Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 DOI: 1.7/s13131-17-11-1 http://www.hyxb.org.cn E-mil: hyxbe@263.net The sptio-temporl vrition of wintertime subtidl currents in the western Tiwn Strit SHE Junqing 1, QIU Yun 1, 2 *, GUO Xiogng 1, PA Aijun 1, ZHOU Xiwu 1 1 Third Institute of Ocenogrphy, Stte Ocenic Administrtion, Ximen 365, Chin 2 Lbortory for Regionl Ocenogrphy nd umericl Modeling, Qingdo tionl Lbortory for Mrine Science nd Technology, Qingdo 266237, Chin Received 27 Mrch 17; ccepted 4 My 17 The Chinese Society of Ocenogrphy nd Springer-Verlg Berlin Heidelberg 17 Abstrct A new dt set of observtions by six cruises of ship-mounted coustic doppler current profiler (SADCP) nd three d long bottom-mounted ADCPs (BADCPs) is employed to revel the sptiotemporl vribility of tidl nd subtidl currents in the western Tiwn Strit (TWS) during winter seson. The results confirm the existence of intense cotidl lines for M 2 tidl current, which is locted north of 25. In this cse, no existence of n mphidromic point cn be identified. It is lso reveled tht the counter-wind current (CWC) cn extend through the whole western TWS nd even occupy the entire wter column during winter monsoon relxtion. However, this CWC is observed to be thoroughly overwhelmed by the downwind Chin costl current (CCC) during the two big monsoon bloom events in the winter of 7, nd the CCC consequently extends southwrd throughout the western TWS insted. Most importntly, the vrition of the sptil extent for the CWC nd the CCC in the western TWS is found to be well explined by the first two modes of the vector empiricl orthogonl function (VEOF) nlysis, tht is, it is minly controlled by wind-driven qusi brotropic current s the first mode nd slightly modulted by reltively wek bckground current with first-order broclinic structure s the second mode. Key words: counter-wind current, Chin costl current, M 2 tidl current, Tiwn Strit, winter seson Cittion: Shen Junqing, Qiu Yun, Guo Xiogng, Pn Aijun, Zhou Xiwu. 17. The sptio-temporl vrition of wintertime subtidl currents in the western Tiwn Strit. Act Ocenologic Sinic, 36(11): 4 13, doi: 1.7/s13131-17-11-1 1 Introduction The Tiwn Strit (TWS), locted between the Est Chin Se nd the South Chin Se, is crucil pssge for wter nd nutrient exchnge between these two se res. The strit is generlly shllower thn 6 m except for the deep Penghu Chnnel, nd is chrcterized by complex bottom topogrphy. The circultion in the TWS, primrily forced by monsoons, the bottom topogrphy nd remote forces (e.g., the South Chin Se Wrm Current nd the Kuroshio brnch current in the estern TWS), shows significnt sesonl vrition (Gun nd Fng, 6; Hu et l., 1). Unlike summertime when the entire strit is occupied by uniform northwrd/northestwrd flows, the wintertime circultion is much more complicted with severl unique flows. The Tiwn Strit Wrm Current proposed in the pioneering work of Gun (1986) nd Gun nd Fng (6) is ctully counterwind current (hereinfter CWC), which origintes from the se off Gungdong Province, Chin s n extension of the South Chin Se Wrm Current nd connects with the Tiwn Wrm Current north of the strit. The follow-up studies using in situ observtions (Chung, 1986; Hu et l., 199) nd numericl models (Su nd Wng, 1987; Wng et l., 1) confirmed the existence of the CWC. Although the sptil extent of the CWC on the surfce nd intermedite lyers chnges gretly in response to the intrsesonl vrition of locl wind during winter (Gun nd Fng., 6; Hu et l., 199), the current in the bottom lyer is stble throughout the whole western TWS (Zhng et l., 1991; Sun et l., 1996). The northestwrd pressure grdient long the western boundry is proposed s the min forcing mechnism for the CWC (Chung, 1985; Yng, 7). Another importnt current in the western TWS during wintertime is the Chin costl current (CCC), which is downwind costl current locted closely djcent to the lndwrd side of the CWC (Jn et l., 2; Zhng et l., 5; Hu et l., 1). It crries colder nd less sline costl wter southwrd nd extends s fr s to n o Islnd in Gungdong Province (Xio nd Ci, 1988). During the cold-ir outbrek, the CCC ws reported to intrude southestwrd into the Penghu Islnd re from the western strit (Lio et l., 13). Hydrogrphic observtions suggested tht the internnul vrition of this downwind current ws significnt; the strength nd sptil extent of this current re lrgely enhnced (reduced) under strengthened (wekened) northest monsoon during L iñ (El iño) events (Wu et l., 7; Zhu et l., 13). The monsoon forcing (Chen, Foundtion item: The tionl turl Science Foundtion of Chin under contrct os 414, 427 nd 4127634; the Scientific Reserch Foundtion of Third Institute of Ocenogrphy, Stte Ocenic Administrtion under contrct o. 1711; the Stte Ocenic Administrtion Progrm on Globl Chnge nd Air-Se Interctions under contrct os GASI-IPOVAI-2, GASI- IPOVAI-3 nd GASI-3-1-1-4; the Chinese Acdemy of Sciences Strtegic Leding Science nd Technology Projects under contrct o. XDA1114. *Corresponding uthor, E-mil: qiuyun@tio.org.cn
SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 5 3; Zhng et l., 5; Jn et l., 6) nd the costlly trpped wves tht propgte southwrd (Ko et l., 3; Pn et l., 13) re considered s two mjor driving mechnisms of the CCC. During the pst decde, mny studies contributed to the understnding of wintertime current chrcteristics in the TWS bsed on vrious vluble observtions, including bottommounted nd ship-mounted coustic doppler current profilers (hereinfter BADCP nd SADCP, respectively) (Ling et l., 3; Wng et l., 3; Lin et l., 5), surfce drifters (Tseng nd Shen, 3; Qiu et l., 11) nd high-frequency ground wve rdrs (Zhu et l., 13). However, most of the in situ current dt during winter were obtined under clm se conditions; the dt under rough se conditions re rrely vilble. It is still difficult to explicitly depict the sptil pttern of current response to the evolving northesterly wind. In this study, we minly focus on the flow field response to n evolving winter monsoon in the western TWS using set of new in situ dt obtined under both clm nd strong wind conditions. The reminder of this pper is orgnized s follows: in Section 2, we describe the dt nd the nlyticl methods used in this study; in Section 3, we present the fetures of tidl currents; in Section 4, we nlyze the vrition of subtidl currents s response to vrying locl wind; nd summry is provided in Section 5. 2 Dt nd methods 27 26 25 24 23 22 1 118 1 122 E Chin s minlnd DS TWB B3 D1 2.1 Dt The SADCP current dt used in this study were collected in the winter sesons from 5 to 12. During the study period, there were six cruises in totl, out of which three lsted for more thn 1 d (Fig. 1 nd Tble 1). The ship velocity ws clculted from the bottom trcking, nd ws deducted from the rw SAD- CP dt by using the dt processing softwre VmDs. The verge time intervl of the dt ws 5 1 min, nd two types of frequencies were set up for the current-profiler observtions, with bin length of 2 m for -k ADCP nd of 4 m for 1-k ADCP. Three criteri were used to chrcterize the cceptble dt, nmely, the percentge of good dt should be greter thn %, the error of the velocity nd the verticl velocity should be less thn.15 m/s, nd the ship speed should be slower thn 6 m/s. Liner interpoltion ws then used to fill the gps in the cceptble profiles. Subtidl currents were extrcted from depth-verged SADCP dt following the method of Cndel et l. (1992), which re used to nlyze the sptil pttern of winter currents. Three BADCPs (i.e., B3, B4 nd B6 in Fig. 1), which were locted t wter depths of 41, 41 nd 43 m respectively, were deployed in the western TWS during 3 December 7 to 12 Jnury 8. Current velocity profiles nd ner-bottom temperture were mesured t ech mooring. Given the ADCP instlltion height (bems surfce from the sebed being bout.5 m) nd the resulting blind zone (upper 2.5 m), the deepest current observtions were bout 5 m bove the sebed. The vlid mesured depth nerest to the se surfce ws 4 m, becuse the ner-surfce bins were contminted by side-lobe reflection from the se surfce. Then, the current velocity dt t 4 36, 4 36 nd 4 38 m were collected with 2-m nominl bin, while the temperture ws mesured t.5,.5 nd 42.5 m for B3, B4 nd B6, respectively. All moorings were recovered successfully with record lengths of bout d nd ensemble verged t time intervl of 1 h. To exclude high-frequency fluctutions of semidiurnl nd diurnl tidl currents from the velocity observed, digitl symmetric low-pss filter (i.e., PL33) (Limeburner, 1985) ws pplied to removing the fluctutions with periods less thn 33 h. Similrly, the temperture dt were smoothed by 11-point running verge to reduce the noise. The filtered current velocity (which is the subtidl current) together with the filtered temperture dt re used to explore the verticl structure of currents in the western TWS. For surfce wind, we used hourly in situ dt from 3 December 7 to 12 Jnury 8 collected by the mooring buoy D1, positioned closely to B3 (Fig. 1). Before nlysis, high-frequency fluctutions in the wind speed dt were filtered out by pplying the sme low-pss filter employed in processing the current velocity of the BADCPs. Furthermore, the 6-hourly ERA- renlysis wind velocity field (with speed bis of 1.36 m/s nd direction PT B4 PHC CYR B6 Tiwn Islnd Fig. 1. Bthymetric chrt (contours in m) of res round Tiwn. The cruise trcks of the SADCP observtions from 5 to 12 re denoted by the dotted lines. Three BADCP sttions nd the mooring buoy re mrked by the red tringles nd red str, respectively. The blck circles denote the nodes, nd the solid lines re isobths. The mjor geogrphic loctions involved in this pper re lbeled, including the Chngyuen Ridge (CYR), the Penghu Chnnel (PHC), the Tiwn Bnk (TWB), Pingtn (PT) nd Dongshn (DS) Islnd, etc. Tble 1. The informtion of SADCP current dt set Yer Time (dt rnge) Instrument setup Version Month (Dy) Bin size/m First bin depth/m Ensemble intervl/min 5 Mr. (7 17) WHM-k 2 7 1 6/7 Dec. (24 31), Jn. (1 1, 14, 19 31), Feb. (3 6) WHM-k 2 7 5 7/8 Dec. (1 4), Jn. (5 8) WHM-k 2 7 5 7 Dec. (1 22) WHM-1k 4 15 5 12 Feb. ( 24, 27 29), Mr. (1) WHM-k 2 7 5 12 ov. (19, 25) WHM-k 2 7 1
6 SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 root-men-squre error of 33 in the TWS, s noted by Kung et l., 15) on.125 grid ws lso obtined by verging in the domin from 22 to 26 nd from 117 to 121 E. The smple intervl is improved to 1 h using liner interpoltion, nd thus the obtined dt mtched those of the SADCP dt t every integrl point. The renlysis wind field is used to explore the wind chrcteristics during the period of the SADCP observtions, while the in situ wind dt re nlyzed to understnd the response chrcteristics of subtidl currents to the short-term fluctutions of winter monsoon. 2.2 Lest squres method As intense tidl currents re ubiquitous in costl nd shelf wters, snpshot of the SADCP dt is unvilble when investigting the subtidl current field unless the observed current is detided with relible process. Therefore, methods hve been developed to remove tides from the SADCP mesurements, such s the trditionl hrmonic nlysis method (Geyer nd Signell, 199; Tkikw et l., 3), the method bsed on tidl forecsting model (Isobe et l., 7) nd the sptio-temporl fitting by the lest squres method (STF-LSM) (Cndel et l., 1992; Münchow, ; Vzquez et l., 11). Among these methods, the STF-LSM is most widely used to seprte the men flow from tidl currents, becuse it cn extrct those sptilly evolving tide currents by fitting rbitrry bsis functions (Cndel et l., 1992), which does not require repeted trnsects nd hence hs logistic dvntge. A brief summry of the methodology involved is presented here, while more detiled explntion cn be found in Wng et l. (4). The fitting scheme is implemented to the verticlly integrted velocity field from the SADCP mesurements. The irregulrly smpled (both in spce nd time) velocity components re represented s follows: 8 KX < MX u(r; t) = : k + k=1 j =1 9 j k cos( j t) + j k sin( j t) = ; (jr r k j) + Ã (r; t) ; (1) where u(r, t) is the zonl component of verticlly integrted velocity t time t nd given loction r; φ( r r k ) is the bse function; ψ(r, t) is the residul term including the mesurement noise nd unresolved tidl constituents; α k is the men flow coefficient; β jk nd γ jk re tidl component coefficients; K is the number of nodes; M is the number of tidl constituents used in the nlysis; ω j is the frequency of the jth tidl constituent; r is the loction of the observtions; nd r k is the kth nodl loction. A similr eqution is used for the meridionl component v(r, t). The coefficients α k, β jk nd γ jk re determined by lestsqures fitting of the SADCP dt using Eq. (1). In the TWS, the semidiurnl tidl current, especilly the M 2 constituent, is dominnt (Fng et l., 1985; Jn et l., 4). Since it is difficult to seprte M 2 from other semidiurnl tides like S 2, 2 nd K 2 becuse the durtion of ech individul cruise ws reltively short, only M 2 is considered in Eq. (1) (i.e., M=1) in this study. A simple Gussin interpoltion function proposed by Wng et l. (4) is selected s the sptil bsis function s shown s follows: (jr r k j) = exp( jr r k j 2 =2L 2 ); (2) where L is the spce impct fctor intimtely ssocited with node-ffected re. Since the sptil weighting of the Gussin function decreses with incresing distnce, the interpoltion is dominted by locl influence from nerby dt points. Here, the number of node is ssumed to be 8 nd the spce impct fctor is set to 11 km. The distribution of the nodes is shown in Fig. 1. Though the tidl currents nd the men flow in the study re vry sptilly nd the observtion errors re rndom in the SAD- CP dt, the results from this clcultion re not sensitive to the distribution of nodes. 2.3 Brotropic tidl model To evlute the tidl currents extrcted from the SADCP mesurements, two-dimensionl brotropic costl tidl model, nmed ADCIRC (Westerink et l., 1993), is used. The model foring term is obtined from the tidl potentil tht is derived from estern Chin s ses using the Oregon Stte University tidl model (TPXO) with eight tidl constituents (i.e., K 1, K 2, M 2, 2, O 1, P 1, Q 1, nd S 2 ). In the computtion, the unstructured grids re employed, nd the globl bthymetry is subsmpled from 1 gridded erth topogrphy (ETOPO1) dt. This model reproduces the M 2 tidl ellipse prmeters obtined from the BADCP mesurements t B3, B4 nd B6 very well (Tble 2), though the observtion results (e.g., the mjor xis of M 2 ellipse) re bised upwrd s they do not resolve ll the semidiurnl tidl components due to the short time spn of just.2 d. Overll, the excellent greement between the ADCIRC model results nd the BADCP mesurements demonstrted tht the true M 2 currents could be well represented by the modeled M 2 tidl constituent in the TWS. 3 Tidl currents Figure 2 shows the M 2 tidl current ellipses obtined from the SADCP observtions nd clculted from the ADCIRC model output. Within most of the region of our concern, both tidl ellipses were quite similr in terms of current speed, direction, phse, ellipticity, nd rottion. The verged vlue (.44 m/s) of the mximum tidl current mplitude by the SADCP observtions is rther close to tht (.43 m/s) given by the ADCIRC model, nd the correltion coefficient between them is.73 with stndrd devition of.13 m/s. Obviously, with sufficient dt, the SADCP-derived M 2 tidl current should be rther relible, whose ccurcy is t lest comprble to tht from the ADCIRC model. Consistent with previous studies (Fng et l., 1985; Wng et l., 3; Jn et l., 4), both SADCP nd model results (Fig. 2) show tht the inclintion of M 2 ellipses is generlly in the E SW direction, which is thought to be primrily constrined by isobths, but it turns into the W SE direction in the shllow Tiwn Bnk nd the northwest Penghu Chnnel. Though the model results gree well with the observtions in generl, we Tble 2. The elements of tidl ellipses of M 2 constituent obtined in the observtion nd the numericl tide model (bbrevited s Obs nd Mod, respectively) t three current sttions Sttion Mjor xis/cm s 1 Smll xis/cm s 1 Inclintion/( ) Phse/( ) Obs Mod Obs Mod Obs Mod Obs Mod B6 33.8 35. 23.8 14.9 48 73 86 6 B4 32.2 26. 14.1 13.4 27 17 195 199 B3 45.7 36.9 2.9 7.3 44 42 232 215
.6 SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 7 118 119 1 121 E 26 cm/s 25 24 23 118 119 1 121 E 118 119 1 121 E 118 119 1 121 E 26 b c d.3 25 1 8.4 24 23 7 2 18.2.4. 2.3 Fig. 2. The brotropic tidl current ellipses (M 2 ) derived from the SADCP dt (red) nd the brotropic tidl model (blue) (); nd complitude (cm/s) (b), co-phse ( ) lines (subject to time of 1 E) (c) nd ellipticity (d) for M 2 constituent derived from the SADCP dt. The bold line in Fig. 2d represents the boundry of different rottion motions of M 2 currents. should mention tht in some plces, e.g., the TWS center close to the minlnd cost nd the northwest Tiwn Bnk, the model results of ellipse inclintion devite wy from the mesured ones. This devition is probbly becuse the model cn djust well with the chnging isobths on the premise of not missing locl fluctutions; menwhile, the STF-LSM tht emphsizes the smoothness of interpoltion (Münchow, ) probbly neglects the fluctutions of locl signls (Chio nd Wng, 4). Figures 2b d revel severl mjor chrcteristics of the M 2 tidl current. One cn see tht the strong tidl currents ppered ner the two ends of the TWS nd diminished grdully towrds the centrl strit (Fig. 2b). The mximum M 2 current mplitude is bout.8 m/s on the Tiwn Bnk (22.8, 118.4 E), wheres its minimum mplitude is less thn.3 m/s in the centrl strit. From the co-phse lines for M 2 currents (Fig. 2c), it is found tht the mximum velocity first ppered in the northestern prt nd then propgted southwrd. This result is consistent with the hrmonic nlysis of se level records (Jn et l., 4), suggesting tht the M 2 tide is progressive wve in the western TWS. Obviously, there exist intense cotidl lines north of 25, where the M 2 current mplitude is lmost minimum (Fig. 2b). Within this region, the time difference between the north nd the south is bout 3 h (bout 9 ). This ws lso reported in previous studies (e.g., Fng et l., 1985; Li nd Wng, 199). However, no mphidromic point proposed by Ye et l. (1985) is found in our study. From the distribution of the M 2 ellipticity shown in Fig. 2d, t pproximtely 23.5 there exists ltitudinl isoline. South of the isoline there re negtive vlues, which suggest the clockwise rottionl motion of the M 2 currents, wheres the cse for the north of the isoline is exctly the opposite. 4 Subtidl currents during winter monsoon 4.1 Sptil distribution of subtidl currents during winter monsoon relxtion Figure 3 shows the depth-verged men subtidl currents over the entire SADCP observtion periods nd the stndrd devition of residul currents. The stbility of the men subtidl currents cn be exmined by compring their mgnitude with their stndrd devition. Generlly, the stndrd devition of the residul currents is significntly smller thn the men subtidl currents, except for the sitution of the minlnd cost, where the two re comprble. The flow field in the western portion of the TWS is dominted by northestwrd currents ginst the northesterly wind (i.e., the CWC). The strong CWC is ner the Chngyuen Ridge (the mximum current is.38 m/s) nd extends northwrd to Pingtn Islnd, while the wek CWC generlly occurs off the minlnd cost. Additionlly, very wek southwestwrd current ws found north of Pingtn Islnd, which ws chrcterized by the nrrow minstrem close to the cost, nd it hs been reported s the CCC (Jn et l., 2; Pn et l., 13).
8 SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 26 117 118 119 1 121 E cm/s b orth 1% % % West Est 25 South Wind speed/m s -1 8 6-8 4-6 <4 24 8 c 23 24 32 cm/s B3 B4 Sttion B6 Fig. 3. The distribution of the men subtidl currents (blck rrows) over the entire SADCP observtion period, the stndrd devition of residul currents (blck ellipses), nd the men depth-verged subtidl currents (red rrows) with the corresponding stndrd devitions (red ellipses) from BADCPs mesurements obtined with se surfce wind speed less thn 8 m/s (); the rose digrm (b) ws obtined from the ERA renlysis se surfce wind over the sme period of SADCP observtions; nd the sme men subtidl currents (rrows) nd stndrd devitions (ellipses) s those in Fig. 3, displyed with verticl levels (c). During the SADCP observtions, the northesterly wind previled in the TWS, which ccounted for up to 83.9% of the totl surfce wind smples (Fig. 3b), but the wind ws generlly wek, with the wind speed less thn 8. m/s ccounting for 95.2% of the totl smples. In ddition, the verge wind speed ws only bout 4.6 m/s, which ws fr less thn the generl men strength of wind (bout 8.6 m/s; see Tble 1 in Li (1986)) during the sme winter months when the SADCP dt were collected. The monsoon ws considered s being in relxed stte during the SAD- CP observtions. Therefore, the forementioned men subtidl currents shown in Fig. 3 only present the sptil pttern of the flows under clm se conditions. The current mesurements during winter monsoon relxtion stge (i.e., when the se surfce wind speed ws less thn 8. m/s) from the three BADCPs re used to revel the verticl structure of flows in the western TWS (Figs 3 nd c). Consistent with the flow pttern from the SADCP observtions, the men depthverged flows t ll sites show primrily northestwrd currents (Fig. 3). However, by exmining the mjor xis of the stndrd devition ellipse (red nd blck ellipses, Fig. 3), one cn see tht the mgnitudes of the flow fluctutions t the three sttions were ll fr lrger thn those of the SADCP observtions t the corresponding loctions, prticulrly t B4. ote tht between the SAD- CP nd BADCPs mesurements, only the ltter mesurements cn revel the short-term vrition of the flows. Therefore, the forementioned lrge discrepncy in the flow vrition between the BADCPs nd the SADCP suggests tht very strong flow components for the short-term vrition existed t the three sites, which cnnot be resolved by SADCP since it is kind of irregulrsmpling mesurement. Figure 3c shows the men subtidl current profile for these three sttions verged during the winter monsoon relxtion stge. The result indictes tht the men subtidl currents for ll lyers t B3 nd B4 re northestwrd, with the mximum of ech site ll locted in the middle level (up to.13 nd. m/s for B3 nd B4, respectively). In contrst, the men subtidl currents t B6 re southestwrd from the surfce to the 8 m lyer with its mximum (bout.13 m/s) locted in the surfce lyer, wheres it turned to northestwrd in the lyers below 8 m. The mjor xis of ellipses t B3 indictes tht t this sttion the fluctution mplitude is lmost uniform mong ll lyers, wheres for B4 nd B6, significnt decresing trend in the fluctution mplitude is observed from the top to the bottom lyer. The fct tht the uniform northestwrd flows from the middle to the bottom lyers t ll three sites suggests tht the CWC cn extend throughout the whole western strit during winter monsoon relxtion stge. However, in the surfce lyer, due to the dominting southestwrd CCC in the northern prt (i.e., the loction of B6), the CWC only extended from the south to the middle prt long the western strit (i.e., from the loction of B3 to tht of B4). 4.2 Sptio-temporl vrition of subtidl currents The northest monsoon begins in mid-september nd peks from October to Jnury in generl (Jn et l., 2). It hs strong short-term vrition with period of 3 1 d induced by successive cold fronts pssing over the Chin s continentl shelf (Hsueh nd Rome, 1983). Consequently, the flows in the strit re lrgely ffected by the short-term fluctution of winter wind. As shown in Fig. 4, though the fluctution of the depth-verged flows t the three sttions during the whole observtion periods ws s strong s tht observed under clm se conditions (see Fig. 3), the flow ptterns t ll sites verged for these two periods were quite different (Figs 4b nd 3c). By compring Fig. 4b with Fig. 3c, one cn see tht the CCC derived from the men
SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 9 117 118 119 1 121 E b 15 m/s 26 cm/s B6 5 1 25 15 24 B4 25 B3 23 35 B3 B4 Sttion B6 Fig. 4. Sctter digrm of depth-verged subtidl current fluctutions t three sttions superimposed on bthymetric contour mp from 3 December 7 to 12 Jnury 8 (), nd the verticl distribution of men subtidl currents t the three sttions (b). subtidl currents for the whole periods ws fr stronger thn the counter-prt obtined for the monsoon-relxtion periods. Wht is more, it hs lrger sptil extent, which occupied the whole western strit, covering the full depth in the northern strit (i.e., t the loction of B6) nd the upper lyers (bove m) in the centrl nd southern prts (i.e., t the loctions of B3 nd B4). In contrst, compred with tht during the monsoon relxtion stge, the strength nd sptil extent of the men CWC during the full period were drmticlly reduced. As shown in Fig. 4b, the men CWC over the whole period only ppered in the lyers below m in the centrl nd southern prts of the western TWS (i.e., the loctions of B4 nd B3). The lrge difference in men flow pttern over the forementioned two monsoon stges suggests tht the vrition of the two mjor flows (i.e., the CWC nd CCC) is sensitive to the short-term vrition of the winter monsoon. Figure 5 is comprehensive depiction of the chrcteristics of the forementioned two flows (i.e., the CWC nd CCC) in response to different stges of winter monsoon (i.e., relxtion or pek stge). The in situ se surfce wind (Fig. 5) indictes tht the northesterly wind previled in the TWS during the periods of BADCPs mesurements, with strong short-term vrition. Among the short-term vrition, there were four big monsoon relxtion events (shown s red-shded res in Fig. 5) nd two big monsoon bloom events (shown s blue-shded res in Fig. 5). The men surfce wind speed during these relxtion events rnged from 1.4 to 8. m/s, nd those during the bloom events vried from 1. to 15.6 m/s. Consistent with the surfce wind (Fig. 5), the long-shore nd cross-shore current components showed notble short-term fluctutions (Figs 5b g), which were lmost in phse with the vrition of the surfce wind. Under the intense northesterly wind, the previling current direction of the longshore component t ll sites ws primrily towrd the southwest, while the northestwrd CWC ws very wek nd only existed in the subsurfce nd bottom lyers (Figs 5b d nd 5h j). Prticulrly, during the two big monsoon bloom events, the whole wter column t ll sites ws dominted by the strong southwestwrd CCC, nd the CWC ws hrdly detectble (Figs 5b d). In contrst, during the wek winter monsoon, the bove processes were exctly the opposite (Figs 5b d nd 5h j), which re consistent with those shown in Fig. 3. The northestwrd CWC ws evidently displyed in the whole wter column of ll sites for most of the big monsoon relxtion events. Although the CCC still existed t B3 nd B4 during the third monsoon relxtion event, its strength ws significntly wekened in the subsurfce nd bottom lyers (Figs 5c nd d). Similr to the forementioned longshore current, the crossshore current component t ll sites showed nerly in-phse fluctutions with the surfce wind (Figs 5e g). It ppered to be in the onshore direction during the two monsoon bloom events, wheres it becme offshore during the four monsoon relxtion events when the surfce wind remined northesterly with profoundly diminished strength in most of the cses. Within the wind-driven Ekmn regime, the cross-shore se level slope induced by the Ekmn dvection ws formed during the wintertime monsoon outbreks, wheres the longshore se level slope cused by the mighty Kuroshio (Yng, 7) ws present during the monsoon relxtion stge. Such scenrios were probbly the mjor fctors leding to the vrition of the cross-shore currents. The ner-sebed tempertures t the three sites provide dditionl evidence for the vrition of two mjor flows (i.e., the CWC nd the CCC) in the western strit (Figs 5b d). Overll, the tempertures rose during the monsoon relxtion stge. At Sttion B6, which ws locted in the northern TWS, the temperture rose from ~18 to 21 C on 23 December 7 when the northesterly wind wekened. The wrm wter crried by the extension of the CWC to the north of Pingtn Islnd probbly contributed to the rising of the temperture t B6 (Pn et l., 13). When the monsoon ws persistently intensified (e.g., the second monsoon bloom period), the CWC lmost disppered from the western strit. Consequently, insted of wrm wter, the southwrd extension of the cold costl wter ws brought in by the CCC long the whole western strit, which resulted in 4. 5. C decrese in bottom wter temperture t B6. The nerly identicl vrition of the ner-sebed temperture with smller mplitude thn tht of B6 ws detected t B4. onetheless, the ner-sebed temperture of B3, which ws locted in the southern strit, re-
1 SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 15 1 18 b. B6.V h 19 18 17 1 c. B4.V i 19 18 17 1 d. B3.V 1 j - -1 1 Vm/cm s-1 65 55 e. B6.U 45 f. B4.U 1 1 35 25 15 5 V, U/cm s-1 er-sebed T/ C Speed/m s-1-5 -15-25 -35 g. B3.U 9 3 15 28 22 Dec. 7 3 Jn. 8 9-45 -55-65 Dte Fig. 5. Se surfce wind vectors (). The red (blue) solid line corresponds to the period of monsoon wind speed less thn 8 m/s (lrger thn 1 m/s), nd the red (blue) shdow covers the time xis during the monsoon relxtion (prevlent), which re displyed t the top. The verticl profile of longshore subtidl speed V (whose positive nd negtive vlues correspond to the northest nd southwest directions, respectively) t B6 (b), B4 (c) nd B3 (d); nd the synchronous ner-sebed temperture T is shown. The verticl profile of cross-shore subtidl speed U (whose positive nd negtives vlues corresponds to the offshore nd onshore directions, respectively) t B6 (e), B4 (f) nd B3 (g). The verticl distribution of men V(Vm) is displyed by the red lines (blue lines) t B6 (h), B4 (i) nd B3 (j), under the condition of se surfce wind speed less thn 8 m/s (lrger thn 1 m/s) t the top. ote tht, the so-clled principl coordinte system proposed by Kundu nd Allen (1976) is used to divide the subtidl current into the longshore nd cross-shore components, nd the relevnt principl directions clculted re 33, 27 nd 12 for B3, B4, B6, respectively. After clockwise rottion of the Crtesin coordinte by the bove principl directions, the longshore current, V, could be obtined whose positive nd negtive vlues correspond to the northest nd southwest directions; it lso gives the cross-shore current, U, whose positive nd negtives vlues correspond to the offshore nd onshore directions, respectively. mined t 17. 19.5 C no mtter whether the monsoon ws in its relxtion or onset stte. This indictes tht the bottom lyer of the corresponding re, rther thn being ffected by the cold CCC, is minly controlled by the wrm wter of the CWC during the observtion periods. 4.3 Forcing mechnism of short term vrition of subtidl currents Since previous studies (Jn et l., 2; Lin et l., 5; Zhu et l., 13) generlly consider tht circultions in the western TWS re minly controlled by the monsoons, here we use vector empiricl orthogonl function (VEOF) method (Hrdy nd Wlton, 1978) to explore the connection between subtidl currents nd wind. The subsequent discussion pertins to the first two modes of the VEOF nlysis. This is primrily becuse the first two modes ccount for 88%, 93% nd 91% of the totl vrince in the VEOF nlysis for B3, B4, nd B6, respectively. Figure 6 displys the sptil structures nd time-dependent
SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 11 b Current/cm s -1 - - 1 m/s 8 24 32.5 cm/s c d Current/cm s -1-3 Dec 7 9 15 22 28 3 Dte Jn. 8 9 8 24 32.5 cm/s B3 B4 B6 Sttion Fig. 6. Temporl () nd sptil (b) mode of VEOF 1, nd temporl (c) nd sptil (d) modes of VEOF 2. The longshore component of the wind is shown in Fig. 6 s red curve. The green, blck nd blue curves correspond to Sttions B3, B4 nd B6, respectively. coefficients for the first nd second modes of the VEOF decomposition. The first VEOF mode (VEOF1) contins 6%, 77% nd 79% of the totl vrince t B3, B4 nd B6, respectively (Figs 6 nd b). The sptil mp for the VEOF1 (Fig. 6b) shows generlly uniform sher flow t ll the sites, with decline in velocity from the surfce lyer to the bottom lyer (i.e., qusi-brotropic structure of flow profile). The time coefficient for the VEOF1 (Fig. 6) displys tht it fluctutes with the northesterly wind, with the correltion between it nd the longshore wind being.63,.62, nd.76 t B3, B4, nd B6, respectively. The spectrl nlysis shows n identicl brod pek t subtidl period round 2. 5. d for the time coefficient of the VEOF1 nd the longshore wind (not shown). A significnt coherence is found between them t this subtidl period with the flows lgging wind by bout 5.7 h t the three sites (Figs 7 nd b). This short, lmost instntneous, response of the flow to the longshore wind Phse/( ) Coherence 18 1 6-6 -1-18 1..8.6.4.2 b 95% wind-b3 wind-b4 wind-b6.1.2.3.4.5.6.7.8.9 1. Frequency/cpd Fig. 7. The phse () nd the coherence squred (b) between the first temporl VEOF mode nd the longshore component of the wind. The red dotted line in Fig. 7b represents the 95% significnce level (.61). The green, blck nd blue curves correspond to B3, B4 nd B6 sttions ssocited with the wind, respectively. 1 cpd=hz/86. is typicl cse of locl, directly forced, wind-driven response (Chung, 1985). Therefore, the temporl behvior for the VEOF1 (Figs 6 nd b) minly contributed to wind-driven qusibrotropic subtidl currents, showing very strong short-term vrition with reltively strong northestwrd flow (i.e., the CWC) over the whole wter column in the western strit during the winter monsoon relxtion, which reverted to succession of southwestwrd flow (i.e., the CCC) events s the northesterly wind intensified, similr to the behvior shown in Fig. 5. Obviously, the 2. 5. d fluctution of the northesterly wind for driving the subtidl currents in the western strit is thought to be relted to the periodic monsoon frontl pssges (Chung, 1986). The second VEOF mode (VEOF2) contins 28%, % nd 12% of the totl vrince t B3, B4 nd B6, respectively (Figs 6c nd d). Unlike the VEOF1, the sptil structure of the VEOF2 shows typicl broclinic structure (Fig. 6d). At B3 nd B4, the velocity vectors were southwestwrd (northestwrd) bove (below) the m lyer. Similr to B3 nd B4, the velocity vectors t B6 were minly southestwrd from the surfce lyer to the m lyer, but were northestwrd below the m lyer. As the VEOF2 time series show lmost positive vlues in spite of very strong fluctution t ll sites during the observtion periods, consequently, the second mode minly contributed to persistent bckground currents with two-lyer structure in the western strit. Tht is to sy, in the upper lyer, there ws uniform southwrd bckground current driven by the winter monsoon; in the bottom lyer, there exists stedy northestwrd current throughout the western strit, which is thought to be primrily cused by n longshore se level slope (Fng nd Zho, 1988; Yng, 7). Therefore, the vrition of sptil extension for the CWC nd the CCC in the western strit shown in Fig. 5, ws minly controlled by the first mode nd slightly modulted by the second mode. As the northesterly wind intensified, the wind-driven strong downwind currents (Figs 6 nd b) overlpped with the reltively wek bckground current (Figs 6c nd d), leding to stronger long-strit downwind current (i.e., the CCC, Figs 5b d), with lrger extension in spce in the western TWS. In contrst, when the monsoon becme wek, similr processes operted but in n opposite direction.
12 SHE Junqing et l. Act Ocenol. Sin., 17, Vol. 36, o. 11, P. 4 13 5 Conclusions In the pst few decdes, the current chrcteristics of the TWS in winter hd received much ttention nd there re mny chievements (e.g., Lin et l., 5; Gun nd Fng, 6; Yng, 7; Lio et l., 13; Oey et l., 14). However, due to the pucity of synchronous or qusi-synchronous mesurements corresponding to different sttes of the winter monsoon, the sptio-temporl vrition of current ptterns in the strit is still poorly understood. In the present study, the subtidl currents re explored by new set of the SADCP nd BADCP observtions, which were collected in the winter sesons of recent yers. To extrct the subtidl currents, the lest squres method is employed to seprte tides from the SADCP mesurements. It is worth mentioning tht the sptil distribution chrcteristics of M 2 tidl current obtined from the SADCP observtions gree well with the results of the ADCIRC model, which re lso consistent with previous works. In this study, we confirm the existence of the intense cotidl lines for M 2 tidl current, which is locted north of 25 ; menwhile, it should lso be noted tht in our cse no existence of the mphidromic point is observed. Our results indicte tht the flow pttern in the western portion of the TWS is rther different between the relxtion nd brek stges of the winter monsoon. The CWC could extend through the whole western strit nd even occupied the entire wter column during the winter monsoon relxtion stge. However, when the winter monsoon outbrek strted, the intense northesterly wind forced the long-strit current to flow downwind in the western strit, while the northestwrd CWC ws very wek nd only existed in the subsurfce nd bottom lyers. Prticulrly, the CWC ws lmost overwhelmed during the two big monsoon bloom events in the winter of 7, nd s result, the whole wter column ws dominted by the strong CCC throughout the western strit. The VEOF result indicted tht the vrition of sptil extent of the CWC nd the CCC in the western strit ws minly controlled by wind-driven qusi-brotropic current s the first mode nd slightly modulted by reltively wek bckground current with first-order broclinic structure s the second mode. Although the locl wind plyed dominnt role in the fluctution of the subtidl currents in the western TWS, it could not be the only importnt fctor. It shll be noted tht since the CWC is ginst the winter monsoon, the CWC is most likely driven by the pressure grdient, s noted in some previous studies (Chung, 1985; Yng, 7). However, up to dte it remins uncler whether the corresponding cross-strit component or the long-strit one plys the dominting role. To nswer this question, in ddition to the simultion method, synchronous in-site se level records observed in CWC-bloom event during the winter monsoon, if there re ny, re expected to contribute lot to explining the driving mechnism of the CWC nd will be reported in our future work. Acknowledgements The uthors cknowledge the officers nd crew of the mooring cruises nd the mngers of the ships for their support. They re grteful to the Europen Centre for Medium-rnge Wether Forecsts (ECMWF) for providing their renlysis wind product (http://pps.ecmwf.int/dtsets/dt/interim_ full_dily/). References Cndel J, Berdsley R C, Limeburner R. 1992. Seprtion of tidl nd subtidl currents in ship-mounted coustic Doppler current profiler observtions. Journl of Geophysicl Reserch, 97(C1): 769 788 Chen C T A. 3. Rre northwrd flow in the Tiwn Strit in winter: note. Continentl Shelf Reserch, 23(3 4): 387 391 Chio L Y, Wng Yuhui. 4. Multiresolution interpoltion nd detiding of the ADCP dt. 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Winter counter-wind currents off the southestern Chin cost: review. Journl of Ocenogrphy, 62(1): 1 24 Hrdy D M, Wlton J J. 1978. Principl components nlysis of vector wind mesurements. Journl of Applied Meteorology, 17(8): 1153 12 Hsueh Y, Rome R D. 1983. A comprison of observed nd geostrophiclly clculted winter time surfce winds over the Est Chin Se. Journl of Geophysicl Reserch, 88(C14): 9588 9594 Hu Jinyu, Fu Zilng, Wu Linxing. 199. Studies on the wintertime current structure nd T-S fine-structure in the Tiwn Strit. Chinese Journl of Ocenology nd Limnology, 8(4): 319 327 Hu Jinyu, Kwmur H, Li Chunyn, et l. 1. Review on current nd sewter volume trnsport through the Tiwn Strit. Journl of Ocenogrphy, 66(5): 591 61 Isobe A, Kurmitsu T, ozki H, et l. 7. Relibility of ADCP dt detided with numericl model on the Est Chin Se shelf. Journl of Ocenogrphy, 63(1): 135 141 Jn S, Chern C S, Wng J, et l. 4. The nomlous mplifiction of M 2 tide in the Tiwn Strit. 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