THE SOUTH CHINA SEA MONSOON
DING Yihui1，LI Chongyin2 and LIU Yanju1
1. National Climate Center, Beijing 100081, China
2. Institute of Atmospheric Physics, CAS, Beijing 100029, China
ABSTRACTThe 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
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 NetworkSCSMEX 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 5—25 and June 5—25.
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°50‘E, 6