Prediction of Slope Water Intrusion Into the Kii Channel in Summer

Journal of Oceanography, Vol. 62, pp. 105 to 113, 2006

Prediction of Slope Water Intrusion into the Kii Channel in Summer

1,2 1 3 2 TOSHINORI TAKASHI *, TATEKI FUJIWARA , TOSHIAKI SUMITOMO and WATARU SAKAMOTO

1Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Kyoto 606-8561, Japan 2Fisheries Laboratory of Kinki University, Wakayama 649-2211, Japan 3Fisheries Division, Tokushima Prefectural Government, Tokushima 770-8570, 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, nutri- Keywords: ent-rich bottom slope water (BSW) from the continental slope influence the annual ⋅ Slope water variations in water temperature and nutrient concentrations in the Kii Channel in intrusion, ⋅ August. In order to evaluate the relationships between both these intrusions and the nutrient, ⋅ distance of the Kuroshio axis from Cape Shionomisaki (Kuroshio distance), a Dis- temperature, ⋅ Kii Channel, tance-Intrusion-Diagram (DID) for temperature, which can reproduce the vertical ⋅ Kuroshio, temperature profile of the channel, was constructed by analyzing the temperature ⋅ annual variation. and Kuroshio distance records in August for 1967Ð2001. 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 al- most the entire water column when the Kuroshio approaches Cape Shionomisaki (Kuroshio distance = 18.5 km). When the Kuroshio distance lies in the range 18.5Ð74 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.

1. Introduction The Kii Channel is located in the western part of Oceanic water intrusion has been observed in conti- Japan (Fig. 1(a)). It has mean depth 56 m, surface area nental shelf seas. The intrusion affects the physical, 1554 km2, and volume 870 km3. This channel connects chemical, and biological processes in the shelf region. In with the Seto Inland Sea through the Kitan Strait and the the South Atlantic Bight, the intrusion of the cold, nutri- Naruto Strait in the north of the channel, and its southern ent-rich Gulf Stream waters accompanies the passage of part faces the Pacific Ocean. Fresh water flows into Osaka the Gulf Stream frontal eddy (Lee et al., 1981). During Bay and the north of the channel and flows out to the this passage, a large amount of nitrate is transported onto Pacific Ocean through the Kii Channel, while oceanic the shelf (Lee et al., 1981), stimulating biological pro- water flows from the Pacific Ocean into the Kii Channel. duction (Yoder et al., 1981). Two types of slope water Therefore, both fresh water and oceanic water influence intrusions occur in Georges Bank (Churchill et al., 2003): the physical, chemical, and biological conditions in the the warm, high salinity surface slope water intrudes into Kii Channel. Two distinct types of oceanic waters intrude the near-surface layer, while the cold, high salinity sub- into the Bungo Channel, which is another entrance of the surface slope water intrudes into the bottom layer. 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 * Corresponding author. E-mail: [email protected] Kyushu. Warm water intrusion tends to occur during the Copyright © The Oceanographic Society of Japan. neap tide in the Bungo Channel (Takeoka et al., 1993).

105 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 Chan- The above studies indicate that the SSW and BSW nel (Takeuchi et al., 1997). intrude into the Kii Channel in relation to the Kuroshio The Kuroshio Current flows eastward off the Kii path variance. Furthermore, the intrusions affect the nu- Channel. It is well known that the path of the Kuroshio trient flux in the channel. However, these studies raise varies widely with time and space. Takeuchi et al. (1997) two questions. First, what are the reasons for the classifi- revealed that oceanic waters intruding into the Kii Chan- cation of the slope water intrusions into two modes in nel are divided into two types, one being the cold, nutri- relation to the Kuroshio distance? Second, how much do ent-rich bottom slope water (BSW), while the other is the SSW and BSW water intrusions affect annual varia- the warm, oligotrophic surface slope water (SSW), which tions in nutrient concentrations in the Kii Channel? In originates from the Kuroshio surface water. They sug- this study we examine the relationship between the gested that slope water intrusions can be classified into Kuroshio distance and the intrusions of slope waters, two modes in relation to the Kuroshio path: the cold, nu- namely, the SSW and BSW, by analyzing a 35-year water trient-rich BSW that intrudes into the Kii Channel when temperature record measured in the month of August. The the distance of the Kuroshio axis from Cape Shionomisaki relationship is modeled as a Distance-Intrusion-Diagram (hereafter referred to as Kuroshio distance) is greater than (DID) for temperature, which can reproduce a vertical 55 km (=30 n. miles), the BSW intrusion rarely occurs temperature profile estimated from the Kuroshio distance. and the warm, and oligotrophic SSW that frequently flows We have applied the DID to the historical data of the into the Kii Channel when the Kuroshio distance is less Kuroshio distance and hindcast past temperature in order than 37 km (=20 n. miles). to evaluate the accuracy of the DID. DIDs for nutrients Fujiwara et al. (1997) measured the nutrient flux at are also constructed. These DIDs are used to investigate the southern end of the Kii Channel in August 1995 and the relationship between the Kuroshio distance and an- demonstrated that a large amount of nutrient flows into nual variation in the nutrient concentrations. the Kii Channel from the outer ocean in relation to the BSW intrusion. The nitrogen and phosphorus fluxes were 2. Data 206 ton dÐ1 and 34 ton dÐ1, respectively, which is compa- The Kobe Marine Observatory (KMO) conducted rable to the loadings to Osaka Bay from the land. Kasai hydrographic observations at 18 stations from Osaka Bay et al. (2001) estimated the nutrient flow using a numeri- to the Pacific Ocean in July 1989 and 1990 on board the cal model and presented the annual nutrient flux varia- R/V Shumpu Maru (Fig. 1(a)). Water temperature, salin- tions in summer. In their estimation, the nutrient flux was ity, nitrate and phosphate data from the observations were large when the BSW intrusion occurred (the Kuroshio was used to confirm the oceanic water intrusions into the Kii separated from the Kii Channel), while it was small when Channel. the BSW intrusion was weak (the Kuroshio flowed near The Fisheries Research Institute, Tokushima Agri- the channel). culture, Forestry and Fisheries Technology Center (FRIT)

106 T. Takashi et al. 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 Feb- ruary, May, August, and November, at depths of 0 and 50 m at 15 stations since 1977. 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 2001. The water temperature and salinity were measured with CTD. Nutrient (nitrate, nitrite, ammonium, and phosphate) µ 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 µ Conditions” published by the Japan Coast Guard.

3. Intrusion of Slope Water Fig. 2. Longitudinal distributions of temperature, salinity, ni- 3.1 Kuroshio distance and intrusion trate, and phosphate from Osaka Bay to the open ocean in Vertical sections of water temperature, salinity, ni- July 1990. Downward arrow on the top panel indicates the trate, and phosphate from Osaka Bay to the outer ocean position of the Kuroshio axis. OB: Osaka Bay and KC: Kii in July 1990 and July 1989 are shown in Figs. 2 and 3, Channel. respectively. The Kuroshio axis is known to coincide with the isothermals of 16Ð16.5°C at a depth of 200 m near Cape Shionomisaki (Kawai, 1969). The Kuroshio dis- surface layer of Osaka Bay. This less-saline water spreads tances in July 1990 and 1989 were around 85 km and 35 to the area off the Kii Channel in the surface layer. km, respectively. Comparison of the two periods revealed dissimilar The surface water off the Kuroshio area (depth < 100 distributions of temperature and nutrient concentrations m) was stratified in temperature and salinity (Figs. 2 and in the Kii Channel (Figs. 2 and 3). In July 1990, when the 3). Below this depth, the salinity exceeded 34.8, which is Kuroshio separated from the channel, the cold (T < 20°C) identified as the North Pacific subtropical mode water water intruded into the Kii Channel and Osaka Bay from (Masuzawa, 1969). The water exhibited a continuous tem- off the channel (Fig. 2). The cold water included a high perature and salinity stratification between the Kii Chan- nutrient concentration, the maximum nutrient concentra- nel and the Kuroshio. The cold water (T < 20°C) between tions being 8.5 µM for nitrate and 0.62 µM for phosphate the Kii Channel and Kuroshio axis was distributed in a in the Kii Channel. In contrast, the warm water (T > 20°C) shallower layer than that off the Kuroshio. The concen- occupied the channel and the nutrient level was relatively trations of nutrients off the Kii Channel increased with low in 1989 the period when the Kuroshio flowed near the decrease in water temperature. The waters in Osaka the Kii Channel (Fig. 3). This difference is related to the bay and the Kii Channel were stratified in both salinity difference in the Kuroshio distance (Takeuchi et al., and temperature. The lowest salinity water existed in the 1997). Although the oceanic water which intrudes into

Slope Water Intrusion into the Kii Channel in Summer 107 µ µ

Fig. 4. Relationship between the temperature and nutrient (top: nitrate, bottom: phosphate) concentrations at the shelf edge µ (stns. G0 and G1) in July 1989 and July 1990. Circles and crosses indicate values for 1989 and 1990, respectively.

Fig. 3. As Fig. 2, but showing the distributions in July 1989.

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 Fig. 5. (a) Annual variation in the temperature in the Kii Chan- nel in August. (b) Annual variation in the Kuroshio distance. the colder water has high nutrient concentration. Thus, Shaded areas indicate the BSW. the cold water and the warm water, which distribute off the channel, intrude into the Kii Channel.

3.2 Annual variation in the SSW and BSW intrusions fer to the cold (T < 22°C) water as the BSW and the warm The differences in the Kuroshio distance induced (T > 22°C) water as SSW. changes in the oceanic water intrusions (Figs. 2 and 3). The annual variations in the horizontally averaged The vertical water temperature distribution indicates temperature at four stations (denoted by triangles in Fig. whether the SSW or BSW intruded into the Kii Channel 1) in the Kii Channel in August and those in the Kuroshio in relation to the Kuroshio distance. Therefore, to inves- distance at the end of July are shown in Fig. 5. The tem- tigate the relationship between the intrusions and the perature shows annual fluctuations (Fig. 5(a)). The warm Kuroshio distance, we analyzed the historical tempera- SSW occupied the entire water column continuously from ture record and the Kuroshio distance. Hereafter, we re- 1970 to 1974 and from 1996 to 1999 when the Kuroshio

108 T. Takashi et al. 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 277.5 km, respectively.

approached the Kii Channel (Fig. 5(b)). In contrast, the the Kuroshio distance, in intervals of 9.5 km. For exam- cold BSW intruded into the Kii Channel from 1975 to ple, the average temperature and its standard deviation in 1995 when the Kuroshio distance was large. The tempera- each bin at a depth of 0 m are plotted against the Kuroshio ture fluctuations were in accordance with the variance in distance in Fig. 6(a). The temperature decreases to a mini- the Kuroshio distance. mum with the increase in the Kuroshio distance up to 74 km; thereafter, it rises gradually with a further increase 4. Distance-Intrusion-Diagram (DID) in distance. Since the relationship can be segregated into two ranges of the Kuroshio distance—A (from 18.5 km 4.1 Construction of DID for temperature and nutrients to 74 km) and B (from 74 km to 277.5 km)—two approxi- levels mation formulas relating the temperature to the Kuroshio The water temperature data for August acquired by distance can be estimated (Fig. 6(a)). The Kuroshio dis- FRIT at the four stations (denoted by triangles in Fig. tance for the lowest temperature is 74 km. The resultant 1(b)) were horizontally averaged at six depths (0, 5, 10, approximation lines can be used to predict the water tem- 20, 30, and 50 m). Using these values, a DID was con- perature at a depth of 0 m, from the Kuroshio distance. structed for the temperature, which predicts the vertical The same procedure was applied to the remaining data profile of the temperature in the Kii Channel from the sets of the Kuroshio distance and water temperatures at Kuroshio distance. The water temperature in the Kii Chan- depths of 5, 10, 20, 30, and 50 m (Figs. 6(b)Ð(f)). The nel at a given depth is divided into 13 bins, depending on Kuroshio distance at which the minimum temperature

Slope Water Intrusion into the Kii Channel in Summer 109 Fig. 8. Relationship between temperature and nutrient concentrations ((a): nitrate and (b): phosphate) in the Kii Channel in July and August from 1999 to 2001. 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. Fig. 7. Distance-Intrusion-Diagram of (a) temperature (°C), (b) nitrate (µM), and (c) phosphate (µM) in August. Shaded area indicates BSW.

trations decrease linearly with increasing temperature. This indicates the conservative nature of the nutrients. occurs was the same for all the data sets (74 km). However, the nitrate concentration is very low and nearly The procedure for constructing the DID for water below the detectable limit in the range of temperatures temperature is as follows: using the approximation for- higher than 25°C. This suggests nitrate uptake by mulas in Figs. 6(a)Ð(f), the water temperatures at six phytoplankton and its depletion in the upper, warmer depths were calculated from the Kuroshio distance. These layer. The DID for temperature was converted into DIDs calculations were carried out over a Kuroshio distance for nitrate and phosphate by using approximation lines, range of 18.5 km to 277.5 km. The DID was constructed which are shown in Fig. 8(a). The obtained DIDs for ni- using the results of these calculations by vertical linear trate and phosphate are shown in Figs. 7(b) and (c). interpolation (Fig. 7(a)). This DID can be used to predict the vertical profile of the temperature in the Kii Channel 4.2 Hindcast of the historical records of temperature and at an arbitrary Kuroshio distance. nutrient concentrations The DID also indicates the thickness of the intruded Historical records of the Kuroshio distance (Fig. SSW and BSW. The SSW occupies the Kii Channel from 5(b)) were substituted in the DID for temperature, result- the top to the bottom in a Kuroshio distance range of 18.5 ing in a hindcast water temperature of the Kii Channel km to 37 km. The thickness of the SSW decreases and for August from 1967 to 2001. The observed and calcu- that of the BSW increases according to the Kuroshio dis- lated water temperatures at a depth of 50 m are shown in tance increment up to 74 km. The thickness of the BSW Fig. 9(a). The annual variation in the calculated water is at a maximum at 74 km. The BSW thickness gradually temperature coincides with that in the observed tempera- decreases as the Kuroshio distance exceeds 74 km. ture. Furthermore, the calculated vertical profiles of the The DIDs for nitrate and phosphate were constructed temperature also correlate well with the observed one using the relationships between water temperature and (Figs. 5(a) and 10). The SSW over the entire water col- nutrient concentrations; these relationships are shown in umn from 1970 to 1974 and from 1996 to 1999, when the Fig. 8. The water temperature shows a high correlation Kuroshio approached Cape Shionomisaki (Kuroshio dis- with the nutrient concentrations in July and August in the tance = 18.5Ð37 km), is reproduced well by the DID. The Kii Channel (Fig. 8). The nitrate and phosphate concen- frequent intrusions of the BSW from 1975 to 1995 are

110 T. Takashi et al. 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 1967Ð2001.

5. Discussion and Conclusions We have elucidated the relationship between the BSW and SSW intrusions into the Kii Channel and the Fig. 9. Annual variations in the calculated (circles and thin Kuroshio distance in August. The thickness of the intrud- lines) and observed (thick lines) (a) temperature, (b) nitrate ing BSW (T < 22°C) and SSW (T > 22°C) depends on the concentration, and (c) phosphate concentration in the Kii Kuroshio distance (Fig. 7(a)). The SSW occupies the Channel at 50 m in August. 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 Fig. 10. Annual variations in the calculated temperatures in the fact that they used the temperatures recorded during the Kii Channel in August. Shaded areas indicate the BSW. 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 also reproduced. In particular, the strong intrusions of the BSW intrusions are not observed in the Kii Channel. BSW in the 1980s were well expressed when the Kuroshio Kuroshio distances of 18.5 km and 74 km were the distance was around 74Ð120 km, which is favorable for most favorable for the SSW and BSW intrusions, respec- BSW intrusions. A scattering diagram of the calculated tively, and that of 203.5 km was the largest distance ob- temperatures vs. the observed temperatures is shown in served during the analysis periods. Figure 12 shows the Fig. 11. The Kuroshio distance can explain 70% of the typical Kuroshio paths with three different Kuroshio dis- observed temperature variation. tances. The variation in the Kuroshio path probably in- The nutrient variations at 50 m depth are also calcu- fluences the slope water intrusions. A similar phenom- lated using the DIDs for nitrate and phosphate (Figs. 9(b) enon was observed in the Bungo Channel, which is an- and (c)). The variations in the calculated and observed other entrance to the Seto Inland Sea. When the Kuroshio nutrients are well correlated. In this case, too, the flows near the Kyushu, the cold water intrudes into the Kuroshio distance can explain 35% and 30% of the ob- bottom of the Bungo Channel (Kaneda et al., 2002). They served nitrate and phosphate variations, respectively. suggested that this cold water intrusion is related to the

Slope Water Intrusion into the Kii Channel in Summer 111 (a) 28 ) Cape 1976 26 Shionomisaki 1990 24 1989 22

Temperature ( 20 Observed 18 19701980 1990 2000 (b) 28 ) 1989 26 1976 24 1990 22

Fig. 12. Paths of the Kuroshio in the south of Japan in August. Temperature ( 20 The Kuroshio distance is 18.5 km in 1989, 74 km in 1990, Calculated 18 and 203.5 km in 1976. 19701980 1990 2000

Fig. 13. 5-year running mean of (a) observed and (b) calculated temperatures at depths of 0, 5, 10, 20, 30, and 50 m in boundary current along the coast and bottom friction. August from 1977 to 2001. 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 exist- nel, Bungo Channel, and Sagami Bay, including other ence of this phenomenon has been indicated in other shelf regions where the Kuroshio flows offshore, water tem- seas (e.g., Merio, 1997; Oke and Middleton, 2001). How- peratures change abruptly within a few days, affecting ever, the BSW intrusion in the Kii Channel is probably with the frontal disturbance of the Kuroshio (Matsuyama not related to the bottom Ekman transport because the et al., 1992; Takeoka et al., 1993; Takeuchi et al., 1997). BSW intrusion occurs when the Kuroshio detaches from These short-period variations might explain some of the the coast. prediction errors of the DID. A bifurcation current, which is the alongshore cur- The results from the DIDs for nutrients show that rent diverging at a point along the southwest coast of the the change in the Kuroshio distance explains 35% and Kii peninsula, is often observed when the Kuroshio flows 30% of the variations in nitrate and phosphate, respec- near Cape Shionomisaki (Takeuchi et al., 1998). A west- tively. These values are lower than that of temperature ward current appears at the southern entrance of the Kii (70%). The prediction error of the nutrient concentration Channel in relation to the bifurcation current, as reported arises from two factors: the relationship between the by Takeuchi (2001), who suggested that the SSW and Kuroshio distance and the temperature, and the relation- BSW intrusions were related to the occurrence of the ship between the temperature and nutrient concentrations. westward current at the entrance of the channel. How- An error in the prediction of temperature is manifested in ever, the dynamic mechanism that explains the SSW and a prediction error of the nutrient concentration. In fact, BSW intrusions is unknown, and further research would upon eliminating the data that exhibit a temperature dif- be required in order to elucidate it. ference greater than 2°C between the calculated and ob- The DID for temperature can explain 70% of the his- served values, the determination coefficients increased torical temperature variance with only the Kuroshio dis- to 0.52 (nitrate) and 0.42 (phosphate). tance as an explanatory variable. However, some differ- The Kuroshio distance indicates a decadal change ences exist between the calculated and observed tempera- (Fig. 5(b)). The Kuroshio approached Cape Shionomisaki tures (in 1968, 1977, 1991, 1992, and 1996). The posi- in the early 1970s and frequently detached from the chan- tion of the Kuroshio distance was calculated using the nel until the mid 1990s. It tended to flow near Cape measured data of around two weeks. Therefore, the ob- Shionomisaki in the late 1990s. A 5-year running mean tained Kuroshio distance represents an average distance for temperature shows the decadal changes of the SSW for a 2-week interval. In shelf seas such as the Kii Chan- and BSW intrusions (Fig. 13(a)). Water temperatures for

112 T. Takashi et al. depths shallower than 30 m show a V-shaped variation Channel. Umi-to-Sora, 73, 63Ð72 (in Japanese). with a minimum in the late 1980s. The temperature at 50 Kaneda, A., K. Norimatsu, K. Watanabe, Y. Koizumi and H. m depth shows a distorted V-shaped change with a steep Takeoka (2002): Influence of onshore/offshore movements decline in the 1970s and a gradual rise from the late 1970s of the Kuroshio on the water temperature in the Bungo Chan- to the late 1990s. The decrement in the steep decline pe- nel, Japan. Bull. Coast. Oceanogr., 39, 181Ð188. Kasai, A., T. Fujiwara and M. Tada (2001): Flow structure and riod reaches 7°C within an 8-year period. These long- nutrient transport in the Kii Channel. Proc. Coas. Eng., term variations are also described by the DID model (Fig. JSCE, 48, 436Ð440 (in Japanese). 13(b)). The V-shaped variation in the upper layer and the Kawai, H. (1969): Statistical estimation of isotherms indica- distorted V-shaped change in the lower layer are also re- tive of the Kuroshio axis. Deep-Sea Res., 16 (Suppl.), 109Ð produced. 115. The decadal change in the BSW and SSW intrusion Lee, N. T., L. P. Atkinson and R. Legeckis (1981): Observa- affects the biological variation in the Kii Channel. tions of a Gulf Stream frontal eddy on the Georgia conti- Takeuchi (2001) and Ozaki et al. (2003) demonstrated nental shelf, April 1977. Deep-Sea Res., 28, 347Ð378. that the intrusions of the SSW and BSW influenced the Masuzawa, J. (1969): Subtropical Mode Water. Deep-Sea Res., variation in the plankton biomass in the Kii Channel. 16, 463Ð472. Takeuchi (2001) suggested that the nutrient-rich BSW Matsuyama, M., S. Iwata, A. Maeda and T. Suzuki (1992): The Kyucho in Sagami Bay. Bull. Coast. Oceanogr., 30, 4Ð15 intrusion causes interdecadal variations in the plankton (in Japanese). biomass. Ozaki et al. (2003) also found that the quantity Merio, M. (1997): Upwelling on the Yucatan Shelf: of zooplankton is large during the BSW intrusion. These Hydrographic evidence. J. Mar. Sys., 13, 101Ð121. studies indicate that intrusion of the BSW is important Oke, P. R. and J. H. Middleton (2001): Nutrient enrichment off for the long term variation in biological production in the Port Stephens: the role of the East Australian Current. Cont. Kii Channel. The DID for temperature indicates that the Shelf Res., 21, 587Ð606. variations in water temperature in the Kii Channel are Ozaki, K., S. Uye, T. Kusumoto and T. Hagino (2003): governed by only one parameter: the Kuroshio distance. Interannual variability of the ecosystem of the Kii Chan- It is also one of the main contributors to control the nu- nel, the Inland Sea of Japan, as influenced by bottom intru- trient level in the Kii Channel. sion of cold and nutrient-rich water from the Pacific Ocean, and a recent trend of warming and oligotrophication. Fish. Oceanogr., 13, 65Ð79. Acknowledgements Takeoka, H., H. Akiyama and T. Kikuchi (1993): The Kyucho We are especially grateful to Dr. Y. Ueta, Mr. in the Bungo Channel, Japan—Periodic intrusion of oce- Yoshihisa Kaneda, and the crew of the R/V Tokushima anic warm water. J. Oceanogr., 49, 369Ð382. (FRIT) for providing the data and helpful comments. We Takeuchi, J. (2001): Interannual and interdecadal variations in thank Mr. J. Takeuchi and Dr. A. Kasai for their valuable Plankton Biomass and intrusion of Bottom Cold Water into comments. We also thank to the editor and two anony- Kii Channel. Umi-to-Sora, 77, 91Ð98 (in Japanese). mous reviewers for useful suggestion on improving manu- Takeuchi, J., Y. Nakaji and T. Kokubo (1997): Intrusion of Sur- script. This work was partly supported by a Grant-in-Aid face Warm Water and Bottom Cold Water into the Kii Chan- for the 21st Century COE program, “Center of aquaculture nel. Umi-to-Sora, 73, 81Ð92 (in Japanese). science and technology for Bluefin tuna and other culti- Takeuchi, J., N. Honda, Y. Morikawa, T. Koike and Y. Nagata vated fish”, from the Ministry of Education, Culture, (1998): Bifurcation Current along the Southwest Coast of the Kii Peninsula. J. Oceanogr., 54, 45Ð52. Sports, Science and Technology. Yoder, A. J., L. P. Atkinson, T. N. Lee, H. H. Kim and C. R. McClain (1981): Role of Gulf Stream frontal eddies in form- References ing phytoplankton patches on the outer southeastern shelf. Churchill, H. J., J. P. Manning and R. C. Beardsley (2003): Slope Limnol. Oceanogr., 26, 1103Ð1110. water intrusions onto Georges Bank. J. Geophys. Res., Yoshioka, H. (1988): The coastal front in the Kii Channel in 108(C11), 8012, doi:10.1029/2002JC001401. winter. Umi-to-Sora, 64, 79Ð111. Fujiwara, T., N. Uno, M. Tada, K. Nakatuji, A. Kasai and W. Sakamoto (1997): Nutrient flux and residual current in Kii

Slope Water Intrusion into the Kii Channel in Summer 113