Current observations in the southern Yellow Sea in ... - Springer Link

Chinese Journal of Oceanology and Limnology Vol. 22, No. 3, P. 217-223, 2004

Current observations in the southern Yellow Sea in summer* TANG Xiaohui 0~U~5~) *'*t, WANG Fan (:k~)~L)t, CHEN Yongli ([~7~IJ)*, BAI Hong ( l~t ~t0~)t, HU Dunxin (di~~0]:~.)* (~lnstitute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China) (trGraduate School of the Chinese Academy of Sciences, Beijing 100039, China) Received May 20, 2004; revision accepted June 1, 2004 Abstract

Current data from three moored Acoustic Doppler Profilers (ADPs) deployed in the southern

Yellow Sea at sites A (124.17 ~

34.82 ~

B (122.82 ~

35.65 ~

in summer 2001 and site C (120.85 ~

34.99 ON) in summer 2003 were analyzed in this paper. Features of the tidal and residual currents were studied with rotary spectral and cross-spectral methods. Main achievements were as follows: 1) Tides dominated the currents. At sites A and B, the semidiurnal tidal current was basically homogeneous in the whole depth, taking a clockwise rotation at site A, and near-rectilinear counterclockwise rotation at site B; while the diurnal tidal current was strong and clockwise near the surface, but decreased and turned counterclockwise with depth; at site C, semidiurnal tidal current dominated and diurnal current took the second, both of which were counterclockwise and vertically homogeneous. Inertial motion contributed to the clockwise component of diurnal fluctuations; 2) The 3-5d fluctuation of residual current was found at site C and attributed to the response of current to meridional wind, with a lag time of approximately 1.8d; 3) Mean residual flows at sites A and B in 2001 probably suggested an anticyclonic inner circulation in the middle of the southern Yellow Sea in summer.

Key words: current observation, the southern Yellow Sea, Acoustic Doppler Profiler, spectral analysis, tidal current, residual current

1 INTRODUCTION Currents in the southern Yellow Sea (SYS) in summer are generally determined by circulation structure of the Yellow Sea Cold Water Mass (YSCWM) and the Yellow Sea Warm Current (YSWC). It has been widely accepted that the YSCWM is formed by the remnant cold water from previous winter (He et al., 1959), but its circulation structure has yet been disputable. Guan (1963) presented a cyclonic circulation model in the YSCWM with maximum upwelling velocity in the thermocline, but was later negated by the discovery of maximum dissolved oxygen near the thermocline which suggested little vertical movement (Gu, 1980). Thereafter, different schemes with no vertical movement through thermocline were brought forward. Hu et al. (1991) suggested a two-gyre circulation structure with an outer cyclone and an inner anticyclone at surface, and an upwelling rising from the bottom diverges below the thermocline. Li and Yuan (1992) created a model that vertical flow occurs only in a thin shell above the thermocline. Su

and Huang (1995), however, constructed a reverse circulation model against Hu's, with no vertical flow through thermocline either. Yet there are still no unified believes on the circulation pattern of the YSCWM. Further evidence from observation is needed. The YSWC is a northward flow along the Yellow Sea (YS) trough first suggested by Uda (1934). Before the 1980's, it has been generally thought of intruding into the YS all the year round. Observation ofZhao et al. (1991), Teague and Jacobs (2000), and Tang et al. (2000) showed that in summer the YSWC does not enter the YS. Due to difficulties with current measurement, direct current observations in the YS were limited. The U.S. Naval Oceanographic Office utilized three moored ADPs to measure current along the YS trough from July 1995 through January 1996, focusing on the onset o f Y S W C (Teague and Jacobs, 2000). Korea Ocean Research and Development * Contribution No.4608 from the Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. Supported by the National Basic Research Program of China (No. G1999043803).

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C H I N . J. O C E A N O L . L I M N O L . , 22(3), 2 0 0 4

Institute conducted a mooring measurement at southwest of the Korea Peninsula in summer 1983, and found steady northeastward flows at 20m depth (Tang et al., 2000). Yet current observation in the middle and west of the SYS is very rare. To gain a better understanding of YSCWM related circulation, the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) conducted several direct current measurements in the SYS in summers of 2001, 2002 and 2003. These measurements were made with moored Sontek Acoustic Doppler Profilers (ADPs). Currents at levels near the surface, at middepth, and near the bottom were analyzed in conjunction with synchronous wind data from QSCAT/NCEP blend winds field in the present study.

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provided in Table 1. Three levels in the upper mixed layer (9m at site A, 10m at site B, and 4m at site C), in the thermocline (41m at site A, 40m at site B, and 14m at site C), and near the bottom (75m, 60m, and 29m for sites A, B, and C respectively) were selected according to the hydrographic structure. For further comparison with current, wind data from QuickSCAT/NCEP blend winds field at 10m altitude were employed, which was provided on a 0.5~ ~ grids at 0.25 day intervals. 42 N

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2 OBSERVATION AND METHOD There were total five ADPs deployed in our observations. Data retrieved from two ship-aboard ADPs (one in July 2001, the other in July 2002) were relatively unstable, so data from another three moored ADPs were analyzed in this paper. As shown in Fig. 1, mooring A and B were deployed in July 2001 using 250 kHz three-beam ADPs, and mooring C in July-August 2003 using a 250 kHz four-beam ADP. Effective data length was 4.2d, 3.7d, and 25.8d respectively. The instrument error was 0.5 cm/s or 1% of measured velocity. Geographical positions, deployment periods, ADP sample intervals, water depth, and sample levels are

36

34

32

118

120

122

124

126E

Fig. 1 Bathymetry of the area and location of Acoustic Doppler Profilers (A, B, and C; stars)

Table 1 ADP summary ADP A ADP B ADP C

Lon.(~ 124.17 122.82 120.85

Lat.(~ 34.82 35.65 34.99

Start Da~r July 17, 2001 July 16, 2001 July 16, 2003

Profiles of u (zonal) and v (meridional) components of velocity were used to form patterns of horizontal current. To remove the influence of tides, residual flows were acquired by using a Butterworth low-pass filter with a 40 hour cutoff frequency. Features of the currents and winds were studied with rotary power spectral analysis method developed by Gonilla (1972). This method has some advantages over traditional spectral analysis methods: 1) the separation of a velocity vector into oppositely rotating components can simplify analysis in the case of, for example, inertial motions which are almost entirely clockwise rotary in the northern hemisphere; 2) the rotary spectral properties are invariant under coordinate rotation (Emery and Thomson, 2001).

End Day July 21, 2001 July 20, 2001 Aulg. 11, 2003

Interval(rain) 20 20 30

Depth(m) 5-77 6-62 1-29

Bin Size(m) 2 2 1

3 ANALYSES ON THE TIDAL CURRENT Tidal features of the currents were examined by rotary spectral analyses (Fig.2). As seen from Fig.2a, b, c, obvious periodical fluctuations of velocity were attributed to diurnal and semidiurnal tidal currents at site A. The semidiurnal tidal current was basically homogeneous in the whole depth, and formed the most dominant fluctuation at site A except at the depth of 9m, where the clockwise component of the" diurnal current appeared more significant. Spectral density of the clockwise semidiurnal fluctuation was greater than the counterclockwise, which meant the semidiurnal tidal current rotated mainly in clockwise direction. The diurnal current was strong and

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TANG et al.: Current observations in the southern Yellow Sea in summer

clockwise near the surface, but decreased and turned counterclockwise with depth. Note that the local inertial period is about 21h, close to the period of diurnal fluctuations, and an inertial motion is clockwise. So the diurnal fluctuation here was probably composed o f both diurnal tidal current and inertial motion. Rotary spectra of the currents at site B also showed significant peaks at semidiurnal and diurnal periods (Fig.2d, e, f). The counterclockwise component of the semidiurnal tidal current was a little

stronger than the clockwise component, which meant a counterclockwise and near-rectilinear flow. As to the diurnal fluctuation, the counterclockwise component was sizable in the whole depth, while the clockwise component exceeded the power of the semidiurnal flow near the surface and decreased distinctly with depth, analogical with that of site A. The peak of the clockwise diumal fluctuation fell on a period shorter than 24h, which implied a combination of diurnal current and inertial motion.

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Fig.2 Rotary Spectra of current vectors at: (a) 9m, site A; (b) 41m, site A; (c) 75m, site A; (d) 10m, site B; (e) 40m, site B; (f) 60m, site B; (g) 4m, site C; (h) 14m, site C; (i) 29m, site C (solid line: counterclockwise spectra; dashed line: clockwise spectra). The 95 o%confidence interval is indicated. Figures near the peaks indicate periods Currents at site C were mostly dominated by counterclockwise semidiurnal tidal current in the whole depth (Fig.2g, h, i). Diurnal current, which took the second, was also vertically homogeneous and counterclockwise. Besides the main fluctuations above, a peak at 6h was found mainly in the clock-

wise component, and fluctuations of longer period of 4.6-5.2d and 10-14d could be recognized. These directly measured semidiurnal and diurnal tidal currents were compared with cotidal charts by Zhao (1994), and turned out to be basically consistent at sites B and C, but somewhat ambigu-

220

C H I N . J. O C E A N O L . L I M N O L . , 22(3), 2 0 0 4

ous at site A, which is far from nodal points. 4 ANALYSES ON THE RESIDUAL CURRENT

4.1

Temporal

residual

and

vertical

structures

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The filtered residual currents were subsampled at 0.25 day intervals for comparison with winds (Figs. 3, 4). The residual currents at site A showed

complicated vertical structures. Current flows in the whole depth were generally toward the south, opposite to the direction o f the wind. Current directions near the surface and bottom changed with depth complying with the Ekman spiral. The maximum residual velocity below the thermocline was as strong as 14.8 crn/s (Fig.3a, 50-60m depth), comparable with that o f the upper layer. Surface current kept turning clockwise during the observation period.

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Vol.22

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Day Fig.4 Vector stick diagrams of wind speed and residual current velocities at 4, 14, and 29m for site C in 2003 The residual currents at site B (Fig.3b) were relatively homogeneous in depth, with a generally

eastward flow at surface. Currents were turning counterclockwise during the observation days.

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TANG et al.: Current observations in the southern Yellow Sea in summer

4.2 Cross-spectral analyses o f currents and w i n d s

The residual currents at site C (Fig.4) were strong near the surface, and homogeneously weak below 10m. For a better view on the temporal variation, vector sticks o f currents at 4m, 14m and 29m were presented instead o f that at all the depth. In accordance with the southerly prevailing winds, surface current generally flowed northwards. Obvious low-frequency fluctuation o f the surface currents could be seen at a period o f about 5d, which was consistent with the result o f rotary spectral analysis (Fig.2g). The residual currents at 29m also ran northwards, but the current direction at 14m alternated between south and north. From both a fluctuation period o f 4-5d can be seen. t" "t

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To gain further understanding on the low- frequency fluctuation at site C and its relationship with winds, cross-spectral analyses between current and wind were conducted. As the major characteristic o f the prevailing winds in summer, only the meridional component o f the wind (vw) was considered for analyses. Figure 5 depicted the squared coherence and phase lag between zonal current (u) and meridional wind (vw) with 95% confidence level, where negative phase lag means that wind leads current. Despite noise at high-frequency band which was

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Fig.6 Coherence and phase diagrams for the meridional residual current velocity v and meridional wind speed vw at: a) 4m; b) 14m; c) 29m for site C. The 95 oVoconfidence level is indicated. Negative phase indicates that the currents trail the wind

222

CHIN. J. OCEANOL. LIMNOL., 22(3), 2004

introduced by filtering, high coherences (>0.8) stood ing phase was about one third of the period, whichmeans -1.8 days phase lag with the winds. Relatively high coherence (>0.75) could be seen at 14m and 29m at the peak of 3.5d, with the same lag time of about 1.8-2d. Cross-spectral analysis between meridional current (v) and meridional wind (vw) yielded similar result at 4m level (Fig.6a) that 0.79 squared coherence at the period of 5.3d with lag time of about 1.8d existed, but showed no evident coherence in this band at 14m and 29m. On the other hand, self spectra of the winds showed peaks at 3-6d, 12.6d and 16d. These results suggested that the 5d fluctuation of the surface current was in response to fluctuations in the wind with response time of 1.5-2d, while the response at middle and bottom levels were not so apparent. 38 N

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4.3 Characteristics o f m e a n flow

Mean residual current velocities during each observation were calculated at several levels (Fig.7). There were some common features at all the levels: currents flowed southwards at site A, eastwards at site B, and generally northwards at site C; current velocities at site A were always the largest among the three sites. For each site, currents were strongest at the upper levels (10.14 cm/s at 15m for site A, 3.75 cm/s at 14m for site B, and 5.56 cm/s at 6m for site C), and weakest at the middle levels (3.94 cm/s at 39m for site A, 0.48 cm/s at 40m for site B, and 0.87 cm/s at 14m for site C). Hence the vertical structure of the current velocity may be closely linked to the hydrographic structure.

38 N

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Fig.7 Mean residual flow during observation at the depth a) 6m; b) 14m; c) 40m; d) bottom (75m for site A, 60m for site B, and 29m for site C)

5 DISCUSSION There has been little study on the correlation between current and wind in the SYS, except for the work of Teague and Jacobs (2000), which indicated high coherence between currents along the YS trough and wind stress at semiweekly and weekly periods, and increasing lag time with depth. On the

current analyses at site C, it was found that correlations between currents at different levels and winds fell on different periods (5.3d for 4m, 3.5d for 14m and 29m), but with similar lag time (about 1.8d). Different regimes were suggested for the correlations at different depth, which calls for more measurements and further studies. No clue of the northward YSWC was observed at site A in the YS trough that was thought as path

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TANG et al.: Current observations in the southern Yellow Sea in summer

of the YSWC when it exists (Tang et al., 2000). Actually, the observed residual currents in summer 2001 flowed southwards at site A, and eastwards at site B. It seemed to suggest an anticyclonic inner circulation in the middle o f the SYS in summer. 6 CONCLUSION Obvious periodical fluctuations o f the studied currents were attributed to diurnal and semidiurnal tidal currents. At sites A and B observed in July 2001, the semidiurnal tidal current was basically homogeneous in the whole depth, taking a clockwise rotation at site A, and near-rectilinear counterclockwise rotation at site B; while the diurnal tidal current was strong and clockwise near the surface, but decreased and turned counterclockwise with depth. At site C observed in the summer 2003, semidiurnal tidal current dominated and diurnal current took the second, both o f which were counterclockwise and vertically homogeneous. Inertial motion contributed to the clockwise component o f diurnal fluctuations. Residual currents at the surface o f site C responded to meridional winds at a significant period of 5.3d, and the response time was approximately 1.8d; zonal residual currents at and below the thermocline made weaker response with meridional winds at a period o f 3.5d, but had similar response time with that of the surface. Average residual currents at site C flowed northwards near the surface and bottom, and turned weak in the depth o f the thermocline. Mean residual currents observed in 2001 were southward at site A, and eastward at site B, which probably suggested an anticyclonic inner circulation in the middle o f the SYS in summer. Still more measurements are needed to fully understand the current structure and circulation pattern in the SYS. 7 ACKNOWLEDGEMENTS Thanks are due to Prof. Maochang Cui for his helpful suggestions, to Drs. Xuezhi Bai, Enbo Wei, Mr. Qilong Zhang, Mingkui Li and our entire research group for considerable amount o f help and

223

cooperation, and to crews on R/V Science I and Venus II for their work in the deployment and recovery operations. References

Emery, W. J. and R. E. Thomson, 2001. Data Analysis Methods in Physical Oceanography. second and revised edition, Elsevier Science B. V., p. 409-500. Gonilla, J., 1972. A rotary-component method for analysing meteorological and oceanographic vector time series. Deep-Sea Research 19: 833-846. Gu, H., 1980. Maximum vertical distribution of dissolved oxygen in the Yellow Sea. Acta Oceanol. Sin. 2(2): 70-80. (in Chinese) Guan, B., 1963. A preliminary study of the temperature variations and the characteristics of the circulation of the Cold Water Mass of the Yellow Sea. OceanoL et Limnol. Sin. 5(4): 255-284. (in Chinese with English abstract) He, C., Y. Wang, Z. Lei and S. Xu, 1959. Preliminary study of the formation of Yellow Sea Cold Water Mass and its property. Oceanol. et Limnol. Sin. 2(1): 11-15. (in Chinese with English abstract) Hu, D., M. Cui, 3(. Li and T. Qu, 1991. On the Yellow Sea Cold Water Mass-related circulation. Yellow Sea Research 4: 79-88. Li, H. and Y. Yuan, 1992. Theoretical study on the thermal structure and circulation pattern related to cold water mass of Yellow Sea. Oceanol. et Limnol. Sin. 23(1): 7-13. (in Chinese with English abstract) Su, J. and D. Huang, 1995. On the current field associated with the Yellow Sea Cold Water Mass. Oceanol. et Limnol. Sin. Suppl., 26(5): 1-7. (in Chinese with English abstract) Tang, Y., E. Zou, H. Lie and J. Lie, 2000. Some features of circulation in the southern Huanghai Sea. Acta Oceanol. Sin. 22(1): 1-16. (in Chinese with English abstract) Teague, W. J. and G. A. Jacobs, 2000. Current observations on the development of the Yellow Sea Warm Current. J. Geophys. Res. 105(C2): 3401-3411. Uda, M., 1934. The results of simultaneous oceanographical investigations in the Japan Sea and its adjacent waters in May and June, 1932. J. Imperial Fish. Exped Stn. 5: 57-190. Zhao, B., G. Fang and D. Cao, 1994. Numerical simulation on tides and tidal currents of the East China Sea. Acta Oceanol. Sin. 16(5): 1-10. (in Chinese) Zhao, B., R. Limeberner, D. Hu and M. Cui, 1991. Oceanographic characteristics of the southern Yellow Sea and the northern East China Sea in summer. OceanoL et Limnol. Sin. 22(2): 132-138. (in Chinese with English abstract)