Key Lab of Ocean Dynamic Processes and Satellite Oceanography, SOA,Hangzhou 310012,China
Second Institute of Oceanography, SOA, Hangzhou 310012,China
On the study of the circulation in the East China Sea, the Kuroshio and the currents east of the Ryukyu Islands, there was a lot of work to be made by Chinese scientists during 1998-2002. The work can be classified into three aspects, i. e., the circulation and air-sea interaction in the southern Huanghai Sea and East China Sea, the Kuroshio and the currents east of the Ryukyu Islands.
I. CIRCULATION AND AIR-SEA INTERACTION IN THE SOUTHERN HUANGHAI SEA AND EAST CHINA SEA
The two joint cruises on the project “The air-sea interaction process of cyclone outbreak over the Huanghai Sea and East China Sea” carried out by Chinese, Korea and Japanese scientists in June of 1999(Yuan et al., 2000a,2002a). This project was supported by the National Natural Science Foundation of China (Grant No.49736200) and the State Oceanic Administration. On the basis of these two joint cruises and historical data, the combination of the Physical Oceanography with the Meteorology is studied for this project (Yuan et al., 2000a, 2002a). The hydrographic features and circulations in the Huanghai Sea and East China Sea are analyzed and computed by using the modified inverse method (Yuan et al., 2002b,c), P vector method (Lou et al., 2002), 3D diagnostic, semidiagnostic and prognostic models (Wang et al., 2002), MOM-2 model (Xu and Yuan, 2000; Xu et al., 2002) and so on. Such important phenomena have further been studied as the Huanghai Sea Cold Water Mass, the Changjiang River plume, the Huanghai Sea Coastal Current, the Taiwan Warm Current, the Kuroshio and its adjacent cold and warm eddies and so on.
(1) Based on the CTD data obtained by R/V <Xiangyanghong No.14> in the cruise of June 1999, the hydrographic characteristics are analyzed and the velocity field is computed by the P-vector inverse method (Lou et al., 2002). The results are as follows: ① The stratified water occurs in this continental area during summer. The Changjiang River diluted water (CRDW) spreads towards the Cheju Island, whose thickness is about 10m. If we consider the 31 isohaline as the CRDW's boundary, it reaches the region east of 124°E. ② The Yellow Sea Coastal Current flows to the southeast. Then it makes a cyclonic turn along the boundary of the Yellow Sea Cold Water (YSCW), and flows northeastward. ③ A cyclonic eddy occurs southwest of the Cheju Island, at 125°—126°30′E, 30°40′—31°50′N. The hydrographic distributions show it has high dense and low temperature. Its center moves to the east in the lower layer.
(2) On the basis of the hydrographic data and observed current data with the vessel-mounted ADCP and toward ADCP data obtained in June 1999, the circulation in the southern Huanghai Sea and East China Sea is computed by using such models as the modified inverse method (Yuan et al., 2000b, Yuan et al., 2002b,c), the P vector method (Lou et al., 2002), the 3D diagnostic model, semidiagnostic and prognostic models (Wang et al., 2002). The following results have been obtained. ① The inshore branch of Taiwan Warm Current (TWCIB) through the study region is about 0.4×106m3/s.The Taiwan Warm Current much affect the currents in the continental shelf. ② The Yellow Sea Coastal Current flows southeastward and enters into the northwestern part of the study region, and after that it flows to turn cyclonically, then it flows northeastward, which is due to the influence of the Taiwan Warm Current and topography. ③ There is a cyclonic cold eddy southwest of the Cheju Island, and it is located at the north of area, in which the Yellow Sea Coastal Current flows to turn cyclonically. It has the high dense and cold water. ④ The cold and higher density water appeared in the layer from about 30 m depth to the bottom between stations C3-6 and C3-11 of the north most section C3.It is a southwestern part of the Yellow Sea Cold Water Mass (YSCWM).
(3) On the basis of hydrographic data obtained in June 17 to 25, 1999 on borad the R/V Eardo, Korea (hereafter the second cruise), the circulation in the southern Huanghai Sea and East China Sea is computed by using the modified inverse method (Yuan et al., 2002c). The following results have been obtained. 1) The comparison between two results computed on the basis of the first cruise which were carried out in June 4 to 19, 1999 on board the R/V <Xiangyanghong No. 14>, China and the second cruise shows the following variablities (Note that the time difference for two cruises is about two weeks). ① The position of the Kuroshio in the second cruise is slightly more east than that in the first cruise; ② The high dense water (HDW) with a cold core is located in the region south of Cheju Island between 125°30′E and 127°E at Sections D and C. The circulation in the region of HDW is cyclonic. Comparing the position of HDW during the second cruise with that during the first cruise, its position in the second cruise moves slightly northward. 2) The cold and uniform mixing layer is found in the layer from 30 m depth to the bottom of the middle part of Section A and in the layer from 20 m depth to the bottom of the middle part of Section B, respectively. They are both a southern part of the Huanghai Sea Cold Water Mass (HSCWM). 3) It has higher temperature and lower density with a weaker anticyclonic circulation in the southwestern part of the computed region. Its center is located at the western most point of Section E.
(4) Direct measurements of current velocity and water temperature were undertaken at the mooring station M (125o 29.38' N, 31o49.70' E), which is located over the continental shelf of the East China Sea during June 1999 by the R/V <Xianyanghong No.14>. The relationships between oceanic fluctuations at different depths are calculated by the spectrum analysis (Liu et al., 2002). The major results are as follows. 1) The mean flows are southeastward at both 30 and 45m depths. The currents become stronger gradually during the observation period. This may be mainly attributed to the transition of the tidal currents from neap to spring. 2) The semidiurnal fluctuations are the most dominant part in the current fluctuations, and rotates mainly clockwisely. And the diurnal fluctuations take the second. The local inertial period is close to the period of diurnal fluctuations, and an inertial motion is clockwise. Besides the fluctuations of above main periods, there is a peak at period 3d for counterclockwise components in the upper and lower layers. 3) A calculation of the cross spectra between two time series of current velocities at 30 and 45m depths shows that both the current fluctuations at both 30 and 45m depths are synchronous.
(5) A theoretical solution of one-dimensional heat transfer equation and a numerical simulation of 3D baroclinic circulation by MOM-2 are given to analyze the influence of bottom boundary mixing and the Topographic Heat Accumulation Effect (THAE) on baroclinic structure of the Huanghai Sea Cold Water Mass (HSCWM) in summer (Xu and Yuan, 2000; Xu et al.,2002).
(6) In combination with the observations of the mooring current system and the ADCP current measurement, the computed results showed the changes of each current system with time and space and the interactions between them in the Huanghai Sea and East China Sea. The changes of each current system with time and space and the interactions between them have important effect on the developing process of cyclone over the Huanghai Sea and East China Sea (Yuan et al., 2002a).
(7) The distributions of velocity and volume transport of each current system are computed in the study region (Yuan et al., 2002b, c; Wang et al., 2002; Lou et al., 2002).
(8) The effect of the developing process of cyclone on the ocean is explained (Yuan et al., 2002a).
(9) It can be found that there appeared negative latent and sensible heat fluxes near the center area of cyclone in the June of 1999 cruise, and their positions are located in the cold eddy region west of the Kuroshio and the region where the Huanghai Sea Coastal Current flowing southeastward and then making a cyclonical turn , the cold and cyclonic eddy region southwest of the Cheju Island and so on, respectively (Yuan et al., 2002a, b; Qian et al., 2002; Zhou et al., 2001, 2002a,b). From the observed results, large and positive latent and sensibl e heat fluxes are found in the Kuroshio and the warm eddy east of the Kuroshio core, where are the high temperature region. This shows that the distributions of latent and sensible heat fluxes are closely related to the hydrographic characteristics and circulation in the Huanghai Sea and East China Sea. The above relation of air-sea interactionis is found for the first time in the Huanghai Sea and East China Sea (Yuan et al., 2002a, b; Qian et al., 2002; Zhou et al., 2001,2002a, b).
(10) The numerical simulation and analyses of developing process for two examples of cyclones in June of 1999 are carried out (Zhou et al., 2002b). The simulated results of distributions of latent and sensible heat fluxes in this period are basically similar to the relevant, observed results . It may be remarked from the numerical simulation about developing process of cyclone moving eastward that the heat fluxes in front of the cyclone may be one of important causes leading cyclone to moving eastward , since there are the very large and positive heat fluxes in the Kuroshio and the region southwest of Japan, which support the enough heat fluxes transferred from the ocean to atmosphere (Zhou et al., 2002b; Yuan et al., 2002a).
(11) The model-computed result shows that the monthly mean net flux has distinct seasonal variation and the heat flux is transferred from ocean to atmosphere in January, and from atmosphere to ocean in July in the Huanghai Sea and East China Sea (Zhou et al., 2001,2002a).
(12) It plays a very important role for early developing process of cyclone in the East China Sea that the heat fluxes is transferred from ocean to atmosphere, in which the latent heat flux is more important than the sensible heat flux and is about 20 times of the sensible heat flux. The heat fluxes transferred from ocean to atmosphere accelerates the instability of the atmosphere in the lower layer, which is one of important causes leading to a cyclone developing in the East China Sea (Ma et al., 2002a,b).
(13) It is explained that the several main dynamical mechanisms for a developing physical process of cyclone in the East China Sea, which is necessary for the forecast of developing cyclone (Ma et al., 2002a,b; Qin et al., 2002).
On the seasonal variability of the sea water exchange between the Huanghai Sea and the East China Sea, Guo et al. (2000a) discussed it by using the CTD data in recently many cruises. For example, the Subei coastal water runs southeastward into the northern East China Sea, and the corresponding Huanghai Warm Current intrudes into the southern Huanghai Sea as a compensation current in water. The process of water exchange becomes gradually weak from the beginning of spring and so on. Guo et al. (2000b) also discussed seasonal variations of the Taiwan Warm current and the Kuroshio water intruded over the continental shelf, based on the hydrographical survey data measured in autumn of 1997, spring and summer of 1998 and winter of 1999 and so on. They mainly discussed the different causes to form of the Taiwan Warm Current in different seasons.
On the study of numerical simulation of the circulation in the Huanghai Sea and the East China Sea besides the above works, there were the following works. A 3D baroclinic model of wintertime circulation was presented by Wang and Feng (2000). The numerical results gave the general circulation pattern in this area in winter, especially, the main upwelling (downwelling) areas. Zhu et al. (2002) established a three-dimensional higher resolution turbulent closure model with σcoordinate applying the Huanghai Sea and the East China Sea. Their calculated results are fairly consistent with the previous observation and studies. This shows that the established model can be applied successfully to study the circulation in the Huanghai Sea and the East China Sea. Finally, Su (2001) made a review of circulation dynamics of the coastal oceans near China.
II. THE KUROSHIO
In the study area, the Kuroshio region can be divided into two parts, namely the Kuroshio east of the Taiwan and the Kuroshio in the East China Sea. Yuan and Su (2000)had reviewed the works of the Kuroshio and the currents east of the Ryukyu Islands, which had been carried out by Chinese scientists during 1995-2000. Now we keep discussing as follows.
1. The Kuroshio East of the Taiwan
There have been many studies on the Kuroshio and its variations east of Taiwan, especially the works on the program of China-Japan joint Research of the Subtropical Gyre from 1995 to 2000.There are four cruises of China-Japan joint research of the Subtropical Gyre from 1995 to 1998, i.e., a cruise of October of 1995 (Yuan et al., 2000e), a cruise of early summer of 1996 (Yuan et al., 2000f), and two cruises of summer and winter of 1997(Yuan et al., 2000h, i). On the basis of the hydrographic and observed current data obtained from above four cruises, the modified inverse model (e.g. Yuan et al., 2000e), P-vector method (Bu et al., 2000), the 3D diagnostic, semidiagnostic and prognostic models (e.g. Wang et al., 2000a,b) have been used to compute the Kuroshio and currents east of the Ryukyu Islands, and the hydrographic analysis also made. There are the important results and progress as follows.
(1) The net northward VTs of the Kuroshio through Section K2 southeast of Taiwan were about 57.8×106 and 44.6×106m3/s, respectively, in October of 1995(Yuan et al., 2000e) and early summer of 1996(Yuan et al., 2000f). However, it were about 37.5×106 (Yuan et al., 2000h) and 27.6×106m3/s(Yuan et al., 2000i), respectively, in July and December of 1997. This means that VT of the Kuroshio and its maximum velocity southeast of Taiwan obviously decreased during the 1997 stronger E1-Niño year as first reported by Yuan et al.(2000c,d,h,i). The decrease of the Kuroshio VT through Section K2 in July 1977 may be due to the following two reasons (Yuan et al., 2000d). 1) The North Equatorial Current (NEC) weakened during the El Niño event. As 1977 is the strong E1-Niño year, the Kuroshio VT at Section K2 may have decreased with the weakening of NEC. 2) Comparing the strength of the anticyclonic recirculating eddy east of the Ryukyu Islands in July 1997 with that in the other year, such as May-June 1996,the decrease of the Kuroshio VT through Section K2 may be associated with the weakening of anticyclonic recirculating eddy east of the Ryukyu Islands in July 1997.
(2) Yuan et al. (2000d) showed that the classical Sverdrup relation is not valid in the region of the Kuroshio. However, it is still valid in the broad ocean interior east of the Kuroshio Countercurrent. This means that we can not explain the above seasonal variation of the Kuroshio VT by using the classical Sverdrup relation.
(3) As far as the position of the Kuroshio southeast of Taiwan is concerned, it was more far from Taiwan during December of 1997 than that during other cruises (Yuan et al., 2000i, and Wang et al.,2000a).
(4) During October of 1995 and early summer of 1996, there should be two or three branches of the Kuroshio east of Taiwan (Yuan et al., 2000e,f, g, Yu et al., 2000a,Wang et al., 2000b). The main branch of the Kuroshio rode on a submarine ridge east of Taipei and Suao, and was deflected anticyclonically (Yuan et al., 2000f). The easternmost branch of the Kuroshio flowed northeastward to region east of Ryukyu Islands during October of 1995 and early summer of 1996, which was a first report (Yuan et al. 2000e,f,g). However, there was no branch of the Kuroshio east of Taiwan to flow northeastward to the region east of the Ryukyu Islands during July and December of 1997(Yuan et al., 2000c,d,h,i,j; Wang et al.,2000a,b, Bu et al., 2000). This showed that the patterns of circulation during October of 1995 and early summer of 1996 were different from these during July and December of 1997.
(5) The current data observed directly at 290m and 594m depths of the mooring station west of Yonakuni Island show that the Kuroshio was quite steady during the periods of observation from May 18 to June 1(Yuan et al., 1999b). The rotary spectral estimates of the current data by the maximum entropy method showed that there were the peaks of periods at 3-day to 7-day. There is a significant coherence between time series of currents at 290m and 594m depths in the range of 3-day to 5-day (Yuan et al., 1999b).
(6) There were some cyclonic and anticyclonic eddies east of Taiwan. Yuan et al.(2000e,f,g) further pointed out that the above patterns of the Kuroshio current were closely related to the strengths and positions of cyclonic and anticyclonic eddies in the adjacent region.
(7) The T-S curve of Kuroshio water was in the S-shape in the region east of Taiwan. There are four kinds of water masses from the surface to the lower layers, i.e., the high-temperature and subhigh-salinity surface water, the high-salinity subsurface water, the low-salinity mid-layer water and the low-temperature water. However, the water temperature and salinity properties of the 4 water masses vary with seasons (Yu et al., 2000a-e).
(8) In the region south of Taiwan and near the Bashi Channel, the Kuroshio water and the South China Sea water intruded alternately, resulting in a complicated distribution of the water masses there, which vary with seasons (Yu et al., 2000a-e ).
(9) A countercurrent was found below the 500m level in the region near the southeastern part of Taiwan. Thus, the position of the main branch of Kuroshio was further to the east here than that in the upper 500m during early summer of 1996 (Yuan et al., 2000f,g).
A Modular Ocean Model (MOM-2) is used to simulate the circulation of the North Pacific, which is driven by climatological wind stress from HERLLERMAN and ROSENSTEN (1983) (Xu et al., 2000; Xu and Yuan, 2001). Their results showed that the Kuroshio east of Taiwan separates into two branches: the main branch flows through the ridge northeast of Taiwan and then flows along the continental slope of the East China Sea, and the other eastern branch flows to the east of the Ryukyu Islands. These two branches were jointed together in the region south of Japan (Xu et al., 2000; Xu and Yuan, 2001).2. The Kuroshio in the East China Sea
Recently there were some numerical studies on the Kuroshio in the East China Sea, which included, for example, the 3D nonlinear diagnostic, semidiagnostic and prognostic models (Wang and Yuan, 2001), the modified inverse model (e.g. Yuan et al., 1999a, 2000c,d,f, 2001, 2002b,c; Liu and Yuan, 1999a,b,c) , the P-vector method ( Bu et al., 2000) , the MOM-2 model (Xu et al., 2000; Xu and Yuan,2001), a 3D baroclinic model of winter circulation (Wang and Feng ,2000), a 3D higher resolution turbulent closure model with σcoordinate. (Zhu et al., 2002) and so on. They discussed the variability of the Kuroshio and the eddies in the East China Sea from 1992 to 2002 in detail (such as Yuan et al., 1999a, 2000c,d,f, 2001, 2002a,b,c; Liu and Yuan, 1999a,b,c and so on). The above-mentioned studies shows the following results.
(1) There existed one core or multi-cores of the Kuroshio from 1992 to 2002. This means that it had significant variations with seasons. The main core always lies over the shelf break. Sometime the Kuroshio undergoes small turns several times (Bu et al., 2000).
(2) There are some anticyclonic and cyclonic eddies east and west of the Kuroshio, respectively, all the time (e.g. Yuan et al., 1999a, 2000c,d,f, 2001, 2002b).
(3) The net northward volume transport (VT) through section PN in the East China Sea is largest in summer and smallest in autumn with an average of 28.0×106m3/s in the four cruises of 1992, and the VT at Section TK (at Tokara Strait) also largest in summer of 1992 (Liu and Yuan, 1999a). However, the VT of the Kuroshio in the East China Sea was smallest in spring and largest in summer during 1993 and 1994. The average net northward VT through Sections PN and TK were 27.1×106 and 25.0×106m3/s, respectively, during the eight cruises of 1993 and 1994 (Liu and Yuan, 1999b).
(4) As for variability of the Kuroshio in the East China Sea in 1995, the VT through Section PN was largest in spring but smallest in summer of 1995. However, from the statistical average results, its VT is largest in summer but smallest in autumn. This means that 1995 is an anomalous year, as also showed from T-S distributions and reported firstly by Liu and Yuan(1999c).
(5) Based on the CTD data of the ten cruises in 1997and 1998 in the East China Sea, Yuan et al. (2001) pointed out the following facts.1) When the El Niño event occurred in May of 1997, both VT of the Kuroshio in summer of 1997 and the average VT of the Kuroshio in 1997 decreased.2) There appeared two different patterns of the circulation in the East China Sea in January and June-July of 1997 corresponding to the periods before and after the El Niño event, respectively.3) The Kuroshio through Section PN has multi-current cores during the period from April to November of 1997, especially it has three current cores in October and November of 1997. The position of the main current core of the Kuroshio moves eastward in the autumn.4) Both 1995 and 1998 are an anomalous years for the Kuroshio in the East China Sea, which may be due to the following two reasons: a) the increase of the Kuroshio VT may be associated with the strengthen of the anticyclonic recirculating eddy south of Okinawa Island; b) the transform from the El Niño event to the La Niña event was occurred in summer of 1995 and 1998 (Yuan et al., 2001).
(6) The volume and heat transports through Sections PN and TK both were studied from 1992 to 2002 (e.g. Liu and Yuan, 1999a,b,c; Yuan et al., 1999a,2000c,d,f, 2001, 2002b,c).
(7) As above, the Kuroshio plays a very important role in early developing process of cyclone in the East China Sea, when the heat fluxes is transferred from ocean to atmosphere (Yuan et al., 2002a).
(8) The semidiagnostic computation gives better results of describing the effect of the bottom topography on the Kuroshio in the East China Sea than that by diagnostic computation (Wang and Yuan, 2001).
(9) The results given by the MOM-2 model show that the Volume Transport of Kuroshio ranges in the East China Sea from 27.5×106m3/s in winter to 32.9×106m3/s in summer, which is in contrast with that derived from Sverdrup relation (Xu et al., 2000; Xu and Yuan, 2001). The modified inverse model gave the same result (Yuan et al., 2000c,d).
(10) A modified inverse method is used to compute the volume transport(VT), the heat transport(HT) and the horizontal material fluxes in the East China Sea on the basis of the data of CTD, TCO2, DOC, POC , etc. obtained in April 1994 (Yuan et al., 1999a). In April, 1994 the VT and HT through Section PN in the East China Sea are 30.6×106 m3/s and 2.42×1015 W respectively, and the horizontal fluxes of TCO2, DOC and POC through Section PN are 65×106, 2.2×106 and 0.17×106 mol/s, respectively (Yuan et al., 1999a).
Finally, Li et al. (2000) discussed the origin of mid-layer water and deep water of the Kuroshio in the East China Sea based on the historical data and data obtained from the cruise of July, 1997.
III. THE CURRENTS EAST OF THE RYUKYU ISLANDS
On the currents east of the Ryukyu Islands, especially the western boundary current east of the Ryukyu Islands (hereafter WBCE), there were some works done after the works of Yuan et al. and so on from 1986 to 1998 (Yuan et al., 2000e, f; Yuan and Su, 2000; Liu et al., 2000a, b, c, d ; Liu and Yuan, 2000; Li et al., 2000). There are the following results in their studies.
(1) There are two origins of the WBCE. One came from the eastmost branch of the Kuroshio east of Taiwan during October of 1995 and early summer of 1996 (Yuan et al., 2000e,f, Liu et al., 2000a, c). However, in summer and winter of 1997 there appeared to be no branch of the Kuroshio east of Taiwan to flow northeastward to the region east of the Ryukyu Islands (Yuan et al., 2000c, h, i; Liu et al., 2000d). Another origin comes from the anticyclonic recirculating gyre south of Ryukyu Islands. The anticyclonic recirculating gyre always exists in the region southeast of Okinawa Island in all cruises (e.g. Yuan et al., 2000e, f; Yuan and Su, 2000).