Chapter 5 Monsoon over Eastern Asia (Including Weng/course/monsoon... · Chapter 5 Monsoon over Eastern

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  • Chapter 5Monsoon over Eastern Asia (Including China,Japan, and Korea) and Adjoining WesternPacific Ocean

    5.1 Introduction

    The study of monsoon and related weather phenomena over Eastern Asia has along history. Prior to the 3rd century B.C., it was mostly the farmers who watchedthe weather seriously and maintained some kind of an agricultural calendar ofclimatic events in connection with agricultural operations. In some central parts ofChina, these agricultural calendars are still in vogue, though other parts have optedfor more modern methods.

    The modern instrumental period may be said to have begun about the closeof the 19th century, but the observing network was very limited in the beginningand confined mostly to densely populated areas. Vast areas were uncharted. It isonly recently from about the middle of the twentieth century that the observationalnetwork 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,19491963, the number of meteorological observing stations in China increased30-fold (Cheng, 1963). A Chinese national project on the meteorology of theTibetan Plateau was reported upon by Yeh and Gao (1979) who along with theirmany colleagues carried out excellent studies of the heat budget of the plateau andother related problems of the high-altitude region. The observational network onthe plateau was further improved upon during a special experiment known as theQinghai-Xizang Plateau Meteorology Experiment (QXPMEX) during the summerof 1979 which was conducted by Chinese scientists as part of the Global WeatherExperiment, 19781979. Earlier, during the winters of 1974 and 1975, a GARPfield project under the leadership mostly of Japanese scientists had conducted anAir Mass Transformation Experiment (AMTEX) over the sea areas southwest ofJapan to learn more about the energy and momentum exchanges between the seaand 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 theKuroshio current. Like the Indian Ocean, the South China Sea and the WesternPacific Ocean play important roles in monsoon circulation over Eastern Asia, theMaritime Continent and the Australian region, especially during advance and retreatof monsoon current across these ocean areas. In order to learn more about these

    123K. Saha, Tropical Circulation Systems and Monsoons,DOI 10.1007/978-3-642-03373-5_5, C Springer-Verlag Berlin Heidelberg 2010

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    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 outintensive observations of surface and upper air parameters relating to monsoons. Awealth 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 monsoonover Eastern Asia are, therefore, based on an excellent coverage of data, thoughin some areas long-period data are still lacking. An excellent review of some ofthe recent studies which were carried out on the seasonal march of the East AsianSummer Monsoon has been provided by Ding (2004).

    It is well-known that during the peak summer months of July-August, monsoonover the tropical belt of Eastern Asia suddenly jumps to extratropical latitudes tocover such areas as Northern China, Northern Japan, Korea and Eastern Siberia withits poleward boundary near about 60N. We study monsoon over this extratropicalbelt 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 includessome 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 thesouthwestern boundary of mainland China, the mighty Himalaya mountain com-plex with the worlds highest mountain peak, Mount Everest, rising to an altitude ofabout 8.85 km a.s.l. and associated Tibetan plateau with a mean elevation of wellover 4.6 km a.s.l. stand guard over the extensive lowlands of Northern and EasternChina which have several smaller high ground or hill ranges scattered all over theregion. The Tibetan Plateau descends steeply both northward and eastward to theplains of China and this is clearly indicated by the direction of flow of water ofthe two mighty rivers, the Yantzekiang and the Hwang-Ho which flow in a zig-zagcourse eastward to the China Sea. Also, along the northwestern boundary of Chinalie a series of high-rise mountains, the Tien Shan and the Altay mountains, andseveral other lesser mountain ranges which extend northeastward to as far north as60N or even beyond. A relief map of Eastern Asia showing the above-mentionedtopographic features is at Fig. 5.1.

    Another important physical feature of China which exercises great influenceupon the climate of the region is an extension of the vast Central Asian desert low-lands from Sinkiang in the west to the Gobi desert or even beyond to Manchuria andEastern Siberia in the east.

    The Korean peninsula lies over the northeastern part of the region and juts outsouthward 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 thesouth and east. Besides the mainland, several large and small islands belonging toChina and Japan lie scattered over the western North Pacific Ocean.

  • 5.3 The Winter Season over Eastern Asia (NovemberMarch) 125

    Fig. 5.1 Relief map of Eastern Asia

    The topography and the geographical location of Eastern Asia are responsible fora wide variety of climatic conditions in terms of temperature, pressure, airflow andrainfall. The seasons also are somewhat different here from those over the IndianSubcontinent. On account of more northerly location and greater continental andoceanic influences, the winter season starts early in November and lasts till the endof 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 (NovemberMarch)

    5.3.1 Temperature, Pressure, and Wind

    Mean air temperatures over Eastern Asia start falling rapidly from October onwardand by January extremely low temperatures often dipping to a minimum of < 30Cmay prevail over the Gobi desert of Outer Mongolia and adjoining eastern Siberiaas well as over the Korean peninsula (Fig. 5.2).

    In response to the temperature distribution, an extremely high pressure cell buildsup over the region with maximum pressure exceeding 1032 hPa centered over the

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    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 thecoast of Eastern Asia. Side by side, a deep low pressure cell develops in the vicinityof the Aleutian Islands area with the subtropical high pressure cell of the PacificOcean lying to its south with ridge along about 25N.

    Consistent with the prevailing temperature and pressure distributions describedin the preceding para, there is strong anticyclonic circulation over a vast region ofEastern Asia and adjoining Pacific Ocean where it appears to merge with the sub-tropical anticyclonic circulation with its axis along about 30N. A strong cycloniccirculation prevails over the Aleutian Islands area. However, the circulations changerapidly with height, with westerlies dominating the flow at 500 and 200 hPa overthe subtropical and midlatitude belts. These aspects of the airflow at low levels andat 500 and 200 hPa over Eastern Asia and adjoining Pacific Ocean during Januaryare 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 theHimalayan Massif and Tibetan Plateau on the upper air circulation over Asia, espe-cially Central and Eastern Asia where the winds are predominantly westerly above

  • 5.3 The Winter Season over Eastern Asia (NovemberMarch) 127

    Fig. 5.3 Streamlinesshowing mean atmosphericcirculation over Eastern Asiaand adjoining western PacificOcean during winter: (a)Low-level (925 mb),(b) 500 mb, and (c) 200 mb.Thick continuous line in (a)shows the NESW orientedconvergence line over theWestern North Pacific Ocean

    the low-level E/NE-ly tradewinds. During winter, upper-level midlatitude wester-lies migrate southward and blow around the Himalayan mountain complex andthe Tibetan plateau. On striking the western side of the mountain barrier, the flowappears to divide itself into two parts, one flowing northward around the north-ern boundary of the mountain block and the other flowing southeastward aroundthe southern boundary. The divided aircurrents appear to merge on the leeside overChina, some distance away from the eastern side of the mountains.

    An interesting aspect of the midtropospheric circulation over the region is thatit is the weakest (

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    Murakami (1981a) attributes this feature to the frictional effects of the elevatedTibetan Plateau. By comparing the geostrophic wind with the observed wind, heestimated the height of the boundary layer over