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PROGRESS IN STUDIES ON THE CIRCULATION

AND REGIONAL AIR-SEA INTERACTION IN THE

SOUTH CHINA SEA DURING 1998-2002

WANG Dongxiao, DU Yan, WANG Weiqiang, CHEN Ju and XIE Qiang

Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology,

Chinese Academy of Sciences, Guangzhou 510301, China

In the present paper, by focusing on the dynamic process of the South China Sea (SCS), we review the progress about physical oceanography in the SCS carried out in the recent years (1998-2002), such as the general circulation and the large-scale air-sea interaction. The issues include the progress on the following topics: the seasonal basin-scale circulation, the mesoscale eddy, the interannual variability of the upper layer, the dynamical adjustment of circulation, the water mass exchange through the Luzon Strait and the circulation in the middle and deep layers. In addition, the progresses on the upper mixed layer and seasonal thermocline are also reviewed.

 

I.  INTRODUCTION

The South China Sea (SCS) is a typical tropical marginal sea, which can exchange water mass with the Pacific and the Indian Ocean via several straits. The semi-enclosed SCS basin spans about 20 degrees in the meridional direction, which makes the dynamics in the SCS similar to the open oceans. However, the curved coastline and numerous islands, as well as the shallow continental shelves and bays there have significant effects on the interior oceanographic process in the SCS. As a result, the characteristics of temperature and salinity distributions, tidal mixing process, mesoscale and small scale eddies as well as the meridional circulation are different from those of the open ocean. The spatial structure of the SCS circulation is of multiple scales, which includes the basin-scale structure, sub-basin scale structure and mesoscale structure. Moreover, the evolution of the SCS circulation is corresponding to multiple time-scales variations, such as the interannual variability, seasonal variability and some transient variations. To a certain extent, such kind of multi-scale feature on the spatial and temporary structures can be attributed to the regional air-sea interaction process, nevertheless, the monsoon forcing and the air-sea heat exchange should have tight connection with that too.

The SCS, which locates in the central region of the Asian-Australian monsoon, has a typical seasonally reversed feature. In recent years, many relevant studies suggest that the monsoon in the SCS has important regional characteristics. That is to say, there exists a tropical sub-system of the East Asian monsoon in the SCS, namely the SCS monsoon. The large-scale monsoon and its corresponding vorticity mainly contribute to the basin-scale circulation, which generates Ekman pumping and significantly influences the thermocline depth through the mass convergence or divergence. And the strong short-term weather phenomenon, such as typhoon, will form several strong mesoscale eddies in the upper ocean, which always leads to cooling of the upper ocean with the associated wind-induced evaporating at the sea surface. The Luzon Strait, which locates in the northeastern SCS and is 2500 m deep, is the mainly passage to the open sea. The Kuroshio and water exchange in the lower layers significantly influence the vertical structure and the interannual variability of the SCS circulation. Besides that, the surface heat fluxes and the sea surface fresh water fluxes related to monsoon variation have important influences on the thermodynamical and dynamical processes in the upper SCS.

By focusing our attention on the fields of the circulation dynamics and air-sea interaction, we will review the recent progress on the SCS oceanography made by the Chinese oceanographers from 1998 to 2002.

 

II.  THE SCS REGIONAL OCEANOGRAPHY

Up to now people have already had numerous knowledge on the evolution of the upper layer circulation in the SCS (Su et al., 1999). When the large-scale circulation is concerned, the northeast monsoon will drive a basin-scale cyclonic circulation with each cyclonic gyre in the northern and southern SCS (NSCS, SSCS), respectively, and the circulation displays a double-gyre structure. In summer, the circulation in the NSCS will still maintain a weak cyclonic gyre. However, due to the southwest monsoon, the circulation in the SSCS displays an anti-cyclonic gyre, and the general circulation over the entire basin shows a dipole structure. The basin-scale circulation is of significant western intensification, which was bore out by many observations and numerical simulations (Qu, 2000; Liu et al., 2000; Cai et al., 1999; Wang et al., 2000).

The promotion of the satellite altimeter technique is worthy of noting. The altimeter sea surface height anomaly (SSHA) data have been used to study the spatial and temporary variability of the large-scale circulation in the SCS on both seasonal and interannual time scales (Mao et al., 1999; Shaw et al., 1999; Ho et al., 2000; Hwang et al., 2000; Wang et al., 2000; Liu et al., 2000, 2001).

1.  Dynamical Interpretation to the SCS Circulation Pattern

When the mean circulation pattern is concerned, Wyrtki suggested that it should be mainly attributed to the broad topography and wind stress vorticity as early as 1961. Zeng et al. suggested that the SCS circulation can be taken as barotropic currents due to the coastline constrain and wind stress forcing. The result obtained by Wang et al. through an enclosed boundary simulation agrees well with the large-scale circulation pattern, as a consequence, monsoonal forcing plays a dominant role.By assuming the SCS as an enclosed basin, Liu et al. (2000), found that the basin-scale circulation pattern can be gotten from the Sverdrup stream function. And from the Sverdrup stream function in the interior region, the mass transport of the western boundary current (WBC) is 5-6Sv and 3-4Sv in winter and summer, respectively. They suggested that the upper layer basin-scale circulation can be mainly regarded as a wind-driven circulation forced by the wind stress vorticity, which indicates the SCS circulation having a strong regional characteristics.

2.  Analysis of the New Cruise Observations

The direct current observation data still remains shortage in the SCS. Qiu et al. (2000) decompose the ADCP data into the barotropic and baroclinic components, which had been observed for 28.6 days running and at 27 levels (from 11, 12, 115m) over the continental slope in the NSCS from March to April 1996. The harmonic analysis and energy spectrum method are applied to deal with the time series of the baroclinic current of each level.

Based on the data observed by two cruises of R/Vs <Kexue No. 1> and <Shiyan No. 3> from May to August 1998 during the SCSMEX-IOPs, Wang et al. (2001) analyzed the structure and variation of temperature and salinity at different sections prior and after the summer monsoon onset. The observation indicates that the central SCS is mainly dominated by a typical SCS water mass, however, in the NSCS, especially around the Luzon Strait, water mass in the surface and subsurface layers is influenced significantly by the western Pacific water mass. After the summer monsoon burst, the sea surface temperature (SST) in the NSCS increases clearly, whereas its variation in the middle and southern regions can be negligible. At the same time, the mixed layer in those areas in the NSCS develops. By applying the P-vector method to deal with the SCSMEX data, Pu et al. (2001) found that the Kuroshio intrudes westwards into the SCS; then it forms an anticyclonic loop current; and finally it shows the trend to flow out of the SCS. They also pointed out that the cool center of a cyclonic gyre located in the NSCS, whose strength decreases and whose spatial domain shrinks with the increasing depth, moves southwards with the increasing depth. Off the middle and east Vietnam coast there exist a significant anticyclonic eddy and a cyclonic eddy, which are very stable at these layers shallower than 200 m. Depending on the spectrum of the mooring ADCP data sampled in the northeastern SCS, which span 77 days in the 450-m level and 7 months in the 2000 and 2300 m levels respectively, Yuan et al. (2002) addressed that the upper layer currents in winter is stronger than that in summer and autumn.

Wang et al. (2002) analyzed the data sampled in the central and southern SCS in summer 2000. Their result indicates that the distributions of temperature and salinity in the upper ocean varies with the depth increasing. They also noticed that the temperature and salinity distribution in the middle layers (from about 250 to 400 meters) is much different from that in any other layers above and below. During the survey period, the deep circulation in the SSCS showed a weak cyclonic trend, however, the circulation in the upper layer displayed an anti-cyclonic trend. The circulations by the data-diagnostic method agree well with the geostrophic velocities derived from the Topex/Poseidon SSH during the survey, that is to say, there exist many mesoscale eddies.

3.  Oceanography in the Southern SCS

The circulation in the SSCS is much different from that in the NSCS due to the existence of the Nansha Islands. Based on the previous observed data, the analytical results indicate that the circulation in the SSCS mainly consists of some cyclonic and anticyclonic eddies, which shows a multi-eddy structure. In general, a large scale gyre consists of two or more small scale eddies (Fang et al., 1998). He et al. (2000) calculated the circulation in the SSCS by using the temperature and salinity data from 1959-1988, and found that an anticyclonic circulation in winter dominates the region southeastern of the Nansha Islands, and the cyclonic circulation rest of area dominates. Guo et al. (2000) analyzed many in situ data and suggested the following three characteristics. The circulation in the upper layer in the region around the Nansha Islands has an independent enclosed structure. The deep circulation in the SSCS together with the central SCS forms an enclosed circulation system. The direction of circulation in the upper layer is always against that in the lower layer. The reversal of the upper and lower circulations indicates that the SSCS is dominated by a typical baroclinic structure; as a consequence, the stronger vertical motions will occur.

 

III.  EDDIES IN THE SCS

The gyre and eddy structures in the SCS are reported by many hydrographic investigations, no matter how the data were obtained by scientists in China or foreign countries. Wang and Chern (1997) found that there was a cyclonic eddy located off the northwestern Luzon. Later, using his numerical model, Fang et al. (1998) also indicates that this eddy exists all the year round. Chu et al. (1997) mentioned that there obviously existed a cold eddy off the southeast Vietnam coast, which was named as the Vietnam cold eddy. The cross correlation analysis carried by them shows that the local air-sea feedback mechanism is the major cause to form the Vietnam cold eddy. Based on the P-vector method, Chu et al. (1998) obtained the basic characteristics of the multi-eddy structure of the SCS circulation. There exist cold eddies in the region east of Vietnam in the central SCS and in the region off the northwest Luzon Strait, and perennially exists a cold eddy in the region around Dongsha Islands. There also exist some stable eddies in the regions around the Nansha Islands and southeast of the Mekong estuary. On the basis of the altimeter data, Wang et al. (2001) pointed out that there exist some clear seasonal SSH signals of the Kuroshio's intrusion in the northeastern SCS. He et al. (2001) compared the geostrophic velocities derived from the altimeter SSHA data with that from the Argos buoy trajectories. They found that the buoy trajectories could be interpreted by the geostrophic current; as a consequence, by using the diagnosed geostrophic current we can explain most of the transient mesoscale eddy activities marked by the buoy trajectories with good validation.

Regarding the relationship between the SCS eddy and the seasonal adjustment of the SCS circulation, Wang et al. (1996) suggested that the appearance, decline and migration of the sub-basin scale and mesoscale eddies lead to the seasonal adjustment of the large scale circulation, which has been verified partly by the data assimilation result and complex ocean model simulation (Yang et al., 2000). Wang (2001) suggested that the research into the SCS circulation have been promoted from the research into the seasonal mean circulation to the reveal of the circulation energetics and the interaction between the large-scale and mesoscale motions. The Dynamics of generation of the multi-eddy structure connect tightly with the process of the energy transform among the large-scale circulation, sub-basin scale circulation and mesoscale eddy. The mesoscale and small scale eddy, which are mainly generated by the monsoon forcing, lateral boundary and topography restrict have important role in energy transform of the larger scale gyre system. There include the Kuroshio forcing at the Luzon Straits and the local and remote effects by the Rossby waves generated. However, till now, the knowledge on the vertical structure of the eddy and the possible role of the thermocline dynamics in the generation, maintenance and development of the SCS multi-eddy structure, especially in the understanding on the energy transform among those eddies is deficient. There is not sufficient observation to uncover the transformation of the kinetic energy, momentum and vorticity on different spatial and temporal scales.

 

IV.  DYNAMICS OF THE SCS CIRCULATION ADJUSTEMNT

The ocean circulation will respond to any change of the wind fields, which usually occurs in terms of the large-scale planetary wave adjustment. This kind of process can be simplified as the follows. The wind stress acts on the ocean and generates a barotropic Rossby wave at first, which migrates through the SCS basin in several days, and then generates a barotropic velocity fields, which has uniform property in the whole water column. It will take about 1 month (in the SSCS) and 3 months (in the NSCS) for the first mode of baroclinic Rossby wave propagates from east to west, which enhances the baroclinic structure in the Sverdrup stream field. After the coastal trapped Kelvin waves (CTKW) propagates along the lateral boundary completely, and the Rossby wave initiated by the CTKW at the tip of the eastern coast reaches the western coast, the adjustment of the SCS basin-scale circulation will be accomplished as a preliminary stage. As a consequence, the Sverdrup transport mainly concentrates in the upper layers, and the full Sverdrup relationship degenerates to an upper layer Sverdrup balance (Yang, 2000).

Theoretically, the observed SSH seasonal cycle represents the variation of the mean thermocline in the SCS, when the response to Ekman pumping is concerned (Yang, 2000; Wang, 2002). The circulation in the NSCS is relatively stable, however, the SSCS circulation has a strong annual variation. One reasonable interpretation is that the seasonal signal in the SSCS is much stronger than that in the NSCS. In winter, there exists a cyclonic vorticity center over the both SSCS and NSCS. In summer the vorticity center over the NSCS weakens, whereas wind stresses over the SSCS reverse to an anticyclone. Another reasonable interpretation is that the propagation of the Rossby wave in the SSCS is much faster than that in the NSCS. As a result, the SSCS can take a response to the external forcing more quickly than that in NSCS (Yang, 2000). Based on a spin-up result of the 1/6o resolution OGCM model, Wang et al. (2002) revealed the role of the Kelvin and Rossby waves in the build-up process of the SCS circulation. They approached the characteristic time scale of adjustment of wind-driven circulation in the SCS. It will take about one month to propagate around the SCS basin by Kelvin waves. Initiated at the northernmost tip of Philippines, it will take about 3 months to propagate from the eastern boundary to the western boundary by Rossby waves. In the view of dynamics, their work presents us a reason of the large difference between the winter and summer circulation patterns.

In winter, the cyclonic wind stress in the northwestern SCS forces a cyclonic gyre to the west of the Luzon Strait, whose typical spatial scale is similar to the Rossby deformation scale. Yang (2000) and Liu et al. (2001) suggested the westwards propagation of the wind force leads to the similar shift of SSHA, which often are regarded as a Rossby wave signal. In the matter of the wave propagation in the interior SCS, the systematic eastward migration of wind stress curl will drive a westward-propagating forced Rossby wave. Thus it presents an eastward migration of SSHA, whereas the westward-propagating signal carried by free Rossby waves always obscured by the forced eastward-propagating Rossby wave. It is much different from that in the open oceans. As a consequence, the seasonal variation of the SCS basin-scale circulation is mainly attributed to the internal dynamical process. Cai et al. (2001) simulated a westward propagating anticyclonic eddy within the evolvement of the winter circulation in a two-layer model, which has a period of 50 days, analogous to the westward propagation Rossby wave. Gao et al. (2002) analyzed the temperature variation of the three mooring buoy data located in the central SCS, and found out that the thermocline had a significant intraseasonal signal.

Recently, besides these, there is some other works about the planetary waves in the SCS. Hu et al. (2002) documented that SSHA migrates from the Luzon Strait to the western boundary of the NSCS with a speed of about 11-12 cm/s, which resembles the Rossby wave propagating from the Pacific Ocean to the SCS. Gan et al. (2001) found that, the calculated velocity of the baroclinic Rossby wave with interannual time scale is about 30 cm/s.

 

V.  WATER EXCHANGE VIA THE STRAITS

Several straits connect the SCS with the surrounding waters. However, water exchange at the Taiwan, Kalimantan and Palawan Straits is mainly concentrated in the surface layer, whose effect on the intermediate circulation can be negligible. In the Luzon Strait, which connects the SCS with the western Pacific in the northern tip of the Luzon Island, waters in the NSCS are influenced by the Kuroshio directly. The Strait's water depth and geographical location, the outside western boundary current (WBC) and the western boundary countercurrent (WBCC) (Qu et al., 1998, 1999) makes it necessary that the transport at the Luzon Strait has significant variation in the vertical direction.

1.  Water Exchange at the Luzon Strait

In early stages, the discussion about water exchange between the SCS and Pacific Ocean was mainly limited to the surface layer. First of all, Wyrtki related the monsoon with the exchange between the Kuroshio water and the SCS water. He suggested that the winter monsoon force the Kuroshio water to enter the Luzon Strait and even to reach the western coast of the NSCS. However, the summer monsoon drives the SCS water into the Pacific. Based on the observed data, he thought that the Philippines seawater entered the SCS perennially above the submarine sill in the Luzon Strait. By the aid of different data sets, which including in situ data (such as temperature, salinity and currentmeter data), remote sensing data (such as buoy trajectory data, SSH, SST data) and the model simulation, it has generally been accepted that the Kuroshio surface water does enter the SCS.

How does the Kuroshio intrude the SCS? Li suggested that the Kuroshio enter the SCS in the way of loop current. After shedding off from the Kuroshio, water mass intrudes into the NSCS in an eddy style all the year round, which is analogous to the Mexico Gulf Stream. Li et al. (1997, 1998) discovered a cyclonic ring away from the Kuroshio by using in situ data, whose horizontal scale is about 150 kilometers, and vertical scale is about 1000 meters. The maximum surface velocity is about 1 m/s.

It is very difficult to observe the exact intrusion of Kuroshio into the SCS directly. As an alternative way, when and how the Kuroshio intrudes into the SCS is approached mainly from the diagnostic analysis or from numerical modeling. The geostrophic velocity field at the isopycnal surfaces, calculated by the P-vector method on the basis of temperature and salinity data, indicates that the back and forth of Kuroshio waters (also named as the Philippine Sea waters), which pass through the Luzon Strait at the intermediate layer, can be well confined by the 34.6 isohaline. The maximum (minimum) intrusion of the salinity tongue arises in June (October). The warm, saltier Kuroshio water enters the SCS through the Luzon Strait from October to March, whose maximum (minimum) intrusion arises in February (September) with a volume transport of about 13.7 Sv (1.4 Sv), and the annual mean volume transport is about 6.5 Sv (Chu et al., 2000). Similar studies show that the saltiest North Pacific Water (NPW) enters the SCS all the year round with a semi-annual period, strong intrusion occurs in winter and summer, and weak intrusion occurs in spring and autumn. On the other hand, the freshest NPW enters the SCS only in spring, when the saltiest NPW enters the SCS very little. However, the annual mean mass transport through the Luzon Strait is 3 Sv, and the maximum (minimum) arises during January and February (June and July) with a volume transport of about 5.3 Sv (0.2 Sv) (Qu et al., 2000). Metzger et al. (1996) pointed out that the volume transport of through the Luzon Strait is about 4.4 Sv as annual mean with an amplitude of 2.5 Sv from a coarse resolution modeling result. Based on the observing result of the SCSMEX in 1998, Su et al. (1999) didn't find the evidence of significant Kuroshio intrusion. Note that the survey period of the SCSMEX is from April to July, when the minimum Kuroshio intrusion occurs in the study by Qu et al. (2000).

2.  Dynamic Interpretation of the Kuroshio Intrusion

Most of the work did not reveal how the Kuroshio water enters into the SCS, and what factors control the intrusion. From the observed data, Farris et al. (1996) find a loop-current development process, which is largely determined by an integrated supercritical wind stress parameter. The loop current grows when a four-day average of the local wind stress component directed to the south exceeds 0.08 N m-2. When this average wind stress component drops below the critical value, the Kuroshio returns to its northward path. In fact, the Kuroshio, South China Sea gyre, monsoon and local topography all influence circulation in the Luzon Strait area. Chern et al. (1998) applied a numerical model to examine the influence of the difference in the vertical stratification between the SCS and the Kuroshio waters on the loop current of the Kuroshio in the Luzon Strait during summer. According to their model results, the loop current's strength in the strait reduces as the strongly stratified South China Sea water is driven northward by the southwest winds. Numerical results also indicate that the Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. Moreover, the water flowing from the SCS contributes primarily to the near shore part of the Kuroshio.

Numerical modeling indicates that the volume transport through the Luzon Strait is mainly attributed to the pressure gradient fed by the mass convergence due to the monsoon forcing. Ekman pumping driven by wind stress curl doesn't affect the transport through the Luzon Strait. Topography also plays an important role in the transport. There is no evidence for any relationship between the interannual variability of the Kuroshio intrusion and the meridional or zonal components of wind or wind stress strength or wind stress curl in this area. However, it can be found that the seasonal cycle of the transport through the Luzon Strait has tight relationship with seasonal reversal of the northeasterly and southwesterly monsoon. Some other independent researches also suggested that pressure gradient be the major factor influencing the Kuroshio intrusion (Qu et al., 2000), with the enhanced influence concerning the β effect (Yuan, 2002).

3.  Vertical structure of Water Exchange Through Luzon Strait

An analysis of the climatological temperature and salinity data set indicates that the water property below the 3500 m depth in the SCS is similar to that of the Philippine Sea water in the layer from 1900 m to 2000 m, which shows that the bottom water in the SCS originates from the western Pacific (Nitani, 1970). Same result also could be gotten from the chemical tracer analysis. Using a box model, Han (1998) suggested the renewal time of the deep-water below 2000 m is about 76 years. Fang et al. (2002) pointed out the circulation between Pacific Ocean and Indian Ocean through the SCS as a branch of the Throughflow. From their numerical results, the annual mean transport is about 4±1.5 Sv, and the renewal time of the SCS water is 40±15 years. All the evidences mentioned above show that the renewal rate of the SCS water is much faster than that of the open ocean.

The regular observations indicate that the Kuroshio water (SCS water) flows into  (out of ) the SCS basin changed seasonally in the upper layer (from surface to 350 m). In the intermediate layer, the SCS water flows to the northwestern Pacific all the year around (Chen et al., 1998). In the deeper layers, what has been confirmed by distribution of isothermals and relationship between temperature and salinity, is that the Northern Pacific Bottom Water enters the NSCS in the layer from 1500 meters to the bottom of the Luzon Strait. Therefore, the SCS water originates from the Pacific can be inferred. The intruded bottom water acts as an anticyclonic circulation (Wang et al., 1986). By analyzing the observed data sampled at the east part of the Luzon Strait, Qu et al. (1997, 1998, 1999) also pointed out that the SCS water had a close relationship with the water outside the Luzon Strait. From the temperature and salinity observation, Wang et al. (1986) estimated the transport of the bottom water (1500 m to 2500 m) into the SCS at about 0.7 Sv. However, the direct observations show that the transport of the deep water through the Luzon Strait into the SCS is about 1.2 Sv (Liu et al., 1998). The vertical distribution of its currents can be also found in some numerical results (for instance, Yuan, 2002).

Some researches have been done about water exchange between the Kuroshio and the SCS in the upper layer. However, the issue about how and why the deep and intermediate waters go through the Luzon Strait is an open question. Based on the analysis of the temperature and salinity data, salinity of the saltiest SCS subsurface water is saltier than that in Philippine Sea, while salinity of the freshest intermediate SCS water is fresher than that in Philippine Sea (Li et al., 2002). This kind of salinity distribution hints that there exist strong mixing processes in the interior SCS. Yuan (2002) argued that the SCS works as a “mixing mill” that stirs the surface and deep waters to return them to the Luzon Strait at the intermediate depth. Based on the assumption above mentioned, he inferred that the volume transport through the Luzon Strait may be dominated by the diapycnal mixing.

 

VI.  INTERANNUAL VARIABILITY OF THE SCS CIRCULATION

The air-sea system over the SCS connects with global climate system to a certain degree. The upper layer circulation in the SCS has significant seasonal adjustment responding to seasonal change of wind stresses. Interannual variation of the large-scale wind field mainly causes interannual variability of the upper layer circulation. Interannual variability of the SCS relates closely with the Southern Oscillation Index, which stands for interannual variability of the tropical ocean and atmosphere. This reveals that the SCS interannual variability may have a planetary scale background associated with the global low frequency oscillation (Wang, 2000). Some researchers found that SST in the SCS can be taken as a kind of forecasting index of Asian monsoon onset, and so much as ENSO events (Wang and Xie, 2002). Hwang et al. (2000) also found interannual variability of SSHA correlating to ENSO in the SCS.

1.  Upper Layer Oceanic Thermodynamics during ENSO

In the view of climatology, the sea waters get heat in the SSCS and lose heat in the NSCS. As an integral, the SCS is a neutral system with the heat exchange balanced between air and sea, till ENSO events destroy this equilibrium. The pronounced anomalies appear in the SCS during ENSO. This response appears not only in the thermal structure of upper layer, but also in the SCS circulation and in wind field. Many authors found that the upper thermal structure in the SCS has significant variations in periods of quasi-two years, 3-5 years and interdecadal oscillation (Wang, 2001). He et al. (1997) found that the upper layer heat content in the SCS increases significantly in the El Niño years, with a delay of 4-12 months compared with the eastern equatorial Pacific. Wang et al. (2000) supported this point and found that whenever a warm/cold event occurred in the SCS, an El Niño/La Niña event with 5 months delay happened in the eastern equatorial Pacific.

2.  Anomalous Wind and Modified Circulation during ENSO

Accompanied with occurrence of ENSO, there are northeasterly anomalies before ENSO events and southwesterly anomalies during ENSO. Wu et al. (1998) decomposed the simulated SCS circulation field in 1992-1995 by the means of EOF, and found that there was a southern center in the signature structure in the first mode, corresponding with a seasonal reversal of circulation in the SSCS, in April and October, respectively. Two centers are found in the second mode in the SSCS and NSCS respectively, corresponding to the seasonal and interannual variations in circulation in the NSCS. The NSCS circulation changed greatly in the El Niño year, for instance, the winter time circulation weakened and shrunk southward, whereas the summer time circulation displayed no dipole structure. The EOF of wind stress curls shows that the first two modes in the SCS circulation can be explained by the two leading modes in the wind stress curl. In the El Niño year the second mode of wind stress curls decreased greatly. In another research, changes of circulation field during El Niño were testified also (Wu et al., 1999). By EOF decomposing TOPEX SSH data of 1992-1995, Shaw et al. (1999) found that the first mode, corresponding to the basin-scale oscillation in the SCS, has weaker interannual signal. The weakened wind stress curl in winters of 1992-1993, 1994-1995 and summer of 1995 resulted in a weak winter and summer circulations respectively, while the reduced low center off Vietnam showing a suppressed eastward Vietnam jet current. These studies show interannual variability in the basin scale-circulation in the SCS connected with the East Asian monsoon. Wang et al. (2001) found that anomalous stream function in summer during the El Niño events mainly strengthen the seasonal mean pattern, e.g., strengthen the anticyclonic gyre in the SSCS and the cyclonic gyre in the NSCS, but weaken the entire cyclonic gyre in winter. In the La Niña case, anomalous stream function in summer mainly weights in the NSCS, i.e., weakens cyclonic gyre. But in winter it strengthens the entire cyclone.

Distinct changes occurred in the vertical mode in the SCS circulation during ENSO. Chao et al. revealed the modification of the SCS circulation in El Niño 1982/1983 by using numerical model. A weaker surface flow leads to a strengthened upwelling in the central basin and a weakened downwelling in the surrounding area during August to November of 1982, which resulted in a weakened vertical advection of heat and warmer SST. Kuo et al. (2001) carried out a study on the SST anomalies in the upwelling region in the western SCS 1997-2000. He found that upwelling in 1997 is stronger than that in other years. So does SSH in the SCS in El Niño 1997/1998. By means of qualitative analysis and quantitative diagnosis of SST, wind stress, SSH and thermocline in the SCS during El Niño 1997/1998 and during La Niña 1998, Wang et al. (2002) found that the positive correlation between SSTA and the meridional wind anomaly only occurred in the stages of onset and development of the 1997/1998 warm event, whereas the maintenance of the warm event is controlled by the downwelling mode. In other words, anomalous downwelling pushes thermocline deeper, so the positive SSTA maintained in the SCS can last longer.

 

VII.  DYNAMICS OF THE MIXED LAYER AND THERMOCLINE OF THE SCS

Driven by monsoon, a significant seasonal reversal occurs not only in the SCS circulation, but also in heat storage. There are significant exchanges of mass and energy between the mixed layer and thermocline via process of entrainment and detrainment due to turbulent mixing in the mixed layer (or subduction/ventilation), which affect the circulation in thermocline and below. It was one of the key contents of ocean circulation theory, with significant implications in marginal seas, say, in the SCS (Wang et al., 2001). The most prominent interannual variability in the SCS lies in the level of 100m instead of 300m, contrast to the Kuroshio. Controlled by internal dynamical process, interannual change at the surface is weaker than that at the subsurface in the SCS (Wang et al., 2000). Intraseasonal variation is usually restrained at the surface and subsurface, and also modified by the dynamic process (Gao et al., 2002).

1.  Climatic Features of the Mixed Layer and Thermocline in the SCS

Previous studies about thermocline in the SCS were mainly limited to its strength and thickness, as well as its classification by using single station data (Qiu et al., 1996). Other studies carried out simple diagnostic analysis or forecasting by a simple, 1D mixed layer bulk model (Jia et al., 2000). Qu et al. (2001) found that the mixed layer depth had the negative correlation with SST, and the surface heat flux played an important role in seasonal circle of SST, whereas, influence of oceanic dynamical process could not be ignored. Shi et al. (2001) and Du (2002) show that the mixed layer is in a good agreement with the large-scale circulation. For example, monsoon imposes on the temporal and spatial variation of the mixed layer via the associated advection in the flow field. They also found that among such dominant factors controlling the mixed layer, as wind stress, surface heat flux and fresh water flux, the wind stress is the most important one. Liu et al. (2001) found that the thermocline became deeper and thinner under the action of surface cooling in winter. It can be also found that intraseasonal variation of thermocline is mainly determined by geostrophic currents. Moreover, in the intraseasonal time scale, thermocline and SSH are out of phase. According to the data analysis and numerical modeling, Du (2002) found that the most leading factors for seasonal dynamics of the mixed layer and thermocline of the SCS were direction, strength of wind and wind stress curl, while the both effects had the same order of magnitude.

Some analyses of the thermal structure revealed the existence of barrier layer in the SCS (Jia 2000; Wu et al., 2001; Zhu and Qiu, 2002). The barrier layer usually weakens the cooling effect entrained at the bottom of mixed layer. Jia (2000) found that there are barrier layers in both the NSCS and SSCS, but they are thinner than that in the western equatorial Pacific. Wu et al. (2001) found that a barrier layer in the SSCS has a seasonal variation. The barrier layer depth has a positive correlation with temperature in the mixed layer. Du (2002) found that the barrier layer often exists in summer and autumn. The structure of barrier layer in the SSCS is significantly modulated by wind field, as well as by development of the mixed layer. In summer, relatively fresher water in the upper layer in the SSCS piles up in the southeast SCS because of the combined action of southeastward Ekman transport and downwelling in the eastern SCS. The high temperature water at the bottom of mixed layer keeps in a thermally uniform layer after separating from the mixed layer. The deepest barrier layer lies in the southeastern SCS, about 30m depth. The location of the thickest barrier layer is almost overlap the SCS warm water, which suggests that the heat barrier effect may stimulate development of the SCS warm water.

2.  Warm Waters of the SCS

It was found in several studies that monsoon has a close relationship to the upper water with high temperature formed in spring in the SCS. By analyzing the MOODS of US Navy, Chu et al. (1997) found that both enhanced radiation in spring and convergence/downwelling induced by the anticyclone circulation in the upper layer can stimulate appearance of warm water, over 29.5°C off the west Philippines. It will disappear after the summer monsoon onset. Li (1999) found that fade of the warm pool during monsoon of the SCS is a main feature associated to monsoon reversal. However, the warm waters mentioned above are formed in short time and located in a small sea area.

According to the definition of warm pool in the western Pacific made by Wyrtki, warm waters of the SCS can be defined as SST over 28°C (He et al., 1999). When the three-dimensional structure concerned, the seasonal cycle in the SCS warm water can be classified as four stages, namely development, maintenance, weakness and disappearance stages. Local surface heating usually causes development. In the maintenance stage turbulent mixing and Ekman effect maintain the SCS warm water, whereas SST decreases rapidly in autumn and winter. Then the weakness and disappearance are caused by entrainment of cold water at the bottom of mixed layer (Jia et al., 2000; Zhang et al., 2001). The upper layer circulation over spring to winter also favors development and maintenance of the SCS warm water (He et al., 2000). Variability in the SCS warm waters feeds back to the air-sea interaction over a broad scale. A coupled teleconnection exists among the SCS warm water, ENSO and subtropical high in the western equatorial Pacific. On the other hand, its variations coincide with SST in the warm pool of the western equatorial Pacific, the central and eastern equatorial Pacific (He et al., 1997; Wang et al., 2000).

3.  Ventilated Thermocline in the NSCS in Winter

Studying about thermocline in the NSCS in winter, Wang et al. (2001) found that the thermocline was ventilated in the NSCS, which was accompanied by occurrence of potential vorticity with a specific circulation pattern. The thermocline ventilation is a seasonal phenomenon in the SCS actually. The potential vorticity in winter has a high value center and its distribution looks like a thin and flat ellipsoid. Around the edge of this center, horizontal circulation movement can be tracked. Water subducted into thermocline from the mixed layer moves southward in the path of cyclone along the edge of seasonal potential vorticity pool. Following researchers show that signals in terms of the monthly temperature increment seems come from intrusion of the Kuroshio into the SCS (Du, 2002). These show that seasonal variation of the SCS can deeply reach to the thermocline. Thus, study of the SCS upper circulation should include the thermocline dynamics. Moreover, research into the dynamics of the thermocline and the mixed layer can help interpret seasonal variation of the SCS circulation (Wang, 2001).

 

VIII.  AIR-SEA INTERACTION OF THE SCS AND ITS CLIMATIC IMPLICATION

On the basis of the NCAR regional climatic model RegCM2 and the ocean model POM, Lu et al. (2000) developed a regional air-sea coupled model applied to the interaction over of the SCS to simulate, for example, the coherent variation in air and sea. The model adopts synchronous coupling scheme. The ocean model provides SST to heat the atmosphere model, while the atmosphere model provides short wave radiation, sensible and latent heat fluxes to the ocean model. The ocean model and the atmosphere model exchange fluxes every 15 minutes without any flux correction. Zhao et al. (2000a) analyzed space-time evolution of the singular vector field of SST anomalies in the SCS and in the eastern tropic Indian Ocean from winter to summer, indicating that the tropical SST anomaly field centered off Sumatra from the end of spring to beginning of summer can be traced to the SST variations of the SCS in winter. The better relationship between the latter and the monsoon break over the SCS means that there is a forecasting index of monsoon break over the SCS.

Using the OISST data provided by NECP/NCAR, Zhao et al. (2000b) analyzed the interannual variability in the SCS warm water and its relationship with the warm pools in both the western equatorial Pacific and the Indian Oceans. They detected the relationship between the interannual variability in an index of the SCS warm water and the monsoon break over the SCS. They found that the SCS warm water, and the warm pools in both the western equatorial Pacific and the Indian Oceans are in a same coupled system with large scale, sharing a long period oscillation about 4.8 years. Climatic implication of SSTA in the SCS and its adjacent seas can be revealed by numerical simulation. The numerical experiments of the IAP-AGCM done by Wang et al. (2001) show that atmospheric circulation over the East Asia has a time-dependent response (or seasonality) to SSTA, with a certain meridional and zonal structure. During the years when colder water occurs in the SCS, an atmospheric anticyclone maintains at the east to Philippines in summer. A low frequency activity associated with the anticyclone resulted in the low frequency oscillation in the precipitation field over this region, whereas the weak subtropical high over the western Pacific travels eastward in summer, which is responsible to anomalous distribution of meridional vapor transport.

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