Journal of Oceanography Vol. 52, pp. 251 to 257. 1996

Short Contribution

Observation of Temperature and Velocity in the Coastal Water off Kuala ,

KEISUKE TAIRA1, MOHD NASIR B. SAADON2, SHOJI KITAGAWA1 and TETSUO YANAGI3 1Ocean Research Institute, University of Tokyo, Minamidai, 1-15-1, Nakano, Tokyo 164, 2Fisheries and Marine Science Center, Universiti Pertanian, Malaysia, Mengabang Telipot, 21030 , Malaysia 3Faculty of Engineering, Ehime University, Bunkyo-cho 3, Matsuyama, Ehime 790, Japan

(Received 29 May 1995; in revised form 4 September 1995; accepted 7 September 1995)

Mooring observation of current and temperature was made at 17.8 m layer of 19 m depth about 8 km east to Kuala Terengganu, . Harmonic analysis was applied to tidal currents for 30 days in September 1993, and to the tides observed at Chendering. The K1 tide was the largest both in tidal currents and the tides. Daily mean temperature, currents, sea level, and winds were analyzed from September 1993 to May 1994. Northeast Monsoon from December to February caused sea level rise of 50 cm and temperature lowering of 1°C.

1. Introduction An electro-magnetic current meter was moored from September 1993 to June 1994 at 17.8 m layer of 19 m depth at 5°18′ N and 103°14′ E, 8 km east off Kuala Terengganu, Peninsular Malaysia (see Fig. 1). Northeast Monsoon over the South Sea blows in winter with a wind speed of 7Ð8 m sÐ1 accompanying waves with wave height exceeding 3 m (Chua, 1984), and oceanographic observations are limited during the monsoon. Oceanographic data are requisite for design of protection plan against coastal erosion, and for preservation of marine environment and ecosystem. The mooring observation was planed to collect records of current velocity and temperature during the Northeast Monsoon. Harmonic analysis was applied to the record of current meter, and to the tide record observed at Chendering. Daily variation of temperature and sea level was analyzed together with wind speed and direction observed at Mahmud . Unfortunately, sensitivity of speed sensor of the current meter was hampered by biological fouling, and a limited conclusion was obtained for the current.

2. Mooring Operation and Data At 12:00 on 1 September 1993, a mooring was deployed from the R/V Unipertama-5 (467 ton) of Universiti Pertanian Malaysia at 5°18′ N and 103°14′ E (see, Fig. 1). Figure 2 shows the whole mooring line. Buoyancy was about 51 kg, and the anchor weight was about 50 kg. An electro-magnetic current meter with temperature sensor, ACM-8 of Alec Corporation, and an acoustic release, S-type of Nichiyu Giken-Kogyo Co., were used. Distance between the sensor of the current meter and the weight was 12 m. The mooring site was chosen about 50 m south to a submarine reef of Fisheries Agency to avoid damages by fishing activities. The water depth was measured by an echo sounder of the ship to be 19.0 m, and sensor depth of the current meter was 252 K. Taira et al.

Fig. 1. Locations of mooring station of the current meter, the tide station at Chendering, and meteorologi- cal observatory at .

Fig. 2. Mooring operation from R/V Unipertama-5 on 1 September 1993. Observation of Temperature and Velocity in the Coastal Water off Kuala Terengganu, Malaysia 253

17.8 m. The position was determined by a portable GPS receiver. On 12 June 1994, an acoustic command for release was sent from the R/V Unipertama-3 (19 ton) of the Universiti Pertanian Malaysia, but the mooring line did not come to the sea surface. About 10 days later, a fisherman found the mooring floating at the site. The release hook had been unlocked but it was covered thickly by barnacles. A storm made the hook to rotate. The sensor of the current meter was also covered by barnacles. The current meter was used for about six months at 50 m in the Shikoku Basin and the sensor was free from biological fouling (Taira et al., 1993). Recording time interval was chosen at 120 minutes. Horizontal two components are detected by a spherical sensing element of 9 cm in diameter with an accuracy of 0.5 cm sÐ1. Azimuth of the sensor was detected by a magnetic compass with a resolution of 0.2 degrees. A burst measurement, eight samples at 1 second interval, was made every 120 minutes. A platinum resistance thermometer with an accuracy of 0.01°C was used. The recovered current meter recorded temperature and velocity from 14:00 on 1 September 1993 to 12:00 on 24 May 1994. The record after 24 May 1994 was erroneous due to a low battery. Hourly data was estimated by interpolation, and nontidal variation was diminished by a tide killer filter (Hanawa and Mitsudera, 1985) for analysis of tidal currents. Hourly records of tide at Chendering, wind speed and direction at Sultan Mahmud Airport were kindly provided for the period from 1 September 1993 to 31 May 1994.

3. Tides and Tidal Currents Detided signals of the tide killer filter were subtracted from the east and north components of current record and from the tide record at Chendering. The start time of the records was 0:00 on 6 September 1993. A harmonic analysis with the least squares method (HARMSHRT by Odamaki, 1994, personal communication; see also, Odamaki, 1981) was applied to determine sixteen constituents from 30 day long records. Figure 3 shows the observed records and estimated wave forms. The upper panel shows the tide at Chendering. The observed one is well described by the estimated constants shown in Table 1. Variances were 1617.1 cm2 for the observed and 1565.1 cm2 for the estimated. A standard deviation of difference between the observed and the estimated was 5.9 cm. Observed tidal currents showed a significant decay with time. Amplitude ratios of the observed to the estimated were 2.4 at the start, 1.0 at 300 hours, and 0.53 at 600 hours. The sensitivity of velocity sensor was lowered by the biological fouling. A correction curve of a quadratic in time was obtained, and the corrected time series were analyzed again. The middle panel of Fig. 3 shows a comparison of the corrected with the estimated for the east component. Variances were 35.6 cm2sÐ2 for the observed and 25.7 cm2sÐ2 for the estimated. The standard deviation of the difference was 3.1 cm sÐ1. The bottom panel of Fig. 3 shows a comparison of the

Table 1. Harmonic constants of tides and tidal currents.

Tide East component of current North component of current

O1 29.1 cm (302°) 5.1 cm s Ð1 (189°) 6.6 cm s Ð1 (220°) K1 49.8 cm (345°) 5.2 cm s Ð1 (189°) 8.1 cm s Ð1 (264°) M2 31.5 cm (217°) 2.7 cm s Ð1 (224°) 5.5 cm s Ð1 (303°) S2 11.7 cm (251°) 1.2 cm s Ð1 (324°) 2.0 cm s Ð1 (328°) 254 K. Taira et al.

Fig. 3. Comparison of the observed (dots) with estimated by harmonic analysis (line) for the tide at Chendering (upper panel), east component (middle panel) and north component (bottom panel) of the current velocity. corrected with the estimated for the north component. Variances were 70.8 cm2sÐ2 for the ob- served and 56.3 cm2sÐ2 for the estimated. The standard deviation of the difference was 3.8 cm sÐ1. Azmy et al. (1990) report that M2 is the most predominant constituent on the coasts of Peninsular Malaysia and that the diurnal tides along the east coast are larger than those along the west coast. Table 1 shows that the K1 constituent is the largest of the four major constituents at Chendering. Azmy et al. (1990) presents a cotidal line by numerical experiment, and the propagation direction is northwest near Kuala Terengganu. Tidal ellipses derived from the constants in Table 1 had major axes in 36°T for O1, 15°T for K1, 7°T for M2 and 31°T for S2. A reanalysis of the tidal records at Malaysian coasts may be necessary. Maged (1994) made current measurements for about 5 days five times in a year from May 1992 at 2 m above the bottom of 10 m depth at a station about 1.5 km off the mouth of Kuala Terengganu River, and he showed that amplitudes of tidal currents were more than 1 m sÐ1. Figure 3 shows that the amplitudes are less than 20 cm sÐ1. Chua (1984) describes tidal currents sometimes exceed 1 m sÐ1 at the river mouths on the east coast of Malaysia. This may suggest that the tidal currents are amplified near the river mouth.

4. Daily-Mean Temperature Velocities and Tide Level A diurnal change of temperature was small, and a daily mean was calculated by averaging over 24 hours. Time series of daily means of temperature and velocities were obtained for 226 days from 1 September 1993 to 24 May 1994. The upper panel of Fig. 4 shows daily mean Observation of Temperature and Velocity in the Coastal Water off Kuala Terengganu, Malaysia 255

Fig. 4. Daily mean temperature (upper panel), daily mean east component (middle panel) and daily mean north component (bottom panel) of the current velocity. temperature. The mean was 28.4 ± 0.7°C. The highest was 29.9°C on 24 April 1994, and the lowest 27.3°C on 27 November 1993. Iwasaka and Hanawa (1990) and Tanimoto (1993) estimate monthly sea surface temperature in a square 5 × 5 degrees centered at 7.5°N and 102.5°E from ship reports in the period from 1950 to 1990. The mean over 492 months is 28.68 ± 0.95°C with the highest 30.05 ± 0.43°C in May and the lowest 27.24 ± 0.59°C in January. The daily temperature revealed several events such as a rapid cooling of 2°C in 10 days in September 1993. A cool event occurred in May 1994 with a lowering of about 2°C in 20 days. The middle and bottom panels of Fig. 4 show daily mean velocity components. The mean was Ð0.5 ± 4.5 cm sÐ1 for the east component, and 0.4 ± 5.4 cm sÐ1 for the north component. The maximum daily mean was 21.7 cm sÐ1 to the north on 26 March 1994, and the maximum hourly velocity was 31.8 cm sÐ1 to 334 degrees at 12: 00 on 12 March 1994. If the correction curve obtained in Section 3 is applicable, these speeds may exceed 1 m sÐ1. Figure 5 shows the daily means of tides at Chendering, wind direction and wind speed at Sultan Mahmud Airport. The latter were estimated by vector averaging over the hourly winds. The wind direction was steady in the Northeast Monsoon from December 1993 to February 1994, and the wind speeds were more than 5 m sÐ1. The sea level was raised by about 50 cm in the beginning of the monsoon. Azmy et al. (1991) numerically estimates sea level rise at Malaysian coasts caused by the wind, and shows that monthly sea level at Kuala Terengganu is the highest in December and the lowest in June and July. The range of sea level change in the monthly mean 256 K. Taira et al.

Fig. 5. Daily mean tide (upper panel), wind direction (middle panel), and wind speed (bottom panel). is about 40 cm. Figure 5 shows the range is about 80 cm in the daily mean. The monsoon caused lowering of temperature by 1°C.

Acknowledgements The authors are grateful to the officers and crew of Unipertama-5 and Unipertama-3 for their help on mooring operation. This study was supported by the Ministry of Education, Science and Culture, through Grant-in-aid for Scientific Research, International Scientific Research Program- Field Research, No.05041058.

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