2 Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao 266071, China;
3 University of Chinese Academy of Sciences, Beijing 100049, China;
4 Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
The Yellow Sea, located between the east coast of mainland China and the Korean peninsula, is a typical, shallow semi-enclosed basin with an average water depth of 44 m and a maximum depth of 100 m (Fig. 1). It is connected to the Bohai Sea at its northern extremity through the Bohai Strait and separated from the East China Sea by a line connecting the area north of the Changjiang (Yangtze) River estuary with Jejudo (Jeju Island, previously known as Cheju-do). Recently, the western Yellow Sea has been increasingly noted for its environmental degradation, such as that resulting from harmful algal blooms (Enteromorpha prolifera) (Liu et al., 2009; Hu et al., 2010) and jellyfish blooms (Zhang et al., 2012), which have caused serious ecological damage and financial losses. The regional circulation plays a critical role in the cross-shelf exchange of mass, heat and nutrients, resulting in a strong impact on the regional biogeochemical budgets. Therefore, it is of great importance to study, in detail, the circulation in the western Yellow Sea.
The relatively complicated circulation in the western Yellow Sea results from the combined effects of the wind field and tides, as well as remote forcing from the Kuroshio Current and Taiwan Strait. Figure 1 shows the recognized circulation pattern in the Yellow Sea and part of the East China Sea in winter, which is primarily characterized by the Yellow Sea Warm Current (YSWC), the Cheju Warm Current (CWC), the Bohai Sea Coastal Current (BSCC), the Yellow Sea Coastal Current (YSCC), the Changjiang Diluted Water (CDW), the Korean Coastal Current (KCC), the East China Sea Coastal Current (ECSCC), and the Taiwan Warm Current (TWC), which splits into two branches at about 28.5°N (Guan, 1994; Yuan et al., 2008; Bian et al., 2013). Many studies have been performed on the YSCC (e.g. Guan, 1994; Ichikawa and Beardsley, 2002), showing it as flowing southward along the Jiangsu coast year-round. However, based on ocean color images from satellites showing a turbid water plume extending from the Old Huanghe Delta to the southwest of Jeju-do during winter, Yuan et al. (2008) proposed that, in winter, the YSCC flows from the northern Jiangsu coast to the southwest of Jeju-do, and can transport the Old Huanghe Delta sediments to the southwest of Jeju-do to form a mud patch, which is also supported by Bian et al.(2010, 2013). Moreover, Yuan and Hsueh (2010) suggested that the southward wind-driven coastal currents and the northward-flowing YSWC produce a divergence southwest of Jeju-do, which forces the YSCC to move offshore across the Changjiang Bank. Lie et al. (2009) noted that, in winter, the cold coastal water around the Jiangsu coast extends southeastward onto the shallow Changjiang Bank in the form of a wide pocket. Furthermore, the trajectories of drifting floats confirmed the existence of a southeastward outflow along the 50-m isobath, which Lie et al. (2009) proposed is closely associated with the density front parallel to the 50-m isobaths on the Changjiang Bank. However, Shi and Wang (2010) argued that the sediment plume over the Changjiang Bank is attributable to sediment resuspension caused by the strong vertical mixing resulting from the shallow water depths, strong surface cooling and high wind speeds in winter. Furthermore, they denied the existence of cross-shelf currents based on altimeter observations of anomalies of the sea-surface height. Moreover, Shi et al. (2011) demonstrated that the sediment plume shows significant variability within the spring-neap cycle, and proposed that the tidal current, rather than the cross-shelf current, drives the sediment plume changes over the Changjiang Bank. Thus, the existence of cross-shelf currents remains controversial, so that the circulation in the western Yellow Sea needs to be further investigated.
We have identified a winter onshore warm tongue extending from the YSWC to the north of the Changjiang River estuary, and an offshore cold tongue extending from the north of the Changjiang River estuary to the southwest of Jeju-do, based on satellite remote-sensing data. This phenomenon seems to be inconsistent with the previous understanding that the YSCC flows from the northern Jiansu coast to the southwest of Jeju-do in winter.2 DATA AND METHODOLOGY
The sea-surface temperature (SST) data used here were derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra Satellite. The Level-3 Global Daily Mapped 4-km SST and night sea surface temperature products were adopted to generate the 5-day composite infrared images. The datasets are available from NASA's Ocean Biology Distributed Active Archive Center (OB.DAAC; http://oceandata.sci.gsfc.nasa.gov). The thermal fronts were calculated from the 5-day composite SST data using the oceanic front detection algorithm of Belkin and O'Reilly (2009). The truecolor satellite images shown here were obtained from NASA's Earth Observing System Data and Information System (EOSDIS).
The monthly climatology normalized water-leaving radiances at 670 nm recorded by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) were used to describe the horizontal distribution of suspended sediments in the Yellow Sea. While these have been widely used to study the concentration of suspended sediments in the upper ocean, they contain minimal information about the chlorophyll concentration. In contrast, the diffuse attenuation coefficient at the wavelength of 490 nm, which is derived from the MODOS-Aqua observations using a newly-developed algorithm (Wang et al., 2009) applicable for the coastal turbid waters, is also used to detect the ocean turbidity in the Yellow Sea. The datasets are available at https://oceandata.sci.gsfc.nasa.gov.
The sea-surface wind speeds are derived from the QuickSCAT data produced by the NASA Ocean Vector Winds Science Team, and are available at http://www.remss.com/missions/qscat. In addition, the National Center for Environmental Prediction (NCEP) Final Analysis Operational Global Analysis data is adopted because the mission of the QuickSCAT satellite ended in November 2009, with data accessible at dhttp://rda.ucar.edu/datasets/ds083.2/. The sea-surface wind speeds are gridded to a 0.5° resolution and averaged into 5-day and monthly climatology vectors.3 EVIDENCE OF THE ONSHORE WARM TONGUE AND OFFSHORE COLD TONGUE 3.1 Distribution of the sea-surface temperature
In the daily MODIS infrared images during winter since 2000, we find an onshore warm tongue extending from the YSWC to the southern Jiangsu coast, and an offshore cold tongue extending from the southern Jiangsu coast to the southwest of Jeju-do. In addition, the onshore warm tongue and offshore cold tongue also appear every winter in the National Oceanic and Atmospheric Administration (NOAA) Pathfinder SST data (http://data.nodc.noaa.gov/parhfinder).
Four 5-day composite infrared images for 21–25 February 2005, 26–30 January 2006, 24–28 January 2013 and 15–19 January 2015 (Fig. 2) are shown to illustrate our discussion of the onshore warm tongue and offshore cold tongue, as well as the northward intrusion of the TWC and the northwestward extension of the YSWC. In particular, a significant onshore warm tongue (at about 33.5°N, 123°E) extends from the axis of YSWC to the southern Jiangsu coast, crosses the 50-m isobath mainly through the section FG, and reaches as far as the 20-m isobath (see the area between sections BC and FG). The onshore warm tongue develops in a northeast-to-southwest direction, suggesting the existence of onshore currents. As seen from Fig. 2, the wind from the north is relatively strong (greater than 5 m/s) for 21–25 February 2005 (Fig. 2a), with a pronounced onshore warm tongue. In contrast, the wind direction for 24– 28 January 2013 (Fig. 2c) is from the northwest, and the onshore warm tongue is less distinct. Hence, the onshore warm tongue seems to be closely related to the wind direction from the north over the Yellow Sea. The winter distribution of warm-water zooplankton species, as shown in Wang and Zuo (2004, Fig. 3) and Lü et al. (2013, Fig. 4d), are consistent with the area of the onshore warm tongue. In addition, the SST distribution in Fig. 2 indicates that the cold coastal water around the southern Jiangsu coast spreads southeastward over the Changjiang Bank in the form of an offshore cold tongue, crossing the 20-m isobath mainly through section AB. The onshore warm tongue and offshore cold tongue form a double tongue-shaped structure in the form of the letter "S", which is clearly outlined by the isotherm of 7℃ in Fig. 2. This phenomenon suggests that the offshore currents in the western Yellow Sea originate from the Old Huanghe Delta. While one may argue that the offshore cold tongue is the consequence of rapid heat loss in the shallow Changjiang Bank in winter rather than cold water spreading by the offshore currents, this viewpoint is shown to be incorrect in Section 220.127.116.11 Thermal fronts
Figure 3 shows the thermal fronts derived from the MODIS SST data shown in Fig. 2, where five fronts can be clearly identified: (1) the western Jeju-do front; (2) the eastern Changjiang Bank front; (3) the Shandong Peninsula front; (4) the Subei (northern Jiangsu) Shoal front; and (5) the northern Changjiang River estuary front. The thermal fronts are temporally and spatially stable and similar to the front structure given by Hickox et al. (2000) and Wang et al. (2012). The oceanic fronts have a significant effect on the biogeochemical processes in the frontal region. Fisheries research shows that, in winter, anchovies mainly congregate in specific temperature regions around fronts (2) and (4) (Tang et al., 2005).
The eastern Changjiang Bank front (2) extends mainly along the 50-m isobath, which is induced by the YSWC and the offshore cold tongue on the Changjiang Bank. The Subei Shoal front (4) extends mainly along the 20-m isobath; Hickox et al. (2000) reported this thermal front as the Jiangsu Front and Wang et al. (2012) referred to it as the Subei Shoal front. Obviously, the Subei Shoal front is induced by the onshore warm tongue and the cold coastal water in the Jiangsu coast. As the northern Changjiang River estuary front is induced by the offshore cold tongue and the northward intrusion of the TWC, the Subei Shoal front and the northern Changjiang River estuary front (5) are evidence for the presence of the onshore warm tongue and offshore cold tongue. The narrow gap between the Subei Shoal front and the northern Changjiang River estuary front is regarded as the pathway for the offshore transport of cold coastal water.3.3 Concentration distribution of suspended sediments
The monthly mean diffuse attenuation coefficient at the wavelength of 490 nm from MODIS-Aqua observations in January 2015 and the true-color satellite images derived from EOSDIS for 15–19 January 2015 (Fig. 4) provide further visual evidence of the onshore warm tongue and offshore cold tongue. The suspended sediments around the Jiangsu coast extend offshore from the southern Jiangsu coast to the southwest of Jeju-do (as denoted by the blue arrows), which coincides with the extension of the offshore cold tongue discussed above. Consequently, both the extension of the turbid water plume and offshore cold tongue confirm that the offshore currents flow from the southern Jiangsu coast to the southwest of Jeju-do rather than from the Old Huanghe Delta. In addition, a front of suspended sediments can be clearly seen (as denoted by the red dash line) at about (34°N, 123°E), which is induced by the onshore warm tongue, as in the case of the Subei Shoal front in Fig. 3.
As the complete true-color satellite images are unavailable because of cloud cover, we use the monthly climatology averages of the 670-nm normalized waterleaving radiances from the SeaWiFs data to examine the horizontal distribution of the suspended sediments in the western Yellow Sea. Figures 5 and 6 reveal that a turbid plume is generated around the Old Huanghe Delta and spreads southeastward across the Changjiang Bank to the area southwest of Jeju-do during the winter. The turbid plume emerges in September when the monsoon brings northerly winds over the region, reaches its peak in February, and finally becomes coastally trapped in summer when a southerly wind direction prevails. The field investigations of the distributions of the seasonal suspended sediment concentration conducted by Bian et al. (2013) are consistent with the remote sensing observations discussed above. Others (e.g. Yuan et al., 2008; Bian et al., 2010, 2013) have proposed that the southeastward YSCC transports the Old Huanghe Delta sediments to the southwest of Jeju-do to form the mud patch, with part of the suspended sediments transported to the central Yellow Sea by the YSWC. The evolution of the turbid plume in Figs. 5 and 6 seems to be consistent with this viewpoint.
However, it is worth noting that the turbid plume crosses the 20-m isobath mainly through section AB, especially in January and February (Fig. 5), which coincides with the extension of the offshore cold tongue. In contrast, a front of suspended sediments can be observed along the 20-m isobath between points B and C (as in the case of the Subei Shoal front). The suspended sediments are rarely transported across section BC during winter. Consequently, the suspended sediment concentration in the area between sections BC and FG is relatively low, which coincides with the extension of the onshore warm tongue from the YSWC to the southern Jiangsu coast. It can be concluded that, in winter, the Old Huanghe Delta sediments are first transported southward to the north of the Changjiang River estuary by the coastal currents, and then a large portion of suspended sediments are transported offshore to the southwest of Jeju-do by the offshore currents. Bian et al. (2013) simulated the Chinese land-derived sediment transport in the Bohai Sea, Yellow Sea and East China Sea, and their simulation of the transport of sediments from the Old Huanghe Delta (Bian et al., 2013, Fig. 8) supports the conclusion discussed above. In summary, the onshore warm tongue acts as a barrier to the coastal cold water and suspended sediments, while the offshore cold tongue and extension of suspended sediments suggest that the offshore currents flow from the immediate north of the Changjiang River estuary to the southwest of Jeju-do. As this distinctive phenomenon is inconsistent with the previously accepted view that, in winter, the Yellow Sea Coastal Current flows from the Old Huanghe Delta to the southwest of Jeju-do, the circulation in the western Yellow Sea must be investigated further.4 CONCLUSION
The onshore warm tongue and offshore cold tongue in the western Yellow Sea in winter are identified on the basis of MODIS SST data and further confirmed by the true-color satellite images and the distribution of suspended sediments. The concurrence of the onshore warm tongue and offshore cold tongue in the western Yellow Sea in winter suggests that the Yellow Sea Coastal Current originates from the southern Jiangsu coast rather than from the Old Huanghe Delta, which contradicts the previous viewpoint that, in winter, the Yellow Sea Coastal Current flows from the Old Huanghe Delta to the southwest of Jeju-do. The onshore warm tongue and the cold coastal water around the Jiangsu coast form the Subei Shoal front, while the offshore cold tongue and the northward intrusion of the TWC form the northern Changjiang River estuary front. The narrow gap between the Subei Shoal front and the northern Changjiang River estuary front is supposed to be the pathway for the offshore transport of cold coastal water and suspended sediments. The onshore warm tongue and offshore cold tongue may have important impacts on crossshelf mass, nutrient and sediment exchange in the western Yellow Sea.
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