The Study of the Yellow Sea Warm Current and Its Seasonal Variability

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2009,21(2):159-165 DOI: 10.1016/S1001-6058(08)60133-X

THE STUDY OF THE YELLOW SEA WARM CURRENT AND ITS SEASONAL VARIABILITY* XU Ling-ling, WU De-xing, LIN Xiao-pei, MA Chao Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China

(Received March 8, 2008, Revised April 8, 2008)

Abstract: The Yellow Sea Warm Current (YSWC) penetrates northward along the Yellow Sea Trough, and brings warm and saline water towards the Bohai Sea. The YSWC becomes much less intrusive in summer and is limited mostly in the southern trough, contrasting with a deep winter penetration well into the trough. The seasonal variability of the YSWC has prompted a debate regarding which controls the YSWC and its seasonal variability. In this article, the annual mean and seasonal variability of the YSWC was examined by using a 3-D ocean model together with several experiments. The results show that in the annual mean the YSWC is a compensating current firstly for the southward Korea Coastal Current (KCC), which is mainly caused by the Kuroshio Current (KC). The local wind-stress forcing plays an important but secondary role. However, the local monsoonal forcing plays a prominent role in modulating the seasonal variability. A deep northwestward intrusion of the YSWC in winter, for instance, is mainly due to a robustly developed China Coastal Current (CCC) which draws water along the Yellow Sea trough to feed a southward flow all the way from the Bohai Sea to the Taiwan Strait. Key words: Yellow Sea Warm Current (YSWC), Kuroshio, local wind, monsoon

1. Introduction  The Yellow Sea Warm Current (YSWC), which is one of the most important dynamical phenomena in the East China Seas (ECS), penetrates northward along the Yellow Sea Trough, and brings warm and saline water toward the Bohai Sea. Uda (1934) first described the YSWC as a branch of the Tsushima Warm Current (TSWC) moving northward along the Yellow Sea Trough, which were supported by other oceanographers[1-3]. However, based on six moored current-meter measurements in the Yellow Sea Trough during January-April of 1986, Hsueh[4] showed that the  * Project supported by the National Basic Research Program of China (973 programs, Grant Nos.2005CB422303, 2007CB411804), the National Natural Science Foundation of China (Grant No. 40706006), the Key Project of International Science and Technology Cooperation of China (Grant No.2006DFB21250) and the 111 Project (Grant No.B07036). Biography: XU Ling-ling (1981-), Female, Ph. D.

YSWC is highly time-dependent and just a compensating current of the wind-driven coastal currents. Later analyses and numerical studies confirmed that the YSWC is an episodic current forced by the southward pressure gradient during the relaxation of the northerly wind bursts[5-8]. Recent analyses of Acoustic Doppler Current Profiler and pressure gauge data presented by Teague and Jacobs[9] supported Hsueh’s hypothesis. Besides the controversy on the formation mechanism of the YSWC, people also noticed that there exists obvious seasonal variability in the Yellow Sea current system. In winter the circulation in the Yellow Sea is characterized by the strong northward intrusion of the YSWC in the trough of the Yellow Sea and the southward flowing coastal currents along the coasts of China and Korea, including the CCC and the KCC. In summer, observations showed that the YSWC turns eastward into the Cheju Strait between Cheju and the Korea Peninsula instead of intruding into the Yellow Sea[10] and the CCC flows northward[11-13]. Now it is widely accepted that the

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YSWC is stronger in winter and very week or vanishes in summer. The above works in the YSWC were mainly concerned about the local processes such as the monsoonal forcing, but recent studies indicated the strong effect of the KC on the current system in the ECS[14,15]. Yang[16] demonstrated that the KC may play the most important role in determining the annual mean YSWC, and the local wind-stress forcing plays an important but secondary role. The results from those previous studies were largely conceptual and Yang’s work was only based on a simple barotropic model with the forcing of a steady wind-stress field. The KC effect on the YSWC remains to be tested by a more complete 3-D model with both wind stress and buoyancy fluxes. In addition, the seasonal variability of the YSWC may involve different forcing mechanisms than their annually-averaged fields, which can only be addressed when a seasonallyvarying forcing is used in the model. From the above analysis, it can be found that the formation mechanism of the YSWC and its seasonal variability are still not very clear. In this article, we will use a 3-D Princeton Ocean Model (POM) to examine both the KC and monsoonal forcing in the annual mean and the seasonal variability of the YSWC. There are two major objectives of this study: (1) testing the KC forcing mechanism of the YSWC with a three-dimensional model and (2) examining the mechanism(s) for seasonal variability of the YSWC. The article is organized as follows. The model, including its setup and its forcing fields, will be introduced in Section 2. The annual mean and seasonal variability will be analyzed and discussed in Sections 3 and 4 respectively. Section 5 presents the conclusions. 2. Model domain and forcing fields 2.1 Model The model we used in this study is POM97, which has been widely used in the simulation of the ECS. A lot of previous numerical simulations about the YSWC were conducted only in a small model domain and could not correctly include the KC. This may be the reason why they attribute the YSWC mostly to the local wind forcing. Here a relatively large model domain is concerned with, including the whole northwest Pacific Ocean from 99oE-150oE and 0oN-50oN, with a resolution of 1/6o by 1/6o horizontally and 16 sigma levels (Table 1) vertically. The model boundary is far away from the ECS area so that the fake effect of boundary is reduced. The bathymetry used in the model is interpolated from ETOPO5 gridded elevation data and the minimum and maximum depth is set to 10 m and 3000 m separately. The bathymetry in the ECS is shown in Fig.1.

Table 1 Vertical sigma coordinate used in the model Layer

ı

Layer

ı

1

0.00000

9

 0.30000

2

 0.00313

10

 0.40000

3

 0.00625

11

 0.50000

4

 0.01250

12

 0.60000

5

 0.02500

13

 0.70000

6

 0.05000

14

 0.80000

7

 0.10000

15

 0.90000

8

 0.20000

16

 1.00000

Fig.1 Bathymetry in the ECS. Only depth shallower than 200 m is plotted with an interval of 20 m

The model is initialized with temperature and salinity fields for January from the National Oceanographic Data Center, and is forced by monthly mean climatologic atmospheric fields. The data for the wind stress are taken from the National Center for Environmental Prediction and the data for buoyancy fluxes from the Comprehensive Oceanic and Atmospheric Data Sets (COADS) settled by Da Silva[17,18]. The boundary conditions, such as elevation, velocity and temperature, are all given from the 5 d averaged results of a global model run with a horizontal resolution of 1/2o by 1/2 o [19,20]. In this way, both the local monsoonal forcing and the KC forcing could be included in the 3-D model simulation. For

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simplicity, the river flux has not been considered in the model. Velocities normal to land are set to zero and radiation condition is used at open lateral boundaries. 2.2 Validation of the model Firstly, we conduct a control run which is running under the full forcing fields and boundary conditions mentioned above. In the control run, the model is integrated for 6 years until it reaches a statistically steady seasonal cycle. After that, circulations are well established in the ECS. Figure 2 shows the annual mean verticallyaveraged velocity field from the control run. The main circulations are marked with thick arrows. For example, the KC flows from the east of Taiwan Island to the south of Kyushu and the Taiwan Warm Current (TWC) flows northeastward through the Taiwan Strait. The TSWC flows into the Tsushima Strait as a branch of KC. In the Yellow Sea, the YSWC flows to the northwest nearly along the Yellow Sea Trough. The coastal currents KCC and CCC can also be found from Fig.2. The pattern of the circulation is roughly similar to those in some previous studies[2,3,21,22]. The KC flux in the east of Taiwan is about 21.67 Sv (1Sv =106 m3 s-1), which is well within the range of observed data given by Ichikawa et al. [23] (23.7±2Sv) and Gilson et al. [24] (22±1.5 Sv). The annual mean transport of the TWC is 2.09 Sv which is in the range of the results from Bao et al. [25], a maximum flux of 3.1 Sv in July and a minimum of 0.9 Sv in November. The transport of the TSWC through the Tsushima Strait is 2.98 Sv, close to the value of 2 Sv-3S v reported by Isobe[26].

variability of the YSWC discussed. 3. Annual mean of YSWC 3.1 Results of numerical experiments In the first experiment (Exp.1), the role of the KC in determining the YSWC is to be tested. The local wind stress is removed and the only forcing is through the prescribed lateral boundary conditions so that the outer forcing is just from the open ocean where basically the KC exists. The annual mean vertically-averaged velocity field is shown in Fig.3. All the major currents in the ECS can be seen except the CCC. Compared to the control run (Fig.2), the KC, TWC and TSWC are nearly the same without local wind stress. Only the strength of the YSWC and the KCC seems a little weaker than that in the control run. This is consistent with the barotropic model results given by Yang[16], which suggest that these shelf currents, including the YSWC, should be primarily forced by the KC.

Fig.3

Fig.2

Annual mean vertically-averaged velocities of the ECS in control run

The above results show that the presented model could basically reproduce the circulation in the ECS. In the next two sections, two experiments will be described and the annual mean and seasonal

Annual mean vertically-averaged velocities of the ECS in Exp.1, when local wind forcing is removed from the model

In the second experiment (Exp. 2), the role of the local wind stress is to be tested. So the KC is removed by setting the normal velocities to zero at the lateral boundary. The annual mean vertically-averaged velocity field in this experiment is shown in Fig.4. All the currents are quite different from that in the control run except the CCC. The northward transport in the Yellow Sea is much less than that in the control run, which indicates that the local wind stress could only produce a very weak YSWC. 3.2 Contribution of KC and local wind in determining the annual mean of YSWC To further estimate the contribution of the KC and local wind forcing in the annual mean YSWC, the transport of some main currents is calculated for each

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experiment. The YSWC is 0.19 Sv (positive transport means northward flow) in the control run, 0.14 Sv in the Exp. 1 when it is only forced by the KC and 0.05 Sv in the Exp. 2 when it is only forced by the local wind stress. The KCC is  0.13 Sv in the control run,  0.15 Sv in the Exp. 1 and very weak in the Exp. 2. The CCC is about  0.04 Sv in the control run and the Exp. 2, while it nearly vanishes in the Exp. 1.

southward transport of the CCC in both the control run and the Exp. 2 also means that the CCC is motivated by the local wind forcing.

Fig.5 Meridional velocity across section 35oN in control run in winter. The dashed line denotes zero

Fig.4

Annual mean vertically-averaged velocities of the ECS in Exp.1, when the KC forcing is removed from the model

From above, it can be found that in the control run about 2/3 of the YSWC transport is due to the compensation of southward KCC and the remained transport is due to the compensation of southward CCC. Figure 5 visualizes the compensating mechanism of the YSWC by showing the meridional velocity across the section 35oN in the control run. In the middle of the section below surface, it can be clearly found that the YSWC flows to the north. Near the eastern coast is the southward KCC and near the western coast is the southward CCC, and both are concentrated in the surface layer. Besides, there is southward surface flow above the YSWC which transports quite little water, so its impact on the YSWC can be ignored. Here it is concluded that the annual mean YSWC is a compensating current firstly for the southward KCC and secondly for the CCC. According to Yang[16], the KCC is mainly forced by the KC, but rather indirectly. It is directly forced by the TSWC as a source- and sink-driven flow along isobaths while the TSWC is primarily forced by the KC flowing from the east of Japan with the essential dynamics which can be explained in terms of the “island integral constraint”. Here the authors do not want to go into the details about how the KC forces the KCC because they are beyond the scope of this study. The very weak KCC in the Exp. 2, when there is no KC forcing, clearly support the conclusion that the KCC is mostly determined by the KC. The same

Here it is shown that in the annual mean the YSWC is first forced by the KC and secondly forced by the local wind stress. Our conclusion does not contradict to the previous studies, which indicated that the YSWC is an episodic current forced by southward pressure gradient during the relaxation of the northerly wind bursts. When the northerly wind bursts, it usually induces a stronger CCC which transports more coastal water to the south. To compensate it, the northward YSWC becomes stronger. On the other hand, the YSWC is comparatively weaker when there is no northerly wind bursts. 4. Seasonal variability of YSWC Besides the annual mean, there exists obvious seasonal variability of the YSWC. It penetrates deeply to the north in winter and becomes much less intrusive in summer. Is the seasonal variability also controlled by the KC? In this section, we will examine the seasonal variability of the YSWC. Figure 6 shows the vertically-averaged velocity field in the control run in winter 6(a) and summer 6(b) respectively. It can be clearly seen that both the YSWC and the KC in winter are stronger than those in summer. Does this mean that the seasonal variability of the YSWC is also dominated by the KC? Through calculating the transport across section 35°N in the control run, it is found that in winter the transport of YSWC is over ten times more than in summer. But the transport of KC in winter is only 10 percent larger than that in summer. Considering the small seasonal variability of the KC, it is very difficult to imagine the huge seasonal variability of the YSWC is controlled by the KC. Figure 7 is the vertically-averaged velocity field in the Exp. 1 in winter 7(a) and summer 7(b) respectively, in which the only force in the model the KC. From Fig.7 it can be found that the YSWC is steadily flowing northward both in winter and in summer and there is no obvious seasonal variability. The results of Exp.1 further prove that the KC can not be the control factor in the seasonal variability of the YSWC and the possible candidate is the local wind

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forcing. Figure 8 shows the vertically-averaged velocity field in the Exp. 2 when the only force in the model is the local wind stress. The prominent feature in Fig.8 is the obvious seasonal variability. All the coastal currents change their directions following the leading of monsoon. In winter, the CCC contributes to most of the southward flux and induces a northward YSWC. In summer, the CCC flows northward and induces a southward flow along the Yellow Sea Trough. Figure 9 shows the transport of the YSWC in the control run and two experiments during a year round. The transport in the control run varies similarly to that in the Exp. 2 when the only force is the local wind. Both of them decrease from spring to summer and increase from summer to winter. The transport remains almost steady when there is only KC forcing with very little seasonal variability. This figure is a good illustration to show the dominant role of monsoonal forcing in the seasonal variability of the YSWC.

6(a) Winter

6(b) Summer Fig.6 Vertically-averaged velocities of the ECS in control run

7(a) Winter

7(b) Summer Fig.7 Vertically-averaged velocities of the ECS in Exp.1 when the local wind forcing is removed

8(a) Winter

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Acknowledgement The authors greatly thank Dr. Yang Jia-yan in the Woods Hole Oceanographic Institution for his help and comments. References [1]

[2]

[3] 8(b) Summer Fig.8 Vertically averaged velocities of the ECS in Exp. 2 when the KC forcing is removed

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[6] [7] Fig.9 Transport across the Yellow Sea trough in the control run and two experiments during a year round. X-axis denotes January to December, Y-axis denotes flux in unit of Sv (1 Sv =106 m3 s-1)

5. Conclusions Based on several numerical experiments, the mechanism of the annual mean and seasonal variability of the YSWC has been explored. Our conclusions are as follows. (1) In the annual mean, about 2/3 of the YSWC transport is to compensate the KCC, which is mainly determined by the KC. Another 1/3 is caused by the CCC, which is mainly determined by the local wind. The KC is responsible for the most of the annual mean YSWC, while the local wind stress plays an important but secondary role. (2) The local monsoonal forcing controls the seasonal variability of the YSWC mostly via the CCC. When the wind direction changes from winter to summer the direction of the CCC changes correspondingly. To compensate the southward CCC and KCC in winter, a more intrusive YSWC appears.

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