Prediction of Slope Water Intrusion Into the Kii Channel in Summer
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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 1967–2001. 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 18.5–74 km, the BSW thickness increases proportionally to the Kuroshio distance increment while the SSW thickness decreases. The BSW occupies the largest portion of the chan- nel 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 d–1 and 34 ton d–1, 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 av- eraged 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.