Monsoon Over Eastern Asia (Including China, Japan, and Korea) and Adjoining Western Paciﬁc Ocean

Chapter 5 Monsoon over Eastern Asia (Including China, Japan, and Korea) and Adjoining Western Pacific Ocean 5.1 Introduction The study of monsoon and related weather phenomena over Eastern Asia has a long history. Prior to the 3rd century B.C., it was mostly the farmers who watched the weather seriously and maintained some kind of an ‘agricultural calendar’ of climatic events in connection with agricultural operations. In some central parts of China, these agricultural calendars are still in vogue, though other parts have opted for more modern methods. The modern instrumental period may be said to have begun about the close of the 19th century, but the observing network was very limited in the beginning and confined mostly to densely populated areas. Vast areas were uncharted. It is only recently from about the middle of the twentieth century that the observational network over the region as a whole has improved. Since 1959, a network of surface and upper-air observing stations was estab- lished on the highly elevated plateau of Tibet. It is mentioned that during the period, 1949–1963, the number of meteorological observing stations in China increased 30-fold (Cheng, 1963). A Chinese national project on the meteorology of the Tibetan Plateau was reported upon by Yeh and Gao (1979) who along with their many colleagues carried out excellent studies of the heat budget of the plateau and other related problems of the high-altitude region. The observational network on the plateau was further improved upon during a special experiment known as the Qinghai-Xizang Plateau Meteorology Experiment (QXPMEX) during the summer of 1979 which was conducted by Chinese scientists as part of the Global Weather Experiment, 1978–1979. Earlier, during the winters of 1974 and 1975, a GARP field project under the leadership mostly of Japanese scientists had conducted an ‘Air Mass Transformation Experiment’ (AMTEX) over the sea areas southwest of Japan to learn more about the energy and momentum exchanges between the sea and its overlying atmosphere and meso-scale cellular convection and cyclogene- sis that occurs when there is a cold air outbreak over the East China Sea and the Kuroshio current. Like the Indian Ocean, the South China Sea and the Western Pacific Ocean play important roles in monsoon circulation over Eastern Asia, the Maritime Continent and the Australian region, especially during advance and retreat of monsoon current across these ocean areas. In order to learn more about these K. Saha, Tropical Circulation Systems and Monsoons, 123 DOI 10.1007/978-3-642-03373-5_5, C Springer-Verlag Berlin Heidelberg 2010 124 5 Monsoon over Eastern Asia and Adjoining Western Pacific Ocean roles, the International Community led by Chinese scientists mounted an impres- sive array of field experiment called the South China Sea Monsoon Experiment (SCSMEX) in 1998, spanning the period from 1 May to 31 August, for carrying out intensive observations of surface and upper air parameters relating to monsoons. A wealth of new information was collected from this experiment, which became avail- able for further studies (e.g., Lau et al., 1998, 2000). The recent studies of monsoon over Eastern Asia are, therefore, based on an excellent coverage of data, though in some areas long-period data are still lacking. An excellent review of some of the recent studies which were carried out on the seasonal march of the East Asian Summer Monsoon has been provided by Ding (2004). It is well-known that during the peak summer months of July-August, monsoon over the tropical belt of Eastern Asia suddenly jumps to extratropical latitudes to cover such areas as Northern China, Northern Japan, Korea and Eastern Siberia with its poleward boundary near about 60◦N. We study monsoon over this extratropical belt of Eastern Asia in the latter part of this chapter. 5.2 Physical Features and Climate It is not easy to delineate the southern boundary of Eastern Asia which includes some of the most heterogeneous elements of the global terrain and features prac- tically the whole range of global climate from tropical to arctic. Here, along the southwestern boundary of mainland China, the mighty Himalaya mountain complex with the world’s highest mountain peak, Mount Everest, rising to an altitude of about 8.85 km a.s.l. and associated Tibetan plateau with a mean elevation of well over 4.6 km a.s.l. stand guard over the extensive lowlands of Northern and Eastern China which have several smaller high ground or hill ranges scattered all over the region. The Tibetan Plateau descends steeply both northward and eastward to the plains of China and this is clearly indicated by the direction of flow of water of the two mighty rivers, the Yantzekiang and the Hwang-Ho which flow in a zig-zag course eastward to the China Sea. Also, along the northwestern boundary of China lie a series of high-rise mountains, the Tien Shan and the Altay mountains, and several other lesser mountain ranges which extend northeastward to as far north as 60◦N or even beyond. A relief map of Eastern Asia showing the above-mentioned topographic features is at Fig. 5.1. Another important physical feature of China which exercises great influence upon the climate of the region is an extension of the vast Central Asian desert lowlands from Sinkiang in the west to the Gobi desert or even beyond to Manchuria and Eastern Siberia in the east. The Korean peninsula lies over the northeastern part of the region and juts out southward so as to have the Yellow Sea to its west and the Sea of Japan to the east. The Korean Strait separates the peninsula from the Islands of Japan which lie to the south and east. Besides the mainland, several large and small islands belonging to China and Japan lie scattered over the western North Pacific Ocean. 5.3 The Winter Season over Eastern Asia (November–March) 125 Fig. 5.1 Relief map of Eastern Asia The topography and the geographical location of Eastern Asia are responsible for a wide variety of climatic conditions in terms of temperature, pressure, airflow and rainfall. The seasons also are somewhat different here from those over the Indian Subcontinent. On account of more northerly location and greater continental and oceanic influences, the winter season starts early in November and lasts till the end of March. Summer monsoon starts in May and lasts till the end of September. The transition periods are usually April and October. 5.3 The Winter Season over Eastern Asia (November–March) 5.3.1 Temperature, Pressure, and Wind Mean air temperatures over Eastern Asia start falling rapidly from October onward and by January extremely low temperatures often dipping to a minimum of < –30◦C may prevail over the Gobi desert of Outer Mongolia and adjoining eastern Siberia as well as over the Korean peninsula (Fig. 5.2). In response to the temperature distribution, an extremely high pressure cell builds up over the region with maximum pressure exceeding 1032 hPa centered over the 126 5 Monsoon over Eastern Asia and Adjoining Western Pacific Ocean Fig. 5.2 Mean air temperatures (◦C) over Eastern Asia in January (after Watts, 1969) Mongolian region and a steep pressure gradient to the south and east to cover prac- tically the whole of Asia and a good part of northwestern Pacific Ocean close to the coast of Eastern Asia. Side by side, a deep low pressure cell develops in the vicinity of the Aleutian Islands area with the subtropical high pressure cell of the Pacific Ocean lying to its south with ridge along about 25◦N. Consistent with the prevailing temperature and pressure distributions described in the preceding para, there is strong anticyclonic circulation over a vast region of Eastern Asia and adjoining Pacific Ocean where it appears to merge with the subtropical anticyclonic circulation with its axis along about 30◦N. A strong cyclonic circulation prevails over the Aleutian Islands area. However, the circulations change rapidly with height, with westerlies dominating the flow at 500 and 200 hPa over the subtropical and midlatitude belts. These aspects of the airflow at low levels and at 500 and 200 hPa over Eastern Asia and adjoining Pacific Ocean during January are shown in Fig. 5.3 (Crutcher and Meserve, 1970). Several studies (e.g., Yeh and Gao, 1979; Murakami, 1981a,b; Boyle and Chen, 1987) have emphasized the great mechanical and thermodynamical influence of the Himalayan Massif and Tibetan Plateau on the upper air circulation over Asia, especially Central and Eastern Asia where the winds are predominantly westerly above 5.3 The Winter Season over Eastern Asia (November–March) 127 Fig. 5.3 Streamlines showing mean atmospheric circulation over Eastern Asia and adjoining western Pacific Ocean during winter: (a) Low-level (925 mb), (b) 500 mb, and (c) 200 mb. Thick continuous line in (a) shows the NE–SW oriented convergence line over the Western North Pacific Ocean the low-level E/NE-ly tradewinds. During winter, upper-level midlatitude westerlies migrate southward and blow around the Himalayan mountain complex and the Tibetan plateau. On striking the western side of the mountain barrier, the flow appears to divide itself into two parts, one flowing northward around the northern boundary of the mountain block and the other flowing southeastward around the southern boundary. The divided aircurrents appear to merge on the leeside over China, some distance away from the eastern side of the mountains. An interesting aspect of the midtropospheric circulation over the region is that it is the weakest (<10 m s–1) directly over the Tibetan Plateau at about 500 mb.

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