Observation of Oceanic Structure around Tosa-Bae Southeast of Shikoku

Transcription

1 Journal of Oceanography Vol. 50, pp. 543 to Observation of Oceanic Structure around Tosa-Bae Southeast of Shikoku YOSHIHIKO SEKINE, HARUKI OHWAKI and MOTOYA NAKAGAWA Institute of Oceanography, Faculty of Bioresources, Mie University, 1515 Kamihamachou, Tsu 514, Japan (Received 15 November 1993; in revised form 23 May 1994; accepted 25 May 1994) The hydrographic observations in the vicinity of a seamount, the Tosa-Bae, southeast of Shikoku have been carried out two times in summer of 1991 and The temperature, salinity fields are observed by CTD and velocity fields are measured by ADCP. Results of these observation are presented in this paper. It is shown that salinity maximum water at a depth of 100 m is confined to a southeastern area of the Tosa-Bae, however, salinity minimum water is found in northern side of the Tosa-Bae. This indicates the westward intrusion of less saline water over northern slope. A positive correlation is detected between the estimated Rossby height (fl/n) and the observed height of Taylor Column estimated from the vertical change in the isotherms and isohalines. Almost both heights give smaller value than representative depth of bottom topography of the Tosa-Bae, it is indicated that the topographic effect of the Tosa-Bae is not fully reached to the surface. From the correlations between the vertical difference of geostrophic flow and that of ADCP velocity, ageostrophic flow component is detected. 1. Introduction To estimate the topographic effects of seamounts on temperature, salinity, density and velocity fields are important and it has been of interest to oceanographers (e.g., Hogg, 1973, 1980; Johnson, 1977; Roden, 1987). There have been various observational and theoretical studies: if a stationary barotropic geostrophic flow is assumed, a current has no vertical change (Taylor-Proudman theorem) and an isolated eddy with vertically coherent flow (Taylor column) is formed over a seamount. If a density stratification is considered, topographic effect of a seamount is weakened and its topographic effect is confined to a shorter thickness over the seamount, of which the height of Taylor column is estimated from Rossby height defined by fl/n, where f is the Coriolis parameter, L half of representative horizontal scale of the seamount and N is the Brunt-Väisä lä frequency assumed to be constant (e.g., Gill, 1982). In an unstationary state, topographic waves take important roles on the time evolutions of oceanic structure (e.g., Huppert and Bryan, 1976). As oceanic conditions over seamounts may be different in localities, hydrographic observations should be made for each seamount. The present study is directed toward oceanic conditions over Tosa-Bae southeast of Shikoku (Fig. 1). The Tosa-Bae has an elliptic shape with a longer axis in zonal direction. The top of this seamount has a depth of 147 m. It should be noticed that as the Kuroshio flows over or near Tosa-Bae, the intensity of the topographic effect on the flow may be an important factor for the path dynamics of the Kuroshio. It is well-known that the Kuroshio shows bimodal path characteristics between large meander path and non-large meander path south of Japan (e.g., Nitani, 1972; Taft, 1972). In particular, because the Tosa-Bae locates at the separating area of the large meander path of the Kuroshio from the coastal

2 544 Y. Sekine et al. Fig. 1. Location of Tosa-Bae southeastern Shikoku (upper). Isoplethes of depth (in meter) are also shown (after Taft, 1972). Isopleth of depth (in meter) in the vicinity of Tosa-Bae (lower). topography of Japan, the separation of the Kuroshio path is enhanced if the topographic effect of Tosa-Bae is relatively large and the main axis is deflected southward over the Tosa-Bae. From these view points, to estimate the topographic effect of the Tosa-Bae on the Kuroshio flow is strongly needed. Up to now, some observations have been carried out around the Tosa-Bae. Yoshioka et al. (1986) made STD observations over the Tosa-Bae in April of They showed that the temperature field in the layer shallower than 300 m has a frontal structure, whereas a cold domelike structure is observed below 300 m. Sekine and Matsuda (1987a) carried out observations by CTD and GEK over the Tosa-Bae in November They showed that a coupled warm and cold water are observed in upper layer shallower than 500 m and only cold water is observed at greater

3 Oceanic Structure in the Vicinity of Tosa-Bae 545 depth. The coupled warm and cold water are suggested to be almost identical to the results of numerical experiment performed by Huppert and Bryan (1976). It is also pointed out that the thickness of a mixed layer over the Tosa-Bae varied by a small horizontal scale of 5 10 km with a depth range of m (Sekine and Matsuda, 1987b). We have made two more detailed observations by CTD and ADCP around Tosa-Bae in summer of 1991 and Details of the two observations and some noteworthy results are shown in the present paper. The main emphasis is placed on the temperature, salinity, density and velocity fields that indicates the topographic effect of the Tosa-Bae on the mean flow of the Kuroshio. 2. Observations Two observations were carried out on July in 1991 and on July in 1992 by use of Training Vessel Seisui-maru of Mie University. The first and second observations are hereafter referred to as KS-91JUL and KS-92JUL, respectively. Temperature and salinity were observed by CTD at the observation points shown in Fig. 2. Because of the long time lag of the total observation, the observation points were placed to see the vertical field for each observation line rather than over all horizontal field. Unfortunately, because of the rough sea conditions, Fig. 2. Observational points of CTD for KS-91JUL (upper) and for KS-92JUL (lower). Names of straight observation lines are also shown by capitals.

4 546 Y. Sekine et al. observations along the meridional observation line between LINE-B and LINE-C of KS-92JUL were not completed. Density (σ t ) is calculated from temperature and salinity data. The current velocity was measured by acoustic doppler current profiler (ADCP) at the three depths, 50, 100 and 150 m in KS-91JUL and 5, 50 and 200 m in KS-92JUL. The mean ADCP velocity over 10 minutes are used in the data analysis. 3. Results 3.1 Temperature, salinity and density fields The vertical distributions of temperature, salinity and density (σ t ) along zonal observation line A, which is hereafter referred to as LINE-A, of KS-91JUL are shown in Fig. 3. A weak upward shifts of the isotherms of 21 C and 22 C are detected over the top of the Tosa-Bae. However, the upward shifts are unclear for the isotherms warmer than 24 C. In depths below the top of Tosa-Bae, complicated and various vertical displacement of the isotherms is found over the side slope of Tosa-Bae, which is common to the results of previous observations (Yoshioka et al., 1986; Sekine and Matsuda, 1987a). The salinity maximum layer and minimum layer are found at depths 100 db and 500 db, respectively. Saline water which is more than 34.7 psu is found at a depth of 100 db in eastern side of the Tosa-Bae, but such a saline water is not found in western side. In the upper layer, vertical variations in the isotherms are relatively more dominant in down stream (eastern) area rather than those of upstream (western) area. It is suggested that the dominant vertical variations in downstream area are due to the internal lee wave formed by the bottom topography of the Tosa-Bae. Since the salinity difference is relatively small, density fields have a close resemblance to those of isotherms (Fig. 3). Because similar density patterns to those of temperature are commonly observed for all the other observation lines, density distribution are not shown in the following. The vertical temperature and salinity distributions along meridional observation lines of KS- 91JUL are shown in Fig. 4. As for LINE-B, upward shifts of isotherms are detected over the top of Tosa-Bae in depths of db. Although the data of LINE-A is used at station 18 with a relatively large observation time lag from other data, this uplift is also suggested from the data of the stations 23 and 24. However, downward shift of the isotherms over the top of the seamount in LINE-C and LINE-E are due to the one station of LINE-A: 13 and 5, respectively. These downward shifts may be attributed to the time change in the isotherms during the observation periods. Upward shifts of the isotherms of C are depicted over the northern side slope of the Tosa-Bae in LINE-D. Because of the upward shift of isotherms of C with the opposite horizontal gradient in a southern side, the existence of cold-dome is suggested. As for LINE-E, more dominant uplift of isotherms of C is found over the ridge of the top of the Tosa-Bae. Since this upward shift shows no evanescent change from the topography, this upward shift of isotherms are suggested to be due to internal waves. Maximum and minimum salinity layer are observed at depths db and db, respectively (Fig. 4b). Saline water more than 34.7 psu is found at the southern side of the ridge of Tosa-Bae, while such a saline water is not found in the northern side. In contrast to this, less than psu saline water in the salinity minimum layer is found in both sides. Horizontal salinity distribution over isopycnal surface of σ t = 26.8 corresponding to a depth with salinity minimum layer is shown in Fig. 5. Less than psu saline water spreads westward along the

5 Oceanic Structure in the Vicinity of Tosa-Bae 547 Fig. 3. Temperature, salinity and density (σt) sections along observational line A of KS-91JUL. The locations of the observational points are shown on the top.

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7 Oceanic Structure in the Vicinity of Tosa-Bae 549 Fig. 4. Vertical distribution of (a) temperature and (b) salinity along four meridional observation lines B E of KS-91JUL.

8 550 Y. Sekine et al. Fig. 5. Salinity distribution (psu) on the depth with σ t = northern slope of Tosa-Bae. There is a possibility that the less saline water intrudes westward around the northern topography of Tosa-Bae. Almost similar characteristics of LINE-A of KS-92JUL to those of KS-91JUL are observed (not shown). Main characteristics along meridional observation lines of KS-92JUL are also common to those of KS-91JUL, however, some new features are observed (Fig. 6). Vertically coherent uplift of the isotherms is perceived over the ridge in LINE-D. Clear vertical variations of isotherms in the down stream area (Fig. 4) are not detected in Fig. 6. Northward intrusion of saline water with psu is detected in a salinity maximum layer at a depth of 100 db, while more saline water than 34.8 psu is confined to the southern side. An isolated saline water is found at a depth of 500 db on LINES-D and -E. To examine the intensity of the topographic effect of Tosa-Bae, the estimated Rossby height (fl/n) and the height of observed Taylor column are shown in Fig. 7. Here, the Rossby height (RH) is estimated from observed N and evaluated L, while the observed height of Taylor column (OHT) is evaluated by the coherent or evanescent vertical changes of isotherm and isohaline. Since to estimate constant values of L and N is difficult, their possible ranges are evaluated and the resulted height range of RH are shown for maximum, intermediate and minimum values of N and maximum and minimum for L, of which details are tabulated in Table 1. Positive correlation which exceeds 95% confidence limit of which correlation coefficient (r) is 0.63 is detected in Fig. 7. Two large OHT were observed on LINE-D and LINE-E of KS-92JUL, in which depth of the ridge of Tosa-Bae is deeper than 1000 m. Except for these two cases, all OHT and RH are smaller than 300 m, and this height is smaller than the representative depth of the Tosa-Bae. This indicates that the topographic effect of the Tosa-Bae is not fully reached to the surface. 3.2 Velocity fields Observed velocity fields by ADCP is shown in Fig. 8. Large eastward velocity is detected

9 Oceanic Structure in the Vicinity of Tosa-Bae 551 Table 1. Estimates of Rossby height (fl/n) observation line N (10 3 sec 1 ) L (km) Rossby height (m) Minimum Maximum Minimum Maximum A B C D E observation line N (10 3 sec 1 ) L (km) Rossby height (m) Minimum Maximum Minimum Maximum A B C D E

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11 Oceanic Structure in the Vicinity of Tosa-Bae 553 Fig. 6. Same as in Fig. 4 but for KS-92JUL.

12 554 Y. Sekine et al. Fig. 7. Correlation between the estimated Rossby height (fl/n) and the observed Taylor column. (a) Case of possible maximum of Vä isä lä frequency (N), (b) middle N and (c) possible minimum N (for details, see Table 1). Horizontal bars in each data show the possible range of the Rossby height depending on half of the horizontal scale of the Tosa-Bae (L), while vertical bars show the possible range of OHT depending on the estimation of vertical change in isotherms and isohalines.

13 Oceanic Structure in the Vicinity of Tosa-Bae 555 Fig. 8. Observed horizontal velocity by ADCP in KS-91JUL (left) and in KS-92JUL (right).

14 556 Y. Sekine et al. Fig. 9. Correlation between the vertical difference of eastward geostrophic velocity and those of observed velocity by ADCP, which is estimated as a mean value of all the ADCP velocity data observed between the two neighboring CTD points. Two vertical levels which estimate the vertical velocity difference are shown on top of each panel and r on the upper right shows the coefficient of correlation. over the Tosa-Bae. As for KS-91JUL, it is noted that vertically coherent flow is dominant in southern area of Tosa-Bae, while an eastward velocity at a depth of 150 m is weakened in northern side. As for KS-92JUL, large vertical difference is found between 50 m and 200 m. Cyclonic circulation in an eastern area of the Tosa-Bae is suggested at a depth of 200 m from eastern area. In particular, an eastward flow is observed at the northern area of LINE-E. Although the depth is different between 200 m and 500 m, the cyclonic flow in the eastern side of Tosa-Bae supports the westward intrusion of the less saline water along northern slope of Tosa-Bae (Fig. 5). In order to see the attainment of geostrophic flow balance over the Tosa-Bae, correlations between the vertical difference of geostrophic flow and that of velocity observed by ADCP are shown in Fig. 9. Here, because there are several data of ADCP velocity between two neighboring stations of CTD for the geostrophic flow estimation, three different values of ADCP velocity, which is correlated to the geostrophic velocity, are estimated and their correlation coefficients

15 Oceanic Structure in the Vicinity of Tosa-Bae 557 Table 2. Coefficient of correlation between the vertical difference of eastward geostrophic velocity and that of observed ADCP velocity by three different estimation of the ADCP velocity. Cruise Estiamted depth (m) One point* Simple mean** Weighted mean*** KS-91JUL KS-92JUL *ADCP velocity estimated midway between the two neighboring CTD stations used to estimate the geostrophic velocity. **ADCP velocity is estimated as the simple mean of all ADCP velocity data between the two neighboring CTD stations. ***ADCP velocity is estimated as a weighted mean value of all the ADCP data observed between the two neighboring CTD stations. The weight is inversely proportional to the distance of the ADCP station from the mid-point between two CTD stations. are compared in Table 2. Relatively large difference is found between one point estimation and other two mean estimations, however they show no significant difference in correlation coefficient. It is shown from Fig. 9 that although positive correlation is totally found, they do not give common velocity difference and their dispersions are notable. This result agrees with the case of the seamount, Daini-Kinan Kaizan, located in south of Kii Peninsula (Sekine and Hayashi, 1992). It is also recognized from Fig. 9 and Table 2 that significant low coefficient is found in case with data of 5 m. It is suggested that the velocity at a depth of 5 m is more influenced by the wind-drift current, the Ekman flow and motions of wind waves and/or swells. Ageostrophic flow in deeper layers is suggested to be due to internal tidal flow in ADCP velocity and vertical variation in density fields by internal waves that influences the estimation of the geostrophic flow. 4. Summary and Discussion The hydrographic observations around Tosa-Bae southeast of Shikoku were made by the Training Vessel Seisui-maru of Mie University two times in summer of 1991 and Notable results of the observations are summarized as follows. (1) A saline water than 34.7 psu is confined to a southeastern area of the Tosa-Bae. In contrast to this, less saline water than psu is found in northern and southern area. Westward intrusion of less saline water is suggested in the northern slope of the Tosa-Bae. (2) The estimated Rossby height ( fl/n) and the observed height of Taylor Column were compared. They show a positive correlation. Almost both heights are smaller than the representative depth of bottom topography of the Tosa-Bae, indicating that the topographic effect of the Tosa- Bae has not fully reached the surface layer. (3) Large eastward velocity is detected over the Tosa-Bae. It is noted that large vertical difference is found between 50 m and 200 m. Cyclonic circulation in an eastern area of the Tosa- Bae is suggested at a depth of 200 m of KS-92JUL.

16 558 Y. Sekine et al. (4) In order to see the attainment of geostrophic balance, correlations between the vertical difference of geostrophic flow and that of ADCP velocity were examined. Although positive correlation is perceived, they do not give common velocity difference and their dispersion are notable. In particular, coefficient of the correlation decreases significantly in a case with shallower layer of 5 m, resulting that ageostrophic flow component is not negligible in this region. It should be noted that some different characteristics of temperature and salinity fields are observed between the two cruise: vertically coherent uplift of the isotherms is perceived over the ridge in LINE-D of KS-92JUL. Dominant vertical change of isotherms are observed in KS- 91JUL, but enhanced vertical change is not found in KS-92JUL. These observational results strongly suggests that frequent observation is needed to see the detailed oceanic condition over the Tosa-Bae. Existence of ageostrophic flow is resulted from (4). Although the topographic effect of the Tosa-Bae is not fully reached to the surface layer (2), the topographic effect on the distribution of salinity maximum layer denoted in (1) indicates the existence of the batropopic flow, which enhances the topographic effect of the Tosa-Bae. Furthermore, the oceanic conditions around this seamount is influenced by the seasonal variation and also by the distance from main axis of the Kuroshio. Long term direct current measurements which are able to see ageostrophic and barotropic flow component are needed to obtain the real velocity fields around Tosa-Bae. Acknowledgments The authors would like to thank Captain I. Ishikura, officers and crew of the Tranining Vessel Seisui-maru of Mie University for their excellent help in the observations and Miss A. Miyake now at Okayama City Office for her help in drawing some figures. Thanks are extended to Dr. K. Taguchi and Mr. T. Hayashi of the Faculty of Bioresources of Mie University for their help during observation. References Gill, A. E. (1982): Atmosphere-Ocean Dynamics. Academic Press, New York, London, 662 pp. Hogg, N. G. (1973): On the stratified Taylor column. J. Fluid Mech., 58, Hogg, N. G. (1980): Effects of bottom topography on ocean currents. Orographic Effects in Planetary Flows, GARP Publ. Ser., WMO, 23, Huppert, H. E. and K. Bryan (1976): Topographically generated eddies. Deep-Sea Res., 23, Johnson, E. R. (1977): Stratified Taylor column on a beta-plane. Geophys. Astrophys. Fluid Dyn., 9, Nitani, H. (1972): Beginning of the Kuroshio. p In Kuroshio Its Physical Aspects, ed. by H. Stommel and K. Yoshida, University of Tokyo Press, Tokyo. Roden, G. I. (1987): Effects of seamounts and seamount chains on oceanic circulation and thermocline structure. Seamounts, Islands and Atolis. B. Keating eds., Geophys. Monogr., 43, A.G.U., Sekine, Y. and Y. Matsuda (1987a): Hydrographic structure around the Tosa-Bae, the bump off Shikoku south of Japan, in November La mer, 25, (in Japanese with English abstract). Sekine, Y. and Y. Matsuda (1987b): Observation on surface mixed layer around the Tosa-Bae, the bump off Shikoku south of Japan, in November Umi to Sora, 63, 1 14 (in Japanese with English abstract). Sekine, Y. and T. Hayashi (1993): Oceanic structure in the vicinity of a seamount, the Daini Kinan Kaizan, south of Japan. La mer, 30, Taft, B. A. (1972): Characteristics of the flow of the Kuroshio south of Japan. p In Kuroshio Its Physical Aspects, ed. by H. Stommel and K. Yoshida, University of Tokyo Press, Tokyo. Yoshioka, H., T. Sugimoto, Y. Sekine, S. Serizawa and H. Kunishi (1986): Temperature structures and geostrophic current around the Bump, Tosa-Bae off Kii Channel south of Japan. Umi to Sora, 61, (in Japanese with English abstract).