The aim of this investigation is to understand the surface current circulation pattern off Kuala Terengganu. A drifting drogue was used to measure surface current during April 1993, the transitional period between the northeast and southwest monsoon seasons. Our results show that the drifting drogue moves in an almost circular motion, suggesting the

presence of an eddy. However, further studies are needed to confirm the existence of an eddy off Kuala Terengganu during the transitional period

Marghany, Meged Mahmoud

Saadon, Mohd Nasir

Universiti Putra Malaysia Institutional Repository

Pertanika J. Sci. & Techno!. 4(1): 141-148 (1996)ISSN:0128-7680© Penerbit Universiti Pertanian MalaysiaCOMMUNICATIONSurface Circulation offKuala Terengganu in the Transitional Periodbetween the Northeast and Southwest MonsoonsMohd. Nasir Saadon and Meged Mahmoud MarghanyPusat Perikanan dan Sains SamudraUniversiti Pertanian MalaysiaMengabang Telipot, 21030 Kuala Terengganu, MalaysiaReceived 17 October 1994ABSTRAKTujuan penyelidikan ini ialah untuk memahami corak peredaran aruspermukaan di perairan Kuala Terengganu. Drog hanyut telah digunakan,pada bulan April 1993, untuk mengukur arus permukaan. Bulan Apriladalah musim peralihan di antara monsun timur laut dan barat daya.Keputusan kami menunjukkan drog hanyut yang digunakan bergerakdalam satu bulatan. Pergerakan sebegini biasanya menunjukkan ke-wujudan pusar. Walau bagaimana pun, kami berpendapat kajian yanglebih mendalam perlu dilakukan untuk mengesahkan kewujudan pusar diperairan Kuala Terengganu dalam musim peralihan.ABSTRACTThe aim of this investigation is to understand the surface currentcirculation pattern off Kuala Terengganu. A drifting drogue was used tomeasure surface current during April 1993, the transitional periodbetween the northeast and southwest monsoon seasons. Our results showthat the drifting drogue moves in an almost circular motion, suggesting thepresence of an eddy. However, further studies are needed to confirm theexistence of an eddy off Kuala Terengganu during the transitional period.Keywords: surface current circulation, coastal erosion, drogue,eddy, knot, nautical nrilesINTRODUCTIONDue to coastal erosion along the Kuala Terengganu coastline, currentcirculation has been of interest to many researchers. A million dollar projectto prevent further erosion has been initiated. Although many studies havebeen done, the understanding of surface current circulation patterns in thisarea remains unclear.Kawamura (1986) used satellite data to show that circulation in theGulf of Thailand headed southwards, along the east coast of PeninsularMohd. Nasir Saadon and Meged Mahmoud MarghanyMalaysia, to Kuala Terengganu. In another report, Liew et al. (1987) foundthat even at 85 km from the coastline, tides have some influence on both thespeed and direction of surface currents. Saadon and Rosnan (1991), using adrifting drogue, found that tide is the dominating factor shaping the surfacecurrent circulation. A study on the sub-surface current in the same area alsoindicated that tide plays a major role in current circulation in KualaTerengganu waters (Saadon and Baharim 1992).Previous studies were mainly conducted during the southwest andnortheast monsoon seasons. Therefore, current circulation studies duringthe transitional period are still lacking. The aim of this study was to gainsome understanding of the surface current circulation off Kuala Tereng-ganu during the transitional period when the wind is generally weaker andmore variable than during the monsoon seasons (Chua 1984).There are many ways of measuring surface currents in shallow waters.One is by the use of drogues, which are inexpensive and easy to handle.Even with the presence of modern current meters, there are situations wheredrogues are more useful. Our results show that the path taken by the droguewas circular. Because this was the first attempt to study surface circulationduring the transition period, the authors feel that further studies are neededto confirm the existence of an eddy.MATERIALS AND METHODSThe drogue used in this study had two intersecting sails of 2.5 m length and0.6 m width (Fig. 1). A weight below it ensured that it sank until its tetherrope was taut and essentially vertical. The tether rope was polyethylene, 3m in length and 5 mm in diameter. The surface float was a styrofoam boardof size 50 x 50 x 4 cm . The flag mast was made from PVC tubing 2 m inlength and 2 cm diameter. A small weight at the lower portioI1 of the flagmast stabilized the mast, which carried a marker flag of 30 x 20 cm abovethe water surface. It also carried a beacon for night-time observation.The drogue was deployed and its movement observed twice in the studyarea. The first deployment was at Lat. 5° 21.0' N and Long. 103° 14.0' E(about 5 nautical miles from the coastline) on 4 April 1993, at 1200 h. Thedrogue was allowed to drift for 53 hours and was recovered on 6 April 1993,at 1700 h. The next deployment started at 0800 h on 7 April 1993, at Lat. 5°21.0'N and Long. 103° l8.0'E (about 8 nautical miles from the coastline).It was recovered 49 h later on 9 April 1993, at 0900 h.The position of the drogue was taken hourly by bringing the researchboat (UNIPERTAMA III) close to the drogue during each deployment.The exact position of the drogue was determined with the use of a globalpositioning system (GPS) installed in the research boat. All positions werethen plotted on navigational chart No. 771.142 Pertanika J. Sci. & Techno!. Vo!. 4 No.1, 1996Surface Circulation off Kuala Terengganu, I ,_:~:,:_.__ Beacon- Marker Flag,~Styroform Surface FloatWeightFig. 1. The drogue used in this studyRESULTSIn the first deployment (Fig. 2) the drogue moved northward very fastimmediately after release with a speed of approximately 1.5 knot (0.77 m/s).One hour later its speed slowed, but the net direction of movement wasnorthward. During this period the tide was flooding. This northwardmovement continued for 7 hours. At 1900 h on 4 April 1993, it started totake a southerly direction, coinciding with ebb tide. This southerly directionlasted for 5 h before the drogue moved north again and then northeast till1100 h on 5 April 1993. During this period the tide was low at 0148 h, highat 0700 h and low again at 1246 h.After 1100 h on 5 April 1993, and with tide flooding, the drogue movedsouthward until 2000 h. It then moved west for 4 h, north for 1 h, and eastfor 2 h. The tide was ebbing. Then it moved south and westward. At 0800 hon 6 April it moved north until it was recovered. During this last portion ofits journey the tide was flooding. Throughout the whole 53-hour period, thedrogue travelled a distance of about 21 nautical miles with an average speedof about 0.4 knot (0.2 m/s).In the second deployment (Fig. 3) the drogue moved towards north in azigzag manner for 10 h. At the time ofrelease the tide was ebbing. Then itmoved south, also in a zigzag manner, for 14 h, during which time the effectPertanika J. Sci. & Techno!. Vo!. 4 No. I, 1996 143Mohd. Nasir Saadon and Meged Mahmoud Marghany..:-.;,103' 05' E..:;....Universiti Pertanian :~_Malaysia ",:,". 1"03' 1 O' E 103' 15' E 103' 20' E'-. ~-------------,, Tidal heights (m) 5' 30' N'L2 ,C) ..... DO,......"1'<:> V c:> -...,/ <> \oj"':::-:"---:":;;;'05;;--0a-,.-s"7...;-:::,..;:------C..::":/075' 15' NFig. 2: The path taken by the droguefrom 4 April (1200 h) to 6 April 1993 (1700 h). Each smalldot represents the hourly position of the drogue. The data for the tidal curve during the firstdeployment are taken from Anon (1993). The arrow shows the average wind direction during theobservation period. The bathymetric contours are in metres.of tide on its movements was not very obvious. It then meandered eastwardand then slightly south before it was recovered. In this period the tidal effectwas also not very obvious. From 7-9 April 1993, the drogue had drifted for49 h, covering a distance of about 13 nautical miles with an average speed of0.27 knot (0.14 m/s).Table 1 shows that the wind direction throughout the observationperiods was northeasterly. Comparing this wind direction with the routetaken by the drogue In the first deployment, there seems to be no correlationbetween them. Similarly, in the first half of the second deployment there isalso no correlation between the direction of wind and drogue movement.However, in the second half of the second deployment, when the droguedrifted generally westward, the general direction of the drift seems to havesome correlation with wind direction.144 Pertanika J. Sci. & Technol. Vol. 4 No. I, 1996Surface Circulation off Kuala Terengganu.~ 03" 00' E 103'05'E 1030 10' E 103· 15' E 1030 20' E. ";. Tidal heights (m) 5' 30' N..c:~ <l.~ ,'1i'\? .. \' /eEnd: Apr, 3. 1993: Start: Apr, 7.1993:0900 h,., 0800 h,., 5' 20' NI5° 25' N":IiII5' 15' N-312~ r- / ..."'1....'/ =- c=: "J0-04/:;;:;O',-----;O"'"4/=O';-D-a-te-5-:07:"0;:;:'--7.04/'10SOUTH CHINA SEABatu Rakit"..PENINSULAR MALAYSI:'\(TERENGGANU)AF~~uala.. TerengganuIt = '::~".( ~~ \~\'\J\f Chendering Head~ Kapas IslandFig. 3: The path taken by the drogue from 7 April (0800 h) to 9 April 1993 (0900 h). Each smalldot represents the hourly position of the drogue. The data for the tidal curve during the seconddeployment are takenfrom Anon (1993). The arrow shows the average wind direction during theobservation period. The bathymetric contours are in metres.TABLE 1Records of daily mean wind speed and direction inKuala Lumpur during the observation periodsData Mean Speed (mJs) Direction (Degrees)April 4, 1993April 5, 1993April 6, 1993April 7, 1993April 8, 1993April 9, 1993Average1.41.41.21.01.81.31.3570508080309066.67(Source: Malaysian Meteorological Service)Pertanika J, Sci. & Techno!' Vol. 4 No.1, 1996 145Mohd. Nasir Saadon and Meged Mahmoud Marghany:·"',,":-:::"'-::::'05:-:::"'-::::'06,--;;;"";;;;0',-"':::-';;;;0',-;;:-"";;;;0'-;;:"'''0Dates103" 10' E 103- 15' E 1030 20' E1r:T:::"id7a717h--:ei,-g:-ht,-s--;(-m7)--------,s. 30· N~!iI~ '·""1:~/.,..··; /~-':~ \ ..._/'.\./~.SOUTH CHINA SEA103-05'EPENINSULAR MALAYSiA(TERENGGANU) •f~~ j(u~.la T..erengganu~ '\o .:\\"~~\,/. Chendering Head'~'.~ Kapas Island.Fig. 4: The two paths taken by the drogue during theftrst deployment (solid line) and seconddeployment (broken line) are superimposed. Tidal curve during the observation period is taken fromAnon (1993). The arrow shows the average wind direction during the observation period.When the two paths of the drogue in the two deployments are super-imposed (Fig. 4), it is found that the drogue moved in one big circle. Withinthis circle there are two locations (approximately Long. 103° 14'E andLong. 103° l7'E) where the drogue kept moving in a north to southdirection and vice versa, which makes the paths of the drogue very con-fusing and difficult to analyse.DISCUSSIONThe South China Sea has a monsoon climate. To better understand thedynamics of the monsoon seasons, the reader is referred to Morgan andValencia (1983) and Saadon and Camerlengo (1995).The monsoon winds generate the current circulation in the east coast ofPeninsular Malaysia. Coastal currents generally flow parallel to the coast-line. The current flows southward (northward) during the northeast (south-west) monsoon (Saadon and Camerlengo 1995). However, the currentpattern during transitional periods is not clear.146 Pertanika J. Sci. & Techno!. Vo!. 4 No. i, 1996Surface Circulation off Kuala TerengganuThroughout the observation periods, the average wind blew mainlyfrom the northeast. However, the drogue did not move southward, exceptfor the second half of the second deployment (Fig. 3). The wind is generallyweak during the transitional periods, so the current circulation is not welldefined. Our results show that the surface circulation moves in a large circle(Fig. 4). Water moving in a circular motion usually indicates the presence ofan eddy. However, eddies are usually generated by the horizontal shear ofthe mean flow.Eddies extract energy from the mean flow kinetic energy. Thus, energyis supplied to the eddies by the horizontal shear of the mean flow. The studywas conducted during the transitional period, when winds are light andvariable. It is generally admitted that surface ocean currents are driven bywinds. Therefore, it would be premature to suggest (within this transitionalperiod) the existence of a mean current flow off Kuala Terengganucompared to the mean current flow existing during both monsoon seasons.CONCLUSIONThis was the first attempt to study the surface circulation during thetransitional period. Our results show that the drogue took an almostcircular motion, which usually indicates the presence of an eddy. We feelthat further studies are needed to determine the possibility of an eddy offKuala Terengganu during the transitional period.ACKNOWLEDGEMENTSThe authors would like to thank UPM for funding this project and the crewof UNIPERT AMA III for helping in the data collection.REFERENCESANON. 1993. ]adual Ramalan Pasang Surut (Tide Tables) Malaysia 1993. Kuala Lumpur:Jabatan Ukur dan Pemetaan Malaysia.CHUA, T.E. 1984. Physical environments of the east coast of Peninsular Malaysia. InCoastal Resources ojEast Coast Peninsular Malaysia, ed. T.E. Chua andJ,K. Charles, p.1-10. Penang: Universiti Sains Malaysia Press.KAWAMURA, G. 1986. A preliminary survey of the sea surface in Malaysian waters andthe Gulf of Thailand with the aid of satellite data utilization system. In EkspedisiMatahari '85, ed. A.K.M. Mohsin, M.1. Mohamed, M.A. Ambak, p. 45-48 Facultyof Fisheries and Marine Science Occasional Publication No.3. Serdang: UniversitiPertanian Malaysia Press.LIEW, H.C., KHALID SAMO and S. MASUMITSU. 1987. Subsurface currents off thesouth-western portion of the South China Sea. In Ekspedisi Matahari '86, ed.A.K.M. Mohsin, A.R. Ridzwan and M. A. Ambak, p. 17-22 Faculty of Fisheriesand Marine Science Occasional Publication No.4. Serdang: Universiti PertanianMalaysia Press.Pertanika J. Sci. & Techno!. Vol. 4 No. I, 1996 147Mohd. Nasir Saadon and Meged Mahmoud MarghanyMORGA ',JR. and M.J. VALENCIA. 1983. The natural environmental setting. In Atlasfor Marine Policy in Southeast Asian Seas, ed. JR. Morgan and M.J Valencia, p. 4 -17. Berkeley: Univ. of California Press.SAADON, M.. and M.H. BAHARIM. 1992. Current measurements in the coastalwaters off Kuala Terengganu during the northeast monsoon. In Proceedings of theThird ORI-LIPI Seminar on Southeast Asia Marine Science: oceanography for Fisheries, ed.T. Sugimoto, K. Romimohtarto, S. Soemodihardho and H. Nakata, p. 23-30.Tokyo: Ocean Research Institute.SAADO T, M.N. and A. CAMERLENGO. 1995. Response of the ocean mixed layer, offthe east coast of Peninsula Malaysia during the northeast and southwest monsoons.GEOACTA. [in press].SAADON, M.N. and Y. ROSNAN. 1991. The relationship of tide and wind to the coastalwater movements of Kuala Terengganu. In Proceedings of IEM/ICE Joint Conference1991 - Coastal Engineering in National Development, p. 16 - 30. Kuala Lumpur: Board ofEngineers.148 Pertanika J. Sci. & Techno!. Vo!. 4 No.1, 1996

Surface Circulation off Kuala Terengganu in the Transitional Period between the Northeast and Southwest Monsoons

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Surface Circulation off Kuala Terengganu in the Transitional Period between the Northeast and Southwest Monsoons

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