Characteristics of water exchange in the Luzon Strait during September 2006

Chinese Joumal of Oceanology and LimnologyVol. 27 No. 3. P. 650-657.2009DOI: 10.1007/s00343-009-9175-2Characteristics of water exchange in the Luzon Strait duringSeptember 2006*ZHOU Hui (周慧)', NAN Feng (南峰) t* SHI Maochong (侍茂崇)",ZHOU Liangming (周良明) ", GUO Peifang (郭佩芳)"tKey Laboratory of Occan Circulation and Wave Studes, Insitute of Oceanology, Chinese Academy of Sciences, Qingdao,266071. ChinatCollege of Physical and Enwironmental Oceanography, Ocean University of China, Qingdao 266100, ChinaReceived Aug.1, 2008; revision accepted Dec. 16, 2008AbstractThe Luzon Strait is the only deep channel that cnects the South China Sea (SCS) with thePacifc. The transport through the Luzon Strait is an important process infuencing the circultion, heatand water budgets of the sCs. Early observations have suggested that water enters the SCs in winter butwater inflow or outflow in summer is quite controversial. On the basis of hydrographic measurementsfrom CTD along 120° E in the Luzon Strait during the period from September 18 to 20 in 2006, thecharacteristics of temperature, salinity and density distributions are analyzed, The velocity and volumetransport through the Luzon Strait are calculated using the method of dynamic calculation. The majorobserved resuts show that water exchanges are mainly from the Pacific to the South China Sea in theupper layer, and the flow is relatively weak and eastward in the deeper layer. The net volume transport ofthe Luzon Strait during the observation period is westward, amounts to about 3.25 Sv. This result isconsistent with historical observations.Keyword: Luzon Strait; water exchange; dynamic calculation; volume transport1 INTRODUCTIONfrom the Pacific to the Scs in the 0- -300 m layer,The Luzon Strait between Taiwan Island and Luzonwhile from the SCS to the Pacific in the deeper layer,Islands is situated to the west of the north Pacific. Itand in summer (August), the direction is reversed, i.e.the water flows from the SCS to the Pacific in theconsists of narow water passages of different widths,0- 400 m layer, while from the Pacific to SCS inamong which the Babuyan, Balintang and Bashideeper layer. Nitani (1970) analyzed theChannels are the three biggest channels. The Luzonhydrographic data obtained during CSK and pointedStrait with complicated topography (Fig.1) is the generalout that in the 120°10' E section across the Luzonname of these channels and the passage for the waterStrait, the eastward and westward flows appearedexchange between sCS and the westem Pacific with itsalternately, the current speed was obviously changedwidh abut 360 km and the mean depth of 1 400 m.from year to year.The T-S curve of SCS water appears arFor example, the speed measured in summer 1965approximately reversed 'S', similar to the northwestermwas about half of the speed in 1966. Based onPacific water. The salinity of the subsurface andhistorical data, Huang et al. (1983, 1984) calculatedintermediate waters has a maximum value and athe volume transport for all the 4 seasons along aminimum value, respectively, providing an evidencezonal section of the Luzon Strait, and showed that thefor the northwesterm Pacific water inflow to the SCS.volume transport from the northwestern Pacific toHowever, the passway and from which incidence thethe SCS was the largest in winter, smaller in spring,Kuroshio water enters into SCS have not beenand negative in summer. Huang et al. (1986)determined. Based on the limited data ofanal中国煤化rmeasured duringtemperature and salinity, Wyrtki (1961) calculated"SuppProjet of CAS No.the differences of gravity potential between theKZCMYHC NMH Gard be Nanitnn Bainorth side and the south side of the Luzon Strait, anda (973 Program) (No. 403603)indicated that in winter (February), the water flows " Corespnding author: nanfeng0515@126.comNo.3ZHOU et al: Characteristics of water exchange in the Luzon Strait during September 2006651observed data and obtained that LST was 11-12 Svalong the 120° E section. Xu et al. (2004) calculated21.50-LST using the hydrographic data observed duringsummer in 1994 and winter in 1998, and found that(LST was 4.45Sv in winter and 2.0Sv in summer,respectively, accounted for about 15% and 6.9% ofthe Kuroshio's flux, i.e. LST in winter was muchlarger than in summer. Tian et al. (2006) analyzed the20.590temperature, salinity and ADCP data and pointed outthat the strong westward flow appeared at about100 m depth, while the eastward flow was limited inabyssal layer (usually near the 1 000 m depth).During the observation, the total LST was 6+3 Sv.19.50Study results described above show a consistenceabout the water exchange through the Luzon Strait.9oAs they agreed, the water mainly flows from thePacific to the SCs in winter. Whether it flowswestward or eastward in summer is still not clear. In18.59this paper, the observed data are used to analyze theLumonbladcharacteristics of temperature, salinity， density18°L上distributions along the section, and LST through the1199 119.59 120° 120.59121°121.59 E122°section is calculated. Finally the results in this studyFig.1 The topography of the Luzon Strait and the distributionare compared with those reported previously fromof observing stations which are symbolired by“o"。Theother studies.lower left panel is for depth along 120°E2 DATA AND METHODAugust and September in 1983, and thought that the2.1 Datawater exchange in the upper layer was small. Guan(1990) used the data obtained in July and August inIn this study, the data are obtained by means of1971 to analyze the ditributional characteristics ofCTD casts along 120° E section on board of RVthe geostrophic current, temperature, salinity,'Experiment 3'during a cnuise from September 18 tophosphate, silicate and so on near the Luzon Strait,20. There are 6 observation stations along the 120°Eand concluded that the water exchange was weak insection to the west of the Luzon Strait (Fig.1) and thethe upper layer. Above 1 200 m, the water flowedcast depth is 1 500 m (Table 1). The water depth frommainly from the SCS to the Pacific, and the water19°N to 21°20' N is more than 3000 m in the section,with the features of Kuroshio water only appeared inand there is a trough deeper than 4000 mfrom 1915'the sea area between 120° E and 121° E in the west ofN to 1940'N. The depth either to the souththe Luzon Strait. The results from Liu (2000) show(18925'N-18*45' N) or to the north(21°45'N- -22°N)that the volume transport through the Luzon Straitis more than 1 500 m and shallower than 2 000 m.(LST) reached 8.0 Sv (1 Sv=lx10 m/s) during theThe mean and maximum water depths are 3 000 mnortheasterm monsoon period in winter, and about 3.0and 4 000 m respectively (see the lower left panelSv in the other seasons. Qu et al. (2004) calculatedin Fig.1).LST using OGCM model showing that the annual2.2 Methodmean LST was 2.4 Sv. The LST was the largest inwinter about 6.1 Sv (westward), smallest in summerThe method of dynamic calculation is used in theabout 0.9 Sv (eastward). Guo et al. (1988) used thestudy, the formula is given as follows:Table 1 The positions of the observation stations and the maximum observation depths中国煤化工Sation nameEE2Longiude(E)1195511959120-*00HCNMHG120*00LatitudeN)18945'19930200020-3021°0021030ax.depth (m)15001500_652CHIN. J. OCEANOL. LMNOL.. 27(3), 2009Vol.27(2) In the 300-1 500 m layer especially below 700 m,the isotherms reverse tendency: the peak appearsnear 19°30N and the trough is at 20°N (Fig.2b).where ρ=ρ (T, S, P) represents potential density(3) It can be seen obviously that the water moveswhich can be calculated by the UNESCO statewith upwelling at 20930N in the 0- -300 m layer, andequation; g is gravity acceleration; f is Coriolisthe temperature in the north side is lower byparameter; Po is mean potential density; and μo is1°C -3°C than that in the south side at the same level,velocity at reference level Zo Since the observationwhile the water becomes downwelling at 1930N.depth is 1 500 m at the 6 stations, we assume that thevelocity is zero at the 1 500 m reference depth.3.2 The characteristics of salinity distribution3 THE CHARACTERISTICS OFThe salinity in the north side is higher than in theTEMPERATURE, SALINITY, DENSITYsouth. There are two high salinity cores near 21°NFOR THE 120° E SECTIONand at 19°-1930'N respectively in the subsurface3.1 The characteristics of temperature distributionwater (Fig.3a). There is one low salinity core near21°N in the 300-1 500 m layer with a typical featureThe temperature distributions between 18945"Nof the intermediate water (Fig.3b).and 21930N are plotted in Fig.2. From Fig.2 we can(1) Distribution of the subsurface watersee the characteristics as follows:The subsurface water is characterized with high(1) In the 0 -300 m layer, one peak and one troughsalinity, and it is located between the seasonalcan be found in the isotherm patterm. The trough isthermocline and the main thermocline. It is fornedlocated near 19°30N. The temperature reaches itsbecause of strong evaporation at the surface betweenpeak value near 20*30'N, and decreases to the souththe equator and the subtropical zone in the 100- 200 m(Fig.2a).layer.Temperature distributionTemperature distrbution3005C50000 E70~2-22-150900200110050 A一141300一12190 19.59 20° 20.59 210 21.5915009o 19.59 200 20.59 21° 21.50Latitude (N)Fig.2 The distribution of temperature (unit: °C) along 120E sectionSalinity distribution33.634.41-34.433 喜34人元44-C”-34:39-34.4300 t349700= -34.45-144-34534.47-34一 -34.5134.49-34.7-34.53-_4.5550 t4.5~-中国煤化工-34.57-190 19.59 20° 20.50 21° 21.9MHCNMHG21021.50Latiude (N)Fig_3 The dstrbution of slinity along 120°E sectionNo.3ZHOU et al: Characterstics of water exchange in the Luzon Strait during September 2006653It can be seen from Fig.3a that in the north side ofone ridge and one trough. The trough is near 19°30' N,the section, the high salinity core of 34.9 is at aboutand the ridge near 20° N (Fig.4a).120 m depth near 21°N with the typical characteristics(2) In the 300-1 500 m layer especially below theof the subsurface water of Kuroshio, because the700 m, the density ditributions reverse compared tomaximum salinity of the subsurface water in thethe upper layer. The ridge appears near 1930'N, andKuroshio mainstream to the east of the Luzon Straitthe trough is located at 20°N (Fig.4b).is between 34.91 and 35.02. In the south side of the3.4 The characteristics of current velocitysection, the high salinity core of 34.7 is at about thedistribution160 m depth with the transformed characteristics ofthe subsurface water of Kuroshio influenced by theBased on the data from CTD observations andSCS water.dynamic calculation result (choosing 1 500 m as the(2) Distribution of the intermediate waterreference level), the current velocity distributionIt forms at the surface of higher latitude area.along 120°E section and the volume transport profileTherefore both the temperature and the salinity areare shown in Fig.5 and Fig.6 respectively. Thelow. The intermediate water discussed here refers 0characteristics are described as follows.the North Pacific Intermediate Water (NPIW) origins(1) In the layer above 200 m, we can see that thefrom the southem Kamchatka and near the Thousandwater flows into and out of the SCS. BetweenIslands (about north of 45°N), then sinks at the19930N and 20930'N, the sea water flows from theconvergence area between Kuroshio and Oyashio.west to the east, and the maximum speed at theThe minimum salinity is lower than 33.2 at thesurface is over 25 cm/s; either to the south of 19°30'Nsource. With its southward motion, the salinityor to the north of 20*30N, the water flows from theincreases slightly because of mixing. The depth ofeast to the west with their maximum speeds ofthe intermediate water is at about 500 m and the25 cm/s and 15 cm/s respectively.salinity is between 34.17 and 34.32 to the east side of(2) In the layer below 200 m, the velocitythe Luzon Strait.distribution is obviously different from the above.It can be seen from Fig.3b that in the north side ofBetween 19°30'N and 20*30N, the westward flowthe section, the low salinity about 34.39 is at aboutappears in the upper 200 m but eastward in the layer550 m depth near 21°N with the typical characteristicsbelow with the maximum speed over 5 cm/s.of the intermediate water in the Kuroshio area, whileBetween 20°30N and 21°12N, the flow is mainlythe minimum salinity of the intermediate water in thewestward in the layer below 200 m with theKuroshio area to the east of the Luzon Strait is aboutmaximum speed less than 5 cm/s. In the interval of34.17- -34.32.19°30N-2030N, the subsurface water flows intothe SCcs in the south and out of the SCS in the north33 The characteritics of potential densitywith low speed.(3) The volume transportThe distribution of density is basically consistentFig.6 shows volume transport profile vs. depthwith the temperature (Fig4).from the surface to the 1 500 m depth. It can be seen(1) In the 0 -300 m layer, the density contours have that in the layer above the 500 m, the net volumeDensity distribution30-20.9-s 26.650 t21.9500-26.8-26.7.-27-00 E- 23.9-700-27.1.23.4--272--24.950 F-24.4900-273-25.4-27A-110027.5-250 t-25.91300中国煤化工a1500 IC NMH G。1919.59 20° 20.5° 21° 21 .5021.59Latitude (N)Fig 4 The distribution of potential density (unit: kg/m) along 120° E section654CHIN. J. OCEANOL. LIMNOL, 27(3), 2009Vol.27Velocity distribution000055-5----30000 F500 t00 t700复900q250 t11001300booL.150019.2020*20.4020.89 21.2019.2019.6° 20° 20.4°20.8° 21.2°Latitude (N)Fig. 5 The distribution of velocity (unit: cm/s) along 120E setion (the positive values denote eastward flow, thenegative values denote westward flow)still show the clear tendency for the inflow and20outflow of the SCS in September 2006.4004 COMPARING HISTORICALOBSERVATION RESULTS巨600Guo et al. (1988) analyzed the velocity景800distribution in the upper layer by using dynamic1000calculations from observations in September 1985(Fig.7). From Fig.7, we can see that along 120°E1200section in the intervals 19°N-19*30'N and140020°30N-21*30N, the water flows into the SCs,-0.5 0 0.:which is consistent with our results. For the intervalTransport (1x 10*m2/s)of 19930N-20930'N, the water flows out of the SCs,which is slightly different from our results. To theFig.6 Volume transport profile along 120°E sectionsouth of 19°N, the water flows into the SCS andtransport is negative (-4.37 Sv), and the waterthere is a cyclone eddy which does not appear in ourflowing from the Pacific to the SCS is dominated.results because of our data limitation.Using the CTD data observed in the period fromThe maximum volume transport appears at the 150 mlate August to early September 1994 in the Luzondepth, where the strongest inflow is dominated. InStrait and its adjacent region, Xu et al. (2004)the layer below the 500 m, the net volume transport isobtained the velocity distributions along the 120°Epositive (1.12 Sv), smaller than in the layer above thesection from the surface to the 1000 m by means of500 m, and the water flowing from the SCS to thedynamic calculation (Fig.8). In order to comparePacific is dominated. The eastward transport appearswith their s results, we calculated the geostrophicin the 700-1 000 m depths. The total net volumecurrent by choosing 1 000 m as the reference level.transport of the section from the surface to the 1 500The flow pattern is consistent with of choosingm is negative (-3.25 Sv), i.e. the inward transport1 500 m as the reference level (figure is not shown).from the Pacific to the SCS prevails the outward flow.There are some differences between Fig.8 andOf course, there may be some factors causing error inFig.5.the volume transport estimation. Fistly, the observation(1) About one degree latitude difference can bestations are relatively sparse and with some distancesfound for the positions between the inflows andaway from the shore. Secondly, the maximumoutflows.observation depth is 1 500 m, so the volume transportFor the interval of 18*45'N -21930'N, there is a onebelow 1 500 m and the transport adjacent to islets aredegr中国煤化工ard between theneglected. Thirdly, the 1 500 m depth chosen as theposil_w andoutflow.reference level at which the velocity is assumed to beBetwCNM H G sea waer flowszero could also bring some errors. So the estimatedinto the SCS, while between 20930'N and 21°20'N,volume transport maybe not very accurate, but it canthe sea water flows out the SCS (Fig.8).No.3ZHOU et al: Characteristics of water exchange in the Luzon Strait during September 2006655N厂about a half degree wider in latitude in Fig.8 whichTaiwan Islandmeans that the outflow in the middle layer and theinflow in the north side become displaced northward20about half degree. In the lower layer, the velocityPacifie Oceannear 19°N is positive in the 300 -800 m depths; in the21°middle part of the section, the water flows into theSCS in the south side (19930'N- 20°N) and flaws outin the north side (20°N- -20*55'N). To the north of20°55'N, the water flows out of the SCS from surfaceto 1000m, which is, in general, consistent with ourresults.19°-5 THE PRIMARY ANALYSIS OF THEVELOCITY DISTRIBUTIONLuzon Island119° 120°121°12221239 E 1240It can be seen from Fig.5 that the eastward andwestward flows appeared altermately along the LuzonFig.7 The current in the upper layer in the Luzon Straitstrait which is consistent with results from other(Guo et al, 1985)observations, such as Nitani (1970), Guo et al. (1988),Guan (1990), Xu et al. (2004), Yuan et al. (2005),Tian et al. (2006), and so on.To the north of 2030'N along the 120° E section200(Fig.5), the water mainly flows from the Pacific tothe SCS from surface to the 1 500 m depth. We have仓4009discussed above that both the subsurface water andthe intermediate water have the typical characteristics600of the . Kuroshio Current water. It shows that theKuroshio intrudes into the SCS at about 21930'N800north, which is determined by the complicatedtopography in the east of the Luzon strait. Using21° N 220satellite altimeter data from September 18- -20 inFig.8 The velocity (unit: cm/s) distribution along 120°E2006 (Fig.9)， we analyzed the mean absolutesection(Xu et al, 2004; the positive values denotedynamic topography (ADT) around the Luzon Island,eastward flow, and the negative values denoteand found that the Kuroshio water enters into thewestward flow)Luzon strait mainly from the Balintang Channel(2) The depths between the inflows and the(Fig.1), then flows to the northwest and enters intooutflows of the SCS water with the speed overthe SCS to the north of 20930'N in the 120°E section.10 cm/s are different.To the south of 20°30N along the 120°E section,The depth of the 10 cm/s isovelocity is at the :the velocity distribution in the layer above 200 m is200 -300 m depth (Fig.8), while in our result thequite different from that in the layer below 200 mdepth is at the 100 m depth except in the south side ofwhich is related to the local ocean circulation or eddythe section where it is at the 180 m depth.in this area. The results (Fig.7) of Guo et al. (1988)(3) The velocity distribution below the 300 m isshow that the Kuroshio water flows into the SCSalso different.from the north of 20930N near the 120°E section,The speed below the 300m is lower in Fig.8and there is a cyclonic eddy on the south of 20930N.compared with the speed of 5 cm/s in our result.Wang et al. (2004) analyzed the distribution of(4) Volume transportmesoscale eddies in the SCS and found that it is easyThe volume transport for the same area is 2.4xto generate mesoscale cyclonic and anticyclonic10° m/s calculated by Xu et al. (2004), while it iseddies中国煤化工m Island. From-2.2x10 m'/s calculated in this study.Fig.5 .yclonic eddy onIn the upper layer, there are two patches for thethe nolMHcNMHGwithitscoreatinflow into the SCS and one patch for the outflowabout 1189E, 17945'N, and its northeast boundarywhich is similar in the two figures, but the inflow ispasses through the 120°E section. The velocity in the656CHIN, J. OCEANOL. LIMNOL, 27(3), 2009Vol.27ridge and the trough.22°2(2) The salinity in the north of the section is higherthan that in the south and lower in the middle. There210叶are two high salinity cores located at 21°N and at19°- 1930'N respectively in the 0- 300 m layero° twhich marks the core position of the subsurfacewater. There is one low salinity core near 21°N in the90-300-1 500 m layer which is the typical feature of theintermediate water.80-(3) The strong velocity appears in the 0- 200 mdepth. The maximum speed can reach 25 cm/s.Below the 200 m depth the speed is low (less than5 cm/s).116°118120122124E(4) In the layer above the 500 m, the net volume ofwater transport is negative (-4.37 Sv) suggesting thatFig.9 The mean absolute dynamic topography (unit: cm) inthe inflow from the Pacific to the SCS is dominated.Sept. 18 to Sept. 20, 2006In the layer below the 500 m, the net volume of waterlayer above 200m with the maximum value oftransport is positive (1.12 Sv), and the water flows25 cm/s is mainly influenced by the anticyclonicfrom the SCS to the Pacific. The total net volumeeddy. Furthermore, there is an obvious trough neartransport of the section from surface to 1 500 m is20°30'N in either Fig.2a or Fig.3a and the trough isnegative (-3.25 Sv) indicating that the inflow fromcaused by water convergence and downwelling in thethe Pacific to the SCS prevails over the outflow.anticyclonic eddy. The velocity distribution in theMoreover, there are many other factors such as thelayer below 200 m is less obvious and almost appearsspecial location of the Luzon Strait, the complicateda reverse tendency which could be caused bytopography in the strait, the variabilty of thecyclonic eddy in this area. Due to limited observationKuroshio's flux, the seasonal variations of the winddata, we are not able to confirm this. Liang et al.forcing and so on. All these factors can make the(2008) analyzed the data from three acoustic Dopplerwater exchange through the Luzon Strait verycurrent profilers (ADCPs) deployed in the centralcomplex and variable. In this study, there is only oneobservation section with limited stations conducted,Luzon Strait and composite shipboard ADCP data.and the observation time series is also short.Their results show that the Kuroshio water intrudedTherefore, in order to better understand and describeinto the Luzon Strait through the deepest channelsthe special current structure and the mechanisms,(~20930'N). Velocity in the central of the Lazonmore field observation data in a broader area withStrait show less seasonal variations, but largelonger time series are needed.seasonal variations in the south side.It can be concluded that to the north of 20*30N,7 ACKNOWLEDGMENTSthe water flowing from the Pacific to the SCS isThe authors thank the South China Sea Institute ofmainly caused by the Kuroshio intruding. To theOceanology, CAS for the South China Sea opensouth of 2030N, velocity is related to the localcruise by R/V Shiyan 3.ocean circulation or eddy in the area.References6 CONCLUSIONSCai S Q Liu H L, Li w. 2002. Water transport exchangeBy analyzing the characteristics of temperature,between the South China Sea and its adjacent seas.salinity, density and velocity distributions, weAdvances in Marine Science, 20(3): 29 34. (in Chinese)calculated the volume of water transport through theChuPC,Li R F. 2000. South China Sea isopycnal surfacecirculation. Jourmal of Physical Oceanography, 30:Luzon Strait. The main characteristics of the water2419-2 438.exchange in the section can be summarized as:Fang |中国煤化工3. A survey of studies(1) In the 0 -300 m layer, the isotherm has one:an circulation. Actaridge at 20%30' N and one trough at 1930N. In theYHCNMHG300-1 500 m layer especially below the 700 m, theGuan B X.1990. 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Acta Oceanologica Sinica, 21(2): 175-185.English abstract)中国煤化工MYHCNMHG