of the Tsushima Warm Current, geopotential anomaly, northwestward reversal flow. 1. Introduction The Japan Sea is a marginal body in the northwest Pacific Ocean, bordered on the west by the Asian Continent and on the east by the Japanese Islands. The general surface circulation in the Japan Sea is forced primarily by a warm, saline inflow through the Tsushima/Korea Strait from the East China Sea, while the outflow is through the Tsugaru and Soya Straits to join the Pacific. This warm current is called the Tsushima Warm Current (TWC). Although it has been supposed that the TWC is a separate branch of the Kuroshio, Isobe (1999) recently noted the seasonality of the origin of the TWC using a diagonostic model with long-term observed hydrographic * Corresponding author. nakada-s@janus.co.jp Copyright The Oceanographic Society of Japan. and wind data and found that the origin lies around the Taiwan Strait, except in autumn. After passing through the Tsushima/Korea Strait, the TWC along the Japanese shelf area is known to branch into two streams. Numerical and theoretical studies have suggested that these currents are topographically controlled (e.g., Yoon, 1982a, b, c, 1991; Kawabe, 1982b). Their locations west of Noto Peninsula are schematically shown as the first and second branches in Fig. 1(a). The nearshore (first) branch is trapped along the shelf with a strong barotropy throughout the year, as a topographically steered current. The offshore (second) branch develops in early summer, as the volume transport through the Tsushima/Korean Strait increases. This offshore branch is trapped along the continental shelf break and slope, and has a strong baroclinic structure. Recently, this flow structure for the two branches was confirmed in several sections by using 943

2 Fig. 1. (a) Map of the Japan Sea. Two shaded rectangular regions indicate the study areas A (offshore region) and B (around Toyama Bay). Solid arrows show the first, second, and third branches of the TWC schematically. (b) An enlarged map of the study area B with 26 hydrographic stations on the bathymetric chart. Numerals on the solid lines show depth in meters. ADCP direct current measurements (Katoh, 1993, 1994; Katoh et al., 1996; Hase et al., 1999). However, the continental shelf in the passageway of the two branches breaks at Toyama Bay, which is located in the central Japanese islands and has a wide mouth east of the Noto Peninsula. The major geographical features of Toyama Bay include a poor continental shelf, a steep slope, and submerged valleys with depths of more than 1000 m (Fig. 1(b)). Recently, on the basis of long-term hydrographic data analysis by Hase et al. (1999), the distribution of the flow around Toyama Bay was postulated to be as follows. The offshore branch of the TWC separates from the shelf break north of the Noto Peninsula and flows northeastward toward the Tsugaru Strait, where this branch escapes topographic control. A thermal front, which is regarded as the offshore boundary of the TWC east of the Noto Peninsula, is intensified in the offshore region deeper than 1000 m. Such a flow distribution was also suggested through the analysis of the TWC s main heat path by Nakada and Isoda (2000). To investigate the physical processes governing the formation of this offshore thermal front, Nakada et al. (2002) numerically studied the behavior of a constant barotropic flow trapped on the shelf entering Toyama Bay, which they modeled without a shelf. They regarded this flow as the coastal branch of the TWC without the distinction of two branches, because Toyama Bay has no shelf area and the path of the TWC around the bay is frequently influenced by the development of meanders or eddies. It is suggested that this flow experiences a transition regime and becomes a baroclinic flow, which forms a clockwise eddy that gradually moves north along the shelf slope with an offshore boundary front of the interface that is just permanent pycnocline. These studies showed that the main path of the coastal branch at least the offshore branch does not enter Toyama Bay but passes through the bay s offshore region. In these studies, however, the fine flow structure around the bay was not resolved or laterally averaged from the climatological hydrographic data. It is, therefore not clear yet whether the TWC ever flows into the bay or not, nor how its path variations influence the seasonal flow pattern in Toyama Bay. Our goal here is to describe in detail the characteristics of the fundamental hydrographic structure and seasonal flow pattern of the TWC in the upper layer of Toyama Bay. Since we analyzed the hydrographic data at local stations in the bay, the baroclinic flow pattern can only be estimated. To make up for the lost information of unknown barotropic flow, temporal changes of water properties provided important clues. We regarded the described flow pattern as the coastal branch of the TWC for the same reason as Nakada et al. (2002). First, we examined the seasonal variations of water properties and the baroclinic flow pattern around the bay in terms of long-term averages. The southeastward passing through-flow across the bay mouth, suggested by Hase et al. (1999) and Nakada and Isoda (2000), clearly appears from winter to spring. However, the flow pattern in Toyama Bay undergoes characteristic changes during the transition from summer to autumn. These changes consist of a temporary but remarkable flow change from the coastal-trapped, density-driven current into the bay from 944 S. Nakada et al.

3 Fig. 2. Horizontal distributions of the bi-monthly mean maps at 100 m depth; (a) water temperature in study area A, (b) water temperature in area B, and (c) salinity in area B. Contour interval of (a) is 1 C; that of (b) is 1 C; and that of (c) is Local maxima of the water temperature gradient (the southern grid minus a northern grid) are shown by closed circles east of the Noto Peninsula in summer to a northwestward reversal flow in autumn. The seasonal stability of such flow changes was examined interannually in terms of the water temperature difference due to a local warm eddy north of the Noto Peninsula. 2. Data The bottom topography and sites of hydrographic observation in Toyama Bay are shown in Fig. 1(b). In order to describe the flow pattern, in this study we defined regions as either inside or outside Toyama Bay. The Bay s border is denoted by the thick solid line in Fig. 1(b), which links Rokkozaki of the Noto Peninsula tip and the boundary between Toyama and Niigata prefectures. Water temperature and salinity data at 10 standard depths (0, 10, 20, 30, 50, 75, 100, 150, 200, and 300 m) from 1963 through 1994 were used in this work. The data were observed monthly at the 26 fixed stations distributed all over Toyama Bay and recorded as the original data after the removal of error values by Toyama Prefectural Fisheries Research Institute. The data from each station were averaged every two months as bi-monthly mean data. Seasonal horizontal distributions of water properties and flow patterns were then computed without spatial smoothing. Temperature data from 1963 through 1994 obtained by the Japan Oceanographic Data Center (JODC) were used to examine seasonal and interannual variations in the offshore area of Toyama Bay, i.e., the interior region of the TWC. The study area was the eastern part of the TWC region ( N, E), shown by the thick outline in Fig. 1(a) (hatched region A). Using the same quality control method as that given in Nakada and Isoda (2000), any data that exceeded twice the standard deviation from the mean after negative values were removed were eliminated in 2 squares each month. The monthly data were interpolated into grid points with 0.5 intervals in both zonal and meridional directions using a Gaussian filter with an e-folding scale of 25 km. The bimonthly mean values were obtained by averaging the interpolated monthly data from 32 years. 3. Seasonal Variations of Temperature and Salinity at a Depth of 100 m In the past studies (e.g., Tanioka, 1968; Ohwada and Tanioka, 1971; Moriyasu, 1972; Hase et al., 1999), water Seasonal Variations of Water and Flow Pattern in Toyama Bay 945

4 temperature data at the depth of 100 m have generally been analyzed to discover interior TWC flow path variations, because the surface currents found by GEK or ADCP correlate well with the thermal front at the intermediate depth of 100 m. Nakada and Isoda (2000) investigated seasonal changes in water temperature in the northeastern TWC region, including the Toyama Bay area. They showed that the phase of seasonal variation is delayed as depth increases; e.g., the maximum temperature at 10, 20, and 30 m depths appears in September, at 50, 75, 100, 125, and 150 m depths in November, and at 200, 250, and 300 m in May. The values in the surface layer are attributable to summer heating above the seasonal thermocline, while those at m depths can probably be attributed to seasonal change of the permanent thermocline. So, to examine horizontal behaviors in the TWC core water, i.e., water with a salinity higher than 34.5 (e.g., Kawabe, 1982a; Hase et al., 1999) and the thermal front, we first analyzed temperature and salinity data in the intermediate layer of 100 m depth. Figure 2 shows the bi-monthly horizontal distributions at 100 m depth for (a) temperature in the bay s offshore region, and (b) temperature and (c) salinity around Toyama Bay. Although water temperature in the offshore region changes seasonally, a horizontal gradient of spatially smoothed isothermal lines is always seen (Fig. 2(a)). We can see two local maxima of water temperature gradients (the southern grid minus the northern grid) denoted by the closed circles from May through November. The northern local maximum around 39.5 N is probably the polar front. The southern local maximum is the offshore thermal front along the Japanese coast. As mentioned in the Introduction, this thermal front can be seen far off the continental shelf. It seems that the horizontal gradient of the thermal front weakens from November to March and strengthens from May to September. The water temperature outside Toyama Bay in Figs. 2(a) and (b) is highest in November (>15 C) and lowest in March (<9 C). It is also found that a local thermal front exists temporarily between the warmer waters inside and colder ones outside Toyama Bay (Fig. 2(b)). A weak, broad thermal front is formed around the bay mouth in January and March, when water inside the bay is warmer and has relatively less salinity than that outside. The warmer and more saline water (>10 C and >34.2) appears along the west coast of the bay in May. The area of this water increases, exceeding 13 C and 34.3 in most of the region in July. As a result, the thermal front strengthens because of the increase in water temperature inside the bay. However, this front disappears in September, when water with the maximum salinity (>34.4) occupies the whole bay. In November, this saline water seems to be divided into two regions, one inside and the other outside the bay. The water temperature then reaches its maximum, about 16 C, and a weak thermal front around the bay mouth appears again. Thus, we can find seasonal changes in the location and shape of the local thermal front and in the distribution of saline water around Toyama Bay. 4. Seasonal Relationship between Baroclinic Flow Pattern and Saline Core Water To estimate the seasonal variations of baroclinic flow patterns in the surface and the intermediate layers, we calculated the spatial distributions of the bi-monthly mean geopotential anomaly at depths of 10 m and 100 m with reference to 300 dbar as shown in Fig. 3(a), except for two stations (5 and 19) where the depth is less than 300 m. Figure 3(b) shows the vertical cross-sections of bimonthly mean density and salinity along the central bay axis in the northeast-southwest direction, obtained from observation stations 2, 8, 15, 22, 33, 32, 31 and 30, to denote the seasonal relationship between the density structure accompanying the passing through-flow across the bay mouth and the location of the TWC core water. A comparison of the estimated flow patterns in either layer (Fig. 3(a)) shows similar flow patterns, except in September. Moreover, the main flow axis in the intermediate layer is found to correspond roughly to the location of the local thermal front observed at 100 m depth (Fig. 2(b)), except in September and November. In January, the flow axis intersecting the tip of Noto Peninsula suggests that the current flows southeastward across the bay mouth, while a weak vortex pair seems to exist inside the bay. At that time, the pycnoclines at m depth, which includes highly saline waters (>34.3), decline toward the bay head. This southeastward current gradually intensifies in March, accompanied by sharp declines in the pycnoclines at the bay mouth. These features support the view that the TWC does not enter the bay from January through March. Judging from the flow axis, the path of this current may correspond to the passing through-flow across the bay mouth. It is also found that the maximum salinity ( ) near the pycnoclines is lower in March than in January. Since the period from January to March is the winter cooling season (e.g., Hirose et al., 1996), this decrease in salinity may be understood to result from the vertical mixing of isolated water inside the bay. In May, highly saline water begins to appear in the intermediate layer of the bay mouth, and in July extremely high salinity core water (>34.5) occupies the area outside the bay. The location of these saline core waters corresponds closely to the baroclinic flow axis around the bay mouth. In the period from May through November, the current begins to flow from east of the Noto Peninsula into the bay, especially in the surface layer (Fig. 3(a)), and low-density, low-salinity water appears in the surface layer to a depth of about 30 m (Fig. 3(b)). It is in- 946 S. Nakada et al.

5 Fig. 3. (a) Horizontal distributions of the bi-monthly mean geopotential anomaly at the depths of 10 m and 100 m with reference to 300 dbar. Anomaly contour interval is 0.1 m 2 /s 2. (b) Vertical distributions of potential density along the bay axis in the northeast-southwest direction. Its axis consists of the observed stations of 2, 8, 15, 22, 33, 32, 31 and 30, as shown in the lower panel of the geopotential anomaly in November. Density contour interval is 0.2σ θ. Higher-salinity waters, denoted by darker tones with 0.1 intervals, are also superimposed on each density distribution. ferred that this inflow has the characteristics of a coastaltrapped, density-driven current for a rotating fluid, because the thickness of the surface low-salinity water near the coast becomes larger than that in the previous month. Such vertical distribution of surface low-salinity water resembles that of the nearshore branch off the San-in coast described by Kawabe (1982a), who also analyzed summer data (from March to August), although he only described a high salinity water core in the intermediate layer as evidence of a shelf-trapped current. However, the duration of this inflow is short, and it disappears from September to November. In Toyama Bay, the most drastic change in flow pattern in the intermediate layer occurs from July through September. In those three months, the current direction in the intermediate layer of the bay mouth is reversed from southeastward to northwestward (Fig. 3(a)). This northwestward reversal flow in September is clearly attributable to the shallowness of the pycnoclines inside the bay (see the 27.0 σ θ line in Fig. 3(b), σ θ is the poten- Seasonal Variations of Water and Flow Pattern in Toyama Bay 947

6 Fig. 5. Time series of potential density differences between the waters inside and outside Toyama Bay, averaged over three months (August, September, and October) each year at the 200 m depth. Thick solid line denotes the long-term mean value of 0.1σ θ. Maximum and minimum values beyond the mean (0.1σ θ ) ± standard deviation (0.11σ θ ) are shown by closed or open circles, respectively. Fig. 4. Time-depth diagrams of (a) monthly mean potential density inside Toyama Bay, (b) that outside the bay, and (c) their density difference. Contour interval of (a) and (b) is 0.2σ θ while that of (c) is 0.05σ θ. tial density (kg/m 3 )). At that time, a weak inflow through the center of the bay is also formed in both layers, and saline waters in the intermediate layer (>34.5) then spread from outside to inside the bay. This reversal flow is also temporary, and in November the saline water is instantly divided into two cores, i.e., one inside and the other outside the bay. Again, there may be no inflow of the TWC in early winter, from November to January. Thus, there is obvious inflow into Toyama Bay twice in a year. One instance is the surface inflow of less saline water during the transition from May to July. Most likely, this current begins to flow from east of the Noto Peninsula as the coastal-trapped, density-driven flow of the coastal branch in May, but it disappears four to six months later; that is, September is the end of this inflow period. The other inflow, of more saline water in the intermediate layer, occurs in September. One salient feature of the September flow pattern in the intermediate layers of m is the declination of isopycnal lines in the offshore direction, which is related to the northwestward reversal flow at the bay mouth. At that time, it seems that the most saline water of the year effectively spreads into the bay. Such a seasonal change in baroclinic flow pattern can well explain the temporal changes in water properties, i.e., surface low-salinity and intermediate high-salinity waters. It is thus inferred that barotropic flow controlled by bottom topography may make a small contribution to the transport of TWC waters if it exists in the bay. In the next section, to detect the seasonal variation in the declination that causes the flow to reverse in September, we examine density differences between the waters inside and outside the bay. 5. Density Difference inside and outside Toyama Bay Figures 4(a) and (b) show time-depth diagrams of monthly mean density averaged inside the bay (a: Stations 1, 2, 6, 12, 13, 8, 15, 20, 22, and 10) and outside it (b: Stations 27, 28, 29, 30, 31, and 32). Seasonal density variations inside the bay are similar to those outside (Figs. 4(a) and (b)). The seasonal density stratification begins from May, due to sea surface heating, and strengthens through September. From then through November, isopycnal lines ascend toward the surface due to winter cooling, and simultaneously those in the intermediate layers of m depths also ascend. Such shallowness of the pycnoclines in the intermediate layers is more dominant inside the bay than outside. On the other hand, the development of a mixed surface layer continues until March. 948 S. Nakada et al.

7 Fig. 6. Horizontal distributions of geopotential anomaly compositions (m 2 /s 2 ) in August, September, and October at the 100 m depth around Toyama Bay (area B) during (a) positive index and (b) negative index years. Contour contour interval is 0.1 m 2 /s 2. Figure 4(c) shows the density difference at each depth between the inside and the outside waters. A negative value indicates that the density inside the bay is lower than that outside, i.e., a pressure gradient of density toward the offshore direction (after negative pressure gradient) is expected when the southeastward baroclinic flow exists around the bay mouth, and vice versa. The region denoted by negative values continues from February through April at all depths which means that a weak southeastward current passes through the bay mouth. As described in the previous section, the transient change in flow pattern occurs from July through September. In this period, extremely negative values appear in the surface layer of m, but after August positive values begin to appear below the 150 m depth. These negative and positive values are represented, respectively, by the appearance of low surface salinity and the shallowness of pycnoclines inside the bay. It is inferred that the negative pressure gradient in the surface layer generates the density current flow through the bay, and that this current is quickly weakened by the reversal flow in the intermediate layer. Although the fundamental features of the water properties and flow pattern in Toyama Bay depend on the southeastward coastal branch due to the weak thermal or density front across the bay mouth, the transient appearance of a reversal flow in autumn seems to determine seasonal characteristics. Through the collection of 32 years of data, we attempted to confirm the seasonal stability of this reversal flow. As Fig. 4(c) shows, we found that the positive maximum of the density difference causing the reversal flow occurs at 200 m depth in September; hence, we used that value as the interannual index of reversal flow. 6. Interannual Variation of the Reversal Flow in Autumn Figure 5 shows a time series of density differences averaged for three months (August, September, and October) at the depth of 200 m during the 32 years from 1963 through The thick solid line denotes the longterm mean value, about 0.1σ θ. This value is positive, but there are 4 years with negative values. We therefore selected two year-groups using critical values of the mean (0.1σ θ ) ± standard deviation (0.11σ θ ); i.e., the maximum was 0.21σ θ and the minimum was 0.01σ θ. There are local maximum positive values, denoted by the closed circles in 1965, 1966, 1974, 1978, 1986, 1988, and 1991, and negative minimum values, denoted by the open circles in 1969, 1979, 1984, and Next, we compared the composite flow patterns of the monthly geopotential anomaly at 100 m depth from August to October between the two groups. The enhanced reversal flow patterns of the group organized by years with a large positive index are represented in Fig. 6(a). In the negative index years (Fig. 6(b)), a very weak reversal flow appears only in October, and the southeastward flow in August and September exists far northeast of the Noto Peninsula. Such flows around the bay mouth cannot connect horizontally with the shelf- and coastal-trapped branches west of the Noto Peninsula. It is known that the TWC in the interior region, i.e., Seasonal Variations of Water and Flow Pattern in Toyama Bay 949

8 ure 7(c) shows the difference in water temperature between the groups at each grid, i.e., the temperatures in the years with local maximum positive indexes minus those in the years with negative indexes. In the offshore area north of the Noto Peninsula, a cold region ordinarily develops as one of the meandering valleys of the TWC, as shown in Fig. 7(a) (positive index years), while it is weakened in Fig. 7(b) (negative index years). The difference map suggests that a cold region migrates to the north (positive difference north of 39 N) and a local warm region forms north of the Noto Peninsula (negative difference north of 38 N) in the negative index years. We examined the monthly temperature maps for the corresponding years (not shown), and confirmed that warm eddies were frequently observed there in the negative-index years, although the positions and shapes of eddies and meanders vary considerably over time. The path of reversal flow might surely be influenced by the development of warm eddies just north of the Noto Peninsula, but their occurrence was very sparse, i.e., roughly 10% (4y./32y.) 100%. Therefore, we infer that the seasonal characteristics of the reversal flow pattern in autumn are stable. Fig. 7. Horizontal distributions of water temperature compositions averaged over August, September, and October at the 200 m depth in the offshore region (area A) during (a) positive index and (b) negative index years. (c) Horizontal distributions of water temperature difference between positive index and negative index years. Contour interval of (a) and (b) is 0.5 C while that of (c) is 0.1 C. the offshore region deeper than 1000 m, is characterized by strong variabilities connected with numerous meanders or eddies (Toba et al., 1984; Ichiye and Takano, 1988; Isoda and Saitoh, 1993; An et al., 1994; Isoda, 1994; Lie et al., 1995; Shin et al., 1995). Hase et al. (1999) pointed out that the offshore branch could not always be found around the shelf break because its path was frequently influenced by the development of meanders or eddies. The difference in reversal flow patterns between the two year-groups suggests that the main path of the coastal branch may indeed be influenced by meanders or eddies. We examined this possibility by analyzing offshore temperature data at 200 m depth, where distributions represent the warm and cold regions better than those at 100 m depth. Figures 7(a) and (b) show the composite horizontal distributions of the water temperature at 200 m depth, classified into the same two year-groups as in Fig. 6. Fig- 7. Summary and Discussion The oceanographic phenomena and conditions in Toyama Bay might be significantly influenced by seasonal and interannual variations of the coastal branch of the TWC. It is reasonable to suppose that the TWC influences not only water properties but also the distribution and strength of the flow in Toyama Bay because of the geographical feature of its lacking a shelf. In this study, seasonal variations in the water and flow pattern in Toyama Bay were discovered by analyzing temperature and salinity data from the period 1963 through The inflow of the coastal branch into Toyama Bay occurs at least from May through July, during which time it can be detected from the distributions of geopotential anomaly and less-saline surface water. Most likely, this current begins to flow from east of the Noto Peninsula as the coastal-trapped, density-driven flow of the branch in May. However, it is quickly weakened by the northwestward reversal flow in the intermediate layer, and disappears in November. When the reversal flow forms, the intermediate saline water outside the bay, i.e., the TWC core water, effectively spreads into the bay. In other months, from winter to spring, the southeastward flow occurs according to a weak density front formed near the bay mouth. This reversal flow, which was found to appear in autumn, is a stable phenomenon according to our interannual analysis. We therefore believe that the reversal flow is the most remarkable feature in the seasonal variation around Toyama Bay. Although the transient appearance of reversal flow is accompanied by temporary shal- 950 S. Nakada et al.

9 lowness of pycnoclines inside the bay, its exact cause could not be clarified in this study. Based on the results of past numerical studies, however, we may propose two candidates for the mechanism underlying the seasonal shallowness of pycnoclines and the formation of reversal flow. One is a local reversal flow north of Noto Peninsula formed near the coast after a clockwise eddy, generated north of the Noto Peninsula in the spring, gradually moving north along the shelf slope with an offshore boundary front, as suggested by the two-layer model of Nakada et al. (2002). This phenomenon can also explain the appearance of the observed positive heat content off Toyama Bay and its northward movement from summer to winter. The other possible mechanism is an intermediate reversal flow along the whole Japanese shelf area, which is the baroclinic reversal flow under the TWC over the shelf or shelf-break region, as suggested by the multilayer models of Kim and Yoon (1999) and Seung and Yoon (1995). In these models, the reversal flow starts in spring and develops fully in summer. In fact, Hase et al. (1999) observed this flow as the subsurface countercurrent of the TWC. The above mechanisms show that the reversal flow over the shelf can easily exist due to the strong baroclinicity of the TWC. As to understanding the seasonal appearance of the reversal flow across the bay, we may state that the absence of a shelf at Toyama Bay is a key issue. In the future we shall re-examine the seasonal behavior of pycnoclines within the bay and compare it carefully with that of other areas that have a shelf. Acknowledgements The authors express their sincere thanks to the journal reviewers for useful suggestions on improving the manuscript. The supply of data by JODC (Data Online Service System, J-DOSS) and Toyama Prefectural Fisheries Research Institute is gratefully acknowledged. This work was partially supported by the Science and Technology Agency and by the Ministry of Education of the Japanese Government (No ). Thanks are also extended to Dr. Isobe of Kyushu University for the smooth editorial office work. References An, H., K. Shim and H.-R. Shin (1994): On the warm eddies in the southwestern part of the East Sea (the Japan Sea). J. Korean Soc. Oceanogr., 29, No. 2, Hase, H., J.-H. Yoon and W. Koterayama (1999): The current structure of the Tsushima Warm Current along the Japanese coast. J. Oceanogr., 55, Hirose, N., C.-H. Kim and J.-H. Yoon (1996): Heat budget in the Japan Sea. J. Oceanogr., 52, Ichiye, T. and K. Takano (1988): Mesoscale eddies in the Japan Sea. 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