- Email: [email protected]
Role of wind stress in causing maximum transport through the Korea Strait in autumn
Journal of Marine Systems 115–116 (2013) 33–39
Contents lists available at SciVerse ScienceDirect
Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys
Role of wind stress in causing maximum transport through the Korea Strait in autumn Yang-Ki Cho a, Gwang-Ho Seo a,⁎, Chang-Sin Kim a, b, Byoung-Ju Choi c, Dinesh Chandra Shaha d a
School of Earth and Environmental Sciences/Research Institute of Oceanography, Seoul National University, Republic of Korea Department of Oceanography, Chonnam National University, Republic of Korea Department of Oceanography, Kunsan National University, Republic of Korea d Department of Fisheries Management, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh b c
a r t i c l e
i n f o
Article history: Received 1 August 2012 Received in revised form 11 December 2012 Accepted 5 February 2013 Available online 16 February 2013 Keywords: Tsushima Current Korea Strait Transport Seasonal variation Wind stress Ekman transport
a b s t r a c t Observations show that the maximum transport for the Tsushima Current (TC) through the Korea Strait occurs in autumn. For the TC, variation in transport changes the physical properties of the water as well as the distribution of nutrients, plankton, and other materials in the Japan/East Sea. Despite the importance of the TC, research is yet to unravel the cause of the maximum transport for the TC in autumn. In this study, observational data and numerical modeling data were analyzed in an effort to explore this phenomenon. The maximum transport through the Korea Strait was determined to be the result of the maximum onshore transport crossing the shelf break in the East China Sea (ECS); this transport is driven by strong northeasterly wind stress. Ekman transport driven by wind in the ECS is the primary cause of the maximum transport for the TC through the Korea Strait in autumn. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Current systems and their variabilities are important to the marginal seas of the Northwest Pacific (NWP) Ocean because these systems are instrumental in distributing heat, salt, and other materials through the straits connecting the neighboring marginal seas (Cho and Kim, 2000; Moriyasu, 1972; Senjyu et al., 2006; Yanagi, 2002). The Tsushima Current (TC) flows into the Japan/East Sea (JES) through the Korea Strait (Fig. 1) and then flows out to the Pacific Ocean through the Tsugaru and the Soya straits (Moriyasu, 1972; Teague et al., 2003). It is widely accepted that the TC has two sources (Cho et al., 2009; Guo et al., 2006; Isobe, 2008; Kim et al., 2005). One is a branch of the Kuroshio Current, southwest of Kyushu Island, Japan; this current flows onshore across the shelf break in the East China Sea (ECS) (Lie et al., 1998; Nitani, 1972; Uda, 1934). The other source is a continuation of the Taiwan Warm Current (TWC) that originates in the Taiwan Strait and enters the Korea Strait as the TC (Beardsley et al., 1985; Fang et al., 1991; Zhu et al., 2004). Cho et al. (2009) suggested that the volume transport in the Korea Strait is dominated by the Kuroshio Current in winter (83%) and by the TWC through the Taiwan Strait in summer (66%).
⁎ Corresponding author at: School of Earth and Environmental Sciences, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea. Tel./fax: +82 2 880 6749. E-mail address: [email protected] (G.-H. Seo). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.02.002
The hydrography in the Korea Strait exhibits strong seasonal variations attributed to the monsoon (Cho et al., 2009; Isobe, 2008). The maximum and minimum temperatures in the Korea Strait are observed in summer and winter, respectively, whereas the minimum and maximum salinities appear in summer and winter, respectively (Fig. 2). Fukudome et al. (2010) observed that transport through the Korea Strait is at a maximum in autumn and shows an asymmetric seasonal variation (Fig. 3), whereas the transport in the Taiwan Strait, one of the sources of the TC, reaches a maximum in summer and has a symmetric seasonal variation (Cho et al., 2009; Isobe, 2008; Jan et al., 2006). Little observational data are available to explain the seasonal variation in the onshore flow of the Kuroshio branch across the shelf break in the ECS, though several numerical experiments have been performed to examine the onshore flow of the Kuroshio (Guo et al., 2006; Lee and Matsuno, 2007; Yang et al., 2012). The transport variations in the Korea Strait may be affected by surface forcing such as wind stress, heat flux, atmospheric pressure, sea level difference, and river discharge. In contrast, temperature and salinity variations (Fig. 2) have little significant relationship with the seasonal variation in transport. Despite the importance of transport variability in determining the physical properties of the water, in addition to the distribution of nutrients, plankton, and other materials in the JES, research has not yet provided any clear explanation for the occurrence of maximum transport in the TC in autumn and its asymmetric seasonal variation. In this study, we propose the cause of the autumn maximum transport for the TC through the Korea Strait based on data analysis and
34
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 1. The current system in the study area. KC, TWC, and TC represent the Kuroshio, Taiwan Warm Current, and Tsushima Current, respectively. Dashed and dotted boxes represent the areas from which the mean wind stress for the East China Sea and the Korea Strait were calculated, respectively.
numerical modeling. Numerical modeling successfully simulates the asymmetric seasonal variations in volume transport, with maximum transport for the Korea Strait observed in the autumn. Our modeling reveals the relationships between transport variations and seasonal wind fields in the ECS. 2. Data and numerical model Hydrographic data in the Korea Strait were obtained from the World Ocean Atlas (WOA; www.nodc.noaa.gov). Fukudome et al. (2010) reported and analyzed the monthly mean transport through the Korea Strait from February 1997 to February 2007. Their volume transport data were derived from observations recorded by an acoustic Doppler current profiler (ADCP) mounted on a vessel that crossed the Korea Strait thrice in a week. Isobe (2008) calculated a sinusoidal variation in the transport based on all ADCP observations recorded in the Taiwan Strait. Wind data from February 1997 to February 2007 from the European Centre for Medium-Range Weather Forecasts (ECMWF) were analyzed.
Fig. 2. Mean seasonal variation in temperature (solid line) and salinity (dashed line) in the Korea Strait. Temperature and salinity data for the Korea Strait were obtained from the World Ocean Atlas (WOA).
The model domain ranges from 18.5° N to 48.5° N in latitude and from 117.5° E to 154.5° E in longitude, and includes the ECS, Yellow Sea (YS), JES, and the northwestern region of the Pacific. The horizontal grid has a nominal resolution of 0.1° with 20 vertical sigma levels. The open boundary data of the model were provided from a regional NWP model (Cho et al., 2009). The NWP model has a resolution of 0.25° and the domain ranges from 15° N to 53° N and from 115° E to 160° E. The NWP model is nested within a data assimilative global model known as Estimating the Circulation and Climate of the Ocean (ECCO; www.ecco-group.org). The initial data for temperature, salinity, velocity, and sea surface height were obtained from the NWP model (Cho et al., 2009) from January 1996. The monthly mean data of the ECMWF reanalysis were used for the surface forcing, and bulk-flux formulae (Fairall et al., 1996) were used for the calculation of surface flux and wind stress. Tidal forcing was applied along the open boundaries using 10 major tidal components in order to include the tidal mixing effect on sea surface temperature (Egbert and Erofeeva, 2002). Vertical mixing was calculated by the Mellor–Yamada turbulence closure scheme (Mellor and Yamada, 1974). Chapman, Flather, and clamped boundary conditions were used for free surface elevation, barotropic momentum, and baroclinic momentum, respectively (Marchesiello et al., 2001). The horizontal viscosity coefficient was set to 300 m 2/s. Further details on the model can be found in Cho et al. (2009). A two-year spin-up run was performed using climatological monthly mean atmospheric forcing, which were obtained from monthly mean ECMWF reanalysis data from February 1997 to February 2007. The wind forcing was then changed in the third year. One simulation was performed with realistic wind and the other simulation without wind for one year. Wind speed was set to zero over the whole model domain for the no-wind simulation. Given the uncertainty in the observed transports through the Korea Strait, the amplitude and phase of its seasonal variations were comparable in the simulation and observations (Fig. 4). A sinusoidal variation curve (dashed line) of transport through the Taiwan Strait was fitted
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
35
3. Seasonal variation in current system and wind stress
Fig. 3. Mean seasonal variation in volume transports in the Korea Strait. Vertical bars represent the standard deviation of the volume transport. Volume transport was determined on the basis of observations in the Korea Strait from February 1997 to February 2007 (Fukudome et al., 2010).
from the estimates from various research projects (circle mark) in Fig. 4b. The correlation coefficient of the simulated monthly mean transport in this study with that of Fukudome et al. (2010) was 0.75 (Fig. 4 (c)). The model simulated relatively well the large transports in summer and autumn, which is the season of interest, but there were some differences in transports in winter. The discrepancy between the simulated and the observed transports results partly from some uncertainty in the model forcing and grid resolution. The observed transport for the Taiwan Strait was 1.16–2.34 Sv between March and August, whereas there was no persistent northward flow in winter (Isobe, 2008; Jan et al., 2006). The correlation coefficient of the simulated monthly mean transport with the transport estimated by Isobe (2008) in the Taiwan Strait was 0.93 (Fig. 4(d)). No observed data were available for comparison of the onshore flow transport across the shelf break in the ECS with the model result.
The simulated monthly mean surface velocity shows seasonal variation in the current system (Fig. 5) in the ECS upstream of the TC. The months of February, April, August, and October were selected to represent winter, spring, summer, and autumn, respectively. The current in the Taiwan Strait shows southwestward movement in winter and autumn but flows northeastward in spring and summer. The northeastwardflowing current passing through the Taiwan Strait flows into the Korea Strait. The TC entering the JES through the Korea Strait is relatively weak in winter but strong in other seasons, which is consistent with the observed seasonal variation. The Kuroshio Current passing east of Taiwan follows the continental slope west of Japan's Ryukyu Islands and flows south of Japan. The onshore flow across the shelf break (200-m isobaths) persists in all seasons. The onshore flow is strong in autumn despite the opposite flow near the Chinese coast. The monsoon results in strong seasonal variation in the wind fields in the study area. These changes are plotted in Fig. 6 using the monthly mean winds to represent each season. In winter, a strong northwesterly wind blows in the YS, Korea Strait, and JES, while a northeasterly wind blows in the ECS. The wind weakens and changes its direction in spring. The southerly wind becomes dominant in summer; however, its speed is relatively low. It is notable that the northeasterly wind in the ECS is strongest in autumn. 4. Contribution of wind stress to seasonal variation in currents The seasonal variation in transports in the Korea and Taiwan straits may be related to the variation in wind forcing in the marginal seas (Cho et al., 2009; Moon et al., 2009). Moon et al. (2009) suggested that the along-strait wind in the Korea Strait induces decreased transport through the strait in September. The along-strait and the across-strait wind stresses around the Korea Strait (Fig. 7) do not exhibit the same seasonal variation as that of the volume transport through the Korea Strait; they are at their maximums
Fig. 4. Comparison of the monthly mean model transport from February 1997 to February 2007 with observations in the (a, c) Korea Strait and (b, d) Taiwan Strait. Transports are in Sv (106 m3/s).
36
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 5. Monthly mean sea surface currents from the model in February, April, August, and October. The thick, solid line represents 200-m isobaths.
in September and December, respectively. In contrast, the wind stress in the ECS exhibits strong seasonal variation corresponding to that of the volume transport through the Korea Strait (Fig. 8). In particular, the northeasterly wind stress, which drives the onshore transport in the ECS, has its maximum in October and minimum in July. The seasonal volume transports through the Korea Strait and onshore transport across the shelf break are compared in simulations with realistic wind stress and without wind stress (Fig. 9). Time-varying wind stress modulates seasonal variation of the volume transport through the Korea Strait (Fig. 9). Simulated volume transports of both the Korea Strait and the onshore region with realistic monthly wind stress, averaged from February 1997 to February 2007, show large seasonal variation. The maximum transport commonly occurs in autumn; however, the minimum transport occurs in different seasons. In contrast, the simulated transports from the simulation without wind stress did not produce any clear seasonal variation. The annual mean volume transports in both simulations are similar regardless of the presence of wind. The mean transports through the Korea Strait and for the onshore flow are approximately 2.2 Sv and 1.2 Sv, respectively. The onshore transport was calculated from the velocity across a section from Taiwan to Kyushu (see Fig. 10). On the basis of the results of this model, we can quantitatively understand the effect of wind stress on transport variation in October, when the volume transports in the Korea Strait and the onshore flow are at their maxima. The wind stress increases the onshore transport to approximately 0.93 Sv in October. The increased onshore transport causes a transport increase through the Korea Strait of approximately 0.63 Sv, and a transport decrease in the Taiwan Strait of approximately
0.24 Sv. This modeling experiment and the relationships between the transport though the Korea Strait and wind stresses suggest that the maximum transports of the onshore flow and the Korea Strait in autumn are ultimately caused by strong northeasterly wind stress in the ECS. 5. Discussion Model simulations show that the onshore transport is strongly correlated (R= 0.94) with the averaged northeasterly wind stress in the ECS (Fig. 11). The maximum and minimum onshore transports correspond to the maximum northeasterly wind stress in October and the minimum in July, respectively. It is clear that the northeasterly wind stress is the primary factor that drives seasonal variation in the onshore transport, which determines the maximum transport for the Korea Strait. The onshore Ekman transport (VE) crossing the shelf break into the ECS can be estimated by considering the northeasterly wind stress VE ¼
−τ x ; ρf
ð1Þ
where ρ is the water density (1025 kg/m3); τx, the northeasterly wind stress that drives the northwestward onshore transport in the ECS; and f, the Coriolis frequency of 7.29 × 10−5 s−1. The effective length of the shelf was assumed to be 1000 km. The calculated onshore Ekman transport using Eq. (1) for each month is shown as the dashed line in Fig. 12. For comparison, the onshore transport crossing the shelf break
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
37
Fig. 6. Mean surface wind in February, April, August, and October from February 1997 to February 2007. Wind data were supplied by the European Center for Medium-range Weather Forecasting (ECMWF).
in the ECS was calculated from the model-simulated transport with wind stress, indicated in the figure by a solid line. Both transports correspond well in terms of seasonal variation, with a maximum in autumn and a minimum in summer. The transport difference in autumn and winter is negligible; however, Ekman transport estimated using Eq. (1) is larger than that from the numerical model for spring and summer. This difference, approximately 1.0 Sv in summer, likely reflects the failure of the estimated transport using Eq. (1) to consider the effects of advection and pressure gradients, which induce onshore flow (Guo et al., 2006). The simulated onshore transport shown by the model in winter is slightly smaller than that estimated by Eq. (1). The strong northwesterly wind stress in the ECS may decrease the TC through the Korea Strait and, in turn, the onshore transport in winter.
Our results are consistent with Tsujino et al.'s (2008) suggestion that the seasonal variation of monsoonal wind stress over the North Pacific is responsible for the seasonal variation of transports through the straits in the marginal seas. They emphasized the role of wind stress over the Okhotsk Sea, which we do not include in our model domain. This may cause some discrepancy in modeled transport. The changing sea level difference inside and outside the JES may also contribute to the transport variation in the Korea Strait (Lyu and Kim, 2005; Nof, 1993, 2000). Ohshima (1994) suggested that the volume transport of the TC depends on the sea level difference between the Korea Strait and the Tsugaru Strait, which is determined by the large-scale wind-driven circulation in the Pacific. However, Guo et al. (2006) found no relationship between the TC transport and the sea level difference between the Korea and the Tsugaru straits from realistic model results in autumn and winter,
Fig. 7. Monthly mean along-strait (dashed line) and across-strait (solid line) wind stresses averaged over the Korea Strait (dotted box in Fig. 1) from February 1997 to February 2007.
Fig. 8. Monthly mean northeasterly (U′: solid line) and northwesterly (V′: dashed line) wind stresses averaged over the East China Sea (dashed box in Fig. 1) from February 1997 to February 2007.
38
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 9. Seasonal variation in transport in the Korea Strait (blue line) and onshore flow in the East China Sea (red line) calculated by numerical modeling. Dashed lines represent the no-wind case; solid lines represent actual wind conditions. Fig. 11. Relationships of the model onshore transport with the wind stress in the East China Sea. Transports are in Sv (106 m3/s).
and suggested local wind stress as a possible cause for the seasonal variation in the transport. Other dynamics such as geostrophic balance and strait constraints could also induce transport variation (Yuan and Hsueh, 2010). 6. Conclusions Numerical modeling successfully simulated the observed transport through the Korea Strait, including asymmetric seasonal variation in transport with the maximum occurring in the autumn. Analysis of the observed data and model results suggests that the maximum transport for the TC through the Korea Strait in autumn is associated with the onshore transport across the shelf break and ultimately, with the strong northeasterly wind in the ECS. Wind stress in the ECS is a major factor controlling seasonal variation in onshore transport. The seasonality of the onshore transport crossing the shelf break is strongly related to the northeasterly wind stress in the ECS. The Ekman transport, driven by wind in the ECS, is a primary factor in causing the maximum transport of the TC through the Korea Strait in autumn. Although our modeling experiment has many limitations, our numerical model simulation confirmed that the seasonal variation in transport in the Korea Strait and the onshore flow depend on wind stress variation. The strong northwesterly wind stress in the ECS may decrease the TC through the Korea Strait and the onshore transport in winter. Other factors such as the sea level difference inside and outside the JES and the geostrophic balance may also contribute to the seasonal variation in transport in the Korea Strait. Additional work is necessary to explain the dynamics of seasonal variation in the TC and onshore transport and to further understand the importance of the effects of
Fig. 12. Seasonal variation in the onshore transport (solid line) calculated by numerical modeling and the estimated Ekman transport driven by northeasterly wind stress in the East China Sea on the basis of Eq. (1).
wind stress suggested here. This study suggests that wind stress also has an effect on circulation in other marginal seas adjacent to open oceans. Acknowledgments This research was supported by the Korea Meteorological Administration Research and Development Program under the grant CATER 2012-2080.
Fig. 10. Calculated mean volume transports in the straits and inferred volume transport in the section between Taiwan and Kyushu for actual wind (left panel) and no wind (right panel) in October. Transports are in Sv (106 m3/s).
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
References Beardsley, R.C., Limeburner, R., Yu, H., Cannon, G.A., 1985. Discharge of the Changjiang (Yangtze River) into the East China Sea. Cont. Shelf Res. 4, 57–76. Cho, Y.-K., Kim, K., 2000. Branching mechanism of the Tsushima Current in the Korea Strait. J. Phys. Oceanogr. 30, 2788–2797. Cho, Y.-K., Seo, G.-H., Choi, B.-J., Kim, S., Kim, Y.-G., Youn, Y.-H., Dever, E.P., 2009. Connectivity among straits of the northwest Pacific marginal seas. J. Geophys. Res. 114, C06018. http://dx.doi.org/10.1029/2008JC005218. Egbert, G.D., Erofeeva, S.Y., 2002. Efficient inverse modeling of barotropic ocean tides. J. Atmos. Ocean. Technol. 19 (2), 183–204. Fairall, C.W., Bradley, E.F., Rogers, D.P., Edson, J.B., Young, G.S., 1996. Bulk parameterization of air–sea fluxes for tropical ocean-global atmosphere coupled-ocean atmosphere response experiment. J. Geophys. Res. 101 (C2), 3747–3764. http://dx.doi.org/10.1029/ 95JC03205. Fang, G., Zhao, B., Zhu, Y., 1991. Water volume transport through the Taiwan Strait and the continental shelf of the East China Sea measured with current meters. In: Takano, K. (Ed.), Oceanography of Asian Marginal Seas. Elsevier, Amsterdam, pp. 345–348. Fukudome, K.I., Yoon, J.H., Alexander, O., Takikawa, T., Han, I.S., 2010. Seasonal volume transport variation in the Tsushima Warm Current through the Tsushima Straits from 10 years of ADCP observations. J. Oceanogr. 66, 539–551. Guo, X., Miyazawa, Y., Yamagata, T., 2006. The Kuroshio onshore intrusion along the shelf break of the East China Sea: the origin of the Tsushima Warm Current. J. Phys. Oceanogr. 36, 2205–2231. Isobe, A., 2008. Recent advances in ocean-circulation research on the Yellow Sea and East China Sea shelves. J. Oceanogr. 64, 569–584. Jan, S., Sheu, D.D., Kuo, H.-M., 2006. Water mass and through flow transport variability in the Taiwan Strait. J. Geophys. Res. 111, C12012. http://dx.doi.org/10.1029/ 2006JC003656. Kim, K.-R., Cho, Y.-K., Kang, D.-J., Ki, J.-H., 2005. The origin of the Tsushima Current based on oxygen isotope measurement. Geophys. Res. Lett. 32. http://dx.doi.org/10.1029/ 2004GL021211. Lee, J.-S., Matsuno, T., 2007. Intrusion of Kuroshio water onto the continental shelf of the East China Sea. J. Oceanogr. 63, 309–325. Lie, H.-J., Cho, C.-H., Lee, J.-H., Niller, P., Hu, J.-H., 1998. Separation of the Kuroshio water and its penetration onto the continental shelf west of Kyushu. J. Geophys. Res. 103 (C2), 2963–2976. http://dx.doi.org/10.1029/97JC03288. Lyu, S.J., Kim, K., 2005. Subinertial to interannual transport variations in the Korea Strait and their possible mechanisms. J. Geophys. Res. 110, C12016. http:// dx.doi.org/10.1029/2004JC002651.
39
Marchesiello, P., McWilliams, J.C., Shchepetkin, A., 2001. Open boundary conditions for long-term integration of regional oceanic models. Ocean Modell. 3, 1–20. Mellor, G., Yamada, T., 1974. A hierarchy of turbulence closure models for planetary boundary layers. J. Atmos. Sci. 31, 1791–1806. Moon, J.-H., Hirose, N., Yoon, J.-H., Pang, I.-C., 2009. Effect of the along-strait wind on the volume transport through the Tsushima/Korea Strait in September. J. Oceanogr. 65, 17–29. Moriyasu, S., 1972. The Tsushima Current. In: Stommel, Henry, Yoshida, Kozo (Eds.), Kuroshio: Its Physical Aspects. University of Tokyo Press, Tokyo, pp. 353–369. Nitani, H., 1972. Beginning of the Kuroshio. In: Stommel, Henry, Yoshida, Kozo (Eds.), Kuroshio: Its Physical Aspects. University of Tokyo Press, Tokyo, pp. 129–163. Nof, D., 1993. The penetration of Kuroshio water into the Sea of Japan. J. Phys. Oceanogr. 23, 797–807. Nof, D., 2000. Why much of the Atlantic circulation enters the Caribbean Sea and very little of the Pacific circulation enters the Sea of Japan. Prog. Oceanogr. 45, 39–67. Ohshima, K.I., 1994. The flow system in the Japan Sea caused by a sea level difference through shallow straits. J. Geophys. Res. 99 (C5), 9925–9940. http://dx.doi.org/ 10.1029/94JC00170. Senjyu, T., Enomoto, H., Matsuno, T., Matsui, S., 2006. Interannual salinity variation in the Tsushima Strait and its relation to the Changjiang discharge. J. Oceanogr. 62, 681–692. Teague, W.J., Jacobs, G.A., Ko, D.S., Tang, T.Y., Chang, K.-I., Suk, M.-S., 2003. Connectivity of the Taiwan, Cheju, and Korea straits. Cont. Shelf Res. 23, 63–77. Tsujino, H., Nakano, H., Motoi, T., 2008. Mechanism of currents through the straits of the Japan Sea: mean state and seasonal variation. J. Oceanogr. 64, 141–161. Uda, M., 1934. The results of simultaneous oceanographical investigations in the Japan Sea and its adjacent waters in May and June. J. Imp. Fish. Exp. Stn. 5, 57–190. Yanagi, T., 2002. Water, salt, phosphorus and nitrogen budgets of the Japan Sea. J. Oceanogr. 58, 797–804. Yang, D., Yin, B., Liu, Z., Bai, T., Qi, J., Chen, H., 2012. Numerical study on the pattern and origins of Kuroshio branches in the bottom water of southern East China Sea in summer. J. Geophys. Res. 117, C02014. http://dx.doi.org/10.1029/2011JC007528. Yuan, D., Hsueh, Y., 2010. Dynamics of the cross-shelf circulation in the Yellow and East China Seas in winter. Deep-Sea Res. II 57, 1745–1761. Zhu, J., Chen, C., Ding, P., Li, C., Lin, H., 2004. Does the Taiwan warm current exist in winter? Geophys. Res. Lett. 31, L12302. http://dx.doi.org/10.1029/2004GL019997.
Contents lists available at SciVerse ScienceDirect
Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys
Role of wind stress in causing maximum transport through the Korea Strait in autumn Yang-Ki Cho a, Gwang-Ho Seo a,⁎, Chang-Sin Kim a, b, Byoung-Ju Choi c, Dinesh Chandra Shaha d a
School of Earth and Environmental Sciences/Research Institute of Oceanography, Seoul National University, Republic of Korea Department of Oceanography, Chonnam National University, Republic of Korea Department of Oceanography, Kunsan National University, Republic of Korea d Department of Fisheries Management, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh b c
a r t i c l e
i n f o
Article history: Received 1 August 2012 Received in revised form 11 December 2012 Accepted 5 February 2013 Available online 16 February 2013 Keywords: Tsushima Current Korea Strait Transport Seasonal variation Wind stress Ekman transport
a b s t r a c t Observations show that the maximum transport for the Tsushima Current (TC) through the Korea Strait occurs in autumn. For the TC, variation in transport changes the physical properties of the water as well as the distribution of nutrients, plankton, and other materials in the Japan/East Sea. Despite the importance of the TC, research is yet to unravel the cause of the maximum transport for the TC in autumn. In this study, observational data and numerical modeling data were analyzed in an effort to explore this phenomenon. The maximum transport through the Korea Strait was determined to be the result of the maximum onshore transport crossing the shelf break in the East China Sea (ECS); this transport is driven by strong northeasterly wind stress. Ekman transport driven by wind in the ECS is the primary cause of the maximum transport for the TC through the Korea Strait in autumn. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Current systems and their variabilities are important to the marginal seas of the Northwest Pacific (NWP) Ocean because these systems are instrumental in distributing heat, salt, and other materials through the straits connecting the neighboring marginal seas (Cho and Kim, 2000; Moriyasu, 1972; Senjyu et al., 2006; Yanagi, 2002). The Tsushima Current (TC) flows into the Japan/East Sea (JES) through the Korea Strait (Fig. 1) and then flows out to the Pacific Ocean through the Tsugaru and the Soya straits (Moriyasu, 1972; Teague et al., 2003). It is widely accepted that the TC has two sources (Cho et al., 2009; Guo et al., 2006; Isobe, 2008; Kim et al., 2005). One is a branch of the Kuroshio Current, southwest of Kyushu Island, Japan; this current flows onshore across the shelf break in the East China Sea (ECS) (Lie et al., 1998; Nitani, 1972; Uda, 1934). The other source is a continuation of the Taiwan Warm Current (TWC) that originates in the Taiwan Strait and enters the Korea Strait as the TC (Beardsley et al., 1985; Fang et al., 1991; Zhu et al., 2004). Cho et al. (2009) suggested that the volume transport in the Korea Strait is dominated by the Kuroshio Current in winter (83%) and by the TWC through the Taiwan Strait in summer (66%).
⁎ Corresponding author at: School of Earth and Environmental Sciences, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea. Tel./fax: +82 2 880 6749. E-mail address: [email protected] (G.-H. Seo). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.02.002
The hydrography in the Korea Strait exhibits strong seasonal variations attributed to the monsoon (Cho et al., 2009; Isobe, 2008). The maximum and minimum temperatures in the Korea Strait are observed in summer and winter, respectively, whereas the minimum and maximum salinities appear in summer and winter, respectively (Fig. 2). Fukudome et al. (2010) observed that transport through the Korea Strait is at a maximum in autumn and shows an asymmetric seasonal variation (Fig. 3), whereas the transport in the Taiwan Strait, one of the sources of the TC, reaches a maximum in summer and has a symmetric seasonal variation (Cho et al., 2009; Isobe, 2008; Jan et al., 2006). Little observational data are available to explain the seasonal variation in the onshore flow of the Kuroshio branch across the shelf break in the ECS, though several numerical experiments have been performed to examine the onshore flow of the Kuroshio (Guo et al., 2006; Lee and Matsuno, 2007; Yang et al., 2012). The transport variations in the Korea Strait may be affected by surface forcing such as wind stress, heat flux, atmospheric pressure, sea level difference, and river discharge. In contrast, temperature and salinity variations (Fig. 2) have little significant relationship with the seasonal variation in transport. Despite the importance of transport variability in determining the physical properties of the water, in addition to the distribution of nutrients, plankton, and other materials in the JES, research has not yet provided any clear explanation for the occurrence of maximum transport in the TC in autumn and its asymmetric seasonal variation. In this study, we propose the cause of the autumn maximum transport for the TC through the Korea Strait based on data analysis and
34
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 1. The current system in the study area. KC, TWC, and TC represent the Kuroshio, Taiwan Warm Current, and Tsushima Current, respectively. Dashed and dotted boxes represent the areas from which the mean wind stress for the East China Sea and the Korea Strait were calculated, respectively.
numerical modeling. Numerical modeling successfully simulates the asymmetric seasonal variations in volume transport, with maximum transport for the Korea Strait observed in the autumn. Our modeling reveals the relationships between transport variations and seasonal wind fields in the ECS. 2. Data and numerical model Hydrographic data in the Korea Strait were obtained from the World Ocean Atlas (WOA; www.nodc.noaa.gov). Fukudome et al. (2010) reported and analyzed the monthly mean transport through the Korea Strait from February 1997 to February 2007. Their volume transport data were derived from observations recorded by an acoustic Doppler current profiler (ADCP) mounted on a vessel that crossed the Korea Strait thrice in a week. Isobe (2008) calculated a sinusoidal variation in the transport based on all ADCP observations recorded in the Taiwan Strait. Wind data from February 1997 to February 2007 from the European Centre for Medium-Range Weather Forecasts (ECMWF) were analyzed.
Fig. 2. Mean seasonal variation in temperature (solid line) and salinity (dashed line) in the Korea Strait. Temperature and salinity data for the Korea Strait were obtained from the World Ocean Atlas (WOA).
The model domain ranges from 18.5° N to 48.5° N in latitude and from 117.5° E to 154.5° E in longitude, and includes the ECS, Yellow Sea (YS), JES, and the northwestern region of the Pacific. The horizontal grid has a nominal resolution of 0.1° with 20 vertical sigma levels. The open boundary data of the model were provided from a regional NWP model (Cho et al., 2009). The NWP model has a resolution of 0.25° and the domain ranges from 15° N to 53° N and from 115° E to 160° E. The NWP model is nested within a data assimilative global model known as Estimating the Circulation and Climate of the Ocean (ECCO; www.ecco-group.org). The initial data for temperature, salinity, velocity, and sea surface height were obtained from the NWP model (Cho et al., 2009) from January 1996. The monthly mean data of the ECMWF reanalysis were used for the surface forcing, and bulk-flux formulae (Fairall et al., 1996) were used for the calculation of surface flux and wind stress. Tidal forcing was applied along the open boundaries using 10 major tidal components in order to include the tidal mixing effect on sea surface temperature (Egbert and Erofeeva, 2002). Vertical mixing was calculated by the Mellor–Yamada turbulence closure scheme (Mellor and Yamada, 1974). Chapman, Flather, and clamped boundary conditions were used for free surface elevation, barotropic momentum, and baroclinic momentum, respectively (Marchesiello et al., 2001). The horizontal viscosity coefficient was set to 300 m 2/s. Further details on the model can be found in Cho et al. (2009). A two-year spin-up run was performed using climatological monthly mean atmospheric forcing, which were obtained from monthly mean ECMWF reanalysis data from February 1997 to February 2007. The wind forcing was then changed in the third year. One simulation was performed with realistic wind and the other simulation without wind for one year. Wind speed was set to zero over the whole model domain for the no-wind simulation. Given the uncertainty in the observed transports through the Korea Strait, the amplitude and phase of its seasonal variations were comparable in the simulation and observations (Fig. 4). A sinusoidal variation curve (dashed line) of transport through the Taiwan Strait was fitted
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
35
3. Seasonal variation in current system and wind stress
Fig. 3. Mean seasonal variation in volume transports in the Korea Strait. Vertical bars represent the standard deviation of the volume transport. Volume transport was determined on the basis of observations in the Korea Strait from February 1997 to February 2007 (Fukudome et al., 2010).
from the estimates from various research projects (circle mark) in Fig. 4b. The correlation coefficient of the simulated monthly mean transport in this study with that of Fukudome et al. (2010) was 0.75 (Fig. 4 (c)). The model simulated relatively well the large transports in summer and autumn, which is the season of interest, but there were some differences in transports in winter. The discrepancy between the simulated and the observed transports results partly from some uncertainty in the model forcing and grid resolution. The observed transport for the Taiwan Strait was 1.16–2.34 Sv between March and August, whereas there was no persistent northward flow in winter (Isobe, 2008; Jan et al., 2006). The correlation coefficient of the simulated monthly mean transport with the transport estimated by Isobe (2008) in the Taiwan Strait was 0.93 (Fig. 4(d)). No observed data were available for comparison of the onshore flow transport across the shelf break in the ECS with the model result.
The simulated monthly mean surface velocity shows seasonal variation in the current system (Fig. 5) in the ECS upstream of the TC. The months of February, April, August, and October were selected to represent winter, spring, summer, and autumn, respectively. The current in the Taiwan Strait shows southwestward movement in winter and autumn but flows northeastward in spring and summer. The northeastwardflowing current passing through the Taiwan Strait flows into the Korea Strait. The TC entering the JES through the Korea Strait is relatively weak in winter but strong in other seasons, which is consistent with the observed seasonal variation. The Kuroshio Current passing east of Taiwan follows the continental slope west of Japan's Ryukyu Islands and flows south of Japan. The onshore flow across the shelf break (200-m isobaths) persists in all seasons. The onshore flow is strong in autumn despite the opposite flow near the Chinese coast. The monsoon results in strong seasonal variation in the wind fields in the study area. These changes are plotted in Fig. 6 using the monthly mean winds to represent each season. In winter, a strong northwesterly wind blows in the YS, Korea Strait, and JES, while a northeasterly wind blows in the ECS. The wind weakens and changes its direction in spring. The southerly wind becomes dominant in summer; however, its speed is relatively low. It is notable that the northeasterly wind in the ECS is strongest in autumn. 4. Contribution of wind stress to seasonal variation in currents The seasonal variation in transports in the Korea and Taiwan straits may be related to the variation in wind forcing in the marginal seas (Cho et al., 2009; Moon et al., 2009). Moon et al. (2009) suggested that the along-strait wind in the Korea Strait induces decreased transport through the strait in September. The along-strait and the across-strait wind stresses around the Korea Strait (Fig. 7) do not exhibit the same seasonal variation as that of the volume transport through the Korea Strait; they are at their maximums
Fig. 4. Comparison of the monthly mean model transport from February 1997 to February 2007 with observations in the (a, c) Korea Strait and (b, d) Taiwan Strait. Transports are in Sv (106 m3/s).
36
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 5. Monthly mean sea surface currents from the model in February, April, August, and October. The thick, solid line represents 200-m isobaths.
in September and December, respectively. In contrast, the wind stress in the ECS exhibits strong seasonal variation corresponding to that of the volume transport through the Korea Strait (Fig. 8). In particular, the northeasterly wind stress, which drives the onshore transport in the ECS, has its maximum in October and minimum in July. The seasonal volume transports through the Korea Strait and onshore transport across the shelf break are compared in simulations with realistic wind stress and without wind stress (Fig. 9). Time-varying wind stress modulates seasonal variation of the volume transport through the Korea Strait (Fig. 9). Simulated volume transports of both the Korea Strait and the onshore region with realistic monthly wind stress, averaged from February 1997 to February 2007, show large seasonal variation. The maximum transport commonly occurs in autumn; however, the minimum transport occurs in different seasons. In contrast, the simulated transports from the simulation without wind stress did not produce any clear seasonal variation. The annual mean volume transports in both simulations are similar regardless of the presence of wind. The mean transports through the Korea Strait and for the onshore flow are approximately 2.2 Sv and 1.2 Sv, respectively. The onshore transport was calculated from the velocity across a section from Taiwan to Kyushu (see Fig. 10). On the basis of the results of this model, we can quantitatively understand the effect of wind stress on transport variation in October, when the volume transports in the Korea Strait and the onshore flow are at their maxima. The wind stress increases the onshore transport to approximately 0.93 Sv in October. The increased onshore transport causes a transport increase through the Korea Strait of approximately 0.63 Sv, and a transport decrease in the Taiwan Strait of approximately
0.24 Sv. This modeling experiment and the relationships between the transport though the Korea Strait and wind stresses suggest that the maximum transports of the onshore flow and the Korea Strait in autumn are ultimately caused by strong northeasterly wind stress in the ECS. 5. Discussion Model simulations show that the onshore transport is strongly correlated (R= 0.94) with the averaged northeasterly wind stress in the ECS (Fig. 11). The maximum and minimum onshore transports correspond to the maximum northeasterly wind stress in October and the minimum in July, respectively. It is clear that the northeasterly wind stress is the primary factor that drives seasonal variation in the onshore transport, which determines the maximum transport for the Korea Strait. The onshore Ekman transport (VE) crossing the shelf break into the ECS can be estimated by considering the northeasterly wind stress VE ¼
−τ x ; ρf
ð1Þ
where ρ is the water density (1025 kg/m3); τx, the northeasterly wind stress that drives the northwestward onshore transport in the ECS; and f, the Coriolis frequency of 7.29 × 10−5 s−1. The effective length of the shelf was assumed to be 1000 km. The calculated onshore Ekman transport using Eq. (1) for each month is shown as the dashed line in Fig. 12. For comparison, the onshore transport crossing the shelf break
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
37
Fig. 6. Mean surface wind in February, April, August, and October from February 1997 to February 2007. Wind data were supplied by the European Center for Medium-range Weather Forecasting (ECMWF).
in the ECS was calculated from the model-simulated transport with wind stress, indicated in the figure by a solid line. Both transports correspond well in terms of seasonal variation, with a maximum in autumn and a minimum in summer. The transport difference in autumn and winter is negligible; however, Ekman transport estimated using Eq. (1) is larger than that from the numerical model for spring and summer. This difference, approximately 1.0 Sv in summer, likely reflects the failure of the estimated transport using Eq. (1) to consider the effects of advection and pressure gradients, which induce onshore flow (Guo et al., 2006). The simulated onshore transport shown by the model in winter is slightly smaller than that estimated by Eq. (1). The strong northwesterly wind stress in the ECS may decrease the TC through the Korea Strait and, in turn, the onshore transport in winter.
Our results are consistent with Tsujino et al.'s (2008) suggestion that the seasonal variation of monsoonal wind stress over the North Pacific is responsible for the seasonal variation of transports through the straits in the marginal seas. They emphasized the role of wind stress over the Okhotsk Sea, which we do not include in our model domain. This may cause some discrepancy in modeled transport. The changing sea level difference inside and outside the JES may also contribute to the transport variation in the Korea Strait (Lyu and Kim, 2005; Nof, 1993, 2000). Ohshima (1994) suggested that the volume transport of the TC depends on the sea level difference between the Korea Strait and the Tsugaru Strait, which is determined by the large-scale wind-driven circulation in the Pacific. However, Guo et al. (2006) found no relationship between the TC transport and the sea level difference between the Korea and the Tsugaru straits from realistic model results in autumn and winter,
Fig. 7. Monthly mean along-strait (dashed line) and across-strait (solid line) wind stresses averaged over the Korea Strait (dotted box in Fig. 1) from February 1997 to February 2007.
Fig. 8. Monthly mean northeasterly (U′: solid line) and northwesterly (V′: dashed line) wind stresses averaged over the East China Sea (dashed box in Fig. 1) from February 1997 to February 2007.
38
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
Fig. 9. Seasonal variation in transport in the Korea Strait (blue line) and onshore flow in the East China Sea (red line) calculated by numerical modeling. Dashed lines represent the no-wind case; solid lines represent actual wind conditions. Fig. 11. Relationships of the model onshore transport with the wind stress in the East China Sea. Transports are in Sv (106 m3/s).
and suggested local wind stress as a possible cause for the seasonal variation in the transport. Other dynamics such as geostrophic balance and strait constraints could also induce transport variation (Yuan and Hsueh, 2010). 6. Conclusions Numerical modeling successfully simulated the observed transport through the Korea Strait, including asymmetric seasonal variation in transport with the maximum occurring in the autumn. Analysis of the observed data and model results suggests that the maximum transport for the TC through the Korea Strait in autumn is associated with the onshore transport across the shelf break and ultimately, with the strong northeasterly wind in the ECS. Wind stress in the ECS is a major factor controlling seasonal variation in onshore transport. The seasonality of the onshore transport crossing the shelf break is strongly related to the northeasterly wind stress in the ECS. The Ekman transport, driven by wind in the ECS, is a primary factor in causing the maximum transport of the TC through the Korea Strait in autumn. Although our modeling experiment has many limitations, our numerical model simulation confirmed that the seasonal variation in transport in the Korea Strait and the onshore flow depend on wind stress variation. The strong northwesterly wind stress in the ECS may decrease the TC through the Korea Strait and the onshore transport in winter. Other factors such as the sea level difference inside and outside the JES and the geostrophic balance may also contribute to the seasonal variation in transport in the Korea Strait. Additional work is necessary to explain the dynamics of seasonal variation in the TC and onshore transport and to further understand the importance of the effects of
Fig. 12. Seasonal variation in the onshore transport (solid line) calculated by numerical modeling and the estimated Ekman transport driven by northeasterly wind stress in the East China Sea on the basis of Eq. (1).
wind stress suggested here. This study suggests that wind stress also has an effect on circulation in other marginal seas adjacent to open oceans. Acknowledgments This research was supported by the Korea Meteorological Administration Research and Development Program under the grant CATER 2012-2080.
Fig. 10. Calculated mean volume transports in the straits and inferred volume transport in the section between Taiwan and Kyushu for actual wind (left panel) and no wind (right panel) in October. Transports are in Sv (106 m3/s).
Y.-K. Cho et al. / Journal of Marine Systems 115–116 (2013) 33–39
References Beardsley, R.C., Limeburner, R., Yu, H., Cannon, G.A., 1985. Discharge of the Changjiang (Yangtze River) into the East China Sea. Cont. Shelf Res. 4, 57–76. Cho, Y.-K., Kim, K., 2000. Branching mechanism of the Tsushima Current in the Korea Strait. J. Phys. Oceanogr. 30, 2788–2797. Cho, Y.-K., Seo, G.-H., Choi, B.-J., Kim, S., Kim, Y.-G., Youn, Y.-H., Dever, E.P., 2009. Connectivity among straits of the northwest Pacific marginal seas. J. Geophys. Res. 114, C06018. http://dx.doi.org/10.1029/2008JC005218. Egbert, G.D., Erofeeva, S.Y., 2002. Efficient inverse modeling of barotropic ocean tides. J. Atmos. Ocean. Technol. 19 (2), 183–204. Fairall, C.W., Bradley, E.F., Rogers, D.P., Edson, J.B., Young, G.S., 1996. Bulk parameterization of air–sea fluxes for tropical ocean-global atmosphere coupled-ocean atmosphere response experiment. J. Geophys. Res. 101 (C2), 3747–3764. http://dx.doi.org/10.1029/ 95JC03205. Fang, G., Zhao, B., Zhu, Y., 1991. Water volume transport through the Taiwan Strait and the continental shelf of the East China Sea measured with current meters. In: Takano, K. (Ed.), Oceanography of Asian Marginal Seas. Elsevier, Amsterdam, pp. 345–348. Fukudome, K.I., Yoon, J.H., Alexander, O., Takikawa, T., Han, I.S., 2010. Seasonal volume transport variation in the Tsushima Warm Current through the Tsushima Straits from 10 years of ADCP observations. J. Oceanogr. 66, 539–551. Guo, X., Miyazawa, Y., Yamagata, T., 2006. The Kuroshio onshore intrusion along the shelf break of the East China Sea: the origin of the Tsushima Warm Current. J. Phys. Oceanogr. 36, 2205–2231. Isobe, A., 2008. Recent advances in ocean-circulation research on the Yellow Sea and East China Sea shelves. J. Oceanogr. 64, 569–584. Jan, S., Sheu, D.D., Kuo, H.-M., 2006. Water mass and through flow transport variability in the Taiwan Strait. J. Geophys. Res. 111, C12012. http://dx.doi.org/10.1029/ 2006JC003656. Kim, K.-R., Cho, Y.-K., Kang, D.-J., Ki, J.-H., 2005. The origin of the Tsushima Current based on oxygen isotope measurement. Geophys. Res. Lett. 32. http://dx.doi.org/10.1029/ 2004GL021211. Lee, J.-S., Matsuno, T., 2007. Intrusion of Kuroshio water onto the continental shelf of the East China Sea. J. Oceanogr. 63, 309–325. Lie, H.-J., Cho, C.-H., Lee, J.-H., Niller, P., Hu, J.-H., 1998. Separation of the Kuroshio water and its penetration onto the continental shelf west of Kyushu. J. Geophys. Res. 103 (C2), 2963–2976. http://dx.doi.org/10.1029/97JC03288. Lyu, S.J., Kim, K., 2005. Subinertial to interannual transport variations in the Korea Strait and their possible mechanisms. J. Geophys. Res. 110, C12016. http:// dx.doi.org/10.1029/2004JC002651.
39
Marchesiello, P., McWilliams, J.C., Shchepetkin, A., 2001. Open boundary conditions for long-term integration of regional oceanic models. Ocean Modell. 3, 1–20. Mellor, G., Yamada, T., 1974. A hierarchy of turbulence closure models for planetary boundary layers. J. Atmos. Sci. 31, 1791–1806. Moon, J.-H., Hirose, N., Yoon, J.-H., Pang, I.-C., 2009. Effect of the along-strait wind on the volume transport through the Tsushima/Korea Strait in September. J. Oceanogr. 65, 17–29. Moriyasu, S., 1972. The Tsushima Current. In: Stommel, Henry, Yoshida, Kozo (Eds.), Kuroshio: Its Physical Aspects. University of Tokyo Press, Tokyo, pp. 353–369. Nitani, H., 1972. Beginning of the Kuroshio. In: Stommel, Henry, Yoshida, Kozo (Eds.), Kuroshio: Its Physical Aspects. University of Tokyo Press, Tokyo, pp. 129–163. Nof, D., 1993. The penetration of Kuroshio water into the Sea of Japan. J. Phys. Oceanogr. 23, 797–807. Nof, D., 2000. Why much of the Atlantic circulation enters the Caribbean Sea and very little of the Pacific circulation enters the Sea of Japan. Prog. Oceanogr. 45, 39–67. Ohshima, K.I., 1994. The flow system in the Japan Sea caused by a sea level difference through shallow straits. J. Geophys. Res. 99 (C5), 9925–9940. http://dx.doi.org/ 10.1029/94JC00170. Senjyu, T., Enomoto, H., Matsuno, T., Matsui, S., 2006. Interannual salinity variation in the Tsushima Strait and its relation to the Changjiang discharge. J. Oceanogr. 62, 681–692. Teague, W.J., Jacobs, G.A., Ko, D.S., Tang, T.Y., Chang, K.-I., Suk, M.-S., 2003. Connectivity of the Taiwan, Cheju, and Korea straits. Cont. Shelf Res. 23, 63–77. Tsujino, H., Nakano, H., Motoi, T., 2008. Mechanism of currents through the straits of the Japan Sea: mean state and seasonal variation. J. Oceanogr. 64, 141–161. Uda, M., 1934. The results of simultaneous oceanographical investigations in the Japan Sea and its adjacent waters in May and June. J. Imp. Fish. Exp. Stn. 5, 57–190. Yanagi, T., 2002. Water, salt, phosphorus and nitrogen budgets of the Japan Sea. J. Oceanogr. 58, 797–804. Yang, D., Yin, B., Liu, Z., Bai, T., Qi, J., Chen, H., 2012. Numerical study on the pattern and origins of Kuroshio branches in the bottom water of southern East China Sea in summer. J. Geophys. Res. 117, C02014. http://dx.doi.org/10.1029/2011JC007528. Yuan, D., Hsueh, Y., 2010. Dynamics of the cross-shelf circulation in the Yellow and East China Seas in winter. Deep-Sea Res. II 57, 1745–1761. Zhu, J., Chen, C., Ding, P., Li, C., Lin, H., 2004. Does the Taiwan warm current exist in winter? Geophys. Res. Lett. 31, L12302. http://dx.doi.org/10.1029/2004GL019997.