SCIENCE CHINA Earth Sciences

SCIENCE CHINA Earth Sciences RESEARCH PAPER March 2013 Vol.56 No.3: 386 396 doi: 10.1007/s11430-012-4429-9 Responses of the Zhe-Min coastal current adjacent to Pingtan Island to the wintertime monsoon relaxation in 2006 and its mechanism PAN AiJun *, WAN XiaoFang, GUO XiaoGang & JING ChunSheng The Third Institute of Oceanography, Ocean Dynamics Laboratory, Xiamen 361005, China Received October 18, 2011; accepted April 28, 2012; published online September 30, 2012 In conjunction with synchronous remotely sensed winds and sea surface temperature (SST), the spatiotemporal features of the Zhe-Min coastal current (ZMCC), especially responses of the ZMCC adjacent to Pingtan Island (PT) to the wintertime monsoon relaxation in 2006 and corresponding mechanism are investigated based on the field observations. In situ data are acquired from Conductivity-Temperature-Depth (CTD) cruise and Bottom-Mounted Moorings (BMM), which are conducted during a comprehensive survey for the Chinese Offshore Investigation and Assessment Project in winter 2006. It is revealed that the ZMCC is well mixed vertically in winter 2006. The ZMCC (<14 C) recedes during the relaxation of the wintertime monsoon and is accompanied by the enhanced northward shift of the warm, saline Taiwan Strait Mixed Water (TSMW, higher than 14 C and is constituted by the Taiwan Strait Warm Water and the Kuroshio Branch Water). And greatly enhanced southward intrusion of the ZMCC can be detected when the wintertime monsoon restores. Correspondingly, the thermal interface bounded by the ZMCC and the TSMW moves in the northwest/southeast direction, leading to periodic warm/cold reversals of the near-seabed temperature adjacent to the PT. By EOF (Empirical Orthogonal Function) analysis of the large-scale wind fields and wavelet power spectrum analysis of the water level, ocean current and the near-seabed temperature, responses of the ZMCC off the PT to wintertime monsoon relaxation are suggested to be attributed mainly to the southward propagating coastally trapped waves triggered by the impeding atmospheric fronts. As a result, ocean current and near-seabed temperature demonstrate significant quasi-5 d and quasi-10 d subtidal oscillations. By contrast, the onshore/offshore water accumulation resulted from Ekman advection driven by the local winds has minor contributions. Zhe-Min coastal current, Taiwan Strait mixed water, coastally trapped wave, wintertime monsoon, sea surface temperature, water level, near-seabed temperature Citation: Pan A J, Wan X F, Guo X G, et al. Responses of the Zhe-Min coastal current adjacent to Pingtan Island to the wintertime monsoon relaxation in 2006 and its mechanism. Science China: Earth Sciences, 2013, 56: 386 396, doi: 10.1007/s11430-012-4429-9 The Taiwan Strait is the main channel connecting the East China Sea (ECS) and the South China Sea (SCS), where the circulation is driven by the winter and summer monsoon, alternatively [1]. In summer, the mean current speed in the Taiwan Strait is about 90 cm s 1, with the western and eastern side occupied by the northward SCS monsoon drift and *Corresponding author (email: aijunpan@tiosoa.cn) the Kuroshio branch water, respectively. In winter the eastern part of the strait is dominated by the northward Kuroshio branch water against the prevailing monsoon with current speed at 20 cm s 1 [3 8], while the western part is occupied by the southward ZMCC driven by the northeast monsoon [8 11]. As one of the main current systems on the western side in strait, the ZMCC emanates from the Changjiang outlet and the Hangzhou Bay, spanning the coastal regimes of Science China Press and Springer-Verlag Berlin Heidelberg 2012 earth.scichina.com www.springerlink.com

Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 387 Zhejiang and Fujian provinces and extending northward to no farther than Changjiang outlet. The ZMCC is composed mainly of the Changjiang and Qiantangjiang river plumes, and during its southward spreading, the Oujiang and Minjiang river plumes join in. The ZMCC is characterized by its low temperature and low salinity, and it occurs in autumn, winter, and spring seasons. In summer flood season of the Changjiang, large amounts of river plume flow toward the Jizhou Island driven by the southwest monsoon. The southward spanning of the ZMCC is greatly reduced and usually cannot enter into the Taiwan Strait. By contrast, during strong northeast monsoon season, the cold, less saline, and nutrient-enriched ZMCC can reach as far as Shantou, greatly influencing the circulation pattern, water mass, and oceanic ecosystem [9 12]. The strong northeast monsoon not only hinders the northward flow of the strait warm water but is favorable for the southward intrusion of the ZMCC, therefore greatly changing the impacting zone of the ZMCC. For instance, Wu [13] believed that influences of the ZMCC are confined to north of PT in winter due to its small river plume. Other studies suggested the ZMCC can flow southward to Quanzhou in strong northeast monsoon conditions [12, 14]. Presently, it is widely believed that influences of the ZMCC can reach southward to the zone between Dongshan and Nanao Island [9, 15]. Wang and Chen [16] pointed out that the southward spanning of the ZMCC has significant correlations with the speed of the winter monsoon. Changes of the winter monsoon intensity can induce the spatial wave pattern of the ZMCC and in some extreme cases, the ZMCC can intrude eastward across the middle strait. Using four BMM deployed along the line from Wuqiu to Taizhou, Ko et al. [17] investigated the features of water transport in the northern Taiwan Strait during the beginning phase of winter monsoon in 1999. They suggested that the water transport in Taiwan Strait was dominated by not only the local winds but remote wind fields. The latter can impact the water transport by exciting southward propagating coastally trapped wave in the Yellow Sea (YS) and ECS. Recently, Zhang et al. [11] explored the relationship between the short-term variability of the ZMCC and winds, and pointed out that wind stress played a key role in affecting the SST and the impacting zone of the ZMCC on diurnal timescale. Generally, previous studies on the ZMCC are confined to one or several limited cruise observations; though recent satellite remotely sensed monitor has greatly enhanced our knowledge of responses of the ZMCC to overlaying winds, vertical current structure of the ZMCC and its responses to winter monsoon still remains unclear due to data deficit in long-term current observations. As the impacting zone of the ZMCC depends largely on the intensity of the winter monsoon, further explorations are required to improve our knowledge of the variability of the circulation in Taiwan Strait. In this paper, with the help of unprecedented long-term current profiling data obtained from a comprehensive survey carried out by the Chinese Offshore Investigation and Assessment Project in winter 2006, responses of the ZMCC to the relaxation of winter monsoon are thoroughly explored. 1 Data sources Two main types of data were used in this study. First, temperature and salinity data were obtained from SBE917 Plus Conductivity-Temperature-Depth (CTD, US Seabird), including a total of 178 profiles from 9 January to 6 February 2007. After quality control procedures, the vertical sampling rate was 1 m. Secondly, the ocean current, nearseabed temperature, and water level were acquired from four Bottom-Mounted Moorings (BMM) deployed on the ocean floor adjacent to Pingtan (PT), Quanzhou (QZ), Zhangpu (ZP), and Shantou (ST), respectively. The BMM is equipped with a WHS 300 khz ADCP and ATD-HR. The ATD-HR measured the water level (pressure) and the near seabed temperature (about 0.3 m above the ocean floor) with a sampling rate of 10 min from 11:00 on 5 February to 11:00 on 25 March 2007. The WHS 300 khz ADCP measured the ocean current from 1.47 to 41.47 m with a vertical resolution of 2 m, and thus there were 21 levels in total. Given the ADCP installation height and the resulting blind zone (upper 2.23 m), the deepest current observations were about 4.73 m above the ocean floor. The current data were recorded from 11:00 on 5 February to 11:00 on 25 March 2007 with sampling intervals of 10 min. Besides, the synchronous satellite remotely sensed daily winds (QuikSCAT, downloaded from ftp.ssmi.com) and sea surface temperature (OISST) are also taken into consideration. 2 General Feature of the ZMCC in winter 2006 2.1 Distributions of temperature and salinity at 10, 20 and 30 m depths Figure 1 shows the distributions of temperature and salinity at 10, 20, and 30 m depths. CTD measurements are performed from 9 January to 6 February in 2007, which usually corresponds to the most powerful period of the ZMCC [11, 12, 15]. It is found that the ZMCC can extend southward to Shantou in Guangdong province if we follow the definition of Liang and Li [9] and Zhang et al. [11] and take 17 C isotherm and 33.5 isohaline for the outer edge of the ZMCC. And the southern part of the Taiwan Bank is dominated by prominent warm, saline SCS warm water (Figure 1). For simplicity and emphasis, 14 C isotherm is adopted to denote the outskirt of the ZMCC, and water mass with temperature higher than 14 C is defined as the Taiwan Strait Mixed Water (TSMW, constituted by the SCS warm water and the Kuroshio Branch Water (KBW)). Under this as-

388 Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 Figure 1 (a) (c) and (d) (f) show the temperature (unit: C) and salinity at 10, 20, and 30 m, respectively, where 14 C isotherm (purple dashed line), 17 C isotherm (red dashed line), and 33.5 isohaline (blue dashed line) are superimposed. Taiwan Bank is indicated by the 35 m isobath (black dashed line). With anticlockwise rotation of the coordinate by 56.3, the in situ current obtained from the mooring off the Pingtan Is. is divided into the alongshore and crossshore orientation. sumption, the cold, less saline, and vertically well mixed ZMCC is found to be confined adjacent to PT. Moreover, the warm, saline water emanating from northern Penghu Island to Quanzhou is inferred from the northward KBW along the Penghu Channel. 2.2 Near-seabed temperature Figure 2 presents the distributions of the near-seabed temperature obtained from four BMMs deployed adjacent to Pingtan Island (PT), Quanzhou (QZ), Zhangpu (ZP), and Shantou (ST), respectively. In conjunction with Figure 1, two interesting phenomena can be identified. Firstly, the southward spanning of the ZMCC only happens before 9 February, and then the offshore zone of QZ tends to be occupied by anomalous warm water. Thus, on one hand, it seems to be linked with the westward intrusion of the KBW, and on the other hand, it can also be induced by the local

Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 389 winds and air-sea fluxes due to topographic features adjacent to ZP. Secondly, the near-seabed temperature offshore PT has significant periodicity with warm, cold alternating signals. What is the relationship between the warm, cold alternating signal and the ZMCC? Is it induced by relaxation of the ZMCC or else? If so, what is the cause for relaxation of the ZMCC? In present paper, we will only focus on the second phenomenon. 3 Responses of the ZMCC offshore PT to relaxation of wintertime monsoon in 2006 and its mechanism 3.1 Features of the ZMCC offshore PT Background circulation fields are checked at first. Figure 3 shows a vertical profile of the background current offshore PT that denotes the ZMCC through filtering tidal signals at each level and then making a time average. The parallel current oriented in southwest/northeast direction dominates, with current speed from 0.3 to 0.1 m s 1. Southward flowing ZMCC prevails in upper 15 m layers with current speed decreased downward. The maximum speed is found at near-surface layers, representing monsoon-driven feature. Meanwhile, northward flow against the prevailing wind can be detected deeper than 15 m with current speed increasing upward, and the maximum speed is located at about 30 m depth (0.1 m s 1 ). The northward flow deeper than 30 m gradually decreases, and generally the alongshore current demonstrates a first baroclinic mode pattern. By contrast, the crossshore component has weaker amplitude between 0 and 0.13 m s 1, decreasing downward vertically with the offshore feature. Figure 4 presents the vertical profiles of temperature and salinity acquired from a leg of CTDs deployed perpendicular to the ZMCC and nearby the BMM. The isotherms and Figure 2 Time-distance section of the near-seabed temperature acquired from the four bottom-mounted moorings located off PT, QZ, ZP, and ST. Figure 3 Time-averaged vertical profile of the background current, which is obtained by cutting off the tidal components from the in situ mooring current, which ranged from 5 February to 25 March 2007. After anticlockwise rotation of the Cartesian coordinate by 56.3, the alongshore current (solid line) and the cross-shore current (dashed line) are obtained. Figure 4 Depth-distance CTD profiles for temperature (unit: C) (a) and salinity (b), where 14 C isotherm (purple dashed line) and 17 C isotherm (red dashed line) are overlaid.

390 Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 isohalines extend vertically upward from bottom to sea surface, indicating that the water mass is vertically well mixed. Moreover, based on our previous definition, the mooring offshore PT is just located at the outskirt of the ZMCC and under the control of the TSMW. 3.2 Responses of the ZMCC adjacent to PT to Wintertime Monsoon Relaxation Figure 5(c) further shows the complete time series of the near-seabed temperature acquired from the mooring (5 February to 25 March 2007); the alternating feature of the warm/cold near-seabed temperature is even clearer. Generally, the oscillation of the near-seabed temperature offshore PT tends to be intensified from the beginning of February to the end of March, with maximum amplitude of about 6 C. The power spectrum analysis indicates that the near-seabed temperature has significant subtidal periodic oscillations of quasi-10 d, 5 d, and 3 d. Intuitionally, the quasi-10 d subtidal oscillation signal is more apparent after 23 February, whereas the quasi-5d and quasi 3d subtidal signals dominate ahead of that time. In addition, prominent overlaying semi-diurnal and diurnal tidal signals can also be detected (Figure 5(d)). As to whether the anomalous changes of the near-seabed temperature offshore PT is induced by the ZMCC, now the external atmospheric forcing would be checked at first. Due to data deficit of buoyancy fluxes (specifically, the heat fluxes), here only QuikSCAT daily winds (Figure 5(a)) would be analyzed and for convenience, wind vector is further divided into the alongshore and crossshore components (Figure 5(b)). It is easily seen that winter monsoon off the PT has quasi 5 d periodic gust pattern (power spectrum figure not shown), with alongshore wind stronger than the Figure 5 Time series of (a) wind vector (unit: m s 1 ), (b) alongshore wind (purple-solid line) and cross-shore wind (blue dashed line) (unit: m s 1 ) averaged in (119.375 120.625 E, 24.875 26.125 N) from 5 February to 25 March 2007, with dotted-line representing the mean alongshore wind speed, (c) the near seabed water temperature. (d) Power spectrum analysis on the near-seabed temperature. (e) f) presents the depth-time section of the alongshore current (unit: m s 1 ) and the cross-shore current (unit: m s 1 ), respectively.

Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 391 crossshore component. Noticeably, relaxations of the alongshore wind is highly consistent with changes of the near-seabed temperature. In other words, when winter monsoon increases, decreased near-seabed temperature can be detected; in contrast, greatly enhanced near-seabed temperature can be identified during the relaxation period of winter monsoon. Given featured distributions of the temperature and salinity (Figure 4), an assumption would be easily deduced as follows: the periodic cold/warm alternating phenomena offshore PT may be induced by onshore/offshore water accumulation due to Ekman advection (Figure 6). That is, when winter monsoon enhances, onshore Ekman advection would result in onshore flow from middle to top layers and offshore flow from bottom to middle layers, which tends to steepen the ZMCC/TSMW interface, resulting in decreased near-seabed temperature because the mooring is under the control of cold ZMCC, and vice versa. To verify above hypothesis, vertical current profiles obtained from the mooring is further analyzed (Figure 5(e), (f)). The alongshore current is much stronger than the crossshore one, which represents well the background circulation feature offshore PT under wintertime monsoon conditions (Figure 3). Secondly, both of the alongshore and crossshore current respond very quickly to surface winds (<2 d) and they are highly consistent with each other, implying changes of surface winds (for instance, relaxation) could lead to quick responses of the vertical current structure. If we choose 23 February, 4 March, 15 March, and 24 March to denote four peaks of the wintertime monsoon re- Figure 6 Schematic map for the response of ZMCW to the relaxation of the wintertime monsoon. The solid-line and dashed-line represent the ZMCW/TSWC interface under the normal and relaxation period of the wintertime monsoon, respectively. laxation events and use the moment 3 d ahead to represent the beginning of the monsoon relaxation (Figure 5(b)), for each winter monsoon relaxation event, the northward flow with onshore component tends to extend rapidly from bottom to subsurface at about 10 m depth, though the northeast current remains on the sea surface. With intensification of winter monsoon, the southward offshore current emanates from surface downward, and the upwind northward onshore current is substituted more quickly. The high frequency variations of the alongshore wind ahead of 20 February excite quick responses of the whole vertically current structure, which, however, is not the focus of this paper. Through above analysis, we can find that evidence supporting former hypothesis is not that robust because the current shows totally forced responses to overlaying winter monsoon and the Ekman advection/accumulation effects play a minor role. Hence, what is the cause for the periodic cold/warm alternating of the near-seabed temperature offshore PT? Because in situ CTD observations can only capture a snapshot of the water temperature, they are incapable of providing a complete description of responses of the ZMCC to wintertime monsoon relaxation. Thus, we instead use satellite remote sensing SST to consider its spatiotemporal continuity. Then, could the satellite remote sensing SST be used to document variability of the ZMCC? Figure 7(a) provides distributions of the in situ 14 C isotherms at different depths of 2, 10, 20, and 30 m. A significant thermal interface bounded by the ZMCC and TSMW can be identified, which is oriented in south-north direction and spanning from surface to bottom. Water mass offshore PT (where the mooring is deployed) is vertically well mixed and hence, in situ SST (for convenience, the near-surface temperature at 2 m depth is used to denote in situ SST) can be adopted to represent the full depth water. Comparisons between in situ SST and synchronous remotely sensed SST (Figure 7(b)) imply that both of them have very similar spatial pattern, though the remotely sensed SST is slightly higher and has smaller horizontal temperature gradient. Generally, it is plausible to use remotely sensed SST to represent the ZMCC. Adopted to present a whole depiction of the ZMCC responses to monsoon relaxation are two snapshots of the remotely sensed SST representing the relaxation and normal monsoon, respectively (Figure 7(c)), and another three snapshots derived from the near-surface and near-seabed current vectors at 13:00 on 11 March (normal monsoon), 13:00 on 15 March (monsoon relaxation), and 13:00 on 17 March, 2007 (normal monsoon again) (Figure 7(d) (f)). During normal winter monsoon, apparently southward flow dominates surface layers, and near-seabed layers are controlled by southward flow with offshore component (Figure 7(d)). As monsoon relaxes, southward flow from surface to bottom is substituted by the northward flow (Figure 7(e)) and as a result, significant increase of the near-seabed temperature (Figure 5(c)) can be detected due to northward re-

392 Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 Figure 7 (a) 14 C isotherm distribution at 0, 10, 20, and 30 m depths; (b) the in situ temperature at 2 m depth (shaded, 14 and 17 C isotherm is denoted by purple-dashed line and red-dashed line, respectively) and the concurrent remote sensing SST (black-solid line) distribution adjacent to PT (unit: C); (c) SST distribution on 15 March, 2007 (blue-solid line) and on 18 March, 2007 (red dashed line) and the zero line for their difference (black-dashed line) (unit: C). (d) (f) shows the near-surface ocean vector (pink) and near-seabed ocean vector (blue) at 13:00 on 11 March, 2007; at 13:00 on 15 March, 2007; and at 13:00 on 17 March, 2007, respectively. Besides, 20, 40, 60, 80, and 100 m isobaths are overlaid in ((a), (d), (e), (f)). cede of the ZMCC and northward intrusion of the warm, saline TSMW (Figure 7(c)). When the normal monsoon recovers, significantly enhanced southward invasion of the ZMCC (Figure 7(c), (f)) leads to an apparent decrease of water temperature at the mooring locus because the mooring is under the control of the cold, fresh ZMCC in this case (Figure 5(c)). Further, take the winter monsoon relaxation event from 13 to 18 March 2007 (Figure 5(c)) for example, maximum temperature occurs on 15 March, the temperature decreases to normal condition on 18 March, and a thermal difference about 3 C can be detected. The vertical temperature profile in Figure 4(a) shows that the mooring locus corresponds roughly to that of the 14 C isotherm, although it is not synchronous with the monsoon relaxation event mentioned above. If a 3 C increase of the near-seabed temperature at the mooring locus is assigned, 17 C isotherm is needed to move to the mooring locus. The distance between these two isotherms is less than 40 km and if 3 d is taken for the 40 km movement, then the translating speed is estimated to be about 0.15 m s 1, which is highly consistent with the observed current speed (Figure 5(e)). It should be pointed out that if the remotely sensed SST difference (Figure 7(c)) is adopted for assessment, the translating speed would be 0.35

Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 393 m s 1, greatly exceeding the observed current speed due to mild horizontal gradient of the remotely sensed SST (Figure 7(b)). Hence, the remotely sensed SST is qualitatively capable of describing responses of the ZMCC to winter monsoon relaxation; however, quantitative assessment cannot be achieved due to its smaller horizontal gradient. Generally, the periodic cold/warm alternating phenomena of the near-seabed temperature offshore PT should be Figure 8 Schematic map for the response of ZMCW to the relaxation of the wintertime monsoon. The solid-line and dashed-line represent the ZMCW/TSWC interface under the normal and relaxation period of the wintertime monsoon, respectively. attributed to the totally forced responses of the ZMCC to winter monsoon, rather than the onshore/offshore water accumulation induced by Ekman advection. Thus, the plausible scheme for responses of the ZMCC to winter monsoon relaxation can be deduced as Figure 8. 3.3 Mechanism for responses of the ZMCC to winter monsoon relaxation Figure 9 presents the time series of the nontidal water levels acquired from the four BMM deployed adjacent to PT, QZ, ZP, and ST, respectively. Southward propagating wave signals with apparent 2 12 d subtidal periods (Figure 9(b)) can be easily detected along the coast (Figure 9(a)), among which the quasi-4 d and quasi-8 d signals dominate (Figure 9(c)). The translating speed of the subtidal water level is estimated to be 9.5 to 18.5 m s 1 with 14 m s 1 mean speed and is coherent with Ko et al. s [17] result. Chen [18] is the forerunner in studying subtidal oscillations of water level in winter in the strait, and he pointed out that water level in the western Taiwan Strait has significant quasi-4 d oscillation period and the water level, wind stress and alongshore current correlates well with each other during 4 8 d periods. Chen and Su [19] explored the occurrence of the coastally trapped wave along the continent and its mechanism; they suggest there are coastally trapped wave propagating southward along the coast with 4.8 d and 3.1 d period in winter. Li s [20] research suggests that the cold storm surge not only can induce the storm surge for both sides in the Taiwan Strait, but can trigger coastally trapped waves with 4.6 d and 2.9 d period. In a word, despite the northwestsoutheast shift of the whole ZMCC (Figure 8) or the periodic warm/cold alternating phenomena of the near-seabed Figure 9 (a) Time-distance section of the non-tidal water levels acquired from the four bottom-mounted moorings located in the western Taiwan Strait (unit: m); (b) wavelet power spectrum analysis on the averaged-water levels of four moorings, where the white line represents the 95% significance level; (c) global wavelet power spectrum, where the red-dashed line shows the 95% significance level.

394 Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 temperature with quasi-10 d, 5 d and 3 d periods (Figure 5), all suggest that they are closely linked with the coastally trapped Kelvin waves propagating along the coast toward low latitudes [17]. Power spectrum analysis on the near-seabed current provide more evidence for above delusions; for instance, both of the crossshore and alongshore current have significant subtidal oscillation period of about 2 6 d (Figure 10(b) (e)). In addition, both of them have quasi 10 d subtidal period though it does not surpass 95% significance level due to time average (Figure 10(c), (e)). However, if we look in detail, the quasi-10 d subtidal signal greatly exceeds the 95% significance level from 9 March to 19 March 2007 for the alongshore current (Figure 10(d)) and is highly consistent with that of the near-seabed temperature (Figure 5(c)) and water level (Figure 9(b)). Hence, the former delusion is robust (Figure 8); that is, the cold/warm alternating event with quasi-10 d period of the near-seabed temperature adjacent to PT is most likely induced by the northwestsoutheast shift of the ZMCC driven by winter monsoon changes, and the onshore/offshore accumulation triggered by Ekman advection plays a minor role. Although the local parallel wind correlates well with the current and near-seabed temperature (Figure 5(a), (b)), previous analysis has verified that the northwest-southeast shift of the whole ZMCC offshore PT is resulted from the coastally trapped wave. As is well known, the coastally trapped wave is not excited locally but triggered by remotely large-scale winds, moreover, above studies also imply that the northeast gust in the Bohai Sea and Yellow Sea can often ignite coastally trapped wave [17, 21 24]. Hence, whether do former analyses about the locally parallel winds still have credibility to some extent? Given the sea adjacent to China is fully driven by the northeast monsoon in winter with the dominant meridional component, EOF analysis on the synchronous meridional wind covering the whole northwest Pacific (including China adjacent sea) is carried out (Figure 11). The first and second EOF modes account for 60.6% and 28.0% of the total variance, respectively. Both of them indicate that the meridional wind in the Taiwan Strait changes in phase with that of the whole sea adjacent to China (specifically, the Yellow Sea and Bohai Sea), except for a small zone nearby the Hainan Island. In addition, amplitude of the wind in the Taiwan Strait is extremely en- hanced due to the Narrow tube effect. Wavelet power spectrum analysis on the corresponding time series of the Figure 10 (a) Time series of the near-seabed currents adjacent to PT, where the alongshore current and cross-shore current are denoted by solid line and dashed line, respectively. (b) and (d) The wavelet power spectrum of the cross-shore current and alongshore current, respectively, where the rude-black line indicates the cone of influence and the white line represents the 95% significance level. (c) and (e) The corresponding global wavelet power spectrum, where the red-dashed line shows the 95% significance level.

Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 395 Figure 11 Spatial distribution of the first (a) and second (b) EOF mode of the meridional winds in the western Pacific (involving the China adjacent sea) from 5 February to 25 March 2007, where the dashed-line denotes the zero-line. (c) Time series of the first (solid line) and second (dashed line) EOF mode. (d) and (f) The wavelet power spectrum of the first and second mode, respectively, where the white line represents the 95% significance level. (e) and (g) The corresponding global wavelet power spectrum, where the red-dashed line shows the 95% significance level. first and second modes (Figure 11(c)) reveals that the first mode has a significant quasi-5d subtidal oscillation period (Figure 11(d), (e)). Besides, the quasi-14 d subtidal signal of the first EOF mode can also be detected, though it does not exceed the 95% significant level due to time average. The second mode is dominated by the subtidal signal with quasi-10 d period (Figure 11(f), (g)) and corresponds well with the occurrence period of the water level (Figure 9(b)), near-seabed current (especially for the alongshore component) (Figure 10(d)), and the near-seabed temperature (Figure 5(c)). Generally, the quasi-5 d, 10 d, and 14 d subtidal wind disturbances represent well the atmospheric fronts impacting the adjacent sea in southeast China in winter and hence the previous analysis on local winds offshore PT is somewhat representative. However, responses of the ZMCC offshore PT to the relaxation of winter monsoon should be attributed to the southward propagating coastally trapped wave triggered by the atmospheric front influencing the southeast China adjacent sea in winter, rather than the local winds. 4 Conclusions As one of the important branch constituting the winter circulation of Southeast China Sea, the cold, less saline and nutrient-rich ZMCC not only provides favorable conditions for the Marine biological reproduction in the strait, but also behaves as the boundary for many harbors of the southeast China adjacent sea, especially for those in Fujian province. And for the latter, pollutants carried by the southward ZMCC definitely participate in water exchanges between the harbor and offshore regime, so as to cause damage to the Marine ecological environment. Therefore, studying the spatiotemporal evolution feature of the ZMCC, understanding the characteristics of water exchange between the harbor and ZMCC, and revealing the internal mechanism, are in accord with demands for oceanic development on the hydrologic environment and can provide scientific support for government planning in Marine development, utilization, and management. As is well known, the Minjiang River is a mountain

396 Pan A J, et al. Sci China Earth Sci March (2013) Vol.56 No.3 stream with high variability in runoff and ranks the seventh in total yearly runoff in China. The average runoff amounts to 1903 m 3 s 1 and has remarkable interannual variability. The mutual impacts between the ZMCC and Minjiang are not well understood because of data deficiency. In addition, the unique PT capes terrain also hinders the southward flow of the ZMCC and is favorable for the enhanced responses of the current responses to winds, of which further field observations and numerical simulations are needed. In this paper, the three-dimensional spatiotemporal distribution feature of the ZMCC and specifically, responses of the ZMCC adjacent PT to winter monsoon relaxation and its mechanism are explored using in situ data (temperature, salinity, current, water level, and near-seabed temperature) obtained from the comprehensive survey conducted in winter 2006, in conjunction with synchronous remotely sensed SST and winds. During the relaxation phase of the winter monsoon, southward intrusion of the ZMCC is reduced, accompanied by the northward movement of the warm, saline TSMW. When the normal monsoon restores, the southward invasion is significantly enhanced and the ZMCC-TSMW interface moves from northwest to southeast. Hence, intermittent winter monsoon relaxation events tend to induce periodic warm/cold alternating phenomena of the near-seabed temperature with quasi 10 d, 5 d, and 3 d subtidal oscillations. Analysis in conjunction with synchronous local and large-scale winds reveals that responses of the ZMCC offshore PT are resulted mainly from the southward propagating coastally trapped Kelvin wave triggered by the large-scale atmospheric fronts impacting southeast China adjacent sea in winter, rather than the onshore/ offshore water accumulation due to local Ekman advection. However, we also realize that many important scientific issues cannot be fully resolved as yet due to data deficit, for instance, the exchange rules governing the terrestrial material, biochemical elements nearby the ZMCC-TSMW front and the influences of the ZMCC as external boundary on the self-purification of harbors in Fujian province. In the future, targeted scientific survey is desired to be carried out in the key interactive regime between the ZMCC and TSMW, to solve some important scientific problems and improve our knowledge of the shelf circulation dynamics. This work was supported by National Natural Science Foundation of China (Grant Nos. 41176031 and 40806013), Chinese Offshore Physical Oceanography and Marine Meteorology Investigation and Assessment Project (Grant No. 908-ZC-I-01) and National Basic Research Program of China (Grant No:. 2011CB403504). 1 Wyrtki K. Physical oceanography of the southeast Asia waters. Scientific Results of Marine Investigations of the South China Sea and Gulf of Thailand 1959 1961. Naga Report. San Diego: Scripps Institution of Oceanography, 1961. 195 2 Nitani H. Beginning of the Kuroshio. In: Stommel H, Yoshida K, eds. Kuroshio, Its Physical Aspects. Tokyo: University of Tokyo Press, 1972. 129 163 3 Chuang W S. Dynamics of subtidal flow in the Taiwan Strait. J Oceanogr Soc Jpn, 1985, 41: 65 72 4 Chuang W S. A note on the driving mechanisms of current in the Taiwan Strait. J Oceanogr Soc Jpn, 1986, 42: 355 361 5 Wang J, Chern C S. On the Kuroshio branch in the Taiwan Strait during wintertime. 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