Seasonal response of the southern East China Sea shelf water to wind-modulated throughflow in the Taiwan Strait

Progress in Oceanography 121 (2014) 74–82

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Seasonal response of the southern East China Sea shelf water to wind-modulated throughflow in the Taiwan Strait Jae-Hong Moon ⇑, Naoki Hirose Research Institute for Applied Mechanics, Kyushu University, Kasuga-Kouen, Kasuga 816-8580, Japan

a r t i c l e

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Article history: Available online 7 December 2013

a b s t r a c t Seasonal change of the southern East China Sea (ECS) shelf water and its relation to the throughflow in the Taiwan Strait (TS) was examined based on comparative experiments with inserting passive tracers into a regional ocean model. Through analyzing the model output results, we found that from autumn to winter strong northeasterly wind over the TS significantly weakens the outflow from the TS (i.e., flowing into the ECS) and the Kuroshio water intrudes farther shoreward across the northern shelf of Taiwan in response to the weakened outflow. On the other hand, water flowing into the shelf from the TS extends further offshore from spring to summer when the TS throughflow is enhanced by a wind change from northeasterly to southwesterly and the Kuroshio water retreats seaward off the shelf due to the offshore extension of the shelf water. It suggests that the weakening (strengthening) of the TS throughflow could allow (inhibit) shelf-ward Kuroshio water onto the northeastern shelf of Taiwan, emphasizing that the throughflow modulated by a local wind can be an important factor controlling the seasonal variation of the southern ECS shelf water. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The northern shelf of Taiwan is known to be an extremely dynamic region due to the Kuroshio intrusion along its eastern side, the throughflow in the Taiwan Strait (TS) on its western side and their interactions (Tang et al., 2000). Water mass in the southern East China Sea (ECS) shelf closely depends on their interactions. The Kuroshio entering the ECS collides with the steep, zonally running break of the ECS and then a branch current separating from the mainstream flows to the north/northwest just off the northeastern tip of Taiwan, carrying warm and saline Kuroshio water onto the southern ECS shelf. It has been known for decades that the Kuroshio northeast of Taiwan migrates both intra-seasonally and seasonally (e.g. Sun, 1987; Johns et al., 2001; Zhang et al., 2001). Zhang et al. (2001) have reported a large meandering of the current in the East Taiwan Channel (ETC), which is the entrance of the Kuroshio to the ECS, with a dominant period of 100 days from long-term mooring current meter records. Mesoscale eddies propagating westward from the Pacific Ocean were also attributed to the intra-seasonal variation (Zhang et al., 2001; Hwang et al., 2004). At seasonal time scales, Tang et al. (2000) showed a strong seasonal migration of the Kuroshio, which moves closer to and onto the northern shelf of Taiwan in winter and away from the ⇑ Corresponding author. Current address: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. Tel.: +1 8183932232. E-mail address: [email protected] (J.-H. Moon). 0079-6611/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2013.11.009

shelf in summer, using historical ship board ADCP and hydrographic data. The outflow through the TS (flowing into the ECS shelf) is another important factor controlling the flow patterns in the northern shelf of Taiwan. The northeastward transport through the TS has a remarkable seasonal variation; large during summer and small or mostly zero during winter. This outflow completely connects with the Taiwan Warm Current (TWC) flowing northeastward along the ECS shelf from the Taiwan to the Korea/Tsushima Strait. Based on hydrographic and nutrients datasets, Chen and Sheu (2006) demonstrated that the TWC water mainly originates from the Kuroshio water intruding onto the shelf northeast of Taiwan during wintertime when the TS throughflow is significantly weakened. With respect to the seasonal changes of the shelf water, most studies have been focused on the influences of Kuroshio itself associated with monsoonal winds (Chao, 1991), heating (Chern and Wang, 1992), cooling (Guo et al., 2006; Oey et al., 2010) and mesoscale eddies off northeast of Taiwan (Tang et al., 2000). Nevertheless, mechanism of the Kuroshio migration is still a controversy. Unlike seasonal Kuroshio migration, impact of outflow from the TS on the seasonal shelf water patterns has been relatively little studied so far. Tang et al. (2000) showed a relationship between the mesoscale eddies and the Kuroshio intrusion, using four snapshot current observations, and emphasized on the effect of mesoscale eddies on the seasonal shelf water change. However, Mesoscale eddies exist all year round and have shorter their periods than 3–4 months (Wu et al., 2008) that makes it unlikely that

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the seasonal flow patterns north of Taiwan are influenced by the mesoscale eddy activities. The present study deals with the seasonal variation of the southern ECS shelf water and its relation to the throughflow in the TS. We pay special attention to how the seasonal outflow from the TS interacts with the Kuroshio intrusion on the northern shelf of Taiwan, using a regional ocean model with passive tracer experiments. A few studies on the contribution of the Kuroshio water and the TS water to the ECS shelf have conducted, inserting passive tracers into an ocean model. For instance, Jacobs et al. (2000) and Lee and Matsuno (2007) used local ECS models to examine the contribution to the ECS shelf water. However, their local sea models have some unrealistic components, such as artificially closed southern boundary and fixed open boundaries. In particular, the fixed flux through the shallow TS can block a wind-induced southward Chinese coastal current in winter and force to turn to the northeastward joining the TWC. Although Guo et al. (2006) used a nested high-resolution model considering realistic conditions on the open boundaries, they mainly discussed the origin of the Tsushima Warm Current water, which flows into the East/Japan Sea through the Korea/Tsushima Straits. The rest of this article is divided as follows. After a brief description of the model configuration and verification, we examine the spatial and temporal distributions of the tracers in the numerical model. We then discuss the impact of wind-induced outflow from the TS on the seasonal change of the ECS shelf water. Finally, summary is given. 2. Model configuration and comparison to observation The model used in this study is the Regional Ocean Modeling Systems (ROMS) which is a generalized ‘‘S-coordinate’’ primitive equation model and uses high-order time stepping and advection (Shchepetkin and McWilliams, 2005). It is widely used for coastal and continental shelf applications (e.g. MacCready et al., 2009; Moon et al., 2010). The model domain (Fig. 1) covers an extended ECS area from the northern South China Sea to the southern East/ Japan Sea to avoid artificial boundary component concerning the throughflow in the TS and Kuroshio intruding onto the northern shelf of Taiwan. We used spherical grid with 8 km horizontal resolution and 30 layers in the vertical. The model is initialized with, and one-way nested within, the western North Pacific Ocean

Fig. 1. Model domain and bottom topography (m). Thin lines in A, B and I denote the sections that the tracer concentrations are calculated. Thick lines are the sections for releasing the passive tracers (TS and east of Taiwan). Shaded region in the TS indicates the area where 25% weakened wind is applied into the model for Exp. 2a. Additionally, for Exp. 2b 25% weakened wind over the southern ECS shelf region (dotted box) is carried out to more confirm directly the effect of the windinduced onshore Ekman flow on the shelf region.

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model (WNPOM). The WNPOM domain extends from 110° to 165°E longitudinally and from 15° to 60°N latitudinally with horizontal resolution of 25 km and the model hindcasts from 1993 to 2007 using initial and boundary conditions from the data assimilated Simple Ocean Data Assimilation global model (SODA). The present ECS model uses 5-day mean values for temperature and salinity, subtidal surface height and subtidal velocity from the WNPOM, as the lateral boundary forcing. Atmospheric forcing is based on the NCEP/NCAR reanalysis daily mean and QuikSCAT 0.5° daily wind data from 2000 to 2003. Surface fluxes of momentum and heat are calculated in ROMS using bulk formulations (Fairall et al., 1996). Four-year-hindcast (2000–2003) simulations were carried out with original QuikSCAT wind forcing as a baseline integration, and we then computed three comparative experiments for wind forcing to examine the impact of wind-modulated current through the TS on the southern ECS shelf water: (1) 25% weakened QuikSCAT wind over the entire model domain, (2) the weakened wind only over the TS region, and (3) the weakened wind only over the southern ECS region. The description of the comparative experiments is shown in Table 1. Passive tracers with unit concentration were introduced at east of Taiwan (Kuroshio) and the TS to compare with their contributions on the southern ECS shelf water. Two separated tracers are released from surface to bottom at two sections of east of Taiwan and the TS (thick lines of Fig. 1). The tracer concentration at each release section is fixed to 1 during the model integration. The passive tracers could be used as an indicator for the variation of the southern ECS shelf water. The tracers are independent of each other, but they move in the same current field in the model interior. Before proceeding to an analysis of the results, we need to confirm that the modeled circulation has acceptable result with respect to relevant observation. We are able to use sea surface current fields computed from absolute dynamic topography with 0.25° spatial intervals and a repeat cycle of 7 days, provided by Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO, 2008). Data from January 1993 to December 2009 were used here. Fig. 2 compares the monthly mean surface current from the observation (upper) and the baseline integration (lower panels) in January (left) and July (right panels). The observed surface current in the ECS shows a remarkable seasonal variation, particularly along the continental shelf (Fig. 2a and b). Off the northeastern coast of Taiwan, the Kuroshio mainstream turns sharply eastward colliding with the zonally running shelf slope and then northeastward to continue along the ECS shelf break, approximately 200 m isobath. Speed of the Kuroshio along the east coast of Taiwan and along the shelf break increase in summer and decrease in winter. Prominent seasonal differences are seen in the Kuroshio intruding onto the northern shelf of Taiwan, the outflow from the TS and the TWC along the ECS shelf region. In winter, a substantial portion of the Kuroshio flows northward onto the northern shelf of Taiwan, and the current flowing into the ECS through the TS is quite weak. In summer, on the other hand, the Kuroshio intrusion northeast of Taiwan becomes weaker and the TS outflow is significantly stronger. The TWC which flows along the ECS shelf and finally enters the Korea/Tsushima Straits is also strong in summer and weak in winter in response to the outflow from the TS. The surface flow patterns obtained from AVISO are consistent with the observations of current (Lie et al., 1998; Katoh et al., 2000; Tang et al., 2000). Our model result agrees well with the observed surface current patterns and their seasonality. The Kuroshio moves shoreward in winter (Fig. 2c) and retreats seaward with weakened flow on the northern shelf of Taiwan in summer (Fig. 2d). In particular, the seasonal flow patterns on the northern shelf of Taiwan show a good agreement with the observation. This study focuses on the relationship between the outflow from the TS

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Table 1 Summary of comparative experiments in Section 4. Experiments

Description of each experiment

Exp. 1 Exp. 2a Exp. 2b

25% Weakened wind is applied in entire model domain. 25% Weakened wind is applied only in the Taiwan Strait region (shaded region in Fig. 1). Elsewhere, wind forcing is the same as that of baseline 25% Weakened wind is applied only in the southern ECS region (dotted box in Fig. 1). Elsewhere, wind forcing is the same as that of baseline

Fig. 2. Horizontal distributions of monthly mean surface current from the absolute dynamic topography provided by AVISO data from 1993 to 2009 (upper) and the baseline integration (lower panels) in winter (left) and summer (right panels).

and the Kuroshio intrusion in the northern shelf of Taiwan, and so the veracity of the modeled current field in the shelf region is a key requirement. 3. Seasonal variation of the southern ECS shelf water 3.1. Spatial distributions Fig. 3 represents horizontal distributions of the tracer released at east of Taiwan and TS for the baseline integration. As a response to the seasonal current patterns, the tracer from the Kuroshio water shows a completely different behavior in season over the ECS shelf. In winter, the tracer from the Kuroshio intrudes onto the northern shelf of Taiwan and it extends farther northeastward along the continental shelf from north of Taiwan to the Korea/Tsushima Straits (Fig. 3a). In summer, however, the tracer distributed along the inshore side of the Kuroshio almost vanishes and its concentration at the Korea/Tsushima Straits largely decreases (Fig. 3b). As shown in Figs. 3c and d, the tracer from the TS has an opposite seasonal contribution to the Kuroshio water. Another feature of seasonal differences is shown in Fig. 4 that provides the vertical profiles of tracer for two vertical transects A and B, which are marked in Fig. 1. For the two sections the tracer above 60 m depth is well mixed vertically and its concentration increases gradually from inshore side to offshore side in winter (left

panels of Fig. 4). It shows a mixing process among the intruded Kuroshio, throughflow in the TS and Chinese coastal current carrying the ECS coastal water to the TS. The southwestward coastal current along the Chinese coast develops when the northeasterly monsoon prevails in winter. Because of the southwestward coastal water to the TS, the concentration of tracer from the TS is diluted along the Chinese coast (Fig. 3c). In lower layer, the intruded Kuroshio water lies on the bottom layer beneath the mixed shelf water. In summer, on the other hand, high concentration dominating inshore region during wintertime retreats farther seaward (nearly 200 m isobath) in the upper layer and the tracer on the bottom layer intrudes farther shoreward to the Chinese coastal area (right panels of Fig. 4). At transect B, in particular, the summer coastal upwelling brings the tracer to the surface layer that results in the presence of the Kuroshio water in the offshore area of the south coast of the Changjiang River mouth (see in Fig. 3b). This is quite in agreement with the result of Lü et al., 2006. It can be also explained by the bottom Ekman layer that transports the intruded Kuroshio water northward to the Chinese coastal area (Jacobs et al., 2000; Moon et al., 2009). In general, thermal front is dynamically associated with the circulations of the ECS (Park and Chu, 2006). To support the seasonal variation of the southern ECS shelf water, we used here thermal front calculated from the observed data set (GDEM: the U.S. Navy’s Generalized Digital Environmental Model). The horizontal

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Fig. 3. Horizontal distributions of monthly mean tracer at 10 m depth released from the Kuroshio (east of Taiwan) and the Taiwan Strait (TS) in (a, c) winter (January) and (b, d) summer (July) for the baseline integration. The value of tracer is fixed to 1.0 at the releasing place. Dashed lines indicate the 50 m, 100 m and 200 m isobaths.

Fig. 4. Vertical distributions of monthly mean tracer released from the Kuroshio (east of Taiwan) along A (upper) and B (lower panels) sections in winter (January) and summer (July) for the baseline integration. Contour interval is 0.1.

temperature gradient is calculated on each data point at the depth qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of 50 m as follow jDTj ¼ ð@T=@xÞ2 þ ð@T=@yÞ2 , where T is a

variable of temperature, and x and y axes directed toward east and north. The regions where temperature gradient is higher than

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Fig. 5. Temperature gradient distributions at 50 m depth in (a) winter (January) and (b) summer (July) calculated from the GDEM data. Regions where temperature gradient is higher than 2 °C/100 km are shaded with a contour interval of 1 °C/ 100 km. Dotted lines indicate the 100 m and 200 m isobaths.

2 °C/100 km are shaded as shown in Fig. 5. The criterion of 2 °C/ 100 km is evaluated as the most reasonable value to identify the thermal fronts in the ECS by Park and Chu (2006). In winter, strong thermal front is distributed off northeast of Taiwan and it extends northward along the inshore side of the Kuroshio (Fig. 5a). The thermal front is generated between the Kuroshio warm water and the cold water on the shelf region, indicating that the warm Kuroshio water intruded onto the southern ECS shelf in the northeast of Taiwan. Meanwhile, in summer the thermal front northeast of Taiwan shifts seaward and relatively weak thermal front appears along the continental shelf break of the ECS (Fig. 5b). The thermal front analysis supports the result from our tracer experiment that the onshore Kuroshio intrusion northeast of Taiwan becomes intensified during wintertime, while weakened during summertime. 3.2. Temporal distributions The temporal variability in the northern shelf of Taiwan is shown in Fig. 6 in terms of daily kinetic energy (a), and 90-day running averaged velocity and tracer from the Kuroshio (b) along I line. Kinetic energy is defined as Ke ¼ 1=2ðu2 þ m2 Þ. Large kinetic energy is seen in the shelf region at 100–200 m isobaths from autumn to winter (wintertime) and moves to the Kuroshio mainstream region (east of 122.5°E) from spring to summer (summertime). Relatively weak energy in the TS region has an opposite seasonal variation to the shelf region. Power spectra for the kinetic energy contain various peaks on the time scales from a few days to a few months, but the seasonal fluctuation is the strongest among other signals (not shown). North of Taiwan, the Kuroshio water intrudes as far west (shoreward) as 121°E in winter and retreats eastward in summer (Fig. 6b). The Kuroshio flows north/northwestward across the shelf at 100–200 m isobaths in winter but it becomes significantly weaker in summer. On the other hand, the outflow from the TS shows an opposite pattern to the Kuroshio flow; weak in winter

Fig. 6. The temporal variability of (a) daily kinetic energy and (b) 90-day running averaged velocity and the tracer concentration from the Kuroshio at 10 m depth along I line. Dotted lines indicate the 100 m and 200 m isobaths. Contour interval in (b) is 0.1.

and strong in summer. Strong gradient of the tracer concentration on the northern shelf of Taiwan indicates a strong seasonal interaction between the intruded Kuroshio water and the shelf water from the TS. On the southern ECS shelf (Fig. 7), the northward intruded Kuroshio northeast of Taiwan mainly flows northeastward along the shelf break (approximately 200 m isobath) after rejoining the main stream of the Kuroshio. The shelf current width increases towards the central part of the ECS, showing a strong seasonal variability. Despite the fact that the southern ECS shelf current mostly flows along the shelf, the tracer from the Kuroshio water extends shoreward in winter and retreats seaward in summer across the shelf (Fig. 7). It is shown that water masses in the southern ECS shelf change seasonally in response to the interaction between the two water masses north of Taiwan (Fig. 7). In addition, the intruded Kuroshio onto the northern shelf of Taiwan in winter propagates to the central part of the ECS along the ECS shelf and it takes about 3–4 months from north of Taiwan to south of Jeju Island. At seasonal time scale, the volume transport of Kuroshio at east of Taiwan is known to have its maximum in summer and minimum from fall to winter (Tang et al., 2000; Lee et al., 2001; Liang et al., 2003). Our model reproduces well the observed seasonal signal of the Kuroshio transport at east of Taiwan. However, this is quite different seasonal variability from the shelf-ward tracer from the Kuroshio into the southern ECS shelf. It is unlikely that that the volume transport of the Kuroshio at east of Taiwan is closely related to the intruding water onto the ECS shelf. The remarkable seasonal variation of the tracer from the Kuroshio may be associated with the fact that the Kuroshio water is displaced and diluted by more water mass advected from the TS during the summertime when the transport through the TS increases substantially. The throughflow transport in the TS is critically influenced by a strong monsoonal wind. Even southward transport frequently occurs in winter as an episodic event when northeasterly winds prevail (Ko et al., 2003; Lin et al., 2005; Jan et al., 2006). Using some published ADCP-derived estimate, Isobe (2008) evaluated the seasonal volume transport through the TS; nearly zero in winter and around 2.5 Sv in summer. The seasonal difference is close to an estimate of the onshore Kuroshio intrusion northeast of Taiwan,

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Fig. 7. The temporal variability of 90-day running averaged velocity and the tracer concentration from the Kuroshio at 10 m depth along (a) A and (b) B lines. Dotted line indicates the 100 m isobath. Contour interval is 0.1.

using a triply-nested numerical model (Guo et al., 2006). To investigate the effect of the wind-modulated throughflow in the TS on the Kuroshio intruding onto the northern shelf of Taiwan, three comparative numerical experiments for different wind situations were computed and the results will be discussed in the following section. 4. Discussion 4.1. Response to regional wind over the ECS We first carried out an experiment with weakened wind by 25% of the QuikSCAT wind fields over the entire model domain (named Exp. 1 in Table 1). Fig. 8 shows the volume transports in the TS calculated from the baseline and two comparative experiments. For the results of the baseline integration (black lines of Fig. 8), timeaveraged volume transport through the strait is 1.68 ± 1.52 Sv and seasonal variation is significant with a minimum in winter (0.2 Sv) and maximum in summer (3.0 Sv). This is consistent with the variability estimated by recent studies on the basis of current measurements (Fang et al., 1991; Wang et al., 2003; Wu et al., 2008). In addition to the seasonal variability, short-term fluctuations also exist. The volume transport through the TS strongly responds to weakened wind over the ECS, particularly from autumn to winter (see blue lines of Fig. 8). The time-averaged transport is 2.14 ± 0.93 Sv and its difference from the baseline is 0.46 Sv, indicating that the weakened wind over the ECS makes the transport

Fig. 8. Daily (thin) and 90-day running averaged (thick lines) volume transport in the TS calculated from the baseline (black) and two comparative experiments (blue for Exp. 1 and red for Exp. 2a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

increasing during the wintertime. Effect of the winds on the flow patterns is also evident in the velocity difference between the baseline and Exp. 1 (Fig. 9). In winter, strong anomaly current moves northwestward into the shelf region off northeast of Taiwan and then turn clockwise along the 100 m isobath. At the same time, southwestward anomaly flow appears over the TS and the current along the Chinese coast is relatively strong. Meanwhile, the differences in the transport and surface velocity field between the two cases are much smaller in summer than in winter (Fig. 9b) due to relatively weak summer wind over the ECS. As a seasonal response to these flow patterns on the weakened wind, the tracer (from the Kuroshio) anomaly extends further shoreward across the northern shelf of Taiwan from autumn to winter and retreats seaward from spring to summer (Fig. 9c). These differences between the baseline and Exp. 1 show that strong winter northeasterly wind clearly weakens the outflow through the TS and intensifies the Kuroshio intruding onto the northern shelf of Taiwan. Nevertheless, the results are insufficient to draw firm conclusion about the influence of wind-modulated outflow from the TS on the onshore Kuroshio intrusion because monsoonal wind system over the ECS can cause the onshore (offshore) surface Ekman flux across the shelf during the wintertime (summertime) (Chao, 1991; Chen et al., 1996). We therefore need to conclusively confirm the effect of the outflow from the TS excluding the cross-shelf surface Ekman transport. 4.2. Response to local wind over the TS In Exp. 2a 25% weakened wind is only applied over the TS region (shaded region in Fig. 1), while elsewhere the surface flow is driven by the same wind forcing as the baseline integration. It is expected that the surface Ekman flow across the ECS shelf could be canceled each other out when we consider the anomaly fields between the baseline and Exp. 2a. The time-averaged volume transport in the TS for Exp. 2a is 2.08 ± 1.13 Sv (red lines of Fig. 8) and its value is nearly the same as that of Exp. 1, which the weakened wind was imposed over the entire domain. The volume transport fluctuates seasonally with almost the same pattern as the Exp. 1, demonstrating that the transport through the TS is primary driven by a local wind over the strait (Ko et al., 2003; Lin et al., 2005). An interesting result is revealed by velocity and tracer anomalies between the baseline and Exp. 2a shown in Fig. 10. Strong northwestward flow, carrying the Kuroshio water to the shelf,

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Fig. 9. Differences of velocity and tracer from the Kuroshio between the baseline and Exp. 1. (a, b) Monthly mean velocity differences on the northern shelf of Taiwan in winter (January) and summer (July). Dashed lines indicate the 50 m, 100 m and 200 m isobaths. (c) Temporal variability of 90-day running averaged velocity and the tracer differences at 10 m depth along I line. Contour interval is 0.1.

Fig. 10. Same as Fig. 9 except for differences between the baseline and Exp. 2a. In Exp. 2a, 25% weakened wind only blows in the TS region where the western part of dotted line.

appears in the northern shelf of Taiwan during the wintertime, while during the summertime it almost vanishes. The distributions of spatial and temporal anomalies closely correspond to those between the baseline and Exp. 1. Note that in Exp. 2a the weakened wind only blows in the TS region (western part of dotted line in Fig. 10a). Nevertheless, the current anomaly in the northern shelf of Taiwan (especially between 100 m and 200 m isobaths) changes

in response to the outflow through the TS. To quantify and more directly compare the contribution of the tracer from the Kuroshio water to the southern ECS shelf water in each experiment, daily concentrations at two transects of A and B (see Fig. 4) were averaged for the values vertically from surface to 200 m depth and spatially between 50 m and 200 m isobaths. The tracer at the Chinese coastal region shallow than 50 m was not averaged because the

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Fig. 11. Time series of daily and 90-day running averaged concentrations of the tracer from the Kuroshio at sections (a) A and (b) B for the baseline (black) and two comparative experiments (blue for Exp. 1 and red for Exp. 2a). The tracer concentration was vertically and spatially (between 50 m and 200 m depths) averaged at each section. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

southward coastal water from the northern ECS exists along the Chinese coast during wintertime. For the baseline (black lines of Fig. 11), the contributions from the Kuroshio increase from autumn to winter as the Kuroshio intrusion northeast of Taiwan becomes stronger. The averaged concentration reaches about 80% of total amount during the wintertime at both two transects, while during the summertime it decreases to about 60% and 40% at the transect A and B, respectively. As the outflow from the TS extends further offshore in the central ECS shelf during the summertime, the contribution from the Kuroshio decrease at transect B than A. The time-averaged values are 67.3 ± 11.7% and 58.3 ± 15.4% at transects A and B, respectively. In comparison with the baseline, the contributions from the Kuroshio decrease during the wintertime in both two different wind situations, but there is no clear decrease in summer when the transport differences in the TS are small because of relatively weak wind. It indicates that the TS transport variation directly influences the water property of the southern shelf of the ECS.

Fig. 12. Same as for Fig. 11 except for the Exp. 2b (red lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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One additional experiment (Exp. 2b) with 25% weakened wind over the southern ECS shelf region (dotted box of Fig. 1) was carried out to more directly confirm the effect of wind-induced onshore Ekman flow across the ECS shelf region. For the result of Exp. 2b the seasonal variation of the contribution is almost the same as the baseline in both two transects, although it becomes a little smaller during the wintertime at transect B (Fig. 12). It is likely that the wind-driven surface Ekman flow across the shelf is not as great as the impact of the outflow from the TS on the seasonal variation of the southern ECS shelf water. Comparison among the results from the comparative experiments suggests that the onshore Kursohio intrusion northeast of Taiwan can be influenced by seasonally varying outflow from the TS which strongly responds to a local wind over the TS. From autumn to winter, strong northeasterly wind over the TS significantly weakens the outflow from the TS. As a response to the weakened outflow, the Kuroshio water intrudes farther shoreward across the northern shelf of Taiwan. On the other hand, during the summertime the water flowing into the shelf from the TS extends further offshore because the outflow from the TS is strengthened by a wind change from northeasterly to southwesterly. With the offshore extension of the shelf water, the Kuroshio water retreats seaward off the shelf. It is considered that weakening (strengthening) of the outflow from the TS could allow (inhibit) the shelf-ward Kuroshio water, suggesting that the outflow modulated by a local wind could be an important factor controlling the seasonal variation of the southern ECS shelf water.

5. Summary In this study, we have examined the seasonal variability of the southern ECS shelf water and its relation to the TS throughflow, inserting two separate passive tracers into a nested regional ocean model. In addition, the contributions of surface wind forcing to the regional shelf water changes were also investigated through three comparative experiments. We first characterized the surface current pattern in the ECS, by comparing the result from the model with observational current field inferred from satellite altimeters. Model-predicted current field shows remarkable seasonal differences in the onshore Kuroshio intrusion northeast of Taiwan and the outflow from the TS. In winter, a substantial portion of the Kuroshio flows northward onto the northern shelf of Taiwan, and the current flowing into the ECS through the TS becomes quite weak. Otherwise, the shelf-ward Kuroshio intrusion becomes weaker in summer when the current in the TS is significantly stronger. These results are fairly consistent with the observed current patterns calculated from absolute dynamic topography provided by AVISO. We then focused on the seasonal response of the southern ECS shelf water to wind-induced throughflow in the TS using comparative numerical experiments. Through analyzing model output results, we found that the outflow through the TS modulated by a local wind largely contributes to the seasonal variation of water property in the southern ECS shelf. In winter, strong northeasterly wind over the TS considerably weakens the outflow through the strait, while the Kuroshio intruding into the southern ECS shelf is intensified. As a response to these flow patterns, the Kuroshio water extends farther shoreward across the shelf. In summer, however, the outflow from the TS becomes significantly stronger due to changing wind system and extends further offshore. With the offshore extension of the shelf water, the Kuroshio water retreats seaward off the shelf, suggesting that strengthening of the outflow from the TS inhibits the shelf-ward Kuroshio water. It is suggested that the outflow modulated by a local wind could be an important factor controlling the

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