Tidally induced upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer

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1 Science in China Series D: Earth Sciences 2007 Science in China Press Springer-Verlag Tidally induced upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer LÜ XinGang 1,2,3,4, QIAO FangLi 1,2, XIA ChangShui 1,2 & YUAN YeLi 1,2 1 The First Institute of Oceanography, State Oceanic Administration, Qingdao , China; 2 Key Laboratory of Marine Science and Numerical Modeling (MASNUM), State Oceanic Administration, Qingdao , China; 3 Institute of Oceanology, Chinese Academy of Sciences, Qingdao , China; 4 Graduate University of the Chinese Academy of Sciences, Beijing , China MASNUM wave-tide-circulation coupled numerical model (MASNUM coupled model, hereinafter) is developed based on the Princeton Ocean Model (POM). Both POM and MASNUM coupled model are applied in the numerical simulation of the upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer. The upwelling mechanisms are analyzed from the viewpoint of tide, and a new mechanism is proposed. The study suggests that the tidally inducing mechanism of the upwelling includes two dynamic aspects: the barotropic and the baroclinic process. On the one hand, the residual currents induced by barotropic tides converge near the seabed, and upwelling is generated to maintain mass conservation. The climbing of the residual currents along the sea bottom slope also contributes to the upwelling. On the other hand, tidal mixing plays a very important role in inducing the upwelling in the baroclinic sea circumstances. Strong tidal mixing leads to conspicuous front in the coastal waters. The considerable horizontal density gradient across the front elicits a secondary circulation clinging to the tidal front, and the upwelling branch appears near the frontal zone. Numerical experiments are designed to determine the importance of tide in inducing the upwelling. The results indicate that tide is a key and dominant inducement of the upwelling. Experiments also show that coupling calculation of the four main tidal constituents (M 2, S 2, K 1, and O 1 ), rather than dealing with the single M 2 constituent, improves the modeling precision of the barotropic tide-induced upwelling. upwelling, dynamic mechanism, numerical simulation, tidal movement, tidal mixing front Large-scale red tides (harmful algal bloom) of thousands of square kilometers occur in the upwelling area off Yangtze River estuary (YRE) and near Zhoushan in recent consecutive years [1], and these waters have been serious red tide disaster areas in China. Despite the small magnitude, upwelling provides a dynamic background for red tides because it carries nutrients from deep water to the euphotic zone and promotes the growth of planktons. Besides, upwelling is also an important dynamic factor for the formation of fisheries. Therefore the study on the upwelling off YRE and Zhejiang coast is of great ecological and biogeochemical significance. The study area is shown as Figure 1. After its first formal report in 1993 [2], the summer upwelling off YRE is seldom studied for a long time until recently, when Zhu [3] analyzed the mechanism based on field observation and numerical experiments. Zhu [3] suggested that the causations of the upwelling on the north and south sides of the submarine valley off YRE are baroclinic effects and the baotropic effects related to the intrusion of Taiwan Warm Current (TWC) on continental shelf, respectively, and wind has only very small effect on the upwelling. According to Zhao et al. [4], the upwelling off YRE is mainly a result of the interaction between TWC Received February 9, 2006; accepted June 5, 2006 doi: /s Corresponding author ( lxg@fio.org.cn) Supported by the National Natural Science Foundation of China (Grant No ) and the National 908 Project (Contract No ) Sci China Ser D-Earth Sci March 2007 vol. 50 no

2 Figure 1 Map of the study area. Bathymetry contours are given in meters. and topography, while the wind stress is also an important causation for its influences on the upper layer upwelling. The upwelling off Zhejiang coast was reported in as early as the 1960s [5]. In early 1980s, Hu et al. [6] proposed that the ascending of the distal part of Kuroshio northern branch along the continental shelf is the major contributor for the upwelling. Thereafter for all the large number of investigations focusing on the upwelling in Zhejiang coastal waters, the generative mechanism still remains controversial [7,8]. Besides wind and coastal currents, tidal movement induces upwelling in multifold ways. Upwelling can be driven by the centrifugal forces associated with the strong tidal currents past convex peninsula coastline [9]. This mechanism explains the persistent surface cold waters around Shandong and Liaodong Peninsula tips in summer [10]. Resulting in cyclic upwelling event, internal tidal bore is suggested to be responsible for the drops in sea surface temperature (SST) in the Southern California Bight [11]. Tee et al. [12] found that the interaction between strong tide-induced residual currents (TRC) and complex topography accounts for the upwelling off southwest Nova Scotia. In China adjacent seas, the nonlinear effect of M 2 tidal constituent is believed to be an inducement for the upwelling in Taiwan Strait and Zhejiang coastal waters [13 15]. Another upwelling-inducing mechanism is related to the tidal mixing fronts (TMF) and frontal circulation in coastal shallow seas. The TMF is referred to as the transitional zone which marks the boundary between shallower, vertically mixed, inshore regions and deeper, stratified, offshore waters in tidally energetic shelf seas in summer. The formation of TMF is closely associated with tidal mixing [16,17]. The study on TMF circulation begins in the 1970s. As typical areas of frontal system, Georges Bank and the northwest European shelf seas attracted many investigations on TMF in the past decades [17 20]. Simpson et al. [18] pointed out the possible vertical motions near the front on the basis of observed sea surface convergences and SST minima. The previous two-dimensional model studies indicate that typical frontal density structure, which is prescribed in the model, results in cross-frontal cells in the vertical plane [20 23]. The bottom flow of the cell is onshore, while the upper flow is offshore. Studies also show that the frontal circulation can be driven by tidal rectification, which is irrelevant to density field [20,23]. As for the existing researches on the upwelling off YRE and Zhejiang coast, several points are worth to be addressed below. First, most numerical simulations of the upwelling off YRE neglect tidal effects in the models, though the tidal signals are so conspicuous that the maximum possible tidal current reaches cm/s [24]. Second, almost all the studies on tidally induced upwelling (TIU) only focus on the M 2 constituent [13 15], and fail to take the nonlinear interaction among partial tides into consideration. Finally, most studies on TIU are confined to barotropic aspect. In fact, different waters, the Yangtze River Diluted Water (YRDW), the low-salinity coastal current, and the saline TWC of high temperature, coexist and influence each other, so the baroclinicity is quite strong. The intense tidal waves may exert influences on upwelling by altering the thermohaline distributions. Therefore the contributions of the ceaseless tidal movements on the upwelling in the study region may be considerable. It is necessary to restudy the TIU in detail. In this research, we first calculate the four main tidal constituents (M 2, S 2, K 1 and O 1 ) and the upwelling induced by these barotropic tides using POM. Considering complete physical processes, the high-resolution MASNUM coupled model is then employed to simulate the three-dimensional baroclinic circulation in summer. Based on the successful simulation, the roles of tidal mixing and the consequent baroclinicity in inducing upwelling are explored. 1 Phenomenon In summer, upwelling area is often featured by low SST LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

3 center because of the upwelled cold, deep water, and the satellite image of SST is often utilized as a circumstantial evidence for upwelling event. Figure 2(a) shows a 5-day (August 8 12) composite of SST derived from NOAA AVHRR (Advanced Very High Resolution Radiometer) satellite data. Along Zhejiang coast, the cold water belt (lower than 28 ), which extends southwest- ward from the YRE, is very clear. The minimum temperature even reaches 25, and the SST gradient is quite strong. Figure 2(a) implies the probable existence of a strong upwelling belt. In order to further confirm the persistence of this upwelling belt, NOAA satellite SST climatologies ( ) are plotted as Figure 2(b). The cold water belt of Figure 2 Satellite images of SST. (a) Composite image of August 8 12, 1998; (b) JPL pentad SST climatology of August 1 5 over 15 years ( ). temperature lower than 27.6 is still distinct from this climatological chart over 15 years. The cold core is located at Zhoushan islands, corresponding to the Zhoushan Fishery. There is a small low SST center off YRE, the location of which is consistent with the upwelling area reported in ref. [2]. It is suggested from Figure 2(b) that the upwelling off YRE and Zhejiang coast is probably persistent in summer. 2 Methods 2. 1 Numerical models POM and MASNUM wave-tide-circulation coupled model are used in this study. The former is applied to the modeling of barotropic tides, and the three-dimensional baroclinic circulation is simulated using the MASNUM coupled model. As a three-dimensional, primitive equation ocean model, POM was developed in the late 1970s by Alan Blumberg and George Mellor [25], with subsequent contributions from researchers all over the world. In China, many numerical simulations of tides are performed successfully using POM [26,27]. For the wide application of POM, the details of the model will not be described here. In brief, POM implements σ coordinate vertically, and the equations are solved on an Arakawa C grid in the horizontal. Using mode-splitting techniques, POM separates the external mode from the internal mode. In this way, the fast moving external gravity waves and slow internal waves are solved separately to save computing time. A significant feature of POM is the imbedding of the Mellor-Yamada turbulence closure sub-model to provide vertical mixing coefficients [25]. However, this turbulence closure scheme underestimates the upper-ocean mixing [28] and produces a shallower upper-ocean mixing layer (UML) and a weaker thermocline than observation [29]. By introducing wave-induced mixing into POM [30,31], MASNUM coupled model revises the vertical mixing scheme and effectively improves the simulation results of upper-ocean circulation. To study the TIU in the baroclinic seas, the TMF needs to be simulated reasonably. Garrett and Loder [22] point out that the internal friction (mixing) is a key process controlling the frontal circulation and the formation and maintenance of TMF. Comparing with other tide-circulation coupled model, the most prominent superiority of MASNUM coupled model is the greatly improved physics of upper-ocean mixing and the more ac- 464 LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

4 curate modeling of UML and shelf fronts. Study shows that POM is less capable of simulating correctly the UML in the Yellow Sea (YS) and East China Sea (ECS) without considering the wave-induced mixing process [32]. Hence it is advisable to utilize MASNUM coupled model in this study to investigate the mechanism of TIU in baroclinic circumstances. 2.2 Numerical scheme of barotropic modeling To reduce the boundary distortion, the model domain covers most YS and ECS (see Figure 3), much larger than the study area. Real topography is used. The model has a horizontal resolution of (1/12) by (1/12), and 16 sigma levels in the vertical. The four tidal constituents, M 2, S 2, K 1 and O 1, are simulated simultaneously. The model is driven by the open boundary tidal elevation η() t = H icos( ωit gi ), i where H and g are the harmonic constants, ω represents the tidal angular frequency, t is time. The subscript i represents the four tidal constituents. The velocity is set to be zero at land boundaries, and given according to Orlanski radiation scheme at open boundaries. The model is initialized from quiescent ocean state (flow and surface elevation all set to be zero). After spinning up sufficiently, the model is integrated for another month to accumulate tidal currents and elevation data for harmonic analysis based on least square algorithm. In the barotropic modeling, the temperature and salinity fields are hold constant after being initialized as 25 and 33 psu, respectively. 2.3 Numerical scheme of baroclinic modeling To study the effects of tidal movement on upwelling in baroclinic seas, the MUSNUM coupled model includes complete dynamic and thermodynamic processes to simulate the circulation and thermohaline fields. The model domain covers the YS and ECS. The horizontal resolution is enhanced to (1/18) by (1/18). The model is forced by solar radiation, wind stress, Yangtze River runoff, lateral boundary flow, and tide. The wind stress and heat flux fields are from the Comprehensive Ocean-Atmosphere Data Set (COADS), and Haney-type [33] atmospheric feedback adjustment is made on the heat flux. The Yangtze River runoff is included as an inflow boundary condition. The fresh water discharge data are from the monthly mean observation over 35 years. Current velocity is the governing boundary condition. The boundary velocity, U bc, includes the circulation (U c ) and tidal flow (U t ), i.e., U bc =U c +U t. U c is obtained by interpolating the global results by Xia et al. [34] to the model grids; U t is obtained from the boundary harmonic constants of tidal currents. Only the M 2 tidal forcing is included for its predominance in the study area and the convenience of deriving the Eulerian residual currents. The model is initialized with the thermohaline fields from the annual mean Levitus data and a quiescent state with zero velocity and elevation. After 6 years spin-up, the model is integrated for another year, and the outputs of the last August (representing boreal summer) are stored for analysis. 3 Results and analysis 3.1 Model results of barotropic tidal waves Observations of 148 tide gauge stations (see Figure 3(a)) are compared with the model results to validate the numerical simulation (Table 1). The four tidal constituents are satisfactorily predicted with accuracies less than 8% in amplitude and 8º in phase lag. Table 1 Computation errors of tidal harmonic constants a) Tidal Amplitude error Phase lag error constituent ΔH (cm) Relative error (%) Δg (º) M S K O a) The errors are the averages of absolute values at all stations. Co-tidal charts of M 2 and K 1 are shown in Figure 3(a), (b). The co-amplitude and co-phase lag contour distributions of S 2 and O 1 (not shown) are similar to that of M 2 and K 1, respectively. The semidiurnal tidal wave comes from the Pacific Ocean and divides into two parts after entering the continental shelves. The south branch enters Taiwan Strait, and a degenerate amphidromic point is produced off the northeast coast of Taiwan Island. The north branch enters the YS and propagates in the form of a rotary tidal system. High amplitude area includes the west and east offshore waters of the YS, Hangzhou Bay, and the waters off South Zhejiang and Fujian coasts. As for the diurnal tides, the most prominent feature is the existence of a rotary tidal system in the south YS (Figure 3(b)). Figure 3(c) and (d) exhibit the surface distribution of tidal current ellipses of M 2 and K 1 constituents. Basically, the characteristics of M 2 and K 1 tidal currents are similar to those of S 2 and O 1, respectively. The strongest K 1 tidal currents appear in the contiguous waters between the YS and ECS, and the coastal waters off Ko- LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

5 Figure 3 Model results of barotropic tidal waves. (a) and (b) are the cotidal charts of M 2 and K 1 constituent, respectively; (c) and (d) are the surface ellipses of M 2 and K 1 tidal currents, respectively. The solid and dashed lines in (a) and (b) denote distributions of phase lag (in degrees and referred to the Beijing standard time) and amplitude (in centimeters), respectively. The dots in (a) represent tide gauge stations. rean Peninsula (Figure 3(d)), while the strongest M 2 tidal currents appear in the offshore area off YRE. Islands congregate off the Hangzhou Bay, and the semidiurnal tidal currents flow forward and reverse constrained by the coastline. In the wide region off YRE, the speed and ellipticity of tidal currents is pretty high. In the waters deeper than 50 m, the maximum M 2 tidal current is usually cm/s, while in the coastal shallow waters such as near Zhoushan Islands, the velocity is even above 100 cm/s (Figure 3(c)). Many numerical studies can be found on tidal wave modeling in the YS and ECS. Recently, Fang et al. [35] presented a set of high quality cotidal charts for principal constituents based on 10 years of TOPEX/Poseidon altimetry. Their results agree basically with those in this study. 3.2 Upwelling induced by barotropic tides The horizontal and vertical TRC are derived by using harmonic analysis, and the vertical part is just the upwelling induced by barotropic tides. The computed barotropic TIU consists of three areas, which are defined as the Upwelling Area off YRE, Shengsi Upwelling Area, and Zhejiang Coastal Upwell- 466 LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

6 ing Belt (Figure 4). On the basis of field data analysis, Zhao [2] inferred that the upwelling off YRE is located at about ( E, N), and this site agrees with the model result well. The in situ observation conducted in August 2000 exhibits obvious upwelling signals on the north tip and south side of the submarine valley off YRE [3]. Comparing Figure 1 with Figure 4, it can be found that Shengsi Upwelling Area and the Upwelling Area off YRE correspond to the south side and the tip of the submarine valley, respectively. The location of Zhejiang Coastal Upwelling Belt, which lies in the steep slope between 20 m to 50 m, coincides basically with the low temperature stripe shown in both Figure 2 and the satellite SST image given by Huang et al. (their Figure 1) [13]. These comparisons indicate that the numerical results of barotropic TIU are consistent with observations. upwelling resulting from converging between 124 E and 125 E. The velocity divergence of bottom residual current, V b, is calculated to analyze the dynamic mechanism. Figure 5(b) shows that the bottom convergent areas ( V b < 0 ) agree with the upwelling in loca- tion quite well. The three main upwelling areas are all clearly identified in Figure 5(b), and even the relatively weak zonal upwelling stripe along N is also embodied to an extent. The above analyses reveal that bottom convergence of TRC is the primary mechanism of Figure 4 Upwelling induced by four main barotropic tides (M 2, S 2, K 1, and O 1 ) at 15 m depth. Dashed line frames denote three upwelling areas, and solid contours show upwelling velocities in 10 5 m/s. As the vertical component of TRC, barotropic TIU is closely related to the horizontal residual current. A distinct characteristic of the residual current field on the bottom level (σ = 0.889) is that upwelling roughly appears in the convergent zone (Figure 5(a)). The velocity convergence between the two sides of Zhejiang Coastal Upwelling Belt is especially evident. To illustrate the mechanism perceptually, four depth-longitude cross sections of TRC (u-w vectors) are plotted as Figure 6. The four cross sections from A to D are along N, N, N, and N, respectively (Figure 5(a)). On the cross section A, two currents flow towards each other, converge and upwell on the slope near E, and this upwelling reaches its peak value at m depth. On this transect, there also exists weak Figure 5 Model results of horizontal residual currents induced by barotropic tides in bottom layer. (a) Current field. The areas shaded in blue show the upwelling (10 5 m/s) at 15 m depth, and the red lines denote the positions of cross sections shown in Figure 6. (b) Velocity convergence of the residual currents (10 5 s 1 ). LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

7 barotropic TIU in the study area; that is, the convergence of bottom residual currents inevitably leads to upwelling because of the impenetrability of seabed. In addition to the convergence of TRC, another mechanism could be the ascending effect of TRC. Flowing from deep to shallow water, the cross-isobath TRC is rectified by the topography and forced up. This process can be visualized in Figure 6(b), (c): on the cross sections B and C, the climbing of TRC is clear. In some area, such as along cross section D, the convergence and ascending of TRC coexist (Figure 6(d)). 3.3 Upwelling induced by baroclinic tides Figure 7(a) shows the summer upwelling at 10 m depth produced by the MASNUM coupled model in baroclinic circumstances. The upwelling field has been temporally averaged over one M 2 cycle to filter out the periodic tidal currents. The upwelling is in shape of belts. The south part of upwelling extending southwestward from Shengsi is similar to the result of barotropic case shown in Figure 4. There are two minor disparities between barotropic and baroclinic results. The first one is that the upwelling off YRE is closer to the river mouth in baroclinic case, and the maximum velocity exceeds m/s. Besides, an additional southeast-northwest upwelling belt off Jiangsu coast is predicted by the baroclinic model. Note that both shape and location of this upwelling belt coincide with that of the observed belt of low temperature, high density, and high dissolved oxygen in the China-US joint survey in July, 1984 (see Figure 6(a) in ref. [36]). The upwelling as shown in Figure 7(a) comprises contributions of multiple dynamic factors. The purpose Figure 6 Depth-longitude sections of u-w vectors of barotropic tidal residual currents. (a), (b), (c) and (d) denote cross sections of A, B, C, and D. The contour represents upwelling velocity (10 5 m/s). For the vectors, vertical velocity is amplified 400 times. 468 LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

8 Figure 7 Horizontal distribution of upwelling at 10 m depth from baroclinic MASNUM coupled model. (a) Control Test; (b) Exp.T; (c) Exp.NoT. The areas shaded in gray show upwelling in 10 5 m/s. In (a), the contour denotes Simpson-Hunter index; the lines E and F indicate the positions of cross sections shown in Figure 8. See text for experiment details. of this paper is exploring the tidally inducing mechanism of upwelling, so the influences of other factors will not be addressed in detail. Two numerical experiments are designed to diagnose the role of tide in inducing upwelling. In the first one, we try to keep tide as the only dynamic factor inducing upwelling; in the second experiment, tidal forcing is excluded from the model. For convenience, the experiments are referred to as Exp.T and Exp.NoT, respectively, and the basic run of MASNUM coupled model is named Control Test hereinafter. (i) Exp.T (with tide). (1) Experimental scheme. Studies suggest that wind, TWC, and YRDW are the main inducements of the upwelling off YRE and Zhejiang coast [3 8]. To examine the effects of tide, the three factors are removed as a whole from the model, while other model configurations are kept intact. Concretely, the wind stress and Yangtze River discharge are set to be zero, and the two inflow entrances of TWC, Taiwan Strait and the Kuroshio offshoot northeast to Taiwan Island, are closed. (2) Experimental results. Without the three traditionally important upwelling contributors, the basic upwelling pattern in Exp.T still retains well in comparison with the Control Test (see Figure 7(a), (b)). The upwelling off YRE, the upwelling belts off Jiangsu and Zhejiang are legible. The study area can be divided into two parts if the latitude of 30.7 N is taken as a division. By contrast with Control Test, the south part shows only trivial changes, while the shrinkage of north upwelling area is relatively apparent. The difference may be related to the effects of YRDW and summer monsoon: the extension of YRDW is basically confined within the north part; the prevailing wind is from southeast and almost parallel to Jiangsu coastline, so Ekman transport is more favorable for the upwelling off Jiangsu. To quantify the tidal effects on upwelling, we compute the sum of vertical velocities at all upwelling grids on 10 m level. This index can be understood as the upwelling-induced vertical water mass flux in that it reflects the upwelling scope and intensity simultaneously. As for the Zhejiang Coastal Upwelling Belt south to 30.7 N, the total vertical flux in Exp.T accounts for 84% of that in Control Test. As for the upwelling north to the dividing line and in the whole study area, the percentages are 52% and 62%, respectively. It can be inferred from Exp.T that tidal movement is crucial for the generation of upwelling. (ii) Exp.NoT (without tide). (1) Experimental scheme. The only modification to the model is eliminating the tidal forcing at open boundaries, and all other configurations are totally identical with that of Control Test. (2) Experimental results. The upwelling in Exp.NoT, as shown in Figure 7(c), presents a striking contrast to LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

9 the result of Control Test. One can see that the upwelling is so patchy that the original belt structure becomes irregular and almost illegible. Quantitatively, the vertical flux in Exp.NoT amounts to mere 40% of that in Control Test. Although the above diagnosis is pretty rough, the results of Exp.T and Exp.NoT agree with and sustain each other. These experiments confirm the indispensability of tidal movement to upwelling in the study area. It can be estimated that in the study area about 60% of the upwelling is attributed to tide, and therefore tide plays a predominant role in inducing the upwelling. (iii) Mechanism analysis. The position of TMF is calculated to shed light on the mechanism of baroclinic TIU. At the same time, this calculation will provide the third evidence for the importance of tide to upwelling, though the above two experiments have verified tide s role to considerable extent. We use the Simpson-Hunter stratification index [17] to locate the tidal front. This index is defined as H k = Log, 3 U where H is water depth and U is the maximum tidal velocity. The threshold value of k varies slightly in different seas. For Fundy Bay and the shallow seas around England [16], the value is about 1.85; for the Celtic Sea and the northern Irish Sea [37], the value is and 1.9, respectively; Zhao [36] takes k as 1.8 in his study of TMF in the YS; Tang [38] suggests that TMF is usually found to occur when k=2.0. Referring to these examples and considering the thickness of the fronts, we take as the threshold range of k to determine the location of TMF. As shown in Figure 7(a), the location of TMF, k contours from 1.8 to 2.0, is highly consistent with most upwelling areas except for the water near the YRE. This result implies the inherent relationship between tidal mixing and upwelling in the study region. For the estuarine waters, the strong salinity front may account for this disparity. The dynamic mechanism of TMF-induced upwelling consists in the intense density difference and the ensuing pressure gradient across the front. Vertical structure analysis is helpful to expound the mechanism. Cross section E (along N) and F (along 30.5 N, see Figure 7(a)) represent gentle and steep slope, respectively. In the inshore area, the tidal turbulence is so strong that shallow water is mixed from bottom to surface, and the whole water column is heated by solar radiation and hence attaining high temperature and low density (Figure 8(a), (c)). For the deep area, the thermocline is obvious between the upper and bottom mixed layer. The bottom water up to 30 m is controlled by tidal mixing [32], and is quite cold. A sharp boundary (the TMF), which separates the vertically well-mixed and stratified waters, is clearly identified in both temperature and density field. Strong cross-front density variance and the subsequent baroclinic pressure gradient drive the bottom water to move from deep to shallow area along the slope and move reversely in the upper layer. So a secondary circulation is present near the front, and upwelling appears as a vertical branch. In contrast with cross section E, on section F the slope is steeper, the TMF is stronger, and the isotherm domes induced by upwelling is more acute (Figure 8(c)). In the absence of tide, both the front and upwelling almost disappear (Figure 8(b), (d)). Generally, the TMF always occurs on the bottom slope, and for this reason, this TIU is actually a joint consequence of the TMF and topography. Since the TMF is a typical baroclinic phenomenon, we call the upwelling baroclinic TIU. 4 Discussion Tidal mixing plays an important role in determining the density structure of coastal shallow seas [16]. In the coastal seas with strong tidal signals, tidal mixing often results in easily discernable fronts. For the difficulty of field observation [20], two-dimensional models are commonly employed in the studies of cross-frontal circulation [20 23]. Given ideal density fields, the models often predict cell structure on the transect perpendicular to shoreline. Depending on the density distribution, such circulation can be composed of one single cell [21], or two cells including a dominant and a subsidiary component [22,23]. The mechanism of baroclinic TIU described in section 3.3 can be verified by previous studies [20 23]. The recent work of Dong et al. [23] can be taken as a representative of the two-dimensional model studies on cross-frontal circulation. Using an analytical model, they found that the summer density front results in a dominant clockwise cell and a weak anticlockwise cell in the upper layer. Basically, this structure consists with the calculation in this study (see Figure 8(a), (c)), except for the absence of the weak cell in Figure 8. The distinction can be ascribed to the differences in study methods between the two works. In the study of ref. [23], the model 470 LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

10 Figure 8 Depth-longitude sections of u-w vectors and temperature along cross section E (a,b) and F (c,d) from baroclinic MASNUM coupled model. (a) and (c) are from Control Test; (b) and (d) are from Exp. NoT. The contour denotes temperature ( ). The vertical velocity is amplified 500 times. is highly simplified, and the mean flow is sufficiently weak that its influence on tidal flow is negligible. In this paper, complex physics are considered to reproduce the circulation system in summer; the weak signal of the minor cell is masked by strong shelf circulations, and only the dominant clockwise cell is conspicuous. In addition, the simulation of fine circulation structure is restricted by model resolution. In the models used by Loder and Wright [20], James [21], Garrett and Loder [22], and Dong et al. [23], the density field is prescribed and its development is not considered. In contrast, the fronts shown in Figure 8(a) and (c) are modeled rather than given beforehand, and the pattern agrees well with that of the typical front [21,22]. Controlled by physical processes such as solar radiation and vertical mixing, the thermohaline structure is not only set up and developed in the modeling, but also undergoes seasonal variation and annual cycles, showing the well performance of MASNUM coupled model. TMF is believed to be evident in the Yellow Sea [36,38]. Based on field data analysis, Zhao [36] pointed out the existence of upwelling in the frontal zone on the west side of the cold water mass in south YS. Furthermore, he presumed that upwelling might exist at the fringe areas of the whole cold water mass, though no explanation is presented. Afterward, he applied James tidal model into the study of tidal front circulation in north YS [39]. The three-dimensional, high resolution, baroclinic modeling in this study indicates that TMF and the related upwelling occur in the coastal waters off Zhejiang in summer. We believe that TMF and upwelling are closely associated with each other: in the coastal waters where the TMF is strong, upwelling usually appears in the frontal zone. In semidiurnal tidal seas, most previous studies on barotropic TIU focus on M 2 tide only [13 15]. In the study area in this paper, M 2 tide is also dominant. Can we take it for granted that the partial tides other than M 2 exert LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

11 Table 2 Comparison between the upwelling induced by four and three main tidal constituents a) Upwelling area w 3 (10 5 m/s) Averaged upwelling velocity Sum of velocities at all upwelling grids w 4 (10 5 m/s) w 3 /w 4 (%) W 3 (10 5 m/s) W 4 (10 5 m/s) W 3 /W 4 (%) Off YRE Shengsi Off Zhejiang Whole area a) Subscripts 3 and 4 correspond to the results of three (S 2, K 1, O 1 ) and four (M 2, S 2, K 1, O 1 ) tides, respectively. only negligible effect on upwelling? Luo et al. [15] concluded that the maximum upwelling off Zhejiang coast is about m/s, while the number in this study is m/s, with peak value of m/s in specific areas (Figures 4 and 6). Can this larger value be attributed to the coupling of four tidal constituents? An experiment is conducted to examine the contributions of S 2, K 1, and O 1 tide to upwelling. In this experiment, the three tides are simulated simultaneously, while the M 2 tide is excluded from the barotropic model. Similarly to the results of four tides coupling (Figure 4), the calculated upwelling at 15 m depth also includes three areas (Figure 9). Two indexes, the averaged vertical velocity and total velocities at all upwelling grids at 15 m depth, are calculated to quantitatively evaluate the role of S 2, K 1, and O 1 tide in inducing upwelling (Table 2). Notwithstanding the relatively small magnitude the upwelling is still considerable. The weakest averaged upwelling is off Zhejiang coast, accounting for 28% of the upwelling induced by four tides. The highest percentage, over 50%, appears off the YRE. In the entire upwelling area, the percentage is 35%. For the total vertical velocities, upwelling elicited by S 2, K 1, and O 1 tide makes up 27% of the four-tide result. Hence it can be estimated approximately that about 30% of the baro- Figure 9 Upwelling induced by three barotropic tides (S 2, K 1, and O 1 ) at 15 m depth. Contours are from m/s with interval of m/s. tropic TIU results from the joint effects of S 2, K 1, and O 1 tidal constituents, and the contributions of these partial tides are important. One possible interpretation is that the tidal currents of the three tides are fairly strong: in the waters shallower than 50 m (off YRE and near Zhoushan islands) the maximum S 2 tidal flow reaches cm/s. At the same time, there always exists nonlinear interaction among the tidal constituents. 5 Conclusions Owing to the complex hydrography off YRE and in Zhejiang coastal waters, the dynamic mechanism of upwelling may be diverse and complicated. In this paper, the summer upwelling is simulated numerically, and the mechanism is investigated from the point of view of tidal movements. The modeled upwelling basically agrees with field surveys and satellite observations. The main conclusions are as follows: (1) Tidal movement induces upwelling in both barotropic and baroclinic processes. These two processes are related to barotropic tidal currents and tidal mixing front, respectively. Most of the previous studies focused on barotropic aspect and neglected the baroclinic mechanism, so tidal contributions to the upwelling off YRE and Zhejiang coast was likely underestimated in the past. Numerical experiments indicate that in the study area about 60% of the upwelling is attributed to tidal effects, which should be the primary inducement for upwelling. (2) Barotropic tidal residual currents induce upwelling in two ways: bottom convergence and the climbing along bottom slope. The good agreements between the convergent areas of bottom residual currents and the upwelling areas suggest that the bottom convergence is a major cause. In some area, these two effects all act on upwelling. (3) In baroclinic circumstances, strong tidal mixing leads to fronts in coastal waters. The waters are vertically well-mixed and stratified at the inshore and offshore side of the front, respectively. The baroclinic pressure gradient, which stems from the density variance across the tidal front, drives secondary circulation and the consequent upwelling branch clinging to the front. Note that the TMF usually appears on the bottom slope, so the upwelling should be understood as a joint result 472 LÜ XinGang et al. Sci China Ser D-Earth Sci March 2007 vol. 50 no

12 of tidal mixing and topography. (4) For the barotropic process, the upwelling induced by S 2, K 1, and O 1 tides roughly accounts for 30% of the total upwelling driven by the four main tidal constituents (M 2, S 2, K 1, and O 1 ). Although in the study area M 2 tide is predominant, the influences of other constituents are also important on upwelling. Simulating the four main tidal constituents simultaneously is helpful to improve 1 Zhou M J, Zhu M Y, Zhang J. Status of harmful algal blooms and related research activities in China. Chin Bull Life Sci (in Chinese), 2001, 13(2): Zhao B. The upwelling off Yangtze River estuary. Acta Oceanol Sin (in Chinese), 1993, 15(2): Zhu J. Dynamic mechanism of the upwelling on the west side of the submerged river valley off the Changjiang mouth in summertime. Chin Sci Bull, 2003, 48(24): Zhao B, Li H, Yang Y. Numerical simulation of upwelling in the Changjiang river mouth area. Stud Mar Sin (in Chinese), 2003, 45: Mao H L, Ren Y W, Sun G D. 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