Prediction of Slope Water Intrusion into the Kii Channel in Summer

Transcription

1 Journal of Oceanography, Vol. 62, pp. 105 to 113, 2006 Prediction of Slope Water Intrusion into the Kii Channel in Summer TOSHINORI TAKASHI 1,2 *, TATEKI FUJIWARA 1, TOSHIAKI SUMITOMO 3 and WATARU SAKAMOTO 2 1 Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Kyoto , Japan 2 Fisheries Laboratory of Kinki University, Wakayama , Japan 3 Fisheries Division, Tokushima Prefectural Government, Tokushima , Japan (Received 8 March 2005; in revised form 22 October 2005; accepted 22 October 2005) Intrusions of the warm, oligotrophic surface slope water (SSW) and the cold, nutrient-rich bottom slope water (BSW) from the continental slope influence the annual variations in water temperature and nutrient concentrations in the Kii Channel in August. In order to evaluate the relationships between both these intrusions and the distance of the Kuroshio axis from Cape Shionomisaki (Kuroshio distance), a Distance-Intrusion-Diagram (DID) for temperature, which can reproduce the vertical temperature profile of the channel, was constructed by analyzing the temperature and Kuroshio distance records in August for DIDs for nutrients (nitrate and phosphate) are also constructed by using the relationship between the nutrient concentration and water temperature. The only explanatory variable in the DIDs is the Kuroshio distance. The DID for temperature predicts that the SSW occupies almost the entire water column when the Kuroshio approaches Cape Shionomisaki (Kuroshio distance = 18.5 km). When the Kuroshio distance lies in the range km, the BSW thickness increases proportionally to the Kuroshio distance increment while the SSW thickness decreases. The BSW occupies the largest portion of the channel when the Kuroshio distance is 74 km. Further, beyond 74 km, the BSW thickness reduces gradually. Yearly variations in the temperature and concentrations of nitrate and phosphate were hindcast with the DIDs. The results revealed that the Kuroshio distance contributes 70%, 35%, and 30% of the variances in temperature, nitrate concentration, and phosphate concentration, respectively. Keywords: Slope water intrusion, nutrient, temperature, Kii Channel, Kuroshio, annual variation. 1. Introduction Oceanic water intrusion has been observed in continental shelf seas. The intrusion affects the physical, chemical, and biological processes in the shelf region. In the South Atlantic Bight, the intrusion of the cold, nutrient-rich Gulf Stream waters accompanies the passage of the Gulf Stream frontal eddy (Lee et al., 1981). During this passage, a large amount of nitrate is transported onto the shelf (Lee et al., 1981), stimulating biological production (Yoder et al., 1981). Two types of slope water intrusions occur in Georges Bank (Churchill et al., 2003): the warm, high salinity surface slope water intrudes into the near-surface layer, while the cold, high salinity subsurface slope water intrudes into the bottom layer. * Corresponding author. takashi@coral.cypress.ne.jp Copyright The Oceanographic Society of Japan. The Kii Channel is located in the western part of Japan (Fig. 1(a)). It has mean depth 56 m, surface area 1554 km 2, and volume 870 km 3. This channel connects with the Seto Inland Sea through the Kitan Strait and the Naruto Strait in the north of the channel, and its southern part faces the Pacific Ocean. Fresh water flows into Osaka Bay and the north of the channel and flows out to the Pacific Ocean through the Kii Channel, while oceanic water flows from the Pacific Ocean into the Kii Channel. Therefore, both fresh water and oceanic water influence the physical, chemical, and biological conditions in the Kii Channel. Two distinct types of oceanic waters intrude into the Bungo Channel, which is another entrance of the Seto Inland Sea (Fig. 1(a)) (Takeoka et al., 1993; Kaneda et al., 2002). Kaneda et al. (2002) suggested that cold, nutrient-rich water intrudes when the Kuroshio flows near Kyushu. Warm water intrusion tends to occur during the neap tide in the Bungo Channel (Takeoka et al., 1993). 105

2 Fig. 1. (a) Map of the western part of Japan showing the Kuroshio Current. Star and open circles indicate the position of Cape Shionomisaki and those of the KMO observation points, respectively. (b) Map of the Kii Channel and Osaka Bay. The FRIT observation locations are denoted by closed circles and triangles. Similar phenomena have been observed in the Kii Channel (Takeuchi et al., 1997). The Kuroshio Current flows eastward off the Kii Channel. It is well known that the path of the Kuroshio varies widely with time and space. Takeuchi et al. (1997) revealed that oceanic waters intruding into the Kii Channel are divided into two types, one being the cold, nutrient-rich bottom slope water (BSW), while the other is the warm, oligotrophic surface slope water (SSW), which originates from the Kuroshio surface water. They suggested that slope water intrusions can be classified into two modes in relation to the Kuroshio path: the cold, nutrient-rich BSW that intrudes into the Kii Channel when the distance of the Kuroshio axis from Cape Shionomisaki (hereafter referred to as Kuroshio distance) is greater than 55 km (=30 n. miles), the BSW intrusion rarely occurs and the warm, and oligotrophic SSW that frequently flows into the Kii Channel when the Kuroshio distance is less than 37 km (=20 n. miles). Fujiwara et al. (1997) measured the nutrient flux at the southern end of the Kii Channel in August 1995 and demonstrated that a large amount of nutrient flows into the Kii Channel from the outer ocean in relation to the BSW intrusion. The nitrogen and phosphorus fluxes were 206 ton d 1 and 34 ton d 1, respectively, which is comparable to the loadings to Osaka Bay from the land. Kasai et al. (2001) estimated the nutrient flow using a numerical model and presented the annual nutrient flux variations in summer. In their estimation, the nutrient flux was large when the BSW intrusion occurred (the Kuroshio was separated from the Kii Channel), while it was small when the BSW intrusion was weak (the Kuroshio flowed near the channel). The above studies indicate that the SSW and BSW intrude into the Kii Channel in relation to the Kuroshio path variance. Furthermore, the intrusions affect the nutrient flux in the channel. However, these studies raise two questions. First, what are the reasons for the classification of the slope water intrusions into two modes in relation to the Kuroshio distance? Second, how much do the SSW and BSW water intrusions affect annual variations in nutrient concentrations in the Kii Channel? In this study we examine the relationship between the Kuroshio distance and the intrusions of slope waters, namely, the SSW and BSW, by analyzing a 35-year water temperature record measured in the month of August. The relationship is modeled as a Distance-Intrusion-Diagram (DID) for temperature, which can reproduce a vertical temperature profile estimated from the Kuroshio distance. We have applied the DID to the historical data of the Kuroshio distance and hindcast past temperature in order to evaluate the accuracy of the DID. DIDs for nutrients are also constructed. These DIDs are used to investigate the relationship between the Kuroshio distance and annual variation in the nutrient concentrations. 2. Data The Kobe Marine Observatory (KMO) conducted hydrographic observations at 18 stations from Osaka Bay to the Pacific Ocean in July 1989 and 1990 on board the R/V Shumpu Maru (Fig. 1(a)). Water temperature, salinity, nitrate and phosphate data from the observations were used to confirm the oceanic water intrusions into the Kii Channel. The Fisheries Research Institute, Tokushima Agriculture, Forestry and Fisheries Technology Center (FRIT) 106 T. Takashi et al.

3 has been making monthly hydrographic observations in the Kii Channel at the beginning of every month since 1967 on board the R/V Tokushima. The temperature and salinity were observed with a reversing thermometer and a salinometer until 1989, respectively, and thereafter with CTD. We used water temperature and salinity data from depths of 0, 5, 10, 20, 30, and 50 m because data from other depths were too sparse for analysis. In addition, the nutrient concentration (nitrate, nitrite, ammonium, and phosphate) have been measured four times a year, in February, May, August, and November, at depths of 0 and 50 m at 15 stations since The water temperature and nitrate and phosphorus concentration values obtained from four stations along the central longitudinal line (denoted by triangles in Fig. 1(b)) in August were horizontally averaged at each depth and used for analysis. Furthermore, FRIT conducted similar monthly observations at seven stations along the longitudinal line (denoted by closed circles in Fig. 1(b)) in the Kii Channel from 1999 to The water temperature and salinity were measured with CTD. Nutrient (nitrate, nitrite, ammonium, and phosphate) concentrations were measured at depth intervals of 10 m. The data obtained in July and August from 1999 to 2001 were also used to construct the DIDs for nutrient. In this study, the Kuroshio distance is defined as the southward distance from Cape Shionomisaki to the Kuroshio axis. Kuroshio distance data for the end of July since 1967 were obtained from Quick Bulletin of Ocean Conditions published by the Japan Coast Guard. 3. Intrusion of Slope Water 3.1 Kuroshio distance and intrusion Vertical sections of water temperature, salinity, nitrate, and phosphate from Osaka Bay to the outer ocean in July 1990 and July 1989 are shown in Figs. 2 and 3, respectively. The Kuroshio axis is known to coincide with the isothermals of C at a depth of 200 m near Cape Shionomisaki (Kawai, 1969). The Kuroshio distances in July 1990 and 1989 were around 85 km and 35 km, respectively. The surface water off the Kuroshio area (depth < 100 m) was stratified in temperature and salinity (Figs. 2 and 3). Below this depth, the salinity exceeded 34.8, which is identified as the North Pacific subtropical mode water (Masuzawa, 1969). The water exhibited a continuous temperature and salinity stratification between the Kii Channel and the Kuroshio. The cold water (T < 20 C) between the Kii Channel and Kuroshio axis was distributed in a shallower layer than that off the Kuroshio. The concentrations of nutrients off the Kii Channel increased with the decrease in water temperature. The waters in Osaka bay and the Kii Channel were stratified in both salinity and temperature. The lowest salinity water existed in the µ µ Fig. 2. Longitudinal distributions of temperature, salinity, nitrate, and phosphate from Osaka Bay to the open ocean in July Downward arrow on the top panel indicates the position of the Kuroshio axis. OB: Osaka Bay and KC: Kii Channel. surface layer of Osaka Bay. This less-saline water spreads to the area off the Kii Channel in the surface layer. Comparison of the two periods revealed dissimilar distributions of temperature and nutrient concentrations in the Kii Channel (Figs. 2 and 3). In July 1990, when the Kuroshio separated from the channel, the cold (T < 20 C) water intruded into the Kii Channel and Osaka Bay from off the channel (Fig. 2). The cold water included a high nutrient concentration, the maximum nutrient concentrations being 8.5 µm for nitrate and 0.62 µm for phosphate in the Kii Channel. In contrast, the warm water (T > 20 C) occupied the channel and the nutrient level was relatively low in 1989 the period when the Kuroshio flowed near the Kii Channel (Fig. 3). This difference is related to the difference in the Kuroshio distance (Takeuchi et al., 1997). Although the oceanic water which intrudes into Slope Water Intrusion into the Kii Channel in Summer 107

4 µ µ µ Fig. 4. Relationship between the temperature and nutrient (top: nitrate, bottom: phosphate) concentrations at the shelf edge (stns. G0 and G1) in July 1989 and July Circles and crosses indicate values for 1989 and 1990, respectively. µ Fig. 3. As Fig. 2, but showing the distributions in July the channel was different for the two periods, the relationships between the temperature and nutrient concentrations at the shelf edge (stn. G0 and G1 in Fig. 1) are similar (Fig. 4). The nutrient concentrations decreased linearly with the increase in temperature in a range of less than 22 C. The correlations indicate that the nutrients are conservative nature within this range and that the colder water has high nutrient concentration. Thus, the cold water and the warm water, which distribute off the channel, intrude into the Kii Channel. Fig. 5. (a) Annual variation in the temperature in the Kii Channel in August. (b) Annual variation in the Kuroshio distance. Shaded areas indicate the BSW. 3.2 Annual variation in the SSW and BSW intrusions The differences in the Kuroshio distance induced changes in the oceanic water intrusions (Figs. 2 and 3). The vertical water temperature distribution indicates whether the SSW or BSW intruded into the Kii Channel in relation to the Kuroshio distance. Therefore, to investigate the relationship between the intrusions and the Kuroshio distance, we analyzed the historical temperature record and the Kuroshio distance. Hereafter, we refer to the cold (T < 22 C) water as the BSW and the warm (T > 22 C) water as SSW. The annual variations in the horizontally averaged temperature at four stations (denoted by triangles in Fig. 1) in the Kii Channel in August and those in the Kuroshio distance at the end of July are shown in Fig. 5. The temperature shows annual fluctuations (Fig. 5(a)). The warm SSW occupied the entire water column continuously from 1970 to 1974 and from 1996 to 1999 when the Kuroshio 108 T. Takashi et al.

5 Fig. 6. Relationships between the temperature and the Kuroshio distance at 0, 5, 10, 20, 30, and 50 m depths in August. Closed circles and bars represent the average values and standard deviations from 1977 to 2001, respectively. Regression formulas of A and B in each figure were calculated for the ranges of 18.5 km to 74 km and 74 km to km, respectively. approached the Kii Channel (Fig. 5(b)). In contrast, the cold BSW intruded into the Kii Channel from 1975 to 1995 when the Kuroshio distance was large. The temperature fluctuations were in accordance with the variance in the Kuroshio distance. 4. Distance-Intrusion-Diagram (DID) 4.1 Construction of DID for temperature and nutrients levels The water temperature data for August acquired by FRIT at the four stations (denoted by triangles in Fig. 1(b)) were horizontally averaged at six depths (0, 5, 10, 20, 30, and 50 m). Using these values, a DID was constructed for the temperature, which predicts the vertical profile of the temperature in the Kii Channel from the Kuroshio distance. The water temperature in the Kii Channel at a given depth is divided into 13 bins, depending on the Kuroshio distance, in intervals of 9.5 km. For example, the average temperature and its standard deviation in each bin at a depth of 0 m are plotted against the Kuroshio distance in Fig. 6(a). The temperature decreases to a minimum with the increase in the Kuroshio distance up to 74 km; thereafter, it rises gradually with a further increase in distance. Since the relationship can be segregated into two ranges of the Kuroshio distance A (from 18.5 km to 74 km) and B (from 74 km to km) two approximation formulas relating the temperature to the Kuroshio distance can be estimated (Fig. 6(a)). The Kuroshio distance for the lowest temperature is 74 km. The resultant approximation lines can be used to predict the water temperature at a depth of 0 m, from the Kuroshio distance. The same procedure was applied to the remaining data sets of the Kuroshio distance and water temperatures at depths of 5, 10, 20, 30, and 50 m (Figs. 6(b) (f)). The Kuroshio distance at which the minimum temperature Slope Water Intrusion into the Kii Channel in Summer 109

6 Fig. 7. Distance-Intrusion-Diagram of (a) temperature ( C), (b) nitrate (µm), and (c) phosphate (µm) in August. Shaded area indicates BSW. Fig. 8. Relationship between temperature and nutrient concentrations ((a): nitrate and (b): phosphate) in the Kii Channel in July and August from 1999 to Straight lines represent regression lines. Linear regression formulas of A and B in each figure were calculated for the ranges greater than 25 C and lower than 25 C, respectively. occurs was the same for all the data sets (74 km). The procedure for constructing the DID for water temperature is as follows: using the approximation formulas in Figs. 6(a) (f), the water temperatures at six depths were calculated from the Kuroshio distance. These calculations were carried out over a Kuroshio distance range of 18.5 km to km. The DID was constructed using the results of these calculations by vertical linear interpolation (Fig. 7(a)). This DID can be used to predict the vertical profile of the temperature in the Kii Channel at an arbitrary Kuroshio distance. The DID also indicates the thickness of the intruded SSW and BSW. The SSW occupies the Kii Channel from the top to the bottom in a Kuroshio distance range of 18.5 km to 37 km. The thickness of the SSW decreases and that of the BSW increases according to the Kuroshio distance increment up to 74 km. The thickness of the BSW is at a maximum at 74 km. The BSW thickness gradually decreases as the Kuroshio distance exceeds 74 km. The DIDs for nitrate and phosphate were constructed using the relationships between water temperature and nutrient concentrations; these relationships are shown in Fig. 8. The water temperature shows a high correlation with the nutrient concentrations in July and August in the Kii Channel (Fig. 8). The nitrate and phosphate concentrations decrease linearly with increasing temperature. This indicates the conservative nature of the nutrients. However, the nitrate concentration is very low and nearly below the detectable limit in the range of temperatures higher than 25 C. This suggests nitrate uptake by phytoplankton and its depletion in the upper, warmer layer. The DID for temperature was converted into DIDs for nitrate and phosphate by using approximation lines, which are shown in Fig. 8(a). The obtained DIDs for nitrate and phosphate are shown in Figs. 7(b) and (c). 4.2 Hindcast of the historical records of temperature and nutrient concentrations Historical records of the Kuroshio distance (Fig. 5(b)) were substituted in the DID for temperature, resulting in a hindcast water temperature of the Kii Channel for August from 1967 to The observed and calculated water temperatures at a depth of 50 m are shown in Fig. 9(a). The annual variation in the calculated water temperature coincides with that in the observed temperature. Furthermore, the calculated vertical profiles of the temperature also correlate well with the observed one (Figs. 5(a) and 10). The SSW over the entire water column from 1970 to 1974 and from 1996 to 1999, when the Kuroshio approached Cape Shionomisaki (Kuroshio distance = km), is reproduced well by the DID. The frequent intrusions of the BSW from 1975 to 1995 are 110 T. Takashi et al.

7 Fig. 11. Comparison between observed and calculated temperatures at depths of 0 m, 5 m, 10 m, 20 m, 30 m, and 50 m in Fig. 9. Annual variations in the calculated (circles and thin lines) and observed (thick lines) (a) temperature, (b) nitrate concentration, and (c) phosphate concentration in the Kii Channel at 50 m in August. Fig. 10. Annual variations in the calculated temperatures in the Kii Channel in August. Shaded areas indicate the BSW. also reproduced. In particular, the strong intrusions of the BSW in the 1980s were well expressed when the Kuroshio distance was around km, which is favorable for BSW intrusions. A scattering diagram of the calculated temperatures vs. the observed temperatures is shown in Fig. 11. The Kuroshio distance can explain 70% of the observed temperature variation. The nutrient variations at 50 m depth are also calculated using the DIDs for nitrate and phosphate (Figs. 9(b) and (c)). The variations in the calculated and observed nutrients are well correlated. In this case, too, the Kuroshio distance can explain 35% and 30% of the observed nitrate and phosphate variations, respectively. 5. Discussion and Conclusions We have elucidated the relationship between the BSW and SSW intrusions into the Kii Channel and the Kuroshio distance in August. The thickness of the intruding BSW (T < 22 C) and SSW (T > 22 C) depends on the Kuroshio distance (Fig. 7(a)). The SSW occupies the maximum portion of the water column when the Kuroshio approaches Cape Shionomisaki (Kuroshio distance = 18.5 km). The thickness of the SSW decreases and that of the BSW increases as the Kuroshio axis departs from Cape Shionomisaki up to a distance of 74 km. The thickness of the BSW is at a maximum when the Kuroshio distance is 74 km. The BSW gradually thins as the Kuroshio distance exceeds 74 km. Takeuchi et al. (1997) investigated the relationship between the Kuroshio distance and temperature at a depth of 50 m in the center of the Kii Channel. In their analysis, the temperature was at a minimum at a distance of 56 km. This discrepancy from our results seems to result from the fact that they used the temperatures recorded during all months, including the mixing season. During the mixing season, the shelf front is generated in a transition zone between the cold, fresh coastal water and the warm, saline oceanic water (Yoshioka, 1988), and the SSW and BSW intrusions are not observed in the Kii Channel. Kuroshio distances of 18.5 km and 74 km were the most favorable for the SSW and BSW intrusions, respectively, and that of km was the largest distance observed during the analysis periods. Figure 12 shows the typical Kuroshio paths with three different Kuroshio distances. The variation in the Kuroshio path probably influences the slope water intrusions. A similar phenomenon was observed in the Bungo Channel, which is another entrance to the Seto Inland Sea. When the Kuroshio flows near the Kyushu, the cold water intrudes into the bottom of the Bungo Channel (Kaneda et al., 2002). They suggested that this cold water intrusion is related to the Slope Water Intrusion into the Kii Channel in Summer 111

8 (a) 28 Cape Shionomisaki Temperature ( ) Observed (b) Fig. 12. Paths of the Kuroshio in the south of Japan in August. The Kuroshio distance is 18.5 km in 1989, 74 km in 1990, and km in Temperature ( ) Calculated Fig year running mean of (a) observed and (b) calculated temperatures at depths of 0, 5, 10, 20, 30, and 50 m in August from 1977 to boundary current along the coast and bottom friction. Onshore Ekman transport in the bottom boundary layer of the shelf sea can occur as a response to the interior flow along the shelf (poleward along the eastern coast in the northern hemisphere) and bottom friction. The existence of this phenomenon has been indicated in other shelf seas (e.g., Merio, 1997; Oke and Middleton, 2001). However, the BSW intrusion in the Kii Channel is probably not related to the bottom Ekman transport because the BSW intrusion occurs when the Kuroshio detaches from the coast. A bifurcation current, which is the alongshore current diverging at a point along the southwest coast of the Kii peninsula, is often observed when the Kuroshio flows near Cape Shionomisaki (Takeuchi et al., 1998). A westward current appears at the southern entrance of the Kii Channel in relation to the bifurcation current, as reported by Takeuchi (2001), who suggested that the SSW and BSW intrusions were related to the occurrence of the westward current at the entrance of the channel. However, the dynamic mechanism that explains the SSW and BSW intrusions is unknown, and further research would be required in order to elucidate it. The DID for temperature can explain 70% of the historical temperature variance with only the Kuroshio distance as an explanatory variable. However, some differences exist between the calculated and observed temperatures (in 1968, 1977, 1991, 1992, and 1996). The position of the Kuroshio distance was calculated using the measured data of around two weeks. Therefore, the obtained Kuroshio distance represents an average distance for a 2-week interval. In shelf seas such as the Kii Channel, Bungo Channel, and Sagami Bay, including other regions where the Kuroshio flows offshore, water temperatures change abruptly within a few days, affecting with the frontal disturbance of the Kuroshio (Matsuyama et al., 1992; Takeoka et al., 1993; Takeuchi et al., 1997). These short-period variations might explain some of the prediction errors of the DID. The results from the DIDs for nutrients show that the change in the Kuroshio distance explains 35% and 30% of the variations in nitrate and phosphate, respectively. These values are lower than that of temperature (70%). The prediction error of the nutrient concentration arises from two factors: the relationship between the Kuroshio distance and the temperature, and the relationship between the temperature and nutrient concentrations. An error in the prediction of temperature is manifested in a prediction error of the nutrient concentration. In fact, upon eliminating the data that exhibit a temperature difference greater than 2 C between the calculated and observed values, the determination coefficients increased to 0.52 (nitrate) and 0.42 (phosphate). The Kuroshio distance indicates a decadal change (Fig. 5(b)). The Kuroshio approached Cape Shionomisaki in the early 1970s and frequently detached from the channel until the mid 1990s. It tended to flow near Cape Shionomisaki in the late 1990s. A 5-year running mean for temperature shows the decadal changes of the SSW and BSW intrusions (Fig. 13(a)). Water temperatures for 112 T. Takashi et al.

9 depths shallower than 30 m show a V-shaped variation with a minimum in the late 1980s. The temperature at 50 m depth shows a distorted V-shaped change with a steep decline in the 1970s and a gradual rise from the late 1970s to the late 1990s. The decrement in the steep decline period reaches 7 C within an 8-year period. These longterm variations are also described by the DID model (Fig. 13(b)). The V-shaped variation in the upper layer and the distorted V-shaped change in the lower layer are also reproduced. The decadal change in the BSW and SSW intrusion affects the biological variation in the Kii Channel. Takeuchi (2001) and Ozaki et al. (2003) demonstrated that the intrusions of the SSW and BSW influenced the variation in the plankton biomass in the Kii Channel. Takeuchi (2001) suggested that the nutrient-rich BSW intrusion causes interdecadal variations in the plankton biomass. Ozaki et al. (2003) also found that the quantity of zooplankton is large during the BSW intrusion. These studies indicate that intrusion of the BSW is important for the long term variation in biological production in the Kii Channel. The DID for temperature indicates that the variations in water temperature in the Kii Channel are governed by only one parameter: the Kuroshio distance. It is also one of the main contributors to control the nutrient level in the Kii Channel. Acknowledgements We are especially grateful to Dr. Y. Ueta, Mr. Yoshihisa Kaneda, and the crew of the R/V Tokushima (FRIT) for providing the data and helpful comments. We thank Mr. J. Takeuchi and Dr. A. Kasai for their valuable comments. We also thank to the editor and two anonymous reviewers for useful suggestion on improving manuscript. This work was partly supported by a Grant-in-Aid for the 21st Century COE program, Center of aquaculture science and technology for Bluefin tuna and other cultivated fish, from the Ministry of Education, Culture, Sports, Science and Technology. References Churchill, H. J., J. P. Manning and R. C. Beardsley (2003): Slope water intrusions onto Georges Bank. J. Geophys. Res., 108(C11), 8012, doi: /2002jc Fujiwara, T., N. Uno, M. Tada, K. Nakatuji, A. Kasai and W. Sakamoto (1997): Nutrient flux and residual current in Kii Channel. Umi-to-Sora, 73, (in Japanese). Kaneda, A., K. Norimatsu, K. Watanabe, Y. Koizumi and H. Takeoka (2002): Influence of onshore/offshore movements of the Kuroshio on the water temperature in the Bungo Channel, Japan. Bull. Coast. Oceanogr., 39, Kasai, A., T. Fujiwara and M. Tada (2001): Flow structure and nutrient transport in the Kii Channel. Proc. Coas. Eng., JSCE, 48, (in Japanese). Kawai, H. (1969): Statistical estimation of isotherms indicative of the Kuroshio axis. Deep-Sea Res., 16 (Suppl.), Lee, N. T., L. P. Atkinson and R. Legeckis (1981): Observations of a Gulf Stream frontal eddy on the Georgia continental shelf, April Deep-Sea Res., 28, Masuzawa, J. (1969): Subtropical Mode Water. Deep-Sea Res., 16, Matsuyama, M., S. Iwata, A. Maeda and T. Suzuki (1992): The Kyucho in Sagami Bay. Bull. Coast. Oceanogr., 30, 4 15 (in Japanese). Merio, M. (1997): Upwelling on the Yucatan Shelf: Hydrographic evidence. J. Mar. Sys., 13, Oke, P. R. and J. H. Middleton (2001): Nutrient enrichment off Port Stephens: the role of the East Australian Current. Cont. Shelf Res., 21, Ozaki, K., S. Uye, T. Kusumoto and T. Hagino (2003): Interannual variability of the ecosystem of the Kii Channel, the Inland Sea of Japan, as influenced by bottom intrusion of cold and nutrient-rich water from the Pacific Ocean, and a recent trend of warming and oligotrophication. Fish. Oceanogr., 13, Takeoka, H., H. Akiyama and T. Kikuchi (1993): The Kyucho in the Bungo Channel, Japan Periodic intrusion of oceanic warm water. J. Oceanogr., 49, Takeuchi, J. (2001): Interannual and interdecadal variations in Plankton Biomass and intrusion of Bottom Cold Water into Kii Channel. Umi-to-Sora, 77, (in Japanese). Takeuchi, J., Y. Nakaji and T. Kokubo (1997): Intrusion of Surface Warm Water and Bottom Cold Water into the Kii Channel. Umi-to-Sora, 73, (in Japanese). Takeuchi, J., N. Honda, Y. Morikawa, T. Koike and Y. Nagata (1998): Bifurcation Current along the Southwest Coast of the Kii Peninsula. J. Oceanogr., 54, Yoder, A. J., L. P. Atkinson, T. N. Lee, H. H. Kim and C. R. McClain (1981): Role of Gulf Stream frontal eddies in forming phytoplankton patches on the outer southeastern shelf. Limnol. Oceanogr., 26, Yoshioka, H. (1988): The coastal front in the Kii Channel in winter. Umi-to-Sora, 64, Slope Water Intrusion into the Kii Channel in Summer 113