Does the Yellow Sea Warm Current really exist as a persistent mean flow?

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C10, PAGES 22,199-22,210, OCTOBER 15, 2001 Does the Yellow Sea Warm Current really exist as a persistent mean flow? Heung-Jae Lie, Cheol-Ho Cho, Jae-Hak Lee, and Seok Lee National Research Laboratory for Ocean Prediction of the Yellow and East China Seas Korea Ocean Research and Development Institute, Ansan, Korea Yuxiang Tang and Emei Zou Physical Oceanography Division, First Institute of Oceanography, State Oceanic Administration, Qindao, China Abstract. Does saline water of Kuroshio origin intrude into the Yellow Sea interior by a persistent mean current through the Yellow Sea Warm Current or by an intermittently generated wind-driven current? This very fundamental question is discussed by analyzing comprehensive conductivity-temperature-depth data collected over the entire southern Yellow Sea basin and by analyzing satellite-tracked drifter trajectories collected in 1996 and In the southwestern Yellow Sea, relatively saline water ( psu) persists year-round, implying a northwestward intrusion of Cheju Warm Current water (CWCW). This water is a mixture of saline CWCW, with salinity higher than 34.0 psu, and fresh Yellow Sea waters, with salinity <33.0 psu. During the winter monsoon the mixed water distribution is tongue-shaped, apparently intruding from the CWCW area. At the same time a strong thermohaline front with a horizontal form similar to the Greek letter F LtlIU forires acju LII DtJULII III lllldlli,., lot LIIG GGIILI I I 11L)W 0 Llough b l.)ltli:lll b L[l mixed water area from the CWCW. During the summer monsoon the CWCW intrusion is weak, and the mixed water that intruded in the previous winter monsoon remains inside the trough as its salinity decreases. In winter, occasional collapses of part of the front on the western side of the trough are observed, presumably in response to an imbalance between northwest currents generated by strong northerly wind bursts and southeast, tideinduced residual currents. Saline water in the frontal zone may then flush into the southwestern Yellow Sea through the collapsed area of the front. We conclude that the YSWC is not a persistent mean current. We also conclude that CWCW may be transported to the southwestern Yellow Sea in winter by the intermittently generated northwest currents on the western slope of the trough, which override the tide-induced residual currents that flow to the southeast in this part of the basin. 1. Introduction Since Uda [1934] the Yellow Sea Warm Current (YSWC) has been believed to be the only mean flow transporting saline water originating from the Kuroshio and oceanic materials of the East China Sea (ECS) through the Yellow Sea (YS). There are two theories on the origin of the YSWC. The first is that the YSWC separates from a northward Kuroshio branch current at the southeastern area of Cheju-do and flows into the southeastern YS [Nitani, 1972; Guan, 1994]. The second theory claims that the YSWC is a branch of a northeast continuation of the Taiwan Current at the southwestern end of Cheju-do [Beardsley et al., 1985]. In these schematic descriptions the Kuroshio branch current corresponds to a northward current branched from the Kuroshio southwest of Kyushu, while the Taiwan Current denotes the flow passing through the Taiwan Strait that continues to the northeast in the middle continental shelf where the water depth is m. The two theories commonly indicate that the YSWC transports warm, saline water to the southeastern YS. Existence of the YSWC as a mean current in winter has been Copyright 2001 by the American Geophysical Union. Paper number 2000JC /01/2000JC hypothesized using simple dynamical and numerical circulation models. In these studies the northerly wind of the winter monsoon generates an upwind current in the north-to-southoriented central trough of the YS [Park, 1986; Choi, 1982; Lee, 1996; Mask et al., 1998]. Recently, the inflow-outflow numerical model of Jacobs et al. [2000] suggested that both the Taiwan Current water and the Kuroshio branch current water are advected into the YS regardless of wind stress, although the saline water intrusion is modified by seasonal variations of the Taiwan Current and wind stress. Despite this modeling effort the existence of YSWC as a mean current has not been conclusively demonstrated from direct obse wations. After analyzing long-term hydrographic data in the southeastern YS, Lie [1984, 1985] reported that during the winter monsoon from late October to early April the water column is vertically well mixed and a strong thermohaline front forms in the southeastern YS. This front is generally oriented from west to east. Lie [1984, 1985] also claimed that any mean current, such as the YSWC if it exists, should flow along the front rather than across it. Hsueh [1988] col- lected wintertime current data north of the thermohaline front and pointed out that northward currents were intermittently generated by strong northerly winds. Recent measurements of currents in the middle part of the central trough were similar 22,199

2 32 22,200 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? Table 1. Korea-China Joint Surveys in the Yellow Sea During Periods of Research Vessels Number of CTD Seasons Surveys Name/Country Stations Type of CTD Unit Spring April 6-16, 1996 Autumn Oct. 7-21, 1996 Winter Feb , 1997 Summer July 12-18, 1997 Late autumn to Nov , 1997 early winter Late spring to May 12-22, 1998 early summer a Onnuri/Korea 70 Xingyanghong 9/China Haitian 18/China 79 Onnuri/Korea 72 Haitian 18/China 73 Onnuri/Korea 70 Xingyanghong 9/China 68 SeaBird 9/11 SeaBird 25 SeaBird 25 SeaBird 9/11 SeaBird 25 SeaBird 9/11 SeaBird 25 adata in May 1998 are not subjecto analysis because of the malfunction of CTD unit. to Hsueh's measurements [Teague and Jacobs, 2000]. However, Teague and Jacobs' moored current data do not show persistent northward flow in the eastern YS or the central trough that might correspond to the YSWC. A tongue-shaped warm water mass extending from the region west of Cheju-do toward the Shandung peninsula is, however, often detected in wintertime sea surface temperature data and satellite infrared images. The tongue-shaped pattern has been interpreted as proof of the existence of the YSWC [Asaoka and Moriyasu, 1966; Zheng and Klemas, 1982]. During the summer monsoon the water column is strongly stratified with a two-layered structure, and water with salinity S > 34.0 psu appears only in a narrow band close to the west coast of Cheju-do [Chen et al., 1994; Lie et al., 2000a]. Observations suggesthat the saline water near Cheju-do flows into the Cheju Strait after turning clockwise around Cheju-do rather than entering the YS [Lie, 1986; Kim et al., 1991; Lie et al., 1998; Chang et al., 2000]. The clockwise current around Cheju-do is named the Cheju Warm Current (CWC) [Lie et al., 1998]. Recently, Lie et al. [2000a] confirmed the year-round existence of the CWC by analyzing seasonally collected conductivity-temperature-depth (CTD) data and drifter trajectories in the northern ECS. This current transports saline water that originates from the Kuroshio. However, they did not find evidence that the CWC enters the YS interior. The following fundamental questions may be raised: Is the YSWC really a persistent mean current? Alternatively, is it an intermittently generated upwind current as is the case in the eastern YS observed by Hsueh [1988]? If northward or northwestward mean flows do not exist, what dynamics are involved in blocking the continuous advection of saline water from the CWC area by prevailing northerly winds in winter, as expected from various circulation models? These questions are critical to understanding the YS circulation and water mass transport. We attempt to clarify these questions by analyzing CTD data collected over the entire southern YS basin by a joint Korean- Chinese study of the YS circulation and material flux from 1996 to We augment these results with an analysis of trajectories from drifters deployed during the CTD surveys. and pathway of the YSWC, from climatological mean fields and from territorial data is difficult. This task is particularly challenging because the shallow YS with a mean depth of 44 m respond strongly to changes of atmospheric forcing and experiences strong tidal motions. To understand better the YS circulation and material flux, the Korea Ocean Research and Development Institute, Korea, and the First Institute of Oceanography, China, jointly conducted intensive basin-scale CTD surveys on six occasions over the period Two of the principal objectives of this study were to examine the intrusion of saline water from the CWC area into the YS and to trace the main pathway of the YSWC, if it exists. Table 1 presents a brief summary of the joint surveys. CTD casts were made during each survey at about 70 stations comprising six survey lines crossing the YS trough from west to east (Figure 1). The northernmost survey line was located at 37øN and the southernmost lay between the Changjiang estuary and Chejudo, corresponding to the geographic boundary between the YS and ECS. CTD surveys in February and November 1997 were.. o... o ø 2. Data Collection and Treatment 119 ø 121ø ø 125 ø 127 ø E 30ø Most previous oceanographic surveys of the YS were limited Figure 1. The study area showing the location of CTD to the jurisdictional waters of coastal states. Past studies of the stations (diamonds). CTD measurements were made on six YS circulation and hydrography were primarily based on data occasions through a joint Korean-Chinese study on circulation collected during short cruises or on the historical hydrographic and material flux in the Yellow Sea from 1996 to Bottom observations. Deducing physical processes, such as the origin topography is in meters.

3 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 22,201 Table 2. Summary of Drifter Experiments Performed in the Yellow Sea a Float Release Position Mean and rms Currents, cm s - Number/Identification Drogue Duration, Number Depth, m Release Date Latitude, øn Longitude, øe (u u' ( v} v' day 01/27260 b 15 April 8, / April 9, / April 11, / April 13, / Feb. 21, / Feb. 22, /28693 b 15 Feb. 22, /28704 b 30 Feb. 22, / Feb. 22, /27520 b 0.8 Feb. 24, /28708 b 45 Feb. 24, / Feb. 25, /29573 b 15 July 12, / July 12, / July 12, / July 12, / July 16, / July 19, / July 16, / July 19, / July 20, /15361 b 15 Nov. 19, / Nov. 19, / Nov. 21, / Nov. 21, / Nov. 23, /15367 b 15 Nov. 27, amean and root mean square currents are estimated from drifter trajectories during the first 15 days after release. blifetime is < 15 days. completed within 7 days yielding quasi-synoptic snapshots of hydrographic distributions. Synopticity is important because strong northerly wind bursts during the winter monsoon can cause sudden and dramatic changes in the hydrography of the southern YS. Temperature and conuucuwty 5UOS lll191ul1 every 1 m by averaging all data within 20 cm of each computation depth after removing data spikes. Data collected in May 1998 were not subjected to analysis because of malfunction of the CTD underwater unit. Satellite-tracked drifters are a cost effective method to mea- sure currents in the shallow YS, an area of intense fishing activity. We deployed 25 World Ocean Circulation Experiment (WOCE)-type drifters during four of the CTD surveys. Four drifters were deployed in April 1996, six were deployed in February 1997, nine were deployed in July 1997, and six were deployed in November The WOCE standard drifter is equipped with a 644 cm long holey sock drogue centered 15 m below the sea surface. The drifter is known to follow effectively the motion of the water column at its drogue depth; slippage is about 1 cm s - in wind of 10 m s - [Niiler et al., 1995]. Most drifters deployed in the YS had a drogue at 15 m, but some were drogued at 30 or 45 m to measure currents in the lower layer. To trace near-surface motion in February 1997, we deployed two additional surface drifters having a 1 m by 1 m cross-window-shapedrogue, centered 0.8 m below the sea surface (model: ClearSat-1 of ClearWater Instrumentation, Inc). All drifters were set to transmit signals continuously for the first 30 days after release. Thereafter the instruments transmitted 8 hours a day for the remainder of their missions. On average, position fixes were acquired six times a day during the continuous transmission periods. The position fixes were linearly interpolated to hourly values. Daily mean positions were then computed using a 25 hour moving average to remove dominant diurnal and semidiurnal tidal currents. Daily mean current vectors were computed using the detided daily mean positions. A summary of the drifter observations is presented 3. Spatial Distribution of Saline Water During the winter monsoon the water column in the YS is vertically homogeneous, with temperature T and salinity S generally decreasing from south to the north. During the summer monsoon, strong stratification is established, with a seasonal pycnocline formed at depths of m in the central basin where the water depth is >40 m. T and S in the layer below the summer pycnocline vary in the same pattern as in winter. In the surface layer, T and S are spatially uniform, except in the shallow coastal area where tidal mixing and river discharge are important. As can be seen in Figures 2-6, relatively high salinity water with S > 33.5 psu always appears in the area west of Cheju-do, regardless of season. Lie et al. [2000a] classified this saline water into two subtypes: Cheju Warm Current water (CWCW) with S > 34.0 psu and modified CWCW with 33.5 < S < 34.0 psu. The salinities of the CWCW and modified CWCW distinguish these water masses from other water masses in the northwestern ECS. If the YSWC transports these waters to the YS, the intrusion route may be readily traced on the basis of its high salinity. Waters with S > 33.0 psu are always found in the YS below 40 m depth, even in summer (see section 4.2). To describe the seasonal evolution of the distribution of intruded water, we map T and S at 50 m with the objective of avoiding the strong seasonality of the surface waters. In Feb-

4 22,202 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 122 ø 124 ø 126øE Figure 2. Temperature, salinity, and o- t at a 50 m depth in February 1997 with daily mean currents at 5 day intervals. The current vectors are derived from trajectories of satellite-trackedrifters. The pluses and crosses indicate release points of drifters with near-surface drogues ( m) and deep drogues (30-45 m), respectively. Numbers near the drifter release points in the density distribution indicate float number. ruary, isotherms of T > 11.0øC and isohalines of S > 33.8 psu surround Cheju-do rather than extending to the YS interior (Figure 2). The high T and S of the CWCW and its modified water and the low T and S of the YS coastal water form a pronounced thermohaline front northwest of Cheju-do as observed by Lie [1985]. The front extends across the southern entrance of the YS central trough. In the YS interior the isotherm and isohaline patterns are similar. Relatively warm and salty water with 8.0 ø < T < 11.0øC and 33.0 < S < 33.8 psu appears in a tongue shape on the western side of the central trough. This can be compared with the coastal water and the YS cold water in the YS interior, which have salinities <33.0 psu [Lie, 1985] (i.e., 1.0 psu fresher than the CWCW). The tongue-shaped water mass is a mixture of the CWCW and the YS coastal and cold waters. This mixed water extends to 35ø40'N, but the thermohaline front northwest of Cheju-do separates this mixed water from the CWCW in the southeastern part of the basin. The distribution of o't differs from the distributions of T and S because the tongue-shaped mixed water is denser than the Chinese coastal water. The dense water with o't > 25.9 is located to the north of the thermohaline front, with the densest water observed off the southwest tip of Korea. In April the general distributions of T and S are similar to those observed in February (Figure 3). The thermohaline front is located farther to the north, and the tongueshaped region of mixed water extends to 36ø20'N. In April the tongue-shaped region of mixed water is denser due to a slight decrease of T and an increase of S. The heaviest water with o- t > 26.2 occurs in a tongue-shaped region in the southwest- ern YS. The tongue-shaped pattern visible in February and April disappears from the surface layer by early May but persists in the lower layer, although its geographical area is greatly reduced [Korea Ocean Research and Development Institute, 1987]. In addition, the intruded mixed water in the lower layer loses contact with the CWCW to the southeast. In July the isohaline pattern is no longer tongue-shaped but elliptical, with the major axis oriented south to north (Figure 4). This water is completely isolated from the cold, fresh water to the east and the CWCW to the southeast. The mixed water is also displaced farther to the northeast, relative to its position earlier in the year. The displacement may be associated with freshwater input to the Chinese coastal area and with the prevailing southerly wind during the summer monsoon. Abundant freshwater input to the western Chinese coastal area including the Changjiang River estuary induces a west-to-east pressure gradient, possibly forcing the isolated mixed water to the east. The prevailing winds change direction from northerly in the winter monsoon to southerly in the summer monsoon [Han et al., 1995]. Since the southerly wind generates eastward Ekman transport, it may play a major role in displacing the mixed water to the east [Jacobs et al., 2000]. In October the elliptical pattern of isolated mixed water moves farther to the east, with its major axis located near 124ø15'E in the central trough (Figure 5). In mid-november the mixed water maintains an elliptical pattern but is displaced to the western side of the trough, although it occupies a smaller area (Figure 6). The westward migration in November may be associated with a great reduction in the freshwater input and with the wind change from the southerly to the northerly. A conclusion that can be drawn from the five comprehensive CTD surveys is that the CWCW west of Cheju-do does not intrude directly into the YS, at least not during the summer monsoon. The mixed water with 33.0 < S < 34.0 psu is distributed in a tongue shape on the western side of the central trough during the winter monsoon, whereas during the summer monsoon it is completely isolated from the fresh coastal water and the CWCW west of Cheju-do. The mixed water is the densest water type in the YS and is located north of the thermohaline front west of Cheju-do in the winter monsoon. If

5 ,..., LIE ET AL.' IS YELLOW SEA WARM CURRENT A MEAN FLOW? 22, :>2?'- 32 ø Sat,(p su) * 126' E 8igma-t 12W E Figure 3. Temperature, salinity, and rr t at a 50 m depth in April 1996 with daily mean currents at 5 day intervals. The current vectors are derived from trajectories of satellite-trackedrifters. The pluses and crosses indicate release points of drifters with near-surface drogue (15 m) and deep drogues (40-45 m), respectively. Numbers near the drifter release points in the density distribution indicate float number. the CWCW were continuously advected to the northwest toward the Shandung peninsula, this would prevent formation of the strong thermohaline front across the southern entrance of the trough. The seasonal migration of the mixed water from the west in the winter monsoon to the east in the summer monsoon is also an important feature newly revealed by the CTD surveys. 4. Seasonal Evolution of Intruded Mixed Water In this section we discuss in more detail seasonal changes in the location and circulation of the mixed water, its vertical structure, and its volume by analyzing CTD data and drifter data. These results are based on analysis of the CTD data and drifter tracks. We believe that the CTD data used in this study YS9707 o 36 ø ø.. Sal.(.ps U) Sigma-t ø 126' E 124 ø 126 ø E Figure 4. Temperature, salinity, and o- t at a 50 m depth in July 1997 with daily mean currents at 5 days intervals. The current vectors are derived from trajectories of satellite-trackedrifters. The pluses and crosses indicate release points of drifters with near-surface drogue (15 m) and deep drogue (45 m), respectively. Numbers near the drifter release points in the density distribution indicate float number.

6 22,204 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 122' 124' 126øE Figure 5. Temperature, salinity, and % at a 50 m depth in October represent typical seasonal variations since these variations are significantly larger than the interannual variability in the YS formed during a 19 month period; hence our CTD observations were not overly aliased by interannual change. [Lie et al., 1986]. Interannual anomalies of T and S associated with E1 Nino events in 1996 and 1997 were evaluated using 4.1. Seasonal Displacement bimonthly time series data collected over the southern YS during by the Korea National Fisheries Research and Development Institute. These data suggesthat T and S in In order to trace the seasonal location of the mixed water we draw a line connecting the zonal salinity maxima inside the tongue- or ellipse-shaped mixed water at 50 m depth in each of the YS were not strongly affected by interannual variations our sections. This line is referred to as the central axis of the during our study period. Moreover, the five surveys were per- intruded mixed water. Figure 7 presents the seasonal position 3.8" N Figure 6. Temperature, salinity, and % at a 50 m depth in November 1997 with daily mean currents at 5 days interval. The current vectors are derived from trajectories of satellite-tracke drifters. The pluses and crosses indicate release points of drifters with near-surface drogue (15 m) and deep drogue (45 m), respectively. Numbers near the drifter release points in the density distribution indicate float number.

7 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 22, o APR JUL... O eoct '" ' i?nov FEB-'--> 3 8 ø 36 ø 34 ø : ø 124 ø 126 ø E Figure 7. Position of the central axis of the mixed water at 50 m depth as defined by the location of the zonal maximum salinity. Open circles and diamonds indicate the northern limit of the mixed water distribution, where salinity is 33.0 psu, and the location of the highest salinity in the mixed water, respectively. Solid circles show the northwestern limits of the CWCW in February and April (i.e., salinity of 34.0 psu). The vertical distribution of the mixed water changes seasonally in close connection with the west-to-east migration and the seasonal air-sea interaction. Since the midpoint of the central axis is located near 35øN (see Figure 7), we present the seasonal change in salinity along this section. In February the salinity distribution is vertically homogeneous everywhere. With the exception of a narrow band around 124øE the intruded mixed water appears in the central trough to the east of 122ø15'E. A strong salinity front occurs near 122øE near the western boundary of the mixed water, and a local salinity maximum of about 33.8 psu is located at 123øE. In April the water column is weakly stratified, and isohalines near 122øE slope up to the surface in the offshore direction. In July and October the water column is strongly stratified with a halocline near 20 m in July and 30 m in October. The mixed water is confined to the lower layer. In November the surface mixed layer deepens to 40 m, and the vertical extent of the mixed water is reduced. In February and April a salinity maximum is located between 122 ø and 123øE, while in July, October, and November, it moves offshore to between 123 ø and 124øE. The salinity maximum is located farthest to the east in October, when fresh water of S < 32.0 psu occupies the Chinese coastal area. In November the northerly wind begins to blow, and freshwater input is significantly reduced. As a result, the mixed water returns to its wintertime position. This can be compared with conditions during July and October, when southerly winds prevail and freshwater input into the Chinese coastal area is abundant. These data suggest that the seasonal migration may be closely associated with the seasonal change of both the freshwater input and the wind field. It is particularly noteworthy that the mixed water does not penetrate the Chinese coastal area in regions shallower than 50 m. of the central axis at the 50 m depth. Open circles and diamond shapes indicate the northern limit of the mixed water where S is equal to 33.0 psu and the local salinity maxima within the mixed water. Solid circles correspond to the northwestern limits of the CWCW in February and April where S is equal to 34.0 psu. As mentioned earlier, we assume that year-to-year variations are negligibly small as compared to seasonal varia- tions. In February the western limit of the saline CWCW is located at 124ø36'E. The central axis of the mixed water is oriented to the northwest in the southern area around 34%0'N and to the north along 123øE in the northern area. Two local salinity maxima appear along the central axis (see Figure 2). In April the CWCW advances farther to the northwest to 123ø17'E, 34ø05'N, and the northern end of the mixed water is displaced to the north by about 100 km relative to its location in February. The central axis is shifted about 15 km to the west. In July the central axis is shifted eastward to 123ø12'E at 34ø-35øN and to 123ø30'E around 36øN. In October the axis is displaced farther to the east of 124øE and oriented north to south. In 4.2. Circulation of the Mixed Water Circulation of the mixed water is examined by analyzing trajectories of satellite-tracke drifters deployed in the CWCW and its mixed water. Trajectories from the beginning of each drifter record, up to 15 days, are plotted with arrows at 5 day intervals in Figures 2, 3, 4, and 6. Pluses and crosses denote release points of shallow surface drifters having drogues at either 0.8 or 15 m and those of deep drifters having drogues at 30, 40, or 45 m, respectively. Eight of 27 drifters survived for <15 days. Table 2 presents a summary of the drifter experiments and includes release points and mean and root mean square currents for up to the first 15 days of each record. In February 1997 (Figure 2) a surface drifter (float 12) in the CWCW close to the west coast of Cheju-do turned clockwise around Cheju-do at a speed of 13.8 cm s - and accelerated rapidly in the Cheju Strait. Surface and deep drifters (floats 10 and 11) in the thermohaline frontal zone west of Cheju-do also moved clockwise along the front at about 4 cm s-. These three drifters do not suggest a northwest intrusion of the CWCW into the YS. At the same time, at about 35øN, two surface drifters (floats 7 and 5) in the western and eastern sides of the November the mixed water retreats to the west. At its center at trough moved to the west and southwest at 1.3 and 4.0 cm s-, about 35øN it is shifted to its winter position near 123ø20'E. The salinity maximum in November is 33.5 psu, the lowest of respectively, approximately along surface isolines. In addition, one surface and two deep drifters in the central trough moved the five cruises. The central axis therefore migrates seasonally to the south at speeds between 1.1 and 2.4 cm s-. from west to east over a distance of about 100 km in the In April 1996 (Figure 3), four drifters were deployed, two in bathymetric trough north of 34øN. The eastward migration begins in late spring when the northerly wind weakens, while the westward migration begins in autumn at the onset of northerly winds. the CWCW and two in the mixed water. Although a surface drifter (float 1) close to Cheju-do moved to the southwest for its short lifetime of 5.5 days, a deep drifter (float 2) in the frontal zone west of Cheju-do followed the front at a mean

8 22,206 LIE ET AL.' IS YELLOW SEA WARM CURRENT A MEAN FLOW? : 5o 70 N...?F." t 120! Longitude Figure 8. Seasonal patterns of salinity along a section at 35øN (line C in Figure 1). Tick marks at the sea surface show CTD stations. Shaded areas indicate the region where salinity is higher than 33.0 psu. 70 surface circulation computed from a diagnostic model [Yanagi and Takahashi, 1993] that takes into account a dome-like density structure in the YS. In November 1997, when the pycnocline was located near 40 m, surface drifters were deployed in the elliptically shaped region of mixed water (Figure 6). Two surface drifters (floats 26 and 27) at 35øN moved to the southwest at a speed of 3.0 cm s - and to the west at a faster speed of 10.8 cm s -, respectively, over a period of 4 days. On the other hand, a drifter (float 24) at 34øN moved to the east and then turned toward the south, while another drifter (float 25), deployed to the west of float 24, moved to the east at a mean speed of 3.1 cm s -. The trajectories of these four drifters may indicate the existence of a cyclonic circulation in the surface layer of the YS interior at this time. The current measurements from 27 drifters provide two significant clues as to the circulation of the YS mixed water and the CWCW. First, the mean currents in and around the in- --1 truded mixed water are very weak, on the order of a few cm s and smaller than the root mean square (rms) currents. These estimates of mean and rms currents from drifters deployed in the central trough are comparable to estimates from moored current data [Hsueh, 1988; Lie, 1999; Teague and Jacobs, 2000]. The mean and rms currents are weak even for winter. This observation result is different from an anticipation that the prevailing northerly winds in winter may generate stronger currents in the YS interior. The second point is that drifters west of Cheju-do turn clockwise around the island. In particular, in the winter, drifters moved to the northeast approximately along the thermohaline front located there. This clockwise CWC is revealed more clearly in a composite map of drifter trajectories in Figure 9. Thus the drifter motion in west of Cheju-do suggests that the CWCW flows into the Cheju Strait rather than intruding in the YS, a point discussed in detail by Lie et al. [2000a]. KORDI speed of 3.4 cm s-, as in February. A deep drifter (float 4) at 35øN, in the northern part of the tongue-shaped mixed water, moved to the east at a speed of 4.1 cm s -. Meanwhile, a surface drifter (float 3) in the middle of the mixed water at 34øN moved to the west at 1.8 cm s-. In July 1997, when the mixed water on the western trough was completely isolated from the CWCW to the south, we deployed nine drifters in the central YS area: seven in the surface layer and two in the lower layer (Figure 4). Two of the surface drifters (floats 14 and 18), deployed near 35 ø and 36øN in the central trough, moved to the southeast at speeds of 4.2 and 2.3 cm s-. However, the two deep drifters (floats 15 and 20), deployed near the same release points, remained near these sites for 20 days, resulting mean speeds of <1 cm s-. On the other hand, a surface drifter (float 21) at 124øE moved to the northwest at about 3.1 cm s -, and three surface drifters (floats 17, 19, and 16) on the western side of the trough moved to the southeast at speeds of cm s-. The motion of the surface drifters in the surface layer above the mixed water may indicate the existence of a cyclonic surface circulation around the dome-like mixed water in the lower layer (see Figure 8). This inference of cyclonic circulation is consistent with the drogue depth...-{i:' 0-15m m 20 cmos 32 ø 124 ø I t28 ø 130øE Figure 9. A composite map of trajectories of 57 satellitetracked drifters deployed in saline water of S > 33.5 psu from 1991 to 1998 [after Lie et al., 2000a]. Arrows denote daily mean velocity vectors. Thin and thick lines indicate surface drifters with drogues at m and deep drifters with drogues at m, respectively.

9 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 22,207 Table 3. Volume Ratio of Each Water Type, Classified by Salinity, Relative to the Total Water Volume of the CTD Survey Area, Volume Ratio, % Mean Salinity, Month/year <30 psu psu psu psu psu psu psu Feb April July Oct Nov Volume of the Mixed Water mixed water in the winter and spring reflects a salinity source In this section we examine seasonal changes in the volume of somewhere from the CWCW area west of Cheju-do. The the mixed water and its mean salinity. This provides informa- tongue-shaped mixed water in the southwestern trough must result from a northwest intrusion of the CWCW from the west tion about the strength of the intrusion of saline water in the YS. The CTD survey area is 1.62 x 10 TM m 2, and its mean of Cheju-do. How can we explain the northwest intrusion in depth is 61.5 m, leading to a total water volume surveyed of the presence of the strong front across the southern entrance? 9.96 x 10 TM m 3. After interpolating the observed salinity data During the winter monsoon the isopycnals of the mixed onto 0.1 ø by 0.1 ø grids we compute the volume of each water water in the winter monsoon (Figures 2 and 3) have a different type by classification based on salinity (Table 3). The percent- pattern from the isotherms and isohalines. High-density mixed age of mixed water, with 33.0 < S < 34.0 psu, reaches its water is located just to the north of the thermohaline front and maximum of 46.46% in April and its minimum of 23.43% in does not extend to the CWCW area. Such a density distribu- November. The percentage of CWCW is a maximum of tion might produce a cyclonic geostrophic circulation around 15.04% in April and a minimum of 0.09% in October. The the heavy mixed water. Drifters in the thermohaline frontal CWCW in July, October, and November does not exceed 1% zone and the CWCW moved into the Cheju Strait along the of the survey volume. This small ratio of CWCW may result from the southeast migration of the CWCW to Cheju-do during the summer monsoon [Lie et al., 2000a]. The mean salinity of the total survey volume shows a large seasonal change of about 1.0 psu, with a maximum of psu in April and a minimum of psu in October. Despite this large seasonal change the maximum salinity of the mixed water has a smaller seasonal variation. For example, at 35øN the seasonal variation in maximum S is about 0.4 psu (Figure 8), although the volume of the mixed water in autumn is reduced to about half its April value. The small change in maximum salinity may reflect the fact that the mixed water in the YS trough keeps its original salinity characteristics, despite the abundant fresh water introduced into the surface layer during the summer monsoon. From the annual change in salinity and the small ratio of the CWCW in summer and fall we may conclude that in these seasons, salt is not supplied from the front rather than into the southwestern YS. Although daily mean currents from the drifter trajectories are unexpectedly weak, the direction of the current appears to be in good agreement with a cyclonic circulation in the southern YS in winter and early spring (with an exception of one drifter deployed at 35øN in April). The cyclonic circulation does not, however, explain the tongue-shaped saline water in the southwestern YS since salinity in the southeastern YS north of the front is lower than in the tongue-shaped area. Northerly winds prevailing during the winter monsoon may generate a northwest upwind current on the western side of the trough and an anticyclonic circulation in the eastern YS between this upwind flow and the western coast of Korea [e.g., Choi, 1982; Lee, 1996; Mask et al., 1998]. Under the southerly winds prevailing during the summer monsoon a circulation pattern opposite to that in winter would be expected. However, the inflow-outflow model of Jacobs et al. [2000] suggests the CWCW region through the YS interior. existence of a northward mean current in the YS central trough throughout the year, although its magnitude is pre- 5. Discussion dicted to vary seasonally with the monsoon. However, the drifter data in section 4.2 and moored current meter data According to the climatological monthly mean salinity [e.g., China Ocean Press, 1992] the tongue-shaped distribution of high-salinity water is found in the southern YS throughout the year. However, our CTD data show that the tongue shape is developed during the winter monsoon but not during the summer monsoon. During the winter monsoon a pronounced thermohaline front in the form of the Greek letter F is formed approximately along the outer boundary of the CWCW region west of Cheju-do (see Figures 2 and 3). In addition, the horizontal gradient of temperature and salinity across the F-shaped thermohaline front is much steeper in our survey observations than in the monthly mean distributions. The existence of such a strong thermohaline front across the southern entrance of the YS trough implies a mean current flowing along the front. However, an increase in both mean salinity and the volume of [Hsueh, 1988; Lie, 1999; Teague and Jacobs, 2000] do not show persistent northward or northwestward mean flows in winter. Other dynamics must be counteracting the northwest inflow. A possible candidate is the tide-induced residual current generated by nonlinear interaction of tidal currents with sloping topography. The tidal current in the southern YS is of magnitude of cm s - [Lie et al., 2000b]. This is 1-2 orders of magnitude stronger than low-frequency residual currents in winter, which have typical magnitudes of 0-5 cm s- outside of the CWC area. In the boundary zone between the YS and the ECS, well-developed banks are located on the Chinese coastal side, and the YS trough becomes narrower. The tidal current in the boundary region is therefore much stronger than in the interior of the YS, and relatively strong tide-induced residual currents are generated along the western slope of the trough,

10 22,208 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? 32" began at 10:00 on March 29, but the northwest intrusion was not observed until 05:17 on March 30. The northwest intrusion therefore lagged the northerly wind. Similar situations have been observed previously during intermittent events, which generated upwind currents in the central trough north of the front [Hsueh, 1988; Teague and Jacobs, 2000]. In addition to the above explanation, other processes may also be at work. The strong northerly winds will push waters southward, increasing heat loss to the atmosphere, enhancing lateral mixing in the thermohaline frontal zone, and causing a deterioration of the surface signature in the frontal zone. Sea level rise against the Chinese coast in the shallow Changjiang Banks will induce a southward coastal current that may be reinforced by the southeastide-induced current. Appearance of a cold water tongue just west of the warm CWCW (see Plate 1) may be a result of the southeast movement of cold water. 122 ø 124 ø 126' 128øE Figure 10. Schematic regional circulation pattern at the southern entrance of the YS trough in the winter monsoon. CWC, WDC, and TIC denote the Cheju Warm Current, the northwest wind-driven current, and the southeast tide-induced residual currents. The CWC rounds Cheju-do clockwise yearround. A F-shaped strong thermohaline front is formed across the southern entrance of the trough. Both the WDC and TIC are located on the western slope, but they flow in opposite directions. In cases where the WDC prevails over the TIC the front collapses somewhere in its northwestern corner, and saline water in the frontal zone flushes into the southwestern YS. which is characterized by steep and rugged bottom topography. According to tide models [Choi, 1984; Lee and Beardsley, 1999] the residual current induced by the M 2 tide is around 3 cm s- and is directed to the southeast. On the other hand, circulation models predict a northwest upwind mean current of the same magnitude on the western side of the trough but in the opposite direction of the tideinduced residual currents shown in Figure 10. On average, the northwest mean current generated by the prevailing northerly wind might be balanced by the southeastide-induced residual current. The tide-induced current transports cold, fresh water from the Chinese coastal area to the southeast, while the winddriven current transports warm, saline water from the CWC area to the northeast. A F-shaped thermohaline front (see Plate I and Figures 2 and 3) corresponds to the boundary between these two water masses. Its formation may be a good evidence for a balance between the two currents flowing in opposite directions. Moored current meter data [Hsueh, 1988; Lie, 1999; Teague and Jacobs, 2000] show that northward currents sometimes reach 10 cm s- in the central trough to the north of the thermohaline front where tide-induced residual currents are relatively weak. At times when the northwest inflow in the southern entrance prevails over the tide-induced residual current the thermohaline front on the western slope may partly collapse, and the saline water in the frontal zone may flush into the southwestern YS through the collapsed front. Infrared images in Plate 1 show that this type of collapse can occur at the northwestern corner of the front after strong northerly wind bursts. It is also interesting to see a time lag between the intrusion and the northerly wind from images taken at 05:17 on March 30 and 05:06 on March 31, 1997 (Plates le and If). A strong northerly wind exceeding 10 m s - 6. Summary and Conclusions The fundamental question as to whether the YSWC exists as a mean flow has been examined by analyzing a very comprehensive CTD data set collected over the entire southern YS and by analyzing a more geographically limited set of concurrent drifter trajectories. Year-round persistence of mixed water with S = psu on the western side of the YS central trough is indicative of an intrusion of Kuroshio- originated water into the southwestern YS. During the winter monsoon the mixed water distribution is tongue-shaped and apparently separated from the CWCW west of Cheju-do. During the summer monsoon the mixed water in the lower layer is completely isolated from the surrounding waters, with a local salinity maximum somewhere within 34ø-35øN, 123ø-124ø30'E. The elliptical distribution in summer is good evidence for the absence of a CWCW intrusion west of Cheju-do in the YS interior during the summer monsoon. The central axis of the mixed water is located almost parallel to 123øE on the western side of the YS trough in winter and spring, and it migrates to the east beginning in late spring. The west-to-east migration spans a distance of about 100 km in the central YS north of 34øN, possibly in close association with the seasonal changes of surface wind and freshwater input. During the summer monsoon the weak but prevailing southerly wind and an increase of the pressure force imposed by the abundant freshwater input to the Chinese coastal area may push the mixed water offshore. The westward migration takes place during the winter monsoon, mainly because of the northerly wind. During the winter monsoon the central axis of the intruded mixed water is oriented in the southeast-northwest di- rection on the western flank of the trough, suggesting that the intrusion of saline water, if it exists, may occur primarily along this central axis. The CTD surveys and drifter experiments do not supporthe hypothesis that the YSWC is a mean current flowing persistently in the YS. The CWCW west of Cheju-do clearly does not intrude into the southwestern YS during the summer monsoon. The mixed water below the summer pycnocline retains its salinity characteristics formed in the previous winter. The increase in salinity and volume of the mixed water during the winter monsoon implies the salt supply is from the CWCW area. However, the salt supply to the YS is not necessarily transported by a mean flow. The northwest advancement of the CWCW from near Cheju-do in the winter monsoon brings much salt to the southern YS, but it does not explain satisfac-

11 LIE ET AL.' IS YELLOW SEA WARM CURRENT A MEAN FLOW? 22,209 NOAA-14 97/01/I2 EORD! ANSAN, OBEA Sea $urtace?empeta ure (;) MOAA-12 97/02'18 22:57 KOR I, ANSAN KOREA. c Sea Surface Teml:.rat. ure (c) HOAA-14 9?/03/03 17:$5 KORDI, ANSAN, F.9rtgA Wind at Heuksan-do 10m/s 1/Jan l/feb /Mar l/apr Date (1997) Plate 1. Sea surface temperature derived from National Oceanic and Atmospheric Administration advanced very high resolution radiometer infrared images on (a) January 12, (b) January 20, (c) February 18, (d) March 3, (e) March 30, and (f) March 31, Low-passed time series of winds observed at an island of Heuksan-do (see location in Figure 1) during January 1 to April 1, 1997, are plotted on the lower panel. The wind direction is conventional and small letters a-f indicate the dates taken for images. A tongue-shaped thermal front in the form of the Greek letter F is formed in the YS and ECS boundary zone. On February 18, March 3, and March 31 the northwestern corner of the front intermittently collapsed by strong northerly wind bursts, and warm water n the frontal zone flushed into the southwestern YS through the collapsed front. The series of images also show a southeast migration of the front in late March.

12 22,210 LIE ET AL.: IS YELLOW SEA WARM CURRENT A MEAN FLOW? torily the presence of mixed water on the western side of the Seas response to winds and currents, J. Geophys. Res., 105, 21,947-21,968, trough. The existence of the tongue-shaped saline water area Kim, K., H. G. Rho, and S. H. Lee, Water masses and circulation implies a salt supply somewhere from within the F-shaped, around Cheju-do in summer, J. Oceanol. Soc. Korea, 26, , strong thermohaline frontal zone. In other words, the front must collapse during certain periods, allowing the saline water Korea Ocean Research and Development Institute, Oceanographic to intrude into the southwestern YS. From visual inspection of Atlas of Korean Waters, vol. 1, Yellow Sea, 147 pp., Seoul, Lee, H.-C., A numerical simulation for the water masses and circula- National Oceanic and Atmospheric Administration infrared tions of the Yellow Sea and the East China Sea, Ph.D. thesis, images between 1996 and 1999 we find that the collapse occurs Kyushu Univ., Fukuoka, Japan, mainly in the northwest corner of the front as seen in Plate 1. Lee, S.-H., and R. C. Beardsley, Influence of stratification on residual This location is on the western flank of the southern trough tidal currents in the Yellow Sea, J. Geophys. Res., 104, 15, , 701, entrance. Tide-induced residual currents with magnitudes of -3 cm s -1 are directed to the southeast on the western flank. Lie, H.-J., A note on water masses and general circulation in the Yellow Sea (Hwanghae), J. Oceanol. Soc. Korea, 19, , This flow opposes the northwest wind-driven current on the Lie, H.-J., Wintertime temperature-salinity characteristics in the western flank. As a consequence, the frontal collapse may southeastern Hwanghae (Yellow Sea), J. Oceanogr. Soc. Jpn., 41, occur only when intermittently generated northwest currents , overcome the southeast flow of the tide-induced current. Lie, H.-J., Summertime hydrographic features in the southeastern Hwanghae, Prog. Oceanogr., 17, , Lie, H.-J., On the Huanghai (Yellow) Sea circulation: A review by current measurements, Acta Oceanol. Sinica, 18, , Acknowledgments. The data used for this study were collected Lie, H.-J., I. K. Bang, and Y. Q. Kang, Empirical orthogonal function through the Korea-China Joint Study on the YS circulation and maanalysis of seawater temperature in the southeastern Hwanghae, J. terial flux in funded by the Ministry of Science and Tech- Oceanol. Soc. Korea, 21, , nology, Korea, and the State Oceanic Administration, China. The Lie, H.-J., C.-H. Cho, J.-H. Lee, P. Niiler, and J.-H. Hu, Separation of authors appreciate the efforts of the staff and crew of the Korea Ocean the Kuroshio and its penetration onto the continental shelf west of Research and Development Institute and the First Institute of Ocean- Kyushu, J. Geophys. Res., 103, , ography who assisted in field measurements. We thank J. Toole, W. Lie, H.-J., C.-H. Cho, J.-H. Lee, S. Lee, and T. Yuxiang, Seasonal Teague, and an anonymous reviewer, whose constructive criticisms variation of the Cheju Warm Current in the northern East China have helped to improve the original manuscript. We are also indebted Sea, J. Oceanogr., 56, , 2000a. to D. L. Witter, who assisted in revising the manuscript with willing- Lie, H.-J., S. Lee, C.-H. Cho, and S.-K. Kang, An estimation of tidal ness. 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St., 5, , Yanagi, T., and S. Takahashi, Seasonal variation of circulation in the East China Sea and the Yellow Sea, J. Oceanogr. Soc. Jpn., 49, , Zheng, Q., and V. Klemas, Determination of winter temperature patterns, fronts, and surface currents in the Yellow Sea and East China Sea from satellite imagery, Remote Sens. Environ., 12, , C.-H. Cho, J.-H. Lee, S. Lee, and H.-J. Lie, National Research Laboratory for Ocean Prediction of the Yellow and East China Seas, Korea Ocean Research and Development Institute, P.O. Box 29, Ansan , Korea. (hjlie@kørdi're'kr) Y. Tang and E. Zou, Physical Oceanography Division, First Institute of Oceanography, State Oceanic Administration, P.O. Box 98, Qindao , China. (Received September 7, 2000; revised May 25, 2001; accepted May 23, 2001.)