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An asymmetric upwind flow, Yellow Sea Warm Current: 1. New observations in the western Yellow Sea

2011; American Geophysical Union; Volume: 116; Issue: C4 Linguagem: Inglês

10.1029/2010jc006513

2156-2202

Xiaopei Lin, Jiayan Yang, Jingsong Guo, Zhixin Zhang, Yuqi Yin, Xiangzhou Song, Xiaohui Zhang,

Climate variability and models

Journal of Geophysical Research: OceansVolume 116, Issue C4 Free Access An asymmetric upwind flow, Yellow Sea Warm Current: 1. New observations in the western Yellow Sea Xiaopei Lin, Xiaopei Lin linxiaop@ouc.edu.cn Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this authorJiayan Yang, Jiayan Yang Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USASearch for more papers by this authorJingsong Guo, Jingsong Guo Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao, ChinaSearch for more papers by this authorZhixin Zhang, Zhixin Zhang Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao, ChinaSearch for more papers by this authorYuqi Yin, Yuqi Yin Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this authorXiangzhou Song, Xiangzhou Song Physical Oceanography Laboratory, Ocean University of China, Qingdao, China Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USASearch for more papers by this authorXiaohui Zhang, Xiaohui Zhang Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this author Xiaopei Lin, Xiaopei Lin linxiaop@ouc.edu.cn Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this authorJiayan Yang, Jiayan Yang Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USASearch for more papers by this authorJingsong Guo, Jingsong Guo Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao, ChinaSearch for more papers by this authorZhixin Zhang, Zhixin Zhang Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao, ChinaSearch for more papers by this authorYuqi Yin, Yuqi Yin Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this authorXiangzhou Song, Xiangzhou Song Physical Oceanography Laboratory, Ocean University of China, Qingdao, China Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USASearch for more papers by this authorXiaohui Zhang, Xiaohui Zhang Physical Oceanography Laboratory, Ocean University of China, Qingdao, ChinaSearch for more papers by this author First published: 29 April 2011 https://doi.org/10.1029/2010JC006513Citations: 44 This is part of DOI:10.1029/2010JC006514. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] The winter water mass along the Yellow Sea Trough (YST), especially on the western side of the trough, is considerably warmer and saltier than the ambient shelf water mass. This observed tongue-shape hydrographic feature implies the existence of a winter along-trough and onshore current, often referred to as the Yellow Sea Warm Current (YSWC). However, the YSWC has not been confirmed by direct current measurements and therefore skepticism remains regarding its existence. Some studies suggest that the presence of the warm water could be due to frontal instability, eddies, or synoptic scale wind bursts. It is noted that in situ observations used in most previous studies were from the central and eastern sides of the YST even though it is known that the warm water core is more pronounced along the western side. Data from the western side have been scarce. Here we present a set of newly available Chinese observations, including some from a coordinated effort involving three Chinese vessels in the western YST during the 2006–2007 winter. The data show unambiguously the existence of the warm current on the western side of YST. Both the current and hydrography observations indicate a dominant barotropic structure of YSWC. The westward deviation of YSWC axis is particularly obvious to the south of 35°N and is clearly associated with an onshore movement of warm water. To the north of 35°N, the YSWC flows along the bathymetry with slightly downslope movement. We conclude that the barotropic current is mainly responsible for the warm water intrusion, while the Ekman and baroclinic currents play an important but secondary role. These observations help fill an observational gap and establish a more complete view of the YSWC. Key Points New observation in the western Yellow Sea The first direct observation evidence of YSWC The YSWC is located in the western Yellow Sea with on-shore movement 1. Introduction and Background [2] The Yellow Sea (YS) is a broad shelf sea in the northwestern Pacific Ocean. A noticeable bathymetric feature is a trough that extends northwestward from the continental slope toward the Bohai Sea (BS), a semienclosed basin bordered by a coastline with a cluster of major metropolitan centers. In winter the water mass along the central and western side of the Yellow Sea Trough (YST) is distinctly warmer and saltier than the ambient water. This hydrographic feature suggests the existence of a northward winter current which flows against the prevailing northerly monsoonal wind (see Figure 1 for a schematic). This winter current, commonly called the Yellow Sea Warm Current (YSWC), is the main conduit by which the deep Pacific Ocean influences the BS and YS. The YSWC affects not only the hydrography and biogeochemistry of the shallow water in the region, but also maritime conditions, such as sea ice coverage, in some of the busiest harbors in the world [Su et al., 2005]. Understanding the mechanism of the YSWC and its variability therefore has been a major focus of regional oceanographic investigations. Figure 1Open in figure viewerPowerPoint Schematic map of winter circulation in the Yellow Sea and East China Sea. The red arrows denote the warm currents and the blue arrows denote the cold currents. Black lines are the bathymetry with 50, 100, and 200 m labeled. [3] The existence of the YSWC was first suggested by Uda [1934, 1936] based on the tongue-shape distribution of warm and salty water of Pacific Ocean origin. The schematic in Figure 1 is based on a collection of previous studies, including Yuan and Su [1984], Guan [1994], Ichikawa and Beardsley [2002], Yuan et al. [2008], and Isobe [2008]. To compensate YSWC transport, there are two southward coastal currents along the Chinese and Korean coasts. Other prominent currents in the region include the Tsushima Warm Current (TSWC) which flows into the Sea of Japan (East Sea) and the Taiwan Warm Current (TWC) which originates from the Taiwan Strait and the East China Sea shelf region (part of TWC water originates from the Kuroshio). These currents are interconnected and form a coastal warm current system in the East Asian Marginal Seas. In fact, historically, the YSWC was considered either as an upstream branch of the TSWC [Nitani, 1972; Guan, 1994; Guan and Chen, 1964] or as an extension of the perennially northward flowing TWC [Beardsley et al., 1985]. However, the intrusion of the warm water along the YST is a winter-only feature [Lie, 1986; Park, 1986; Beardsley et al., 1992; Lie et al., 2000; Guo et al., 2000; Teague and Jacobs, 2000], while the transport of both the TWC and the TSWC peak in the summer. Thus, in some studies the YSWC has been considered as a locally wind-driven flow or even as a sporadic response to strong northerly wind bursts [Hsueh et al., 1986; Hsueh, 1988]. [4] Lie et al. [2001, 2009] questioned the existence of the YSWC by noting that the current has been inferred from the T-S distribution rather than verified by direct current measurements. They found a thermal front to the west of Cheju Island and argued that this would block the intrusion of warm water into the southern Yellow Sea. A noteworthy result from their study is that the along-trough velocity was very weak in all direct current measurements that were available to them. They concluded, therefore, that the YSWC is an intermittent upwind flow which occurs when the western front of the Cheju Warm Current collapses during northerly wind bursts. Other studies, like Xie et al. [2002], also suggested the tongue shape distribution of warm water is not necessarily due to advection of warm water. They argued that the surface cooling and subsequent convective mixing in winter could lead to a warm water presence along the trough due to the larger depth-integrated heat capacity in deeper regions. [5] Most observations that have been discussed in the literature, including those by Lie et al. [2009], were made in the central YS or on the eastern side of the YST. While in situ data on the western side of the YST are rarely available, the observations have shown that the core of the warm water is distributed along the western YST [e.g., Tang et al., 2001; Huang et al., 2005]. It would be difficult to fully reconcile assessments and hypotheses derived from different observations without data on the western side. So a more systematic description of YSWC needs observations on the western YST. [6] During the winter of 2006–2007, a comprehensive observational program with coordinated cruises of three research vessels was made by Chinese oceanographers on the western side of the YST. This survey was conducted jointly by the Ocean University of China (OUC) and the First Institution of Oceanography, State Oceanic Administration (FIO). The observations filled a large void in the available data and provide a more complete data-based description of oceanic conditions in the YS, especially on western side of the YST. The main purpose of this study is to use this new set of observations, along with previously available in situ and satellite data, to update the characterization of the YSWC. Our study shows that a steady northward YSWC indeed exists in the wintertime, but its axis is shifted upslope toward the western side of the trough. This could potentially explain why the current was weak along the central trough in the data analyzed by Lie et al. [2009]. [7] The paper is organized as follows. The data set used in this study is introduced in section 2. We present analyses the newly observed YSWC features in section 3. A discussion and a summary are given in sections 4 and 5. 2. The Data 2.1. The 2006–2007 Winter Investigation [8] Between 21 December 2006 and 14 February 2007 a large-scale survey involving three research vessels was conducted in a coordinated effort by Chinese oceanographers to study the western Yellow Sea. Three research vessels were used simultaneously. There were 464 stations with interstation distances less than 20 km. During the observations, several outbreaks of strong northerly wind with cold airs occurred, e.g., on 28 December 2006, 8 January 2007, and 28 January 2007. The field work was suspended during each storm event which typically lasted for about 2–3 days. Among all stations, 110 of them were along the coast of Shandong and Jiangsu Provinces. The observation was made prior to 10 January 2007 in these costal stations. We exclude data from those 110 coastal stations in the present study since they were not located in the vicinity of the YST. The remaining 354 stations were located on the western YST (Figure 2) and are used in this study. Observations in all of them were made between 10 January and 4 February 2007. At each station, temperature and salinity were measured with a high vertical resolution of less than 0.1 m using Sea Bird CTDs. All instruments were calibrated prior to and after the deployments following the standard procedures. Data with obvious biases were removed. In addition to CTD casts, 6 moorings with ADCPs were deployed along the possible pathway of the YSWC (Figure 2). FIO deployed moorings M1 to M5 in a section along 34.7°N (section F) in water depths ranging from 50 m to 70 m; the results showed the first direct evidence of the YSWC on the western side of the YST [Yu et al., 2010]. OUC deployed mooring M6 in the northern end of the YST where the depth was 66 m. Unfortunately, the instruments from 2 mooring stations (M1, M3) were lost, likely due to the heavy fishing activity in the area. At each mooring site, a tide gauge and an upward looking 300 KHz RDI ADCP (Acoustic Doppler Currents Profile) or 250 KHz Sontek ADCP were deployed. The sampling interval was 5 min and bin size was 2 m in the vertical. Detailed information about the moorings is listed in Table 1. This observational program was likely the most comprehensive ever conducted on the western side of the YS. Figure 2Open in figure viewerPowerPoint Stations from the 2006–2007 winter survey. Dots denote the field observation stations. Eight sections, named A-H, are selected to show the position of YSWC in different regions. The mooring stations, named M1–M6, are outlined by squares. Table 1. The Mooring Stations Station Latitude (°N) Longitude (°E) Depth (m) Start Day End Day Bin Size (m) M1 34°40.04′ 121°59.84′ 51 9 Dec 23 Jan 2 M2 34°40.35′ 122°15.06′ 55 9 Dec 23 Jan 2 M3 34°40.53′ 122°29.74′ 62 9 Dec 23 Jan 2 M4 34°40.21′ 122°45.48′ 71 9 Dec 23 Jan 2 M5 34°40.01′ 123°00.49′ 73 9 Dec 23 Jan 2 M6 38°00.53′ 123°30.67′ 66 31 Dec 8 Feb 2 2.2. Routine Section Data [9] In addition to the 2006–2007 cruise data, we analyzed routine section observations made along 34°N, 35°N, and 36°N. The State Oceanic Administration (SOA) maintained these sections as routine surveys from 1976 to 2007. Each section was surveyed at least 4 times per year, including one in winter [Guo et al., 2004]. These transects consisted of about 30 fixed stations for a period of more than 30 years. 2.3. Sea Surface Temperature and Wind Stress Data [10] The sea surface temperature (SST) data set used here was the 4 km × 4 km monthly average PFSST based on the Pathfinder algorithm for the period from January 1985 to February 2009. The data were derived from the five-channel advanced very high resolution radiometers (AVHRRs) on board the NOAA −7, −9, −11, −14, −16, −17 series of polar orbiting satellites and combined with the in situ buoy data. This is a newer version of SST product from NOAA/NASA AVHRR Oceans Pathfinder Program. The earlier version on the 9.28 m grid [Kilpatrick et al., 2001] has been applied in the Bohai Sea, Yellow Sea and East China Sea. Gao et al. [2001] compared the 9.28 km PFSST with in situ data in this region and found that in 67.6% of all data pairs, the difference is less than or equal to 0.5 K, and the root mean square error for all pairs is about 0.61 K. The newly published Pathfinder V5 data set provides even higher resolution (4 × 4 km) and presumably a more accurate SST product than the earlier version. For this study, the Yellow Sea surface temperature between 31°N and 38°N and 119°E and 127°E, in each winter (DJF) from 1985 to 2009 were extracted from the PFSST data set. Based on a hierarchical suite of tests [Kilpatrick et al., 2001], the SST quality varies from 0 to 7, with 0 being the lowest quality and 7 being the highest quality. Pixels with quality from 5 to 7 were selected and used to interpolate in those pixels with low-quality data. [11] If the YSWC is a locally wind-driven current, its existence and variability should be highly correlated with wind stress forcing. In this study, we used the QuikSCAT daily sea surface vector wind [Liu, 2002] coincident with the 2006–2007 winter cruises. The wind data have a resolution of 1/4° in both longitude and latitude. 3. The Observed YSWC [12] Observations from the 2006–2007 winter cruises and other data are used here to characterize the structure and variability of the YSWC. These data are presented and discussed in this section. 3.1. Upslope and Westward Shift of the YSWC [13] As noted previously, the intrusion of warm water is not exactly along the central YS trough axis but is displaced westward. This can be seen in the SST field from satellite AVHRR sensors. Figure 3 shows the mean winter SST averaged from 1985 to 2009. The axis of the warm water, denoted by blue dots and defined by the maximum SST across the trough, is clearly displaced westward. South of 35°N, the upslope movement of warm water tongue is very obvious. The axis of warm water (blue dots) is heading northwestward from the water depth of more than 100m in 32°N to about 70m in 33°N. Between 33°N to 35°N, the cross-shelf and onshore intrusion is gradual from 70m to about 50m. The warm water axis reaches the most western position of about 122.5 °E in 35°N. To north of 35°N, the warm water intrusion is basically along the bathymetry with slightly downslope movement. There is another warm water tongue heading northwest toward the Shandong coast line, along the divergence of topography. This westward displacement of the YSWC has been pointed out in several previous studies. For instance, Tang et al. [2001] showed clearly the presence of high temperature and salinity anomalies on the western part of the YST in winter. A more recent study by Ma et al. [2006] found the double warm tongues in the Yellow Sea in winter, with an intermittent tongue along the central trough and a persistent tongue on the western side. Xie et al. [2002] also noticed the westward shift of the warm water and speculated that the westward Ekman flow may be responsible for the shift. Huang et al. [2005] suggested that the westward shift could be due to the surface cooling and self-advection of baroclinic currents even though it was noted that the barotropic current in their 3-D model was responsible for the westward shift of the warm tongue. Figure 3Open in figure viewerPowerPoint The mean SST (°C) in winter (DJF) from AVHRR observations from 1985 to 2009. The blue dots denote the axis of warm water tongue as the pathway of YSWC. The black lines are the bathymetry with the dashed lines representing the 2.5 m interval between 50 and 70 m. [14] The displacement of the warm water occurred not only in the SST field but also over the water column. This was revealed in the hydrographic sections along 34°N, 35°N, and 36°N from the SOA routine surveys from 1976 to 2007. The 31 year average winter observations (Figures 4a, 4c, and 4e) show that the whole water column, not merely the top Ekman layer, was warmer and saltier on the western side of the trough. The warm water column (black bold line denotes the axis) actually slightly moved upslope from 34°N to 35°N but downslope from 35°N to 36°N. This pattern is consistent with the path of the YSWC from satellite observations in Figure 3. Another important evidence for the existence of YSWC is the tongue of high salinity (Figures 4b, 4d, and 4f), which is also located in the western YST along with the warm water core. So the presence of a warm/salty water column on the western side of the YST was evident climatologically in this 31 year record. High salinity in the warm water tongue may compensate the temperature effect and thus reduce the thermal wind shear in the baroclinic geostrophic velocity. It is interesting to note that there was a high temperature core on the bottom just to the west of the trough at 34N, 124E (Figure 4e). This temperature inversion was accompanied by a negative salinity gradient in vertical (Figure 4f) and so water remained stably stratified. While the salinity was distinctly higher in the warm water column in all three sections, there was a considerable vertical stratification in salinity along all three sections. This contrasts with a vertically more uniform temperature profile. The salinity stratification suggests that intense surface-to-bottom vertical mixing, as hypothesized by Xie et al. [2002], does not play the leading role in setting the winter hydrographic structure along the trough. Lateral advection was more likely responsible for the existence of the warm and salty water column. Geostrophic velocity and its vertical shear over the whole water column are strongly steered by bathymetry on a f plane even when the water is strongly stratified [Brink, 1998]. It is therefore expected that geostrophic component of the YSWC could preserve its T-S stratification from the upstream. Ageostrophic velocity components, like surface and bottom Ekman layer velocity, are certainly important as well as to be shown in the velocity data. Figure 4Open in figure viewerPowerPoint The average (1976–2007) winter temperature (°C) from three routine sections, (a) 36°N, (c) 35°N, and (e) 34°N, and salinity (psu) from three routine sections, (b) 36°N, (d) 35°N, and (f) 34°N. The temperature interval is 0.2°C and salinity interval is 0.1 (dashed line). The maximum temperature and salinity are labeled by the black bold line. 3.2. The Winter Observations in 2006–2007 [15] Figure 4, based on three routine hydrographic surveys, clearly shows that the warm and salty water mass is located on the western YST. Prior to the observational program described here, the structure of the western side of the YSWC was ambiguous and the existence of the YSWC remained to be confirmed by direct current measurements. The data from the 2006–2007 cruises helped close some gaps in knowledge of the YSWC. [16] Maps of surface and bottom temperature and salinity during the winter of 2006–2007 are shown in Figure 5. High temperature and salinity water is clearly seen on the western side of the trough from surface to the bottom and this is consistent with the three time average winter vertical sections shown in Figure 4. The main feature in the maps is a warm and salty water tongue trending northward from the southern YS toward the BS, indicating the pathway of YSWC. South of 35°N, the warm and salty water trends northwestward from the central trough to the western slope, indicating cross-isobathic water movement. North of 35°N, the water tongue turns northeastward, basically along the 50–70 m isobaths, eventually reaching the BS. We also notice the existence of a second warm and salty water tongue toward the Shandong coastal line from 35°N to 36°N. This water intrusion is roughly along the divergence of bathymetry and more obvious in temperature than in salinity. Westward and cross-isobathic movement south of 35°N, is also suggested in Figures 6 and 7, which show vertical temperature and salinity sections from the southern to northern Yellow Sea. Along section A, the axis (black bold line) of warm and salty water with temperature greater than 16°C and salinity higher than 34 was located in about 127.5°E, where the water depth is 165 m. Along sections B and C (32.3°N), the warm and salty water core was near the central trough with water depth more than 100m. The easternmost stations along sections A, B, and C were in the shelf break region and can be considered as the entrance of the YSWC into the YST. At those stations with temperature higher than 15°C and salinity higher than 34, the water was most likely from the Kuroshio or Taiwan Warm Currents. Due to the lack of observations east of 124°E along section D, we cannot determine the warm/salty water axis precisely; however it does appear that the core of YSWC water moved westward. The westward and onshore movement of warm/salty water core becomes obvious from sections E to G. The water axis moved from the center to the west slope of the trough where the depth is about 50–70 m. The warm temperature axis reached as far west as 122.7°E along section G, while the high-salinity axis was located in about 123°E. The discrepancy between temperature and salinity distribution in this region was also shown in Figures 4a and 4b (36°N section) and Figure 5. The salinity in this region is likely to be influenced by the fresh coastal water flowing outside of the BS along the Shandong coastal line. Data available to this study are insufficient to quantify the impact from the coastal current. A modeling study would be desirable but is beyond the scope of this study. North of section G, the warm and salty water tongue moved northeastward along the bathymetry. The temperature and salinity distribution were rather similar as that shown in Figure 4. The bottom water was warmer and saltier than the surface water along the trough. This feature can be traced to the upstream region around 31N. This further supports the YSWC moves upstream likely as a stratified Taylor column as discussed by Brink [1998] for a more general coastal flow. There was a second warm/salty water core on sections F and G in the central trough, which is was also reported by Ma et al. [2006]. We speculate that the local topography divergence was responsible for the presence of second warm/salty water core on sections F and G. This could be further investigated with a regional ocean model. Figures 5, 6, and 7 show that the warm and salty water tongue in the YS progressively shifts westward as the intrusion penetrates further onshore in the south of 35°N. This is quite consistent with the satellite SST observations during the investigation. Although patchy, the warm water tongue of AVHRR SST in January 2007 trends northwestward and upslope in the south of 35°N (Figure 8). Figure 5Open in figure viewerPowerPoint Maps of water properties from the 2006–2007 winter survey with temperature in °C and salinity in psu. They are the (a) surface temperature, (b) surface salinity, (c) bottom temperature, and (d) bottom salinity. Dots denote the stations. Water depths of 10, 30, 50, 70, and 90 m are labeled with black lines with the dashed lines representing the 2.5 m interval between 50 and 70 m. Figure 6Open in figure viewerPowerPoint The temperature (°C) in sections A-H with interval of 0.2°C (dashed line). The maximum temperature is shown with the black bold line. The position of the sections is shown in Figure 2. Figure 7Open in figure viewerPowerPoint The same as Figure 6 but for salinity (psu). The interval is 0.1. Figure 8Open in figure viewerPowerPoint The mean SST (°C) in January 2007 from AVHRR observations. The black lines are the bathymetry with the dashed lines representing the 2.5 m interval between 50 and 70 m. 4. Discussion [17] Both the in situ observations and satellite SST measurements indicate the westward and onshore movement of YSWC in the southern YS. These observations imply the existence of an onshore winter current along the western YST. To examine its existence, the residual currents at mooring stations M2, M4, M5, and M6 are used as the direct observation evidence of YSWC. These stations were located in the pathway of the warm and salty water tongue (Figure 2). Tides were removed from the ADCP data by Lanczos low-pass filtering [Duchon, 1979]. 4.1. Mean Current at Morring Stations M2, M4, and M5 [18] Figure 9 shows the time-averaged currents at mooring stations M2, M4, and M5 with the background temperature of section F. The mean currents at stations M2, M4, and M5 are all directed to north or northwest from surface to bottom and support the existence of YSWC in the western YST. At M5, which was located near the warm water core in section F, the northwestward current speed was usually greater than 5 cm s−1. The current speed at M2 and M4 were smaller and around 2–3 cm s−1. There were strong vertical shear of velocity at M2 and M5, with westward in surface layer and northwestward in subsurface layer. This westward velocity component in both M2 and M5 indicates an upslope and onshore movement of YSWC as shown in the in situ and AVHRR SST observations. It is interesting that the currents at M4 had small shear and was basically northward in all layers. Figure 9Open in figure viewerPowerPoint Time-averaged currents at mooring stations M2, M4, and M5 with the background temperature (°C) of section F. 4.2. The Ekman Model Used for Estimating Ekman Current [19] To better understand the vertical structure of observed velocity field, a one-dimensional model is employed here to estimate the Ekman currents at the location of mooring stations. We want to discuss the contribution of Ekman, barotropic, and barocilinic current to the intrusion and shear of YSWC in the western trough. The wind forcing is determined by the mean of adjacent nine grid points of QuikSCAT wind data during observation time in each mooring stations. The governing equation is where f is Coriolis parameter, is the velocity, and ν is the viscosity. The surface boundary condition is where ρ0 is the constant density derived from the mean density of each mooring station, and is the surface wind stress from QuikSCAT observations, which is the same as the classic Ekman theory in the open oceans. In this calculation, ν is taken as 0.01 and other parameters are chosen according to Price and Sundermeyer [1999] as listed in Table 2. The results from the Ekman model will be used in the following discussions. Table 2. Parameters for the 1-D Ekman Model Physical Parameter Presentation Value Vertical resolution z 2 m Air density ρa 1.23 kg m−3 Wind drag coefficient CD 1.2 × 10−3 4.3. The Mean Residual, Ekman, Barotropic, and Baroclinic Components at Mooring Stations [20] The three-dimensional residual current at mooring stations M2, M4, and M5 are shown in Figur