Wind Wave Effects on Hydrodynamic Modeling of Ocean Circulation in the South China Sea

Transcription

1 48 The Open Civil Engineering Jurnal, 2009, 3, Open Access Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin in the Suth China Sea Hng Zhang 1, *, S. A. Sannasiraj 2 and Eng Sn Chan 3 1 Griffith Schl f Engineering, Griffith University, Gld Cast Campus, QLD 45, Australia 2 Department f Ocean Engineering, Indian Institute f Technlgy Madras, Chennai, , India 3 Department f Civil Engineering, Natinal University f Singapre, Singapre 12 Abstract: Wind, wave and current interactins cntrl the bundary fluxes, mmentum and energy exchange between the atmsphere and the cean, and within the water clumn. The wind wave effect n the circulatin is investigated in a threedimensinal time-dependant cean circulatin mdel. This POM (Princetn Ocean Mdel) based mdel is implemented with realistic castlines in Suth China Sea and emphasizes the simulatin f physical parameters in the water clumn. Taking accunt f the wind waves, an increase in air-sea drag cefficient, reflecting an enhanced sea surface rughness due t increased wave heights, is shwn t imprve the simulated surface current and the sea surface elevatin. It is als fund that develping waves with smaller peak perids influenced the surface circulatin mre significantly. The inclusin f the wind wave parameterizatin als affects the current near the seabed in the shallw water. The mdel is validated against current, temperature and salinity data measured in the Asian Seas Internatinal Acustics Experiment (ASIAEX). The simulatin results in the perid f April - May 2001 shw that wave-induced surface stress increases the magnitude f currents bth at the surface and near the seabed. On the ther hand, wave-induced bttm stress retards the near bttm currents in shallw water. Therefre the net effect f wind waves n circulatin depends n the significance f current and elevatin changes due t wind waves thrugh bth the surface and the bttm. Key Wrds: Wave-current interactin, Bttm rughness, Surface stress, Ocean circulatin mdel, Mdel cupling. 1. INTRODUCTION In the cean envirnment, the physical prcesses gverning the water clumn are influenced by atmspheric flw, currents, surface waves, tides and their mutual interactins. A better understanding f the physical prcess is essential fr studying the chemical and bilgical prcesses in scientific and practical applicatins, such as beach ersin, upwelling, strm surges and transprt f varius materials. Cmpared t high cst f field measurements, the numerical mdel fr slving time dependent flws is bth effective and ecnmical. Extensive and intensive studies f cean mdeling have been undertaken in last a cuple f decades. Ocean mdels have becme an imprtant tl fr understanding the seasnal cean circulatin and thermal structure, and fr establishing a nwcast system fr reginal seas. The Suth China Sea (SCS) has cmplex bttm tpgraphy and pen bundaries. The hydrdynamics in the regin is very cmplicated. Metzger and Hurlburt [1] first applied a layered mdel t the SCS and cmpared upper layer currents and sea levels f the mdel with the bserved data. Recently, Cai et al. [2] develped a cupled single-layer/ tw-layer mdel t study the upper circulatin. An enhanced *Address crrespndence t this authr at the Griffith Schl f Engineering, Griffith University, Gld Cast Campus, QLD 45, Australia; hng.zhang@griffith.edu.au understanding f the circulatin characteristics has been achieved. Chu and Chang [3, 4] studied the seasnal thermdynamics in the SCS using the POM with limited bundary cnditins, mnthly mean climatlgical wind stress data set [5, 6] and bi-mnthly variatin f mass transprt at the pen bundaries [7, 8]. Wind wave impact n the cean circulatin is an imprtant aspect f the hydrdynamics. Recent cmputatinal studies by Davies and Xing [9], Xie etc al. [10] and Mn [11] bear this pint. Their studies shw that the wave cntributes t lcal current and sea level changes, and mmentum and stratificatin mixing thrughut the whle water clumn. Mn [11] als investigated the effects f cean waves n sea surface temperature simulatins. Withut cnsidering the wave effect at the surface, the surface stress is a functin f wind speed based n the drag cefficient [12]. Hwever, the actin f wind ver the sea induces the exchange f mmentum between air and cean, leading t wave develpment. Therefre the surface stress wuld be significantly enhanced by the wind waves. Charnck [13], Janssen [14, ] and Dnelan et al. [16, ] presented varius mdels t calculate surface rughness by taking int accunt the effects f the surface waves. Mst recently, Massel and Brinkman [18] presented an analytical slutin fr the wave-induced set-up and flw thrugh simple shal gemetry when water depth is a linear functin f distance. The existing empirical knwledge has shwn that surface / Bentham Open

2 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume 3 49 waves enhance the mixing in the upper cean, which can be applied t the newly derived cntinuity, mmentum and energy equatins fr mre accurate mdeling. Mellr [] and Qia et al. [20] cupled surface wave equatins t mixing equatins in three-dimensinal cean mdels. Their result has cnfirmed a strng wave-induced mixing in bth hydrdynamics and temperature. Graig and Banner [] and Zhang and Chan [22] have suggested that surface waves can enhance mixing in the upper cean. The SCS is mnsn dminated, and surface waves play a significant rle in the circulatin prcess. Wind waves cause an enhancement f the bttm stress encuntered by currents, which has been studied theretically (e.g. []) and experimentally (e.g. [24]; [25]). The rbital mtins f the waves alternate near bttm currents, resulting in a thin bundary layer with intensive turbulence [26]. In cean mdelling, bttm frictin is always cnsidered t be quadratic f current velcities. It was demnstrated that the bttm frictin can be greatly increased by the peridic bttm stress created by cean waves by Grant and Madsen [27, 28], and this result has been tested and applied in a variety f castal situatins [29, 30]. Recently, Zhang and Li [31], Xie et al. [10] and Zhang et al. [32] incrprated the wave-current interactin mechanism prpsed by Grant and Madsen [28] in a hydrdynamic mdel. Their results indicate that the surface waves can significantly affect the bttm currents by mdifying the bttm drag cefficient. Hwever, experimental results [25] shw that the apparent hydraulic rughness prpsed by Grant and Madsen [28] was under predicted. A mdified mdel was prpsed in Madsen [33], which shwed excellent agreement with measurement data. The SCS has a cmplex bathymetry with water depth frm a few meters up t 5000 meters. Previus studies have shwn that the bttm effect f surface waves is significant in shallw regins [11, 32]. Therefre, it is necessary t cuple the wave mdule int circulatin mdeling in the SCS. In the present study, the wind wave effect t the circulatin is investigated using a three-dimensinal timedependant Princetn Ocean Mdel (POM) based mdel. The mdel is cnfigured with realistic castlines in the Suth East Asian Seas and ur emphasis is n the simulatin f physical parameters in the water clumn. A third-generatin wave mdel (WAM) is emplyed t predict the wave parameters. The wind wave effect n the circulatin is examined by applying the thery f Janssen [] t estimate the effect f waves n the sea surface rughness. At the bttm bundary layer, the wave-current interactin mechanisms as develped by the latest Grant-Madsen analytical mdel [33] are applied, which prduce values f the bttm rughness experienced by a current, the apparent bttm rughness, frm knwledge f wind-waves and bttm current shear stress characteristics. The imprved frmulatins f surface stress and bttm stress have been incrprated int the POM mdel. The simulatin results in the perid f April - May 2001 shw that the surface stress with the cnsideratin f waves increases, and as a cnsequence the magnitude f currents bth at the surface and near the seabed has been varied. It is als fund that yung waves with smaller peak perids influenced the surface circulatin mre significantly than in ld waves. On the ther hand, wave-induced bttm stress retards the currents in the water clumn. Therefre the net effect f wind wave n the circulatin depends n the significance f current and elevatin changes due t surface stress and bttm stress respectively. The mdel is validated against the current, temperature and salinity data measured in the ASIAEX field measurements. 2. MODEL DESCRIPTION 2.1. Circulatin Mdel The flw equatins gverning cean circulatin in POM cnsists f the hydrstatic, the Bussinesq Navier-Stkes equatins alng with an equatin f state which incrprates the temperature and salinity f the fluid velcity. The hydrstatic assumptin and the Bussinesq apprximatin are cmmnly used in cean circulatin mdeling based n the premise that the hrizntal extent is much larger than the vertical extent. The gverning equatins f the cntinuity equatin, the Reynlds mmentum equatins, the cnservatin equatins fr ptential temperature and salinity and the turbulent kinetic energy are thus frmulated in rthgnal Cartesian c-rdinates with x increasing in the eastward directin, y increasing in the nrthward directin and z measuring vertically upwards frm an undisturbed water level [] Wave Mdel In the present study, a third-generatin wave mdel, WAM [34] was adpted. WAM estimates the evlutin f the energy spectrum fr cean waves by slving the wave transprt equatin explicitly withut any presumptins n the shape f the wave spectrum. The net surce functins f the whle system takes int accunt all physical prcesses which cntribute t the evlutin f the wave spectrum, representing surce terms due t wind input, nn-linear wave-wave interactin and dissipatin due t wave breaking and bttm frictin. The synthesis f these surce terms as expressed in WAM [34] signifies the current state f understanding f the physical prcesses f wind waves, namely that inputs frm these prcesses balance each ther t frm self similar spectral shapes crrespnding t the measured wind wave spectra. Except fr the nn-linear surce term, all the ther surce terms are individually parameterized t be prprtinal t the actin density spectrum. The nn-linear surce uses the discrete interactin apprximatin (DIA) t simulate a nn-linear transfer prcess, representing the fur-wave resnant interactin Bltzmann equatin and this characterizes the third-generatin mdels Mdels Cupling Cupling Thrugh the Surface Winds blwing at the sea surface cnstitute an imprtant driving frce fr cean currents. Generally, the wind stress at the surface is therefre a necessary frcing parameter fr an

3 50 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al. cean circulatin mdel. The surface wind stress ver the cean is directly crrelated t the wind vectrs. Nrmally, the wave-independent znal ( sx ) and meridinal ( sy ) cmpnents f the wave-independent stress are defined as sx = a C D V 10 u 10 sy = a C D V 10 v 10, (1) where a is the air density; (u 10, v 10 ) are the (x, y) cmpnents f wind speed V 10 at 10 m abve water; C D is the surface drag cefficient. Initially, the drag cefficient frmulatin is based n Large and Pnd [12] mdified fr lw wind speeds as suggested by Trenberth [35]: fr V 10 1 m / s ( ) 10 3 fr 1 m / s <V 10 < 3 m / s (2) C D = V fr 3 m / s V 10 < 10 m / s ( V 10 ) 10 3 fr V m / s The abve equatin has been cmmnly applied in cean mdeling, hwever it desn t include the wave effect. As the wind blws ver the cean, surface waves are develped. The yung waves significantly enhance the surface rughness and hence, surface stress due t waves shuld be cnsidered [, 34, 36, 37]. The surface stress f airflw ver sea waves depends n the sea state. Frm a cnsideratin f the mmentum balance f air it is fund: = a C D V 10 2, (3) where the drag cefficient can be expressed by the shear velcity definitin as: { ( ( ))} 2. (4) C D = / ln z / z 0s Here =0.4 and the surface rughness z 0s = g / ( 1 ( w )/ ) (5) where is the Charnck cnstant [5], is the ttal surface stress and w is the wave-induced stress equals the amunt f the mmentum ging t the waves due t wind. Because nly the develping waves cntribute t the surface rughness, the directin f thse waves fllws the directin f wind clsely. Equatin (2) applies the direct paramerisatin f C D n wind speeds, hwever the imprved frmulatin f equatin (4) which is indirect paramerisatin thrugh a surface rughness height is cupled int the mdel. Drag cefficient in equatin (4) is a mre physically sund parameter. The sur face rughness may vary fr the same wind speed, and als assciate with water depth, wave age r wind directin. The ttal shear stress due t wind and wave has been taken int accunt. Bye et al. [38] als prpsed a frmulatin t calculate the shear stress with mre cmplicated cnsideratin f the mmentum transfer frm the cean t the atmsphere thrugh the swell Cupling Thrugh the Bttm The enhanced near bttm turbulence due t the presence f wind waves influences the flw field. The thery f wavecurrent interactin mechanisms f the Grant-Madsen analytical mdel [33] is applied. If z=z r is clse enugh t the bttm t be cnsidered within the cnstant stress layer, the cncept f a bttm frictin factr can be defined by b = C zr U r U r, (6) where U r is the reference velcity used in drag-law frmulatin, i.e. U r = U(z r ), is the density f water and C z r is bttm frictin cefficient referenced t U at z=z r. The bttm frictin cefficient can be expressed using the shear velcity definitin as fllws, Cz r 2 z r = / ln Z, (7) 0b where =0.4 and Z 0b is ften related t the bttm rughness, k N. Fr example, k N =30 Z 0b, fr fully rugh turbulent flw. Based n skin frictin, the Shields Parameter, m can be calculated fllwing the mdel presented by Madsen [33], ' m ' U 2 *wm = ( s 1)gd, (8) with ' 2 1 ' 2 U * wm = f wu bm and 2 ' U bm f w = exp5.61 dr , (9) where d is sediment diameter and s is the rati f density f sediment t density f water; U bm is the amplitude f the equivalent peridic wave f near-bttm rbital velcity; and U *wm is the maximum frictin velcity due t windwaves. The mvable bed rughness can then be evaluated by the fllwing cnditins [33]: k N = d U bm k N = 4 r ' k = d N where m cr 2 4 Z Z fr fr fr fr ' cr m < 2 Z < (10) < Z < 0.18 ' Z > 0.18 r > 0.35, the critical value f Shields Parameter fr the initiatin f sediment mtin, is btained as a functin f the d fluid-sediment parameter S * = ( s 1) gd frm the 4 mdified Shields Diagram [28] and its extensin [33]; and ' Z = / S ; is the fluid viscsity. m * m

4 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume 3 51 The near-bttm wave velcity and bttm shear stress may be evaluated in terms f the directinal surface wave cmpnents. A wind-wave mdel typically wuld have utput in the frm f the directinal wave spectrum S (,), where is the radian frequency and is directin. The near bttm rbital velcity amplitude U bm, the equivalent peridic wave radian frequency r and the dminant directin, w can be calculated by fllwing the prcedure in Madsen [39]. The directin f the current c can be btained as tan c = by bx frm the circulatin mdel. The angle between waves and current wc is defined as c - w. The maximum wave bttm shear stress can be btained frm 1 2 wm = w f wcu bm (11) 2 where the cmbined wave-current frictin factr, f wc, is a functin f the relative strength f currents and waves, specified by f wc where, = C CμU exp5.61 k n r bm μ (12) μ = c, C μ = { 1+ 2μ cs wc + μ 2 } 1 2. (13) wm Equatins (13) and (14) are slved iteratively, by first assuming μ=0 and C μ =1 t btain an initial estimate f frm (12) and (13). With this value f wm, μ and C μ are updated using equatin (14) and the prcedure is repeated till cnvergence f μ is btained with tw significant digits. Then, the wave bundary layer thickness is given as Cμ wm w wc = A, (14) r wm With the abve results, the crrespnding value f bttm frictin cefficient C can be updated by equatin (7). z r Cupling Prcedure As mentined abve, in this study, there are tw types f wave effects incrprated int the hydrdynamic mdel POM: thrugh surface shear stress and bttm stress. The cupling f the tw mdels takes place in the fllwing sequence. Firstly, wave mdel calculates the directinal wave spectrum and significant wave height with wind inputs. The utputs are then used t estimate the ttal surface stress by Equatins (3) (5). These cupling values are then input int hydrdynamic mdel t mdel the circulatins. In this study, the cupling prcess is ne-way, which takes place every 1800 s. 3. APPLICATION DOMAIN The mdified cean circulatin mdel is applied t the Suth East Asian Seas. It cvers the dmain f 99 E-1 E and 9 S-24 N. A hrizntal grid reslutin f 1/6 degree and 20 Sigma-level are emplyed. As initial 720-day spin-up perid is used, starting frm an initial f statinary state with climatlgical March temperature and salinity field [41], which is driven by the climatlgical mean wind. Frm March 01, 2001 (day 7), the analyzed wind fields frm ECMWF (Eurpean Cmmissin fr Medium Weather Frecasting) are applied t drive the mdel. The results shwn in the fllwing sectins are in the dmain f 1 E- 1 E and 13 N-24 N. The study area cvers the shallw castal waters frm the suthern China castline t the nrthern deep basin f Suth China Sea, as shwn in Fig. (1) m 4300 m 3100 m 1800 m using the final values fr [40], CμU A = exp2.96 k N r bm C μ and wm and the scaling factr () Finally, the apparent rughness is btained by slving the equatin Z c 30 C wm wc μ 0b = wc k. (16) N Lngitude ( E) Fig. (1). The bathymetry in the dmain. 700 m 70 m 10 m

5 52 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al Wind Waves In this study, the wave-induced surface and bttm stresses were cnsidered in additin t the effect f currents. The wave mdel was set up ver a larger dmain f 99 E- 0 and 9 S-52 N with a bathymetric reslutin f 1/6, spectral reslutin f 25 lgarithmically spaced frequency cmpnents with f 1 = Hz. The angular reslutin is 30. ECMWF winds at 0.5 reslutin are used t drive the mdel. The mdel was calibrated and verified by using the available buy measurements lcated at ( E, N) ver the perid f 1 st June 31 st Octber The buy wave data (H s ) are shwn in gd agreement with the mdel predictins fr the five mnths perid in Fig. (2). Our study perid is in April and May 2001, which is the inter mnsn perid, when the wind directin is changing frm the Nrtheast mnsn t the Suthwest mnsn. During this transitin time, a strm ccurred during May 11-13, 2001, as shwn in Fig. (3). A strng nrtheast wind f abut 13m/s was recrded, extending frm the Taiwan Strait t the Luzn Strait, and results the significant wave height and the peak wave perid frm WAM shwn in Fig. (4). On May 12, 2001, the significant wave heights reached 3 m in Sep-2:00 7 Sep-2:00 13 Sep-2:00 Sep-2:00 25 Sep-2: Significant wave height (m) 0 1 Aug-0:00 7 Aug-0:00 13 Aug-0:00 Aug-0:00 25 Aug-0:00 31 Aug-0: , Jul-0:00 7 Jul-0:00 13 Jul-0:00 Jul-0:00 25 Jul-0:00 31 Jul-0: Jun-0:00 7 Jun-0:00 13 Jun-0:00 Jun-0:00 25 Jun-0:00 1 Jul-0:00 Fig. (2). The cmparisn between the predictin and the buy bservatin f significant wave heights ver five mnth s perid frm June Octber, 2001.

6 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume Wind Speed (m/s) Fig. (3). The wind speed at (18N, 118E) in May May 01 May 13 May Jun 1 Fig. (4). The significant wave height and peak wave perid n May 12, the deep central dmain. The peak wave perid reached 7 s in the deep basin where the wind-waves were fully develped. Hwever the peak perid is nly s in the shallw castal water regin. The influence f the windgenerated waves n the current is t be investigated Influence Thrugh the Surface Stress The drag cefficients estimated by equatins (2) and (4) are used t calculated surface shear stress. Equatin (2) des nt taken int accunt wave effects. The drag cefficient indirectly paramterized thrugh surface rughness is pre-

7 54 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al. dicted by equatins (3)-(5), where wave effects have be taken int accunt. Fig. (5a) shws surface shear stress depending nly n the wind speed reaches up t 0.3 N/m 2 in the perid f May 10-13, Hwever, the surface stress including the wave cntributin is illustrated in Fig. (5b); the magnitude f the stress is determined t be up t 0.6 N/m 2. The difference f the surface shear stress is due t wave effects as shwn in Fig. (5c). These results shw that the impacts f waves reach a maximum n May 12, 2001 with the rapid intensificatin f the cyclne. Figs. (5a-c) clearly illustrate that the presence f waves greatly enhance the magnitude f the surface stress. In previus studies, the drag cefficient is shwn t increase with wind speed as lng as strm intensity des nt exceed 30 m/s [, 38, 42]. In the present study, the maximum wind speed is less than m/s. May 10, May 11, May 12, May 13, Fig. (5). The surface stress (N/m 2 ) distributin. (a) wave-independent (b) wave-dependent (c) difference between (b) and (a).

8 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume 3 55 Fig. (6) further shws the cmparisn between the surface stresses calculated by Equatins (2) and (4). Fig. (6a) illustrates that with the greater wind speed the influence t the surface stress is greater. Fr lw wind speed, the wave effect is nt significant, but at wind speeds abve 10 m/s, the magnitude f the surface stress can be dubled. Fig. (6b) indicates that fr develping waves with smaller peak perids the surface stress is influenced mre significantly than fr well develped waves, in agreement with Drennan et al. [43] s five recent field campaigns. It can be seen that Equatins (3)-(5) includes the cntributin f the surface wave t circulatins. Hwever, in the present study, the breaking wave induced current and the wave set-up haven t been investigated, which need t be further studied. Surface Stress (N/m 2 ) (a) The wave-enhanced surface stress results in a greater increase in the surface velcity where the surface stress is enhanced, but the effect n the bttm velcity is nly in shallw regins as shwn in Fig. (7). The maximum difference fr surface currents can reach 0.4 m/s in the central part f the dmain where the strm has its maximum intensity. The surface elevatin als increases in the shallw regin and the variatin can be up t 0.12 m, which reaches 50% f the surface elevatin near the cast Influence Thrugh the Bttm Stress Wave stress, the bttm rbital velcity U bm, directin w and frequency r can be evaluated frm WAM utputs and used in the bttm layer mdule t estimate the bttm Wind Speed (m/s) (b) Surface Stress (N/m 2 ) Wave Perid (s) Wind Speed (m/s) Fig. (6a). The relatinship between the wind speed and the surface stress. (b) The relatinship f the wind speed, the wave perid and the surface stress. ( + withut wave effects; * with wave effects).

9 56 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al. rughness in POM. The prcedure is the same as that described in Sectin 2.3. Fig. (8) shws the distributin f the bttm rbital velcity and the near bttm radian frequency n May 12-13, In the shallw waters near the nrthern pen bundary, the values f U bm are large with a peak value f 0.5 m/s, and the bttm wave perid diminishes t 3 s. Large values fr U bm enhance the turbulence and increase the bttm rughness which can be as high as 0.06 m. The effect f wave-current interactin n the bed frictin cefficient reduces the currents near the cast because f the altering bttm stress. Several authrs have previusly addressed this effect [30, 10, 31]. Intensive turbulence retards the bttm flw. Far frm the cntinental shelf areas, the wave effect n the current is insignificant. Fig. (9) cnfirms that the wave influence thrugh the bttm stress increases in shallw water up t 40 m depth, which reduces the near bttm current up t 40%, but has little effect n the surface current, althugh the surface elevatin is reduced by up t 0.11 m near the nrthern bundary f the dmain. As discussed in Zhang et al. [32], the influence f waves t the near bttm current is assciated t water depths, wave perids, wave numbers and wave heights. In General, with water depth mre than 50 m the wave effect is nt significant. The vertical extent f wave-assciated turbulence is limited by the wave bundary layer. The wave cntributin t the turbulent mixing must decrease with the distance away frm the bttm. It shuld be nted that near-bttm turbulent eddy viscsity, K M =ku z, increases with increasing shear stress. The influence n sediment transprt and the mixing f ther substances is significant near the sea bttm The Ttal Wind Wave Effect and Calibratin using ASIAEX Data We have shwn that surface wave affects the current thrugh bth the surface stress and the bttm stress. Fig. (10) presents the effects f waves thrugh bth the surface Surface Current Bttm Current Surface Elevatin Lngitude ( E) (a) (b) (c) Fig. (7). The influence f wave thrugh the surface t the surface current (m/s), bttm current (m/s) and surface elevatin (m). (a) waveindependent; (b) wave-dependent; (c) difference between (b) and (a).

10 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume 3 57 Fig. (8). The distributin f near bttm rbital velcity and wave perid n May 12 and 13, and bttm shwing that the general flw pattern remains similar, regardless f the wave effect. Taking waves int accunt, an increased energy is input frm the surface and mre energy is dissipated frm the bttm. These tw effects are ppsite. It can be seen frm Fig. (8) that the surface and bttm current differences cmbine the results in Figs. (7 and 9). The surface current increases mainly due t the enhanced ttal surface stress and the bttm current decreases due t the net influence f the surface and the bttm stress. The influence f greater bttm turbulent intensity in the shallw water hwever, is dminant. The influence n elevatin is als the net influence frm bth the surface and the bttm. As the tidal elevatin variatin has been added at the pen bundaries, the diurnal and semi-diurnal cycles can be predicted as bserved in Fig. (11). The ASIAEX field experiment was cnducted frm 25 April t May Measured current data at 8 m depth at lcatin (22.18N, 1.06E) is cmpared with simulated data fr the perid frm April - May, 2001 (Fig. 11). The crrelatins between the measured data and the simulated data are 0.53 and 0.55, respectively, withut and with waves. If we nly cmpare the perid May 10-13, 2001, the crrelatin cefficients are 0.50 and 0.56, which indicates that under strng wind the influence f waves is mre significant. Waves influence surface stress [43] play an imprtant rle in cean mixing prcesses [44], which affects nt nly SST but the three-dimensin temperature and salinity as well [11]. Analysis f CTD data in ASIAEX result in tempral and spatial variatin patterns fr the vertical prfiles f temperature and salinity. Therefre, we cmpare mdel utputs at a given pint with the field measurement data in Fig. 12. Althugh mdel temperature prfiles agree well with bserved data, the predicted salinity is higher than measured salinity at all lcatins. This may due t the fact that the mdel surface salinity is frm climatlgic data which might nt accurately reflect the synptic scale prcesses such as rainfall. The temperature prfile near the bttm als exhibits a difference between the predicted and measured data, which may be due t the seasnal averaged lateral bundary cnditins, which were prescribed. Bth wave enhanced turbulent mixing and depth induced wave breaking influence the hydrdynamic and thermdynamic prcess in the water clumn, but neither is investigated in the study. Further research f the influence f the wave n cean circulatins is t be carried ut.

11 58 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al. Fig. (9). The influence f wave thrugh the bttm t the surface current (m/s), bttm current (m/s) and surface elevatin (m). (a) waveindependent; (b) wave-dependent; (c) difference between (b) and (a). 4. CONCLUSIONS The influence f the wind waves n the cean circulatin was investigated. The imprved frmulatin f the surface stress depends n the wind speed and the rughness f the water surface, which is prescribed t update the surface drag cefficient in the circulatin mdel. The mdified Grant- Madsen analytical mdel [33] fr the bttm rughness is applied t prduce values f the apparent bttm rughness experienced by a current. The knwledge f wind-wave and current bttm shear stress and bttm sediment characteristics was incrprated int the circulatin mdel. Its utility has been examined by presenting results generated in the Suth China Sea. When the wave-enhanced surface stress is accunted fr in the mdel, the impacts f strng winds is significant fr surface current, but nly affects the bttm current in shallw waters. Wind waves have significant bttm rbital velcity in the shallw water, which enhances bttm turbulence and retards the flw near the seabed. Our results suggest that waves have significant impacts in shallw regins, and high wave and lw peak perid cnditins. We cnsider the influence f waves n the circulatin thrugh the surface and thrugh the bttm. Cmparisn with the ASIAEX field measurements shw that taking waves int accunt can imprve the crrelatin between mdeled and measured currents frm 0.50 t 0.56.

12 Wind Wave Effects n Hydrdynamic Mdeling f Ocean Circulatin The Open Civil Engineering Jurnal, 2009, Vlume 3 59 Fig. (10). The net influence f wave thrugh the surface and the bttm t the surface current (m/s), bttm current (m/s) and surface elevatin (m). (a) wave-independent; (b) wave-dependent; (c) difference between (b) and (a). Fig. (11). Current cmparisn between the measured data and simulated data.

13 60 The Open Civil Engineering Jurnal, 2009, Vlume 3 Zhang et al. Fig. (12). Temperature and salinity cmparisn between the measured data and simulated data. In the present study, we examined nly the influence f wind waves n the hydrdynamics. Further study n the sediment transprt and the trajectry f water particles is nging, t btain a better predictin capacity f the cean envirnment. The effect f wind waves n the thermal dynamics will als be cnsidered in a future study. ACKNOWLEDGEMENTS The authrs wish t thank Prf. O. S. Madsen fr his valuable discussins and suggestins n the thery f wavecurrent interactin near the seabed.

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Oceangr., vl. 34, pp , Received: Octber 10, 2008 Revised: Octber 27, 2008 Accepted: Octber 28, 2008 Zhang et al.; Licensee Bentham Open. This is an pen access article licensed under the terms f the Creative Cmmns Attributin Nn-Cmmercial License ( which permits unrestricted, nn-cmmercial use, distributin and reprductin in any medium, prvided the wrk is prperly cited.