Observation of the Kyucho in the Bungo Channel by HF Radar

Transcription

1 Journal of Oceanography Vol. 51, pp. 699 to Observation of the Kyucho in the Bungo Channel by HF Radar HIDETAKA TAKEOKA 1, YOSHIO TANAKA 2, YUICHI OHNO 3, YUKIHARU HISAKI 4, AKITSUGU NADAI 4 and HIROSHI KUROIWA 5 1 Department of Civil and Ocean Engineering, Faculty of Engineering, Ehime University, Bunkyo 3, Matsuyama 790, Japan 2 Tokyo-kyuei Corp., Shiba Tsurugamaru , Kawaguchi 333, Japan 3 Communications Research Laboratory, Nukui-Kitamachi 4-2-1, Koganei Tokyo 184, Japan 4 Okinawa Radio Observatory, Communications Research Laboratory, Aza-kuba, Nakagusuku-son Nakagami-gun, Okinawa , Japan 5 Earth Observation Satellite Department, National Space Development Agency of Japan, Central Bldg Hamamatsu-cho Minato-ku, Tokyo 105, Japan (Received 21 April 1995; in revised form 20 June 1995; accepted 23 June 1995) Observations of sea surface currents by HF radar were carried out in the Bungo Channel in summer The current ellipses of M 2 constituent obtained by the observational results agree quite well with those obtained by the ADCP observations, showing that the accuracy of the HF radar measurements is of the same level as ADCP. The results revealed the current structures and their change with the Kyucho in detail. The Kyucho is influenced by the complicated coastal geometry and does not propagate straightly into the Bungo Channel. It propagates further inward after charging the coastal bays with warm water. The current directions change largely, since the currents turn around the stagnant region in the bay filled with the warm water. The northward intrusion begins to be weakened in the southern part of the channel, while it still persists in the northern part. The northward current speeds of the observed Kyucho are about 50 cm/s and sometimes attain 60 to 70 cm/s. 1. Introduction A high frequency ocean radar (HF radar) is a quite useful instrument which enables longterm observation of sea surface currents in a wide area. In the latest decade, a number of observations by HF radar have been applied to coastal phenomena such as tidal and residual currents (Prandle, 1987), frontal structure (Matthews et al., 1993), upwelling (Shkedy et al., 1995) and so on. Okinawa Radio Observatory of Communications Research Laboratory (CRL) developed the first HF ocean radar system in Japan in 1988 (Iguchi et al., 1989) and the second one in Several current and sea state observations using the radars were made at Tokara Strait, Japan Sea and other places (Ohno, 1991, 1993). We applied current measurements by the radars to the observation of the Kyucho in the Bungo Channel. Kyucho is a sudden and swift current which is usually accompanied by a rise of water temperature. Fundamental features of the Kyucho in the Bungo Channel were revealed by Takeoka and Yoshimura (1988) and Takeoka et al. (1993) through intensive field observations. It is an intrusion of warm water from the Pacific Ocean into the eastern half of the Bungo Channel, being driven gravitationally and advancing with the coast on its right side. It has seasonal and spring-neap periodicities, occurring mainly in summer and in neap tidal periods. The observations of the Kyucho were, however, mainly long term

2 700 H. Takeoka et al. measurements of water temperature (partly including current measurements) in the bays along the coast and CTD casts in the Bungo Channel. Hence, detailed structures of the Kyucho, especially current structure in the offshore region, have not been revealed yet. Observations by HF radar were expected to solve this problem. Moreover, since spatial and temporal changes in current structures due to the Kyucho were supposed to be prominent, the Kyucho seemed to be a good object to test the performance of the radar systems. The present paper reports the results of the observations of the Kyucho in the Bungo Channel by the HF radars carried out in summer in Table 1. Specifications of the HF ocean radar of CRL. Radar type Frequency Modulated Interrupted Continuous Wave (FMICW) radar Center frequency MHz Range resolution 1.5 km Transmit power 100 W (peak), 50 W (average) Antenna type 10-element phased array antenna Beam width 15 Beam directions ±45 (13 directions) Fig. 1. Map of the Bungo Channel. The radars are located at Sata Peninsula and Ohama. S1 to S12 and O1 to O12 show directions of the beams. K1 to K12 are the monitoring stations of water temperature. The closed triangle in Hayasui Straits denotes the station of the tidal current prediction by Maritime Safety Agency.

3 Observation of the Kyucho in the Bungo Channel by HF Radar Specifications of the HF Radar and Observations Table 1 shows the specifications of the HF ocean radar systems of CRL. A phased array antenna of 60 m long is used and the radar beam can be steered electrically within ±45 from the center. Sea echoes over the range of 90 km can be detected in favorable conditions. When wind blows hard, the maximum observable range becomes shorter, since sea waves attenuate the radio wave propagation. On the other hand, the range below 5 km is within the blind range due to the transmit-receive switching and cannot be observed. The radio wave is backscattered by the sea surface waves with half the wavelength of the radio wave. The measured current speeds represents an integrated value over the surface layer effective in advecting the surface waves. The thickness of this effective surface layer is said to be about 1/4π of the surface wavelength and Fig. 2. Original current data at each bin (denoted by range number) in beam S8.

4 702 H. Takeoka et al. hence 1/8π of the radio wavelength (Stewart and Joy, 1974), and it is about 0.5 m for the radars of CRL since the radio wavelength is 12 m. Iguchi et al. (1989) and Ohno (1991) are to be referred for the details of the radar hardware. Observations by these HF radars were made at the two sites shown in Fig. 1. The lines denoted by S1 to S12 and O1 to O12 show the directions of beams from each radar site. Radial currents in each bin of radially 1.5 km and tangentially 7.5 were obtained by analyzing backscattered signals. A ten minute measurement was made in each beam in turn, hence, the interval of the measurements in each bin was 2 hours. These observations were carried out from 4 to 29 July 1992, but data at Ohama were not obtained after 16 July due to some trouble in the radar system. Observations of water temperature were also made at 5 m below the sea surface at 11 stations (K1 to K11 in Fig. 1) along the eastern coast of the Bungo channel. The interval of these measurements was 30 minutes. These observations of water temperature are a part of the surveys to develop a Kyucho prediction system carried out by Ehime Prefectural Fisheries Experimental Station. The surveys were started in 1991 and most of them are now still ongoing. Data at Stns. K2, K3 and K8 were not obtained during the period of the radar observations. 3. Results of the Observations Since the current data obtained by the HF radar measurements are of enormous amount, only those along the beam S8 are shown here as examples. Figure 2 shows the radial current speeds (positive northward) from range 3 to range 40 in beam S8 during the observation period. Prominent semi-diurnal tidal currents appear in this figure, and quality of the data seems rather good. Fig. 3. Records of the water temperature at the stations along the eastern coast of the Bungo Channel.

5 Observation of the Kyucho in the Bungo Channel by HF Radar 703 Figure 3 shows the water temperature at the stations shown in Fig. 1 during the observation period. At Stn. K1, rise of water temperature by 3 to 4 C appears twice; the former occurred 6 to 7 July and the latter 20 to 21 July. These rises of water temperature propagated northward, showing that these events are due to the Kyucho. 4. Analyses and Discussions 4.1 M 2 tidal ellipses Since tidal oscillations dominate the observed surface currents as shown in Fig. 2, we performed harmonic analysis. Here, only the M 2 tidal ellipses are shown to check the accuracy of the observed current speed. The area covered by both beams is divided into 3 km square grids. Radial current speeds at each grid point are obtained by averaging the data in the three nearest bins with the reciprocal of distance as a weight. The resultant data are analyzed by the least square method to obtain harmonic constants of 10 major constituents. Figure 4(a) shows the M 2 tidal ellipses thus obtained, and Fig. 4(b) those obtained by analyzing long-term but intermittent ADCP data (Takeoka and Kikuchi, 1991). Within the area common to both distributions, both tidal ellipses are quite similar in current speed, direction, phase, ellipticity and sense of rotation. To check the relation between both current speeds into more detail, speeds of major axes of the tidal ellipses denoted by 1 to 7 in Fig. 4(b) and those of the tidal ellipses at the grids nearest to the 7 stations are plotted in Fig. 5. At stations 6 and 7, current speeds by the HF radar are rather different from those by ADCP. However, these stations Fig. 4. (a) Tidal ellipses of M 2 constituents determined from the HF radar observations. (b) Those determined from the ADCP observations (after Takeoka and Kikuchi (1991)).

6 704 H. Takeoka et al. Fig. 5. Relation between the speeds of major axis of M 2 tidal ellipses obtained from HF radars and ADCP. Numerals denote locations shown in Fig. 4(b). are close to the islands, and the differences may be attributed to locality and complexity of the tidal currents near islands. Except stations 6 and 7, both speeds agree quite well; the differences are within a few percent. Therefore, the accuracy of the current measurement by the HF radar seems good; at least, it has the same level of accuracy as ADCP. 4.2 Current structures of the Kyucho Tidal signals should be eliminated from the original current data to figure out the Kyucho. This can be done by predicting the tidal currents using the obtained harmonic constants and subtracting them from the original data. However, as shown in the subsequent section, some enhancement of the tidal amplitudes appeared in the neap tidal periods when the Kyucho occurred, and tidal signals cannot be totally eliminated in these periods by the above method. Therefore, we simply take 24-hour running mean to eliminate the tidal signals. In Fig. 6, strong northward currents appear twice; the former 6 to 11 July and the latter 23 to 27 July. These northward anomalies propagate northward taking a few days. Timings of these events agree well with those of the rises of water temperature shown in Fig. 3. Hence, we see that the northward currents are due to the Kyucho, and the HF radar successfully observed it. Horizontal current vectors can be drawn only for the former Kyucho, because the data at Ohama could not be taken during the latter period. Figure 7 shows the time change of 24-hour mean currents in 12 or 24 hour steps during the former Kyucho. In the following, we examine Fig. 7 in detail to see the change of the current field by the Kyucho. Distribution of the currents in Fig. 7(a) is before the Kyucho. Strong southeastward currents appear in the northwest region. They are residual currents usually occur around straits of strong tidal currents, and commonly appear throughout the following panels in Fig. 7. Except this region, the currents are generally weak. Figure 7(b) is a day after Fig. 7(a), and about a half day after the water temperature at Stn.

7 Observation of the Kyucho in the Bungo Channel by HF Radar 705 Fig. 6. Current data processed by 24-hour running mean at each bin in beam S8. K1 began to rise. Rather strong northeastward currents appear in the southeastern end of the observation area, showing that the Kyucho begins to intrude into the observation area. In Fig. 7(c), a half day after Fig. 7(b), the area of strong northeastward currents is extended. In Fig. 7(d) the area of the strong currents is further extended and northward currents appear in the central region of the Bungo Channel, still the area of the strong currents is restricted in the south of Hiburi Island. The rises of water temperature at Stns. K4 and K5 begin around this time. In Figs. 7(e) and (f), the strong northward currents prevail in the eastern part of the Bungo Channel, showing that these are of the matured stage of the Kyucho. However, the intrusion into the bay off Stns. K4 and K5 has almost ceased at this stage, and a stagnant area appears in the bay. The water temperature at Stn. 5 attains maximum around the time of Fig. 7(e). Moreover, the stagnant area in the bay is gradually extended from Figs. 7(e) to (f), and in Fig. 7(f) the strong northward currents form a clockwise curve turning around the stagnant area. These features show

8 706 H. Takeoka et al. Fig. 7. Time change of the 24-hour mean current vectors. The time in each panel denotes the center of the 24-hour mean. that the Kyucho propagates further north after filling the bay along the eastern coast with the warm water. Further, it should be mentioned that, as a result of such a change in the currents, the current directions in the southeastern part of the observation area are almost perpendicular to those in the initial stage of the Kyucho as shown in Fig. 7(b) or (c). In Fig. 7(g), the northward intrusion has almost ceased in the southern half of the observation area, while the strong northward currents still remain in the northern half. In this figure, offshore currents occur in the bay off Stn. K4. Koizumi (1991) shows through intensive CTD observations that a considerable part of the warm water which once intruded into the bay by the Kyucho flows out of the bay after the end of the Kyucho. The offshore currents in the bay off Stn. K4 may be a part of such discharging process of the warm water from the bay. In Fig. 7(h), the strong northward currents have weakened almost everywhere over the observation area, showing that the Kyucho has ceased. The water temperature at Stn. K1 attains maximum around this time. Thus, the HF radar observations revealed the current structures and their change with the Kyucho event into considerable detail. Figures 8(a) to (c) schematically summarizes the time change in the current structure of the Kyucho in the initial, matured and terminal stages, respectively. In the initial stage, the strong currents appear in the southeastern region of the

9 Observation of the Kyucho in the Bungo Channel by HF Radar 707 Fig. 7. (continued). observation area, and the region of the strong currents gradually extends to the north. In the matured stage, the strong currents forms a clockwise curve, turning around the stagnant region in the bay filled with the warm water of the Kyucho. In the terminal stage, the northward intrusion cease in the southern region, while the northward intrusion still persists in the northern region, and the offshore currents appear in the bay. These features, which would be hardly revealed by traditional observation methods, will be useful for further investigations on the dynamical structure of the Kyucho. 4.3 An anomalous feature of the tidal currents related with the Kyucho Although the tidal currents are not our main research subject in the present study, it is worthwhile to mention an anomalous feature of the tidal currents in this section, because it is suggested to be caused by the Kyucho. Figure 9 shows the observed and predicted tidal currents at range 28 in beam S8 during 21 to 25 July. The observed tidal current denoted by solid line is calculated by subtracting the residual current obtained by the 24-hour running mean from the original time series. The predicted tidal current denoted by broken line is calculated from the harmonic constants of 10 major constituents obtained in the present study. Speeds of the observed and predicted tidal currents are rather close in 21 and 22, but, in 24 to 25, the observed current speeds are often much

10 708 H. Takeoka et al. Fig. 8. Schematic current structures of the Kyucho in the initial stage (a), matured stage (b) and terminal stage (c). Fig. 9. Observed and predicted tidal currents at range 28 (42 km from the radar site) in beam S8 during 21 to 25 July. larger than the predicted ones. Such enhancements of the tidal amplitude often occur also in other bins during the periods of the Kyucho events. To show this more clearly, tidal oscillations in each day are compared with those at a reference point. A location where tidal currents are not affected by such disturbances as observed here is preferable as the reference point. Therefore, the station in Hayasui Straits denoted by the closed triangle in Fig. 1 is selected as the reference point, where the harmonic constants obtained by Maritime Safety Agency are available. The observed tidal currents by the HF radar and the predicted tidal currents at Hayasui Straits are compared in the following manner. The original current data are divided into the series of data sets of one-day length, and the currents in each day are approximated by diurnal and semi-diurnal oscillatory components by a least square method. Thus, their amplitudes are obtained for each day and each bin. The same calculations are performed for the tidal currents at Hayasui Straits, and ratios of the semi-diurnal amplitude in each bin to that at Hayasui Straits are obtained. Further, these ratios are averaged over the observation period and the anomalies of the ratio from the average are

11 Observation of the Kyucho in the Bungo Channel by HF Radar 709 obtained. Figure 10 shows the day-to-day change of the anomaly of the amplitude ratio in the bins in beam S8. Quite interestingly, the positive anomalies, filled with black, are arrayed rather regularly. Comparison between this figure and Fig. 6 suggests a close relation of these anomalies with the Kyucho; most of the positive anomalies occur when the residual currents change from southward to northward or just after this change, though there are some exceptions. Therefore, the enhancements of tidal amplitude occur near the leading edge of the intruding warm water. Two possible explanations are supposed for the reason of such enhancements of the tidal amplitude. The first is a technical matter in the Eulerian current observation. If the amplitude of the tidal current is sufficiently larger than the speed of the Kyucho, the leading edge of the Kyucho, which once passed an observation point during a flood period, retreats southward off the observation point during the following ebb. Then it often occurs that the current speed in the flood is enlarged by the speed of the Kyucho, while that in the ebb is kept unchanged. This results in the enhancement of the apparent tidal amplitude, which never occurs in the Lagrangian current speed both in the warm water of the Kyucho and in the pre-existing water. The duration of such Fig. 10. Day-to-day change in the anomalies of the ratio of semi diurnal current amplitude at each bin to that of the predicted tidal current at Hayasui Straits.

12 710 H. Takeoka et al. a process depends on the difference between the speeds of the tidal current and the Kyucho; the larger this difference is, the more often such a process repeats. To estimate the largest duration of the process in the present case, the speeds of the tidal current and the Kyucho are assumed to be (at most) 50 cm/s and (at least) 10 cm/s, respectively. Simple calculations show that the apparent enhancements of the tidal amplitude occur only during two tidal cycles (only one day) in this case. In Fig. 7, however, the positive anomalies continue often two or sometimes three days. Therefore, the above process cannot totally explain the enhancements of the tidal amplitude, though it may contribute to them to some extent. Since stratification is suddenly formed by the intrusion of the warm water of the Kyucho and this structure is oscillated by the tidal currents, it is likely that internal tides are generated in the interface between the warm water and the lower cold water, leading to the enhancements of the tidal current speed. This is the second possible explanation. However, this is merely a hypothesis, and we have no observational evidence for the generation of the internal tides around the leading edge of the Kyucho. Therefore, the causes of the enhancements of the tidal current amplitude around the leading edge of the Kyucho are not totally revealed yet, and require future investigations. 5. Conclusion Observations of the Kyucho in the Bungo Channel by the HF radars were successfully carried out. The current ellipses of M 2 constituent obtained by the observational results agree quite well with those obtained by the ADCP observations, showing that the accuracy of the HF radar measurements is of the same level as ADCP. The current structures and their change with the Kyucho were revealed into considerable detail. The Kyucho is influenced by the complicated coastal geometry and does not propagate straightly into the Bungo Channel. It propagates northward after charging the coastal bays with the warm water. The current directions change largely, since the currents turn around the stagnant region in the bay filled with warm water. The northward intrusion begins to be weakened in the southern part of the channel, while it still persists in the northern part. The northward current speeds of the observed Kyucho are about 50 cm/s and sometimes attain 60 to 70 cm/s. Further, the enhancements of the tidal current amplitude are found. They occur concurrently with the Kyucho, suggesting a close relation of the Kyucho with this phenomenon, but the cause of this phenomenon is not clear. Akiyama and Saitoh (1993) shows by analyzing NOAA satellite imageries that the Kyucho in Sukumo Bay, which is located in the southern end of the Bungo Channel, occurs when a warm filament formed in the Kuroshio front comes across Sukumo Bay. Our recent study, which will be reported in the near future, shows that most of the Kyucho in the Bungo Channel are originated by the warm filament from the Kuroshio. Therefore, if we deploy HF radars in the southern part of the Bungo Channel and monitor the currents associated with the warm filament, it will be quite useful for prediction of the Kyucho. The prediction of the Kyucho is desired for the management of fish and pearl cultures in the bays along the eastern coast of the Bungo Channel, which are at the highest production in Japan. Not only an academic use but also such a practical use of HF radar may be necessary for the development of the HF radar technology. Acknowledgements The authors express their sincere thanks to Prof. T. Yanagi of Ehime University for his helpful discussions and comments. They also give special thanks to Mr. H. Akiyama of Ehime University, Mr. Y. Koizumi of Ehime Prefectural Fisheries Experimental Station and members

13 Observation of the Kyucho in the Bungo Channel by HF Radar 711 of Seto Town Office, Tsushima Town Office, Kitanada Fisheries Cooperative Union for the kind cooperation in the observations. The data processing was carried out on FACOM M-1600/6 of Ehime University General Information Processing Center. References Akiyama, H. and S. Saitoh (1993): The Kyucho in Sukumo Bay induced by Kuroshio warm filament intrusion. J. Oceanogr., 49, Iguchi, T., T. Umehara, Y. Ohno and K. Kozaki (1989): Observation of ocean currents and sea state with an HF ocean radar. Rev. Com. Res. Lab., 35, (in Japanese). Koizumi, Y. (1991): The process of water exchange in Shitaba Bay during the phenomenon of Kyucho. Bull. Coast. Oceanogr., 29, (in Japanese). Matthews, J. P., A. D. Fox and D. Prandle (1993): Radar observation of an along-front jet and transverse flow convergence associated with a North Sea front. Cont. Shelf Res., 13, Ohno, Y. (1991): HF ocean radar observations of ocean current. J. Com. Res. Lab., 38, Ohno, Y. (1993): Present stage of the HF ocean radar at Communications Research Laboratory and results of experiments. Navigation, 116, (in Japanese). Prandle, D. (1987): The fine-structure of nearshore tidal and residual circulations revealed by H.F. radar surface current measurements. J. Phys. Oceanogr., 17, Shkedy, Y., D. Fernandez, C. Teague, J. Vesecky and J. Roughgarden (1995): Detecting upwelling along the central coast of California during an El Ninõ year using HF-radar. Cont. Shelf Res., 15, Stewart, R. H. and J. W. Joy (1974): HF radio measurements of surface currents. Deep-Sea Res., 21, Takeoka, H. and T. Kikuchi (1991): A method of estimating tidal currents using ADCP data. Bull. Coast. Oceanogr., 29, (in Japanese). Takeoka, H. and T. Yoshimura (1988): The Kyucho in Uwajima Bay. J. Oceanogr. Soc. Jpn., 44, Takeoka, H., H. Akiyama and T. Kikuchi (1993): The Kyucho in the Bungo Channel, Japan Periodic intrusion of oceanic warm water. J. Oceanogr., 49,