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THE SOUTH CHINA SEA MONSOON

EXPERIMENT (SCSMEX)

DING Yihui1LI Chongyin2 and LIU Yanju1

1. National Climate Center, Beijing 100081, China

2. Institute of Atmospheric Physics, CAS, Beijing 100029, China

ABSTRACT

The present paper gives an overview of the key project “South China Sea Monsoon Experiment (SCSMEX) operated by the Ministry of Science and Technology of China during the period of 1996-2001. The SCSMEX is a joint atmospheric and oceanic field experiment with aim at better understanding onset, maintenance and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May-August, 1998, a large amount of meteorological and oceanic data have been acquired, which provide excellent datasets for the study of the SCS and East Asian monsoon and their interaction with the ocean. The preliminary research results are very encouraging .The follow-up study is now underway.

Key words:  South China Sea monsoon, field experiment, air-sea interaction

I.  INTRODUCTION

The Asian monsoon has a profound influence on the social and economic condition of over 60% of the earth's population. For centuries, successful forecasting of monsoon rainfall has been a matter of great concern for the people and countries in Asia and surrounding areas. The Asian monsoon can be divided into two systems (Tao and Chen, 1987): the South Asian monsoon and East Asian monsoon systems. The SCS monsoon is one of the important subsystems of the East Asian monsoon. The SCS connects the monsoon originating from the Southern Hemisphere, the monsoon over the western Pacific Ocean and the monsoon over the Indian Ocean due to its unique geographical location as a boundary region between the Asian continent and the western Pacific. The origin, development and evolution of the SCS summer monsoon can greatly influence the weather and climate in East Asia and Southeast Asia (Ding, 1994).

In recent years, the study on the SCS monsoon becomes a focus in the field of monsoon and climate variability again. Through the joint effort of scientists of many countries/regions for years, the unique features of activities of the SCS monsoon have been revealed. Tao and Chen (1987) have put forward that the Asian monsoon broke out earliest over the SCS, usually in the middle May, then moved northward to China mainland and the western Pacific Ocean to the south of Japan, and finally moved northwestward to the Bay of Bengal and the Indian Peninsula. So, the onset of monsoon over the SCS and adjacent areas may be regarded as a signal of the beginning of the Asian monsoon (Ding and Murakami, 1994; Tanaka, 1994). Recently, Wu and Zhang (1998) have further pointed out that the onset of the Asian monsoon consists of three phases: the first onset of the monsoon over the eastern part of the Bay of Bengal during the early days of May, then the onset of the East Asian monsoon on May 20 and the final onset of the Indian monsoon about June10.

The relationship between the Asian summer monsoon and precipitation, especially the linkage of the summer monsoon to Meiyu/Baiu season precipitation has also been investigated by numerous researchers (Yoshino, 1965; Tao and Chen, 1987; Lau, Yang and Shen, 1988; Ding, 1992 ; Yamazaki et al., 1999). The rainy season in China as well as East Asia generally begins with the onset of the summer monsoon and ends with its withdrawal. The major seasonal rain belt over the East Asia moves from low to middle and high latitudes as the summer monsoon develops, and experiences three stable stages and two sudden northward jumps, namely being stationary over South China, the Yangtze-Huaihe River Basins and North China respectively, and then retreats southward rapidly, thus leading to the end of major precipitation season in summer in China (Guo and Wang, 1981).  On the other hand, rainfall intensity and seasonal positions as well as their variations are also significantly modulated by fluctuations of the summer monsoon, especially in the domain of low frequency (Lau, Yang and Shen, 1988). Furthermore, the interannual variability of the Asian monsoon also exerts an important effect on precipitation and climate regimes in East Asia , even at global scale (Yasunari and Seki 1992; Tanaka, 1997). The drought/flood, extreme cold/hot summer and other damaging climate disasters are basically caused by the great interannual variability and intraseasonal variability of the monsoon. Especially, the early or late arrival of the summer monsoon, rapid or slow northward movement and its intensity variation directly influence the temporal and spatial distributions of the major monsoonal rain belt in summer in East Asia and the occurrence of drought/flood during the rainy season (National Climate Center, 1998). Recently, Lau and Weng (2000) have reported their new finding that there are two distinct climate teleconnection patterns that links major U.S. summer droughts and floods to variability in the Asian summer monsoon, mainly in the SCS and the West Pacific. Therefore, the study of the Asian summer monsoon, especially the summer monsoon over the SCS, not only has the regional implication, but also has a global effect. The above-described scientific rationale contributes to the scientific motivation behind launching the South China Sea Monsoon Experiment (SCSMEX).

SCSMEX is a multi-national atmospheric and oceanographic observation plan, and is also the first large-scale joint research project between meteorologists and oceanographers for the South China Sea (SCS). SCSMEX is closely linked to and coordinated with activities of national weather services and research institutions of East Asian countries and adjacent regions as well as ongoing and planned China, US and international field experiments and research programs such as the Global Energy and Water Experiment (GEWEX) and Climate Variability and Predictability (CLIVAR). Many countries or regions were involved in the enhanced and intensive observations of SCSMEX, including 10 provinces in the South China, Taiwan, Hong Kong, Macau; USA, Australia, Thailand, Vietnam, Malaysia, Singapore, Brunei, Indonesia and Philippine.

 

II.  SCIENTIFIC GOAL AND OBJECTIVES

The primary goal of the SCSMEX is to:

Provide a better understanding of the key physical processes for the onset, maintenance and variability of the monsoon over Southeast Asia and southern China leading to improved predictions (SCSMEX Project Office, 1995).

To attain the goal of SCSMEX, we aim at the following specific scientific objectives:

(1) To describe and document the space-time evolution of the large-scale atmospheric circulation, thermodynamic fields, as well as basic ocean flow patterns and thermohaline structures associated with the SCS monsoon.

(2) To identify the influence of heating contrasts between the South China Sea (SCS) and surrounding regions and the role of early monsoon (April-May) convection and multi-scale processes in the SCS in the abrupt transition and subsequent evolution of the East Asian monsoon.

(3) To elucidate physical processes in oceanic response to monsoon forcing and air-sea interaction in the South China Sea (SCS) and relationships with adjacent oceans.

(4) To assess and improve the ability of regional and global models in simulation and prediction of the monsoon onset in Southeast Asia and in southern China.

Three components of SCSMEX were considered:

(1) A pilot-phase component devoted to advanced deployment of observation platforms for enhanced monitoring, testing of observation strategy based on diagnostics and modeling studies.

(2) A field-phase component involving the set up of a multitude of meteorological and oceanic observing platforms and satellite coverage during intensive observation periods (IOP).

(3) A modeling component using a wide range of models from regional to global scales to provide better understanding of physical mechanisms underlying the observations and to augment the field-phase observations through 4-D data assimilation.

 

III.  FIELD OBSERVATIONS

A four-month field phase that covers the period from May 1 to August 3, 1998, is one of the core components of SCSMEX (SCSMEX Project Office, 1998). The observation system consists of the most advanced atmospheric and oceanic observation platforms and instruments including radiosonde, surface observation network, weather radar, scientific research ship, Aerosondes, satellite observation, oceanic boundary layer and flux measurement, integrated sounding system (ISS), radiation, ATLAS moorings, drifting buoys, ADCP, CTD, and air-borne expendable bathythermograph (AXBT) etc. Under the support of participating countries/regions, the four-month field observations have been completed successfully. A wealth of information and data have been obtained on the early onset of monsoon in the northern region of the South China Sea (SCS), the evolution and the northward migration of the monsoon to the Yangtze River Basin in the context of El Nino events, which provides an excellent basis for further research.

1. Observation Network

SCSMEX has designed the large-scale observation region and intensive observation domain. The former includes a large Asian-West Pacific region (70°—150°E, 10°S—40°N) with the focus on conventional observations; the latter is located on the South China Sea (SCS) and its surrounding regions (95°—130°E, 10°S—30°N). Two intensive flux arrays at nearly meso-scale in the intensive observation domain are further designed to carry out sounding, surface observation, ISS, Aerosonde, dual-Doppler radar, radiation, satellite observation, boundary layer flux and oceanic integrated observations.

2. Observation Period

The enhanced phase of field observation for SCSMEX is from May 1 to August 31, 1998, during which there are two intensive observation periods (IOP), May 525 and June 525.

The first IOP focuses on monitoring the onset of the South China Sea (SCS) monsoon and its sudden seasonal change as well as its implication for precipitation in South China and Southeast Asia.

The second IOP focuses on monitoring atmospheric and oceanic conditions over the South China Sea (SCS) during the period of the mature phase and northward migration of East Asian monsoon, and its implication for precipitation in the Yangtze River Basin, the Korean Peninsula and Japan.

3.  Field Observation Platforms

Field observations consist of atmospheric observation network, oceanographic observation network, air-sea interface observation network, and satellite observation network.

(1) Atmospheric observation network

(a) Radiosonde observation

Radiosonde observation is the main observation tool to measure the atmospheric structure below 30 km. 66 land and island sounding stations are involved in SCSMEX. During IOPs period, 36 intensive upper-air sounding stations were in operation, of which 33 stations carried out four times observations every day. There were 11 stations to carry out intensive observation during the ten days without any observation gap between two IOPs. The research vessels R/V Kexue #1 and R/V Shiyan #3 were located in the southern (109°50E, 6°13'N) and northern (116°50E, 20°22'N) South China Sea, respectively, to carry out upper-air sounding observations. The R/V ORI also carried out the intensive upper-air sounding observations during the first IOP in the northern part of the SCS.

(b) Integrated sounding system (ISS) observation

Integrated sounding system is a kind of comprehensive sounding system which integrates surface observation, boundary layer observation and upper-air observation into a whole system, which installs all above-mentioned observation instruments in a container to carry out integrated observations in order to obtain three-dimensional continuous distribution of meteorological elements. The ISS of SCSMEX is installed on the Dongsha Island in the north of the South China Sea (SCS). It consists of UHF wind profiler, OMEGA sounding carried by balloon, RASS system for thermal profile sounding and surface meteorological observation. They monitored the continuous evolution of meteorological elements of the boundary layer and the troposphere during the period of the onset and intensification of the South China Sea (SCS) monsoon. ISS, together with satellite and radar observation network, successively provided information on wind variations, cloud distribution and precipitation time series in the South China Sea (SCS).

(c) Optical rain gauge observation

Three optical rain gauges are separately installed on Dongsha Island, one ATLAS mooring and R/V Shiyan #3. They, together with TRMM observation, may provide high-accuracy precipitation data over the northern part of the South China Sea (SCS).

(d) Wind profiler observation

Wind profilers are installed in Hong Kong, Singapore and Dongsha Island, respectively. They have completed continuous upper-air (below 1215 km) wind observations during the enhanced observation period.

(e) Aerosonde observation

Under the elaborate planning of scientists, Aerosonde, unman-made small airplane on Dongsha Island successfully made 19 flights. Most of them carried out vertical sounding observations at the range of 3001500 m over the sea in the southern SCS, and made great-circle flights around Dongsha airport, C-Pol radar and TOGA radar on R/V Shiyan #3 to observe monsoonal cloud-rain systems over the sea. On May 15, Aerosonde flied 6 hours in thunderstorm cells. The longest flight was 25-hour continuous vertical sounding observation from 9:20 am May 23 to 10:30 am May 24, thus obtaining important data describing atmospheric boundary layer features under stable weather situation over the Dongsha water region.

(f) Surface observation

237 surface stations were involved in SCSMEX. Among them, 105 stations carried out 24-times observations per day during IOPs; others carried out 8-times observations per day. R/V Kexue #1, R/V Shiyan #3, R/V Haijian #74, R/V Xiangyanghong #14 and R/V ORI carried out surface meteorological observations when they undertook cruising missions over the sea. Three ATLAS moorings installed in the northern South China Sea and island stations (Taiping, Yongshu, Xisha etc.) have also obtained important atmospheric surface observation data.

(g) Weather radar observation

Radar observation network consists of 34 digital weather radars, C-Pol radar on Dongsha Island and TOGA Doppler radar on Shiyan #3. C-Pol radar on Dongsha Island successfully observed the structure of linear convective cloud systems and medium-scale convective cloud systems. TOGA Doppler radar on Shiyan #3 and C-Pol radar on Dongsha Island constructed dual-Dual radar array, and successfully obtained information on the evolution of meso-scale convective systems such as tropical squall lines, and water spout during the period of the onset and development of the summer monsoon.

(2) Air-sea interface observation network

(a) Flux observations

The observation of air-sea interface flux is a main way to understand air-sea interaction, which can monitor the exchange of heat, moisture and momentum at the air-sea interface.

The PBL tower with 18 m of height to undertake air-sea flux observation was set on the sea surface, 300 m away from the reef shelf to southwest of Yongxing Island (Xisha Island). Gradient observation system was divided into four layers: 2, 4, 8 and 16 m, including gradient observations of temperature, humidity and wind, fluctuation observations of temperature, humidity and wind speed, long- and short-wave radiation observations and sea surface temperature observation. This system recorded PBL information under different weather conditions including the onset and break of monsoon, the predominance of subtropical high and the incursion of cold air. These data can be used to compute the exchange of momentum, sensible heat and latent heat between sea and air. In addition, high-accuracy instruments measuring air-sea interface flux were also installed on Dongsha Island and oceanographic scientific research ships (R/VKexue#1, R/V Shiyan#3, and R/V Haiyan#1). So, the surface flux datasets at 5 sites in total may be available for SCSMEX scientists.

(b) Radiation measurement

The advanced surface radiation-budget instruments provided by NASA was installed on Dongsha Island. Most instruments can normally receive and record radiation data. The air over Dongsha Island is pretty clean, and the topography there is flat without obstacles surrounding the observation platforms. Therefore, obtained data have a good representativeness. Based on these data, the budget of atmospheric heat can be estimated in order to understand the effects of heating field on the monsoon.

(3) Satellite observation

Satellite observations include GMS-5, NOAA polar-orbiting satellite and TRMM, from which many kinds of products and parameters may be produced. The data from these satellites provide SCSMEX with information on precipitation, cloud, high-resolution blackbody temperature (TBB), outgoing long-wave radiation (OLR), water vapor, sea surface temperature, vegetation types and land surface process etc. Equivalent TBB and OLR data can be used to detect the continuous evolution of convective cloud systems of in the monsoon region.

(4) Oceanographic observation

(a) Oceanic scientific research ships

Oceanic scientific research ships are absolutely necessary platforms for atmospheric and oceanographic observations in recent decades. On these ships, various meteorological elements can be observed, sounding balloon can be launched, radar can be installed, and various elements at air-sea interface and oceanographic elements down to deep sea can be simultaneously observed. A total of 6 ships participated in SCSMEX and successively completed atmospheric and oceanographic observations. A wealth of information has been obtained on atmospheric conditions, air-sea interface flux and sea surface temperature, salinity and ocean current, including hundreds of oceanic profiles from sea surface up to deep sea. Moreover, as a platform of atmospheric observations, about 500 GPS sounding sets were launched on ships to obtain high-resolution atmospheric upper-air elements per 10 seconds.

(b) Buoys

Mooring buoys: Taiwan scientists have launched three large ATALS mooring buoys between Dongsha and Taiping Islands. Among them, the first buoy is located at 18°N, 115°E, the second buoy at 15°N, 115°E, and the third buoy at 12°N, 114°E. Another buoy has been installed near Dongsha Island. In addition, 9 oceanic mooring buoys have been deployed in Bashi straits and the northern South China Sea. These buoys have obtained continuous important atmospheric and oceanic data. 

Drafting buoys: R/V Haijian #74 and R/V Xiangyanghong #14 released 4 ARGOS drafting buoys in the southern South China Sea on April 24 and in the second half of June, respectively.

(c) Airborne expendable bathythermograph (AXBT)

US scientists have released hundreds of bathythermographs in the South China Sea (SCS) in order to obtain high-density coverage SST data, which is of high value for monitoring detailed SST distribution of the South China Sea (SCS).

IV.  ORGANIZATIONAL STRUCTURE AND DATA CENTER

The SCSMEX has set up two committees, the SCSMEX Organizing Committee (SOC) and the SCSMEX Scientific Steering Committee (SSC).

 

Under SCSMEX Organizing Committee. Also, an expert committee for national project of China composed of 11 scientists is set up. Correspondingly, the similar organizations are also established in other countries and regions, which are in charge of the SCSMEX experiment and research. In USA, a liaison Office is established. During SCSMEX field observation period, the SCSMEX Operation Center is established in Guangzhou. During observation period, the operations center is responsible for sending operation instruction, guiding the operations of platforms, coordinating the work tasks of platforms and solving some urgent and contingency matters. Everyday, there was a weather briefing to discuss the activity and evolution of monsoon and to issue the weather forecasting to major observation platforms. The Center is also in charge of publishing Daily Weather and Operation Bulletin via Internet and 9210 communication networks, monitoring the work of platforms through regional network of China Meteorological Administration (CMA) and Guangdong Regional Center.

The SCSMEX has two categories of data centers. The branch centers that include data centers of every participating country (or region) mainly take charge of the collecting and processing of experimental data in their own observation domain. The main center is the SCSMEX Beijing Data Management Center. Its main task is to collect, disseminate, make quality control for experimental data during the experimental period, data assimilation analysis and to establish SCSMEX Database. Besides, there is an atmospheric and oceanic data sub-center in Guangzhou.

 

V.  MAIN RESEARCH RESULTS

1.  Results of Field Observations

A total of 6 ships participated in SCSMEX in 1998 and successively completed atmospheric and oceanographic observations. The advanced dual-Doppler Radar and Aerosonde were also deployed on the northern part of the SCS. At the same time, the large mooring buoys were launched to carry out observation in the hinterland of the SCS. In particular, Chinese scientists set up the first boundary-layer flux tower on the sea surface near Xisha Island. Through successful implementation of the field observation experiment, plentiful and valuable oceanography and atmospheric data have been obtained (SCSMEX Project Office, 1999), which provided a good basis for the follow-up research of this project. (The following results of field observation were all taken from Ding et al .1999)  

(1) Through successful implementation of the experiment, the enhanced observational atmospheric and oceanic datasets at nearly meso-scale (with the resolution of 100200 km) over a large areas of East Asia and the western Pacific Ocean in the summer of 1998 (May to August) have been collected for the first time, which can serve as a precious input for the study on the anomalous behavior and activities of monsoon under the background of El-Nino event.

(2) The fact that the SCS is a region where Asian monsoon broke out earliest has been documented by a great number of observations and the exact onset date, characteristics, evolution of the SCS monsoon and its relationship with flooding, especially the heavy flooding in China in 1998 have been further disclosed and the conceptual model of the SCS monsoon onset has been put forward.

(3) During the SCSMEX, scientists from home and abroad clearly gained a better understanding of the activities of Asian monsoon, especially their influence on the heavy flooding or drought in China. During the observation period this summer, the monsoon experienced about seven significant variations. These variations respectively corresponded to enhancement of South China precipitation, the arrival and intensification of heavy rains in the region to the south of the Yangtze River, then Meiyu onset over the Yangtze-Huaihe River Basin, short appearance of North China rain season and the retreat of seasonal rainbelt; the Second Meiyu process over the Yangtze River and the region to the South China; the rain period of the Yellow-Huaihe River and the seasonal retreat of rain period and rainbelt over North China. So the anomalously southern position and weakening of the Asian monsoon directly led to the prolonged severe flooding disaster in the Yangtze River Basin in the 1998 summer as well as above significant variations. By these studies, the theory and prediction model of relationship between the anomalies of Asian monsoon and the heavy rainstorm in China can be greatly enhanced.

4The features of interaction between the ocean and the monsoon over the SCS have been revealed through ocean observations. The SST over the SCS during the early period influenced the early or late monsoon onset and its intensity. After monsoon onset, the disturbance may affect the SST in the neighboring region, with a warm SST maintaining for the whole summer. By the analysis of observation results of air-sea interaction, a new view can be put forward to explain the fact that the SCS is a region where Asian monsoon broke out earliest, namely the comprehensive theory of air-sea interaction and land-sea interaction.

(5) The comprehensive ocean-atmospheric observational network located in the north region of the South China Sea (SCS) revealed the structure and evolutional process of 3-dimentional convective cloud systems in monsoon current. Another valuable achievement of the SCSMEX in this aspect is that tropical squall-lines and water spouts were observed for the first time over the northern South China Sea (SCS).

(6) By the analysis of air-sea momentum and radiation observation data first obtained during SCSMEX, air-sea momentum, heat and moisture were revealed during the different stage of the monsoon, which is favorable to understand the influence of the SCS ocean on monsoon flow and drought/flood in China.  

(7) The SCSMEX has set up a comprehensive and complete database. The data obtained from the field observation have been put in four-dimensional data assimilation system by China, Japan, America and so on. Also, high quality dataset for SCSMEX has been developed. At present, these data have been widely used at home and abroad. The American scientists have already used these data to carry out theorical and numerical study. They hoped to cooperate with Australia (including China) to verify their operational model, and other countries and regions (especially Japan) also utilized these data to improve and verify their operational model.

2. Scientific Results

The SCSMEX has made about seven preliminary research results.

1The fact that the SCS is a region where Asian monsoon broke out earliest has been documented by a great number of observations and research (Fig.1 ) (Li and Qu 2000). The exact onset date, characteristics and evolution of the SCS monsoon have also been revealed ( Macao Meteorological and Geophysical Bureau 2001). From the viewpoint of synoptic process, its onset is closely related to the early rapid development of a twin cyclone to east of Sri Lanka (Fig.2) (Ding and Liu 2001; Zhang et al. 2001). The 50-year time series of the SCS monsoon onset date has also been confirmed (He and Ding et al 2001). Especially, it pointed out that the SCS monsoon onset in 1998 mainly consisted of two phases (Fig.3, Fig.4) (Ding and Liu 2001). The first onset of the summer monsoon over the northern part of the SCS occurring in the fourth pentad was of baroclinic nature, which was caused by the interaction between the weather systems at middle-latitude and tropical monsoon air current. The second large-scale monsoon onset in the fifth pentad of May was of tropical nature, which was induced by rapid change in tropical circulation features (Fig.5) (Ding and Liu 2001).

(2) The characteristics of low-frequency oscillation of the SCS monsoon have been ascertained. It pointed out that the intra-seasonal oscillation has great influence on the activity of the SCS monsoon. On the one hand, before the monsoon onset, the intra-seasonal oscillation obviously enhanced and expanded from the region to the east of Philippine to the SCS  (Fig.6) (Mu and Li 2000). On the other hand, the anomaly of the intra-seasonal oscillation played an important role in the inter-annual anomaly of the SCS summer monsoon, namely strong/weak intra-seasonal oscillation corresponded to the strong/weak SCS summer monsoon (Figure omitted). The influences of the abnormal SCS monsoon on the precipitation over the eastern China and its mode have been ensured (Fig.7) (Zhang et al., 2001 and Li 2001). Also, the influences of the abnormal SCS monsoon on the atmosphere general circulation and the climate over the Northern Hemisphere, and its basic routes were put forward (Fig.8) (Li 2001).

3A wealth of observation data of the turbulence and radiation was obtained. Based on these data, the parameter of the turbulence structure, air-sea exchange coefficient and the air-sea heating, momentum and moisture exchange were subsequently computed. The results and their comparison with other field experiments were listed in table 1 (Yan et al., 2000). Figure 9 also showed the drag coefficient CD as a function of mean wind U8 for unstable, stable and neutral conditions. When the speed surpassed 3 m s-1, the CD increased linearly with the wind speed (Yan et al., 2000) . The results revealed that the heating and moisture exchange varied abruptly with the SCS monsoon onset. In particular, the net balance of the latent heating flux and oceanic heating varied apparently. The results suggested that the ocean was the process of the energy accumulation before the SCS monsoon onset. During the monsoon onset, it became the process of the energy release. While the SCS monsoon broke, the ocean was the process of the energy re-accumulation (Yan et al.2002).

Table1.  The Comparison with Other Field Experiments (Taken from Yan et al. 2000)

Elements

SCS Xisha

Tower

(5.14-6.22)

Nansha

platform

(9.19-24)

Hakuho

Ship

(11.12-26)

MoanaWave  

 buoy

(11.4-12.3)

Wecoma

buoy

(11.4-12.3)

 

WHOI buoy

(11.4-2.3)

Momentum fluxN/m2

  0.10

0.030

 0.020

0.017

0.020

Sensible flux

W/m2

  7.8

  6.0

 5.5

  6.0

 4.6

 7.0

Latent flux

W/m2

 110-130

 129.2

102.4

 89.7

 86.0

 92.0

Bowen Ratio

0.047-0.071

 0.046

 0.054

 0.067

 0.053

0.076

Atmospheric and oceanic data collected by “Shiyan 3” and “Kexue 1” during the IOP in SCSMEX98 (Ding et al. 2002) were used to calculate the air-sea turbulent flux exchange. Some characteristics of air-sea exchange were analyzed by examining the results (Qu et al. 2000). They found that the onset of the summer monsoon began in the southern region of the South China Sea on May 21 and extended northward to the whole SCS region. There were obvious differences in the characteristics of atmosphere and sea structure, air-sea turbulent flux exchange between south and north. Moreover, the variation of SST is mainly related to the latent flux transfer besides global radiation(Fig.10, Fig.11 ).

(4) Seasonal and intraseasonal variabilities of thermocline and relative surface height in the central South China Sea (SCS) were investigated using time series data of temperature from three buoys of SCSMEX in1998 by Liu et al. (2001). They found that the thermocline becomes deeper and thinner in winter, owing to a great loss of the heat on the sea surface. This feature was more evident in the northern than in the southern part of the central SCS. The intraseasonal variation of the thermocline was mainly controlled by the geostrophic vorticity and was out-of-phase with sea surface height (SSH) (Fig.12) Liu et al. (2001). Liu et al. (2001). Furthermore, a double-thermocline phenomenon occurred in the SCS: In spring, owing to maximum net downward heat flux at the surface, with the new thermocline appearing above 80 m and the old thermocline keeping under 80 m deep (Fig.12, Fig.13) ( Liu et al. 2001).

Besides, the stage features of the distributions and the vertical structure of the sea temperature at the central region of warm waters (113°E, 8°N) were explored in details by Zhang et al. (2001). Based on the 28°C constant temperature envelop surfaces, the evolution processes of warm waters were discovered (Fig.14). It was found that from April to November, the 28°C constant temperature envelop appeared like a pot pattern, with different size and depth, expressing the evolution processes of warm waters Also, the seasonal variations of temperature  above 30 m were consistent. The minimum temperature occurred in January, the maximum temperature in May, the second maximum temperature in October. At the depth of 50 m, the minimum temperature lasted till march; the maximum temperature occurred in July and the second maximum temperature in October Zhang et al. (2001).

(5) The SCS circulation variation around the onset of the summer monsoon was revealed by using the CTD data of two curies of SCSMEX. It suggested that the SCS circulation displayed two high-pressure zones and two low-pressure zones during this period, i.e., there were anticyclonic circulations to the east of Vietnam and to the west of the Philippine, and cyclonic circulations in the north and middle of the SCS (Fig.15) ( Wang and Xu 2001).

6The unique features of the interaction between the ocean and the SCS monsoon were revealed through ocean observations by Taiwan scientists. The SCS and its neighboring region were closely related with the onset and evolution of the SCS monsoon.  The SST over the SCS during the early period influenced the early or late monsoon onset date and its intensity. It could be seen clearly there was obviously warming SST in the ocean before the monsoon onset (Fig.16) (SCCMEX Project Office, 1999). After monsoon onset, the disturbance of the monsoon may affect the SST in the neighboring region, whereas variation of the SST influenced the evolution of the monsoon through the different scale thermodynamic and dynamic processes, thus influencing the weather and climate of the West Pacific Ocean, Southeast Asia and even the North America.  So, the SCS and its neighboring region are the forcing origin and sensitive region that influence the weather and climate of the Northern Hemisphere. Based on these variation (especially the early variation), it would be likely to predict and improve the seasonal, even yearly weather and climate forecast. Recently, the American scientists are beginning to regard the monsoon and the SST of the SCS and its neighboring as an indicator of their mainland drought/flood forecast.

7The moisture transport over the SCS has great influence on the summer precipitation in China. The results suggested that the situation of the moisture transport before and after the SCS monsoon onset was greatly different, which was just the important reason of the increase of precipitation in China after the monsoon onset (Fig.17) (Sun 2002). After the monsoon onset, the SCS is not only the source of the moisture supply over the East Asia ( which means the precipitation of East China in the south of the Yellow River is mainly from the SCS) but also the moisture sink. (Fig.18) (Sun 2002).

Acknowledgements: We are grateful to the Data Center of SCSMEX Project. Sincere thanks also go to all Chinese participants who participated in the SCSMEX and its later research.

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