Current Variation in the Sea near the Mouth of Suruga Bay*

Transcription

1 fl Journal of the Oceanographical Vol.40, pp.193 to 198, 1984 Society of Japan Current Variation in the Sea near the Mouth of Suruga Bay* Hideo Inabat Abstract: In order to investigate the circulation pattern and the characteristics of tidal currents in the sea near the mouth of Suruga Bay, current measurements were conducted at two stations at near-surface levels during the warm months of the year (late May to early October). The mean currents at the eastern and western stations are inversely correlated with each other: when inflow occurs at the eastern station outflow occurs at the western one and vice versa. The circulation pattern, therefore, can be determined from the mean current at either station, e.g., inflow or outflow at the eastern station corresponds to counterclockwise and clockwise circulation, respectively. The predominant tidal constituents of the measured currents are diurnal while those of the sea level along the bay are semidiurnal. The amplitudes of the diurnal constituents of the current show clear long-term variations besides the semimonthly variation corresponding to the phase of the moon. Variation in the path of the Kuroshio off the bay mouth seems to influence not only the circulation pattern but also the tidal currents in the sea near the bay mouth. When the Kuroshio axis is to the north of Zenisu, a shoal off the eastern side of the bay, the circulation pattern is counterclockwise and the amplitude of the tidal current is small. On the other hand, when the Kuroshio is to the south of Zenisu, the circulation pattern is weakly clockwise or stagnant and the amplitude of the tidal current is large. 1. Introduction Suruga Bay is an open and deep embayment on the southern coast of Honshu, Japan. The width and length of the bay are 55 km and 60 km, respectively. A submarine trough deeper than 1,000 m extends from the bay head to the eastern part of the bay mouth and connects to the Nankai Trough. In the area off lit-saki on the eastern side of the bay mouth lies the northern segment of the Izu-Ogasawara Ridge, which includes several islands such as KOzushima and Niijima. The southern end of the northern segment of the ridge is a shoal which is called Zenisu (see Fig. 1). The Kuroshio flows through the area off the bay mouth. Because of the characteristics of bottom topography, the current in the bay is influenced by variations in the path and strength of the Kuroshio. The circulation pattern in Suruga Bay has been studied by many investigators (e.g., Uda Fig.1. Topography of Suruga Bay and adjacent area. Locations of a moored current meter and Zenisu are shown by ci and 0, respectively. Areas shallower than 200 m are shown by vertical lines.

2 Inaba These studies show the predominance of a counterclockwise circulation. However, these studies were not based on direct current measurements with moored current meters, but on estimates from horizontal distributions of oceano graphic properties. The present author (Inaba, 1981) studied the circulation pattern near the bay head using data obtained from moored current meters and demonstrated that the circulation pattern is usually counterclockwise. The study of tidal currents using moored current meters has been limited so far to the sea near the bay head. Inaba (1981) reported that the amplitudes of the diurnal constituents of the measured current near the bay head are much larger than those of the semidiurnal constituents, while the amplitudes of the diurnal constituents of sea level along the bay are much smaller than those of semidiurnal constituents. Inaba also reported that the amplitudes of the diurnal constituents of the current exhibit large seasonal variations. In order to investigate the circulation pattern and the characteristics of tidal currents near the bay mouth, current measurements using a moored current meter were conducted at near-surface levels at two stations. It will be shown in the following sections that the circulation pattern alternates between counterclockwise and clockwise, with each circulation type having a duration of several days to a month, and that the diurnal constituents of the current are predominant and exhibit large long-term variation, just as was found for the sea near the bay head (Inaba, 1981). It will also be shown that the circulation pattern and the characteristics of the tidal current can be related to the location of the Kuroshio axis off the southern coast of Honshii. 2, Methods The current measurements were carried out at two stations in the bay mouth as shown in Fig. 1. The measurements at Stations E and W were made during the warm months (late May to early October) of the years 1977 to 1980 and 1979 to 1980, respectively, as shown in Fig. 2. The durations of the continuous measurements differed widely for each observation period, ranging from 11 days (Seg. 11) to 90 days (Seg. 7). Simultaneous measurements at both stations were made in two cases (Segs. 5 & 8; Segs. 7 Stations E and W were 20 m and 10 m, respectively, where the current velocity associated with surface wind waves can be considered small. Fig. 2. Periods of measurements with a moored current meter. Horizontal lines show the periods of measurements and numerals under the lines indicate the day of the month. Seg. followed by a numeral above the lines indicates the segment number of the record. Fig. 3. Example of current variations. Hourly values (solid lines) and 25-hr mean values (dashed lines) of the eastward and northward components taken 20 m below the surface at Station E from 16 June to 4 August 1977 (Seg. 1, Fig. 2). œand Z indicate the new and full moon respectively.

3 Current Variation in Suruga Bay Data on the semimonthly locations of the current at either of the stations. Kuroshio axis are taken from Kaiy6-sokuheo The long-term fluctuation of the mean current (rapid reports of oceanographic conditions, Maritime Safety Agency, ). various observation periods between 1977 and averaged over 25-hr intervals at Station E for The methods of harmonic analysis and the 1980 are shown in Fig. 5. It can be seen in the figure that the mean current maintained a tidal constituents have been described elsewhere given direction of inflow or outflow for several by the present author (Inaba, 1981). days to a month and the frequency of occurrence of inflow was approximately equal to that of 3. Results and discussion outflow. Considering the inverse correlation between the current at Stations E and W, inflow 3.1. Circulation pattern An example of the measured current variations or outflow at Station E indicates counterclockwise and clockwise circulation, respectively, is shown in Fig. 3 for the case of Station E from 16 June to 4 August, 1977 (Seg. 1). It can therefore, the frequency of occurrence of the two be seen that the mean speed of the northward circulation types is nearly equal. component changed from the beginning of the 3.2. Tidal currents record to the end, indicating that an inflow It can be seen in Fig. 3 that the diurnal (positive value) occurred during the first half of constituents of the current at Station E seem the record and an outflow (negative value) to predominate throughout the record and their occurred during the second half. amplitudes vary widely with a periodicity of In order to see the relationship between the about half a month corresponding to the phase currents in the eastern and western part of the of the moon, being large during spring tides and bay mouth, the mean current averaged over small during neap tides. A long-term variation 25-hr intervals of the record for simultaneous in the amplitudes of the predominant diurnal measurements at Stations E and W are shown constituents can also be seen in Fig. 3 in that in Fig. 4. It can be clearly seen in the figure the amplitudes during spring tides, during the that when an outflow occurs at Station E, an second half of the record, around 18 July and inflow occurs at Station W and vice versa. The 2 August, are much larger than those during correlation coefficients between the currents at the first half, around 19 June and 3 July Stations E and W are for the northward It is noteworthy that the large amplitudes of component and for the eastward component, i.e., the mean currents in the eastern correspond to inflow (positive value of northward the tidal constituents of the current seem to and western parts are inversely correlated with component) and the small amplitudes to outflow each other. The circulation pattern, therefore, (negative northward component). can be determined from the measured mean Fig. 4. Variations of mean currents based on simultaneous measurements at Stations E and W. The mean currents were averaged over 25-hr intervals of the record from 23 August to 12 September 1980 (Segs. 7 and 11, Fig. 2). Fig, 5. Variations of mean current at Station E. The mean current was averaged over 25-hr intervals of the records from 1977 to 1980 (Segs. 1 to 7, Fig. 2).

4 Inaba Fig. 6. Harmonic constants for the four major constituents, M2, S2, K1 and 01. The phases of the maximum eastward and northward velocities are measured from the time of meridian passage (138 30'E) of the fictitious heavenly bodies. Fig. 7. Example of the long-term variation of harmonic constants for the four major constituents of the eastward (full lines) and northward (dashed lines) components. Running harmonic analysis was carried out: harmonic analysis was performed for the first 15-days of the given segment of the record, and then it was repeated for subsequent 15-day periods, each starting 5-days after the start of the previous 15-day period. Fig. 6 shows the results of harmonic analyses for each segment of the record. It can be seen in the figure that the amplitudes of the diurnal constituents of the current are much larger than those of the semidiurnal constituents except in the case of Seg. 11. In contrast, the amplitudes of the diurnal constituents of sea level along the bay are much smaller than those of the semidiurnal ones (Japan Meteorological Agency, 1980). Figure 7 shows an example of the running. harmonic analysis for the case of Station E in 1977 (Seg. 1). It can be seen in the figure that the amplitudes of the tidal constituents of current show clear long-term variation: they are small in the first half of the record and large in the second half. Tidal currents measured in the sea near the bay mouth showed similar characteristics which suggested the occurrence of an internal tide (Inaba, 1981). It is not clear, however, whether the measured currents. near the mouth are also associated with an internal tide or not Relationships betweeen the circulation pattern and the tidal current and the location of the Kuroshio axis Figure 8 shows the relationship between the semimonthly mean current and the semimonthly location of the Kuroshio axis off IrO-saki, and the latter's relationship to the amplitude of the K1 constituent at Station E. The mean current and the amplitude of the K1 constituent were calculated for the semimonthly interval corresponding to that used for determining the location of the Kuroshio axis. It can be seen from the northward component in the figure that when the location of the Kuroshio axis is to the north of or close to

5 Current Variation in Suruga Bay SEMIMONTHLY MEAN CURRENT SEMIMONTHLY AMPLITUDE OF K1 CONSTITUENT Fig. 8. Relationship between the semimonthly mean current at Station E and the semimonthly location of the Kuroshio axis off Ith-saki (left), and the relationship between the latter and the semimonthly amplitude of the K1 constituent at Station E (right). Zenisu is shown by a dashed line in each figure. The east and north values of the semimonthly mean current and of the amplitude of the K1 constituent are shown by œ and, respectively. Zenisu an inflow occurs in the eastern part of the bay, indicating a counterclockwise circulation, and the amplitude of the K1 constituent is small. In contrast, when the location of the Kuroshio axis is to the south of Zenisu a weak or no outflow occurs in the eastern part, indicating a weak clockwise circulation or stagnation, and the amplitude of the K1 constituent is large. 4. Concluding remarks The mean current in the eastern part of the bay mouth is inversely correlated with that in the western part: when inflow occurs in the eastern part outflow occurs in the western part and vice versa. The circulation pattern, therefore, can be determined from the mean current at either the eastern or western station. The circulation pattern is either clockwise or counterclockwise, and the duration of each circulation type is several days to a month, which the frequency of occurrence of the two types is approximately equal. The predominant tidal constituents of the measured current and those of sea level differ: while the predominant constituents of the current are diurnal those of sea level are semidiurnal. The amplitudes of the diurnal constituents of the current show clear long-term variation in addition to the semimonthly variation corresponding to the phase of the moon. The characteristics of the tidal current near the bay mouth are similar to those near the bay head (Inaba, 1981). It was very noticeable that the path of the Kuroshio seems to influence not only the circulation pattern but also the strength of the tidal current near the bay mouth. It can be seen that when the Kuroshio axis is to the north of Zenisu, the circulation pattern is counterclockwise and the amplitude of tidal current is small. On the other hand, when the Kuroshio axis is to the south of Zenisu, the circulation pattern is weakly clockwise or stagnant and the amplitude of tidal current is large. Acknowledgements The author wishes to express his sincere thanks to Drs. Yoshitada A. Takenouti, Mashito Nakano, Ichiro Imai, Masami Hayakawa and Yutaka Nagata for their discussion and encouragement of this work. He is also very gratefull to Messrs. Hiroshi Moriya and Tsuneyoshi Yamanishi for their assistance in field measurements and data analyses and the staff of the Ship Management Section of Tokai University for their cooperation in field measurements. Appreciation is extended to Dr. Harold Solomon for his careful reading of the manuscript. This paper forms part of the author's doctoral dissertation at Tokai University completed in References Inaba, H. (1981): Circulation pattern and current variations with respect to tidal frequency in the sea near the head of Suruga Bay. J. Oceanogr. Soc. Japan, 37, Japan Meteorological Agency (1980): Tide Tables for the year Japan Meteor. Agency, Tokyo, 292 pp.(in Japanese). Maritime Safety Agency ( ): Kaiyo-sokuho (rapid reports of oceanographic conditions). Hydrogr. Dept., Tokyo, (mimeograph)(in Japanese). Nakamura, Y. and F. Muranaka (1979): Temporal Bay and Enshii-nada. Bull. Japan. Soc. Fish. Oceanogr., 34, (in Japanese). Uda, M., T. Nakao and A. Kishi (1977): Marine pollution in Suruga Bay and associated environmental change in relation to fisheries. J. Fac. Mar. Sci. Technol., Tokai Univ., 10, (in Japanese).

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