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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,1949–1963, 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, 1978–1979. Earlier, during the winters of 1974 and 1975, a GARPfield project under the leadership mostly of Japanese scientists had conducted an‘Air 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

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 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 60◦N. 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 world’s 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 as60◦N 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 (November–March) 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 (November–March)

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 < –30◦Cmay 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


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 25◦N.

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 30◦N. 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


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 NE–SW 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 (<10 m s–1) directly over the Tibetan Plateau at about 500 mb.


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 the plateau to be about 1.5 km abovethe plateau. Yeh and Gao (1979) showed that the height of the boundary layer overthe plateau was substantially higher during summer than winter.

Above the boundary layer, the westerly airstream over the plateau appears tostrengthen with height. However, the speed appears to increase downstream to reacha maximum of 60 m s–1 or more on the leeside, about 1000–1500 km eastward of theplateau. This is shown by the 200 mb wind field. The existence of this high-speedJetstream over the eastern part of Asia and adjoining northwestern Pacific Ocean hasbeen known since the days of Second World War (1942–1945) when aerial missionsflying high over Japan encountered these high winds.

5.3.2 Quasi-stationary Wave in Westerlies – Its Interactionwith Traveling Waves – Cold Surges

It is well-known that the low level flow around the northern boundary of theHimalayan mountain complex during the winter maintains a stationary wave in theairflow with a trough along about 70◦E in the west and a trough along about 110◦Ein the east and a ridge of high pressure in between along about 90◦E. This beinga baroclinic zone, normally at midtropospheric height, there is convergence anddescending motion to the west and divergence and rising motion to the east of thetrough axis.

Frequently, however, extraordinary developments take place in the stationarywave when eastward-propagating synoptic-scale baroclinic wave disturbances fromwestern Asia move into East Asia and interact with the stationary wave trough.When the waves are in phase, the interaction leads to an amplification of the station-ary trough, resulting in intense cold surge at low levels from the high pressure areain the rear of the trough.

According to Lau and Chang (1987), it is customary to define a cold surge eventby noting changes of one or a combination of the following indicators within aperiod of 24–48 h:

(i) A drop in surface temperature at a station of 5◦C or more;(ii) An increase of the surface pressure gradient between coastal and central China

of at least 5 mbs; and(iii) A prevalent northerly surface flow over the South China Sea with speed

exceeding 5 m s–1.

Although the above-mentioned criteria are somewhat arbitrary and several varia-tions are also in use, they have been found to be of practical importance in defininga cold surge event.


The southward-moving cold surge usually converges into the circulation aroundthe equatorial trough of low pressure over the Maritime continent where the mois-ture convergence produces heavy clouding and precipitation along the convergencezone (see Fig. 5.3a). However, occasionally, a branch of the southward-moving coldsurge crosses the equator over several segments of longitude between about 105◦Eand the dateline and converges into the ‘heat low’ circulation over Australia.

There is indication in upper-tropospheric flow that air rising in convection overthe equatorial trough zone or the trough zone over Australia diverges in the uppertroposphere and returns to the northern hemisphere after flowing around an anti-cyclonic circulation with its ridge along about 15◦N at 500 mb and converges intothe upper-tropospheric baroclinic trough over East Asia. The convergence appearsto strengthen the pre-existing convergence on the western side of the trough axis,thereby leading to increased subsidence and inducing a fresh low-level cold surge.Also, adding westerly momentum to the flow because of increased Coriolis acceler-ation, results in strengthening the existing jetstream. The cold surge thus appears toconstitute the equatorward-moving lower branch of a vertical circulation which linkstropical convection over equatorial latitudes to midlatitude baroclinic waves. In case,the zone of tropical convection lies in the southern hemisphere, i.e., over Australia,the above circulation may be said to be a truly interhemispheric circulation.

The importance of the meridional-vertical overturning associated with low-level cold surge in producing upper-level W’ly jetstream over northern China and

Fig. 5.4 Ten-year (1961–1970) average rainfall (mm) over Eastern Asia in January. Heavy dashedline indicates the 3 km-height (after Yeh and Gao, 1979)


adjoining Japanese islands has been pointed out by several studies (e.g., Murakami,1981a, 1987b; Lau and Chang, 1987).

5.3.3 Winter Rainfall over Eastern Asia

It appears that winter disturbances moving in the baroclinic westerlies around theHimalayan mountain complex produce appreciable rainfall on the windward slopesof the mountains (see Fig. 5.4).

5.4 Airmass Transformations and Cyclogenesis over the Oceans

Observations indicate that quite a few of the W’ly wave troughs that arrive overEastern Asia move out to the neighboring ocean areas and develop into intensecyclones under favorable conditions. New ones also develop over the oceans. Fora long time, lack of adequate observations over the oceans hindered a thoroughanalysis and understanding of these disturbances. In 1974 and 1975, the WorldMeteorological Organization (1981), with the active participation of Chinese andJapanese scientists, carried out a comprehensive observational program, called theAirmass Transformation Experiment (AMTEX), over the East China Sea with thelong-term objective of learning more about these oceanic cyclones. Figure 5.5, dueto AMTEX (1973) Study Group, shows the distribution of mean ocean surface tem-perature (◦C) off the eastern seaboard of Asia in February from 1953 to 1957, witha temperature maximum along the Kuro-shio current.

When a cold surge flows out over a relatively warm ocean, there is increasedevaporation from the ocean surface and a boundary layer of warm, moist airmassbuilds up downstream due to turbulent mixing. Further, the warming and moisteningof the lower layer lead to increased convective instability and vertical growth of themoist layer and formation of cumulus and stratocumulus clouds and occasional lightrain. But when the aircurrent enters the zone of the warm Kuroshio Ocean current orcomes under the influence of an eastward-propagating disturbance, the developmentsometimes is explosive.

5.4.1 Cyclonic Disturbances over Eastern Asiaand Neighboring Ocean

Several studies (for example, Boyle and Chen, 1987; Lau and Chang, 1987) havetraced the movement of W’ly waves from East Asia across the eastern seaboard ofAsia during northern winter and shown how they develop into explosive cycloneswhen they move out over the East China Sea and the Sea of Japan and neighboringNorth Pacific Ocean.

5.4 Airmass Transformations and Cyclogenesis over the Oceans 131

Fig. 5.5 The mean sea surface temperature (◦C) off the eastern seaboard of Asia in February from1953 to 1957 (Study Group on AMTEX, 1973)

Using a Hovmoller diagram for high-pass-filtered (period less than or equal to5 days) 500 mb height, Lau and Chang (1987) demonastrated how a W’ly wavedisturbance from near Lake Baykal on entering the Sea of Japan developed into anexplosive cyclone.

Here, we present the results of two studies which throw light on different aspectsof these explosive cyclones. These are shown in Fig. 5.6 (Trewartha, 1961), andFig. 5.7 (Sanders and Gyakum, 1980).

The study by Trewartha (1961) reports the frequency of cyclone formation overEast Asia and the neighboring ocean areas, viz., East China Sea, Sea of Japan andthe Northwestern Pacific ocean in the months October through April, 1932–1937.It shows that only a few waves develop into cyclones while they are over land,but the frequency of cyclone formation increases rapidly as they move out to thesea with a maximum over the latitudinal belt, 30–35◦N, the highest maximum ofnear 28 being over the East China Sea. However, the frequency decreases furthereastnortheastward where the ocean surface gets somewhat less warm.

Explosive cyclones form mostly during the winter season and their effects are feltover a wide belt of latitudes poleward of about 25◦N. They are almost non-existentduring the summer when the region comes under the influence of a heat low overNorth China and the tracks of these disturbances shift northwestward over northernSiberia.


Fig. 5.6 Frequency of cyclone formation over ocean areas off the eastern seaboard of Asia. Thevalue of any isoline at any point represents the number of cyclones that formed within a radiusof 2.5 latitude degrees from that point in the months October through April, 1932–1937 (afterTrewartha, 1961)

The study is by Sanders and Gyakum (1980) throws further light on the occur-rence and distribution of explosive cyclones which they call bomb events. Theresults of their study pertain to two selected regions of the globe, viz., the oceanicareas off the eastern seaboards of Asia and North America, during three coldseasons.

Sanders and Gyakum define an explosive cyclone as a cyclone in which the deep-ening rate is, on an average, the geostrophic equivalent of at least 1 mb(h)–1 over a12-h period at 45◦N. After analyzing several cases of such cyclogenesis, which theythought were like bomb explosions, in the northern hemisphere during the periodSeptember through May 1979, they found that the areas where they occurred withthe highest frequency lay over oceans in the vicinity of the east coasts of the conti-nents of North America and Asia, especially where the offshore winds were aboutto enter the zone of warm ocean currents.

Raw non-zero frequencies appear in each 5◦ × 5◦ quadrilateral of latitude andlongitude. Isopleths represent smoothed frequencies, obtained as one-eighth of the

5.5 Transition Period (April) 133

Fig. 5.7 Distribution of explosive cyclones in Pacific and Atlantic basins

sum of four times the raw central frequency plus the sum of the surrounding rawfrequencies. The columns of numbers to the left and right of the heavy line alonglongitude 90◦W represent, respectively, the normalized frequencies for each 5◦ lat-itude belt in the Pacific and Atlantic basins, using a normalization factor of (cos42.5/cos ϕ). Heavy dashed lines represent the mean winter position of the Kuroshioand the Gulf Stream (after Sanders and Gyakum, 1980).

5.5 Transition Period (April)

With gradual warming of the land surface from March onward, the rigour of thewinter monsoon over Eastern Asia lessens and cold surges become less frequent.The NE’ly winds gradually withdraw poleward and are replaced by S/SW-ly windswhich blow around a shallow low pressure that forms over southern China withincursions of warm, moist air from the southern and eastern oceans. In other words,this is the spring season over the region. However, a large part of the ocean sur-face is still cold compared to land surface, and the passage of the southerly windswhich after flowing over the warm Kuro-Siou current blows over this cold waterproduces widespread stratus with drizzle and fog over the coastal waters, which islocally known as ‘crachin’. Crachin may persist over the coastal waters for days and


weeks. In the south, fogs are most frequent during March and gradually become lessfrequent as the ocean surface slowly warms up with the advance of summer.

In April, the eastward-propagating baroclinic waves in the westerlies occasion-ally interact with the low pressure system developing over Southern China and causelight to moderate precipitation over Southern China during the period of interaction.

5.5.1 Development of ‘Heat Low’ over Eastern Asia

Temperatures rise rapidly over Southern China from April onward resulting in theformation of a heat low over the region. However, the formation of a heat low overchina is not an isolated event, nor is it a sudden development. It is part of a chainof heat lows that form during northern summer over all the land sectors of SouthernAsia, extending from the Arabian Peninsula in the west to the Pacific Coast in theeast.

However, as explained in the case of the Indian Subcontinent, heat lows play theimportant role of shifting monsoon from the winter to the summer hemisphere. Inthis regard, Eastern Asia is no exception. However, the actual process in the case ofEastern Asia is explained in the following section.

5.6 Origin of Monsoon over Eastern Asia

The summer monsoon over Eastern Asia appears to originate from movement ofthe equatorial trough of low pressure over the equatorial eastern Indian Ocean, intwo distinct phases, the first from movement of the North Equatorial Trough (NET)during March-April, and the second from movement of South Equatorial Trough(SET) in May.

At the beginning of summer, the NET is located within about 5◦N of the equator,while the SET to the south is located farther away from the equator, leaning towardsAustralia where it links with the heat low over the continent (see Fig. 4.8).

In the first phase, i.e., in March-April, the movement of the heat low over thelandmass of the Sumatra-Malayasia region forces the NET to move northward alongwith it across the narrow landstrips of Malaysia–Thailand–Myanmar complex. As itreaches the northern part of the narrow landstrip, the circulation around it gets bifur-cated into two airstreams, one heading northwestward towards a quasi-stationaryheat low over Myanmar, and the other northeastward towards the heat low overThailand and Vietnam and from there to Southern China.

On arrival over China, the equatorial trough of low pressure merges with thetrough of the seasonal heat low over Southern China and starts the first phase ofmonsoon season over Southern China during April.

Monsoon activity over Southern China gets a boost in early May when in thesecond phase the cold, humid airstream associated with the SET arrives over theregion under similar forcing.

5.6 Origin of Monsoon over Eastern Asia 135

Fig. 5.8 Schematic showing how summer monsoon advances to Southern China and other neigh-boring areas. Thick dark lines show the main supply routes for cool, moist airmass of the southernhemisphere. L denotes Low, H – High. Locations of ITCZ, TCZ, and Meiyu–Baiu front (thicklines) north of the monsoon trough

It is well-known that during southern summer, the SET extends southeastwardover the southern Indian Ocean towards the heat low over Australia. It has then thecool NW-ly monsoon currents on its equatorward side and the cold SE-ly tradewindsof the southern-hemisphere on its poleward side, with the warm trough in between.

With change of season, the heat low over Australia weakens and loses its holdon the SET which then starts shifting equatorward across the Indonesian islands toreach the equator by early May. As the heat low moves across the islands, it narrows


down and the orientation of its trough veers from NW-SE direction to almost W-Edirection, allowing the cool, humid airmass of the winter hemisphere to flow firstas equatorial westerlies and then cross the equator to blow as S/SW-ly winds of theSouth China Sea to usher in monsoon to Southern China.

However, several studies (e.g., Findlater, 1969a,b; Saha, 1970; Wang andLeftwich, 1984; Sun, 2002) have emphasized that the main artery for supply ofmoisture for Southern Asia rainfall including that over Eastern Asia is that via theSomali Monsoon Current in the western Indian Ocean. Additional moisture suppliesfor rainfall over Eastern Asia are those from cross-equatorial airflow via routes overthe Maritime Continent and from the tradewinds originating in the subtropical highpressure over the western Pacific Ocean, as shown schematically in Fig. 5.8.

5.7 Seasonal March of the Summer Monsoon

As in the Indian Ocean region, climatological rainfall has been used as the maincriterion for determining the dates of onset of summer monsoon over China.Figure 5.9, after Tao and Chen (1987), presents the dates of onset of summermonsoon over East Asia along with those over India by Rao (1976).

According to Fig. 5.9, monsoon sets in over Eastern Asia a couple of weeksearlier than over the Indian Subcontinent. Starting on or about 10 May at the south-ern coast of China, it advances slowly and reaches Central China by mid-June. Afaster advance appears to occur thereafter during July when monsoon advances to

Fig. 5.9 Dates of onset of summer monsoon over Eastern Asia (Tao and Chen, 1987) along withthose over the Indian Subcontinent (Rao, 1981)

5.9 Meteorological Developments Associated with the Jump to Central China 137

Northern China and engulfs practically the whole of mainland China, part of Koreaand the islands of Japan and the Kuerile Islands.

However, in recent years, questions have been raised as to the justification forusing rainfall as the sole criterion for determining the date of onset of monsoon,and additional parameters such as airmass properties have been used for this pur-pose. For example, Fong and Wang (2001) used an equivalent potential temperaturevalue of 335 K and a SW-ly wind greater than 2.5 m s–1 at 850 hPa for this pur-pose. Earlier, Ding (1994) had used a value of 340 K for the equivalent potentialtemperature at 850 hPa.

The trend to include airmass properties besides rainfall and wind direction foridentification of monsoon would appear to be in the right direction, since mon-soon in reality is not rainfall alone, nor is it only a reversal of the prevailing winddirection, as ordinarily believed. In the present text, we have defined monsoon asa perturbation of the tradewind circulation associated with the seasonal move-ment of the equatorial trough of low pressure, which converges into the circulationaround the trough, producing rainfall along the ITCZ and the TCZ and several otherchanges in airmass properties.

5.8 Stationary States and Jumps

Several studies (e.g., Fong and Wang, 2001; Wang and Lin, 2002; Sun, 2002; Ding,2004) have identified three distinct stationary states and two significant jumps in theseasonal march of summer monsoon over Eastern Asia.

The stationary states and their approximate periods are:

(1) Southern China (approx. 18–25◦N), from second week of May to middle ofJune;

(2) The Yangtze River basin (25–30◦N) (Meiyu front), from mid-June to mid-July;(3) North China (North of 30◦N), from mid-July to mid-August

The jumps are:

(1) From Southern China to the Yangtze River basin, around the second week ofJune; and

(2) From Yangtze River basin to North China, about mid-July.

5.9 Meteorological Developments Associated with the Jumpto Central China

After monsoon gets established over Southern China during April-May, two impor-tant developments occur in the heat budget of Eastern Asia, which appear toforce a re-organization of the monsoon currents and shift monsoon activity further


northward. These are: (1) formation of large-scale heat source over the elevatedTibetan Plateau and (2) a further deepening and northward movement of the heatlow from Southern to Central China. These developments result in the formation ofa Plateau Monsoon over the eastern part of the Plateau on one side, and develop-ment of the Meiyu–Baiu Front over Central China and adjoining Pacific Ocean onthe other.

5.9.1 Tibetan Plateau Monsoon

The monsoonal characteristics of the boundary layer over the Tibetan Plateau hasbeen brought out by several studies (e.g. Gao and Xu, 1962; Flohn, 1968; Gao, 1976,1979; Yeh and Gao, 1979; Tang, 1979; Tang and Reiter, 1984).

Tang and Reiter (1984) in a comprehensive study compared the monsoonalbehaviour of the atmosphere over the Tibetan Plateau with that over the WesternPlateau of North America and found that inspite of their altitude and latitude dif-ferences, the boundary layer of the two Plateaux behaved in very similar fashion.Both served as a heat source with low pressure, high absolute humidity and lowstatic stability during summer and a heat sink with high pressure and relatively dryconditions during winter. These seasonal reversals had the effect of producing awell-developed warm, wet monsoon during the summer half of the year and a colddry monsoon during the winter half. For their study, they selected 600 mb surface torepresent conditions over the elevated Tibetan Plateau, and 800 mb for the WesternPlateau.

In the case of the Tibetan Plateau, the seasonal variation is well reflected in thedistributions of temperature, pressure, absolute humidity, and atmospheric circula-tion over the plateau and its surrounding areas at the same altitude. As example ofpressure variation, we present in Fig. 5.10 (after Yeh et al., 1979 and Gao et al.,1981), the distribution of monthly mean 600 mb height contours over a 10 yearperiod, 1961–1970, during (a) winter, and (b) summer.

It clearly reveals a high pressure system over the Plateau during January, asagainst a series of low pressure systems during July.

Dashed lines are Troughs, solid lines-ridges, dotted lines – outline of TibetanPlateau (after Yeh and Gao, 1979; Gao et al., 1981; Tang and Reiter, 1984)

An important aspect of the pressure distribution during July is that between thetroughs of low pressure systems over the eastern part of the Plateau and a well-defined ridge of high pressure system which extends eastward to Southern China, asoutherly monsoon current enters the region and rises against the eastern slopes ofthe Plateau, causing extensive rainfall and other types of disturbed weather. A studyby Tao and Ding (1981) shows that with the availability of additional moisture fromthe monsoon flow, the low pressure systems and their attendant shear lines overthe Plateau have the effect of producing severe rainstorms, often accompanied byhail. After formation over the Plateau, many of these rainstorms are found to moveeastward and generate new perturbations to affect low-lying regions, such as theYantzekiang valley and adjoining areas.


Fig. 5.10 Mean 600 hPa height contours (decameters) over the Tibetan Plateau and surroundingareas: (a) January and (b) July

5.9.2 The Meiyu (Plum Rain) Front over China

5.9.2.1 Formation of the Meiyu Front

The merging of the equatorial heat low with the quasi-stationary heat low overSouthern China towards the middle of May prepares the ground for devel-opment of the ITCZ on the equatorial side of the trough of the heat lowand the TCZ on its poleward side. It is along these convergence zones thatthe tradewinds of the two hemispheres meet the heat low circulation overChina. The TCZ which forms over the latitudinal belt 25–30◦N and whichruns along the Yangtze River basin constitutes the Meiyu front over China (seeFig. 5.8).


Monsoon activity with copious rainfall remains largely confined to the ITCZ overSouthern China during the first stationary period from about the middle of May tomiddle of June. Relatively less activity is noticed along the TCZ during this periodexcept when it interacts with eastward-propagating westerly baroclinic waves andproduces heavy rainfall.

However, the situation changes abruptly towards the middle of June when abranch of the Pacific Ocean anticyclonic circulation is drawn into the heat lowcirculation over Eastern Asia and it finds its way to the TCZ over Central China.

The injection of cool, moist air from the Pacific Ocean now shifts the activity tothe TCZ where heavy rainfall occurs whenever eastward-propagating disturbancesin baroclinic westerlies interact with it. It marks the beginning of the Meiyu seasonalong the Yangtze River basin. The other branch of the Pacific Ocean anticycloniccirculation veers eastnortheastward towards the islands of Japan, converging intothe cyclonic circulation around the ‘Aleutian low’ pressure area. Thus, a field ofdeformation forms over Central China and adjoining East China Sea between thevarious aircurrents involved and the TCZ forms a part of the dilatation axis of thisdeformation field. It runs almost zonally along the Yantzekiang river valley and ischaracterized as a zone of heavy rainfall, or ‘plum rain’ during the period frommid-June to mid-July.

The eastern part of the dilatation axis that extends towards Japan is known asthe ‘Baiu’ front. At the Baiu front, the aircurrents that converge are the cycloniccirculation around the Aleutian Low and the anticyclonic circulation around thePacific Ocean subtropical high pressure cell.

5.9.2.2 Interaction of the Meiyu Front with Traveling Disturbances

The intensity of the Mei-yu front and associated rainfall fluctuate considerablywhen eastward-propagating baroclinic wave disturbances interact with it. The inter-action leads to formation of vortices of different scales of motion which travelalong the front and bring about heavy rainfall (Tao and Ding, 1981; Ninomiyaand Akiyama, 1992; Chang et al., 1998, 2000; Ding, 1994; Fong and Wang, 2001;Wang and Lin, 2002). According to Tao (1980), it is the southeast quadrant ofthe vortex where heavy rainfall is concentrated. Several studies (e.g., Chen andTsay, 1978; Tao, 1980) show that meso-scale vortices of wavelengths about 50,80 and 150 km are very effective mechanisms for producing heavy rainfall in theMei-yu front.

Severe rainstorms in the Mei-yu front can cause devastating floods in the YangtzeRiver valley. However, there appears to be an interesting relationship between thestrength of the SW monsoon current over China and Mei-yu rainfall. It is found thatit is only in years of normal or weak monsoon that heavy rainfall occurs along theMei-yu front during early summer. In years of strong monsoon, the moist monsooncurrent rushes to the northern part of China where it rains heavily, while the YangtzeRiver valley is deprived of its normal share of rainfall.


5.9.2.3 Structure of the Meiyu–Baiu Front

Chen and Chang (1980) studied the north-south temperature and humidity gradientsand the vorticity budget of a Meiyu system over Eastern Asia and neighboring oceanduring a period of 6 days, 10 through 15 June 1974, separately for its (a) matureand (b) decaying stages. For this purpose, they divided the front into three zonalsections: a western section (W) over continental China; a central section (C) overEast China Sea; and an eastern section (E) over Southern Japan. They computedthe temperature and vertical motion profiles across these sections and found thatthe structure of the Meiyu front over China (the W section), was substantially dif-ferent from that of the Baiu front which extended over the oceans (the C and Esections) during both mature and decaying stages of the system. Their findings are inFigs. 5.11 and 5.12.

According to their finding, the Baiu front had a typical baroclinic structure withcold air descending behind a sloping frontal surface and warm air ascending in front,as would be found in a typical midlatitude frontal system, while the Meiyu frontbehaved more like a tropical system. The results of their computations are presentedin two vertical cross-sections across the fronts in Fig. 5.11 for the thermal structure,and Fig. 5.12 for vertical motion structure.

Fig. 5.11 Vertical cross-sections of the deviation of temperature (T’) from the cross-sectionalmean at each level in sections, W (west), C (center) and E (east) during (a) the mature stage,and (b) the decaying stage. Dashed lines indicate the trough axes. The abscissa is the distance indegrees latitude from the trough axis (Chen and Chang, 1980)


Fig. 5.12 Vertical cross-sections of pressure velocity, ω (mb s–1), in sections W, C, and E during(a) the mature stage, and (b) the decaying stage. Dashed lines indicate the trough axes. The abscissais the distance in degrees latitude from the trough axis (Chen and Chang, 1980)

The above-mentioned findings appear to be implicit in the deformation fieldleading to the formation of the Meiyu–Baiu front in Fig. 5.8. In the Meiyu frontover Central China, three distinct aircurrents with different airmass properties areinvolved. These are: (1) the Cold, dry NW’ly winds blowing anticyclonically fromthe high pressure cell over Mongolia; (2) the cyclonic circulation around the heatlow circulation over Southeastern China; and (3) the relatively cool, humid SE’lytradewinds blowing out of the Pacific Ocean subtropical high pressure cell. Whilethe cold NW’ly winds tend to converge into the circulation around the heat low pro-ducing the Meiyu front, the Pacific trades also converge into it producing a markedmoisture tongue along the frontal zone. So, structurally, the Meiyu front is charac-terized by conditions which are partly tropical, and partly extratropical. On the otherhand, the Baiu front appears to have different characteristics. Here, from East ChinaSea to Southern Japan and even further eastward, the airmass contrasts across thefront appear to be well-defined and have the characteristics of an extratropical front.

5.10 Jump of East Asian Monsoon to Extratropical Latitudes

As mentioned in Sect. 5.8, the activity of summer monsoon over Central Chinarapidly shifts to Northern China and adjoining land and ocean areas of the extra-tropical belt, including Korea, northeastern Siberia, and Japan, to as far north as

5.10 Jump of East Asian Monsoon to Extratropical Latitudes 143

near latitude 60◦N during the months, July–August. It is interesting to note thatthis is almost the same period when the Indian Summer Monsoon suddenly shiftsto the Western Himalayas. An examination of the NCEP Reanalysis climatologyreveals that the jump involves a simultaneous movement of the monsoon current inmeridional as well as zonal directions.

5.10.1 Evidence of Jump in Climatological Fields

The movement is well reflected in the changes that occur in the distributions ofMean Sea Level Pressure, Mean Air Circulation at 925 hPa, and Mean Rainfall overthe region between the periods DJF and JJA, shown in Figs. 5.13, 5.14 and 5.15respectively.

Fig. 5.13 Mean sea level pressure over Eastern Asia and adjoining Pacific Ocean during: (a) DJFand (b) JJA (from NCEP Reanalysis)

According to Fig. 5.13, the advance is signaled by replacement of a high pressurecell of the winter season over Northeastern Asia by a low pressure cell. In fact,during JJA, two low pressure cells appear over the Chinese Mainland, one overSouthern China and the other over Northern China.


The field of low-level circulation shown in Fig. 5.14 appears to be consistent withthe seasonal change in pressure.

Fig. 5.14 Mean wind circulation (Streamlines-isotach analysis) at 925 hPa over Eastern Asiaduring: (a) DJF and (b) JJA

Figure 5.15 shows that there is more rain over the ocean than over the land in win-ter (DJF). But the field reverses during summer (JJA) when the rainfall maximumlies over the land with lesser amounts over the ocean.

5.10 Jump of East Asian Monsoon to Extratropical Latitudes 145

Fig. 5.15 Mean precipitation rate (mm day–1) over Eastern Asia and adjoining Pacific Oceanduring (a) DJF and (b) JJA

5.10.2 Zonal Anomaly in Seasonal Variations

Whereas the northward jump of the summer monsoon activity from Central toNorthern China has been documented in considerable detail by Chinese andJapanese meteorologists (e.g., Lau and Chang, 1987; Ninomiya and Murakami,1987; Tao and Chen, 1987; Ding, 2004), the evidence furnished in the preceding sec-tion suggests that the jump also involves a zonal movement. This is clearly suggestedby Fig. 5.16 which shows the distribution of seasonal mean sea level pressure P andrainfall rate R during DJF and JJA along latitude 42◦N across North Korea andChina.


Fig. 5.16 Zonal anomaly (deviation from annual mean) of MSLP (P) and precipitation rate (R)along latitude 42◦N across the coast of Eastern Asia

5.10.3 Climatological Rainfall over Eastern Asia During July

In the early stages of summer monsoon over China, rainfall occurs largely along theYangtze River valley. After monsoon shifts to Northern China in mid-July, the mainrainbelt shifts to areas north of the Huang-Ho River valley. Also, during June-July,there is a north-south oriented belt of heavy rainfall along the eastern slopes ofthe Tibetan Plateau caused by Plateau Monsoon. These aspects of monsoon rainfallappear to be brought out by distribution of 10-year mean July rainfall, presented inFig. 5.17.

Fig. 5.17 Ten-year (1961–1970) mean July rainfall (mm) over Central and East Asia. The heavydotted line indicates the 3-km topographic contour height (from Yeh and Gao, 1979)

5.11 Monsoon over Japan 147

5.11 Monsoon over Japan

5.11.1 Geographical Location and Climate

Japan consists of a series of islands, of which four, viz., Kyushu, Shikoku, Honsuand Hokkaido are large and the rest are small. Situated in close proximity ofthe eastern seaboard of Northern China and adjoining Eastern Siberia at thenorthwestern corner of the Great Pacific Ocean, it is separated from Mainland Asiaby the Sea of Japan to the north and the Yellow Sea and East China Sea to the westand south. In view of its geographical location, it is affected by the land-sea thermalcontrast between mainland Asia and the Pacific Ocean in both the seasons. In win-ter, it comes under the influence of the cold, relatively dry NW-ly winds from theland, while in summer it is affected by the relatively cool, moist S/SW-ly winds ofthe Pacific Ocean.

The islands are affected by the eastward-propagating baroclinic wave distur-bances during the winter, while it is the movement of the Baiu front and typhoonsthat brings weather to the islands during the summer.

5.11.2 The Baiu Front – Its Seasonal Movement and Activity

The monsoon over Japan is associated with the movement and activity of the Baiufront which forms the extended part of the Meiyu front over the northwestern part ofthe Pacific Ocean in the vicinity of Southern Japan. As stated earlier in this chapter,this part of the front coincides with the dilatation axis of a deformation field overEastern Asia and the adjoining Pacific Ocean.

During winter, the Baiu front lies way out over the ocean south of the mainislands of Japan and the rainfall activity and cyclogenesis are mainly over theopen ocean where they affect a few small islands only. But in summer it rapidlymoves northwestward towards the heat low over North China and adjoining EasternSiberia and affects both Japan and Korea. According to observation, the southwest-ern part of Japan experiences Baiu rainfall about a month earlier than the centraland northeastern parts. Heavy rainfall occurs along this front whenever extratropicaldisturbances of different scales moving eastward interact with it. During the periodfrom mid-June to mid-July, the disturbances move across Japan and are active overthe islands between latitudes 30 and 40◦N.

The seasonal movement of the front from south to north across Japan is revealedby the dates of maximum rainfall averaged over a 30-year period, 1951–1980, ata few selected stations, as given by Ninomiya and Murakami (1987). It is evidentfrom these rainfall figures that as the front moves northward from the oceanic islandstation of Naha towards mainland Japan, the amount of rainfall increases, becomingmaximum at Kagoshima which is located in the southwestern part of Japan and thenrapidly decreases at stations further north. It is during this period that the southernparts of the Korean peninsula also get their summer monsoon rainfall.


Several observational studies (e.g., Matsumoto and Ninomiya, 1971; Ninomiyaand Akiyama, 1971, 1974; Matsumoto, 1972; Ninomiya,1978; Ninomiya andYamazuki, 1979; Ninomiya et al., 1984) find that the low-level southwesterly jet-stream associated with a well-developed medium-scale depression in the Baiu frontis significantly intensified in the southeast quadrant where the vertical motionis strongly upward and rainfall is maximum. Ninomiya (1978) illustrates thesedevelopments in the Baiu front in the case of a medium-scale disturbance whichdeveloped into a depression at 09 LST on 27 June 1972 near 35◦N, 132◦E, shownin Fig. 5.18.

Fig. 5.18 Southwesterlylow-level jetstream: (a)temperature (dashed lines),geopotential height (fulllines), and winds (arrows) at850 mb. (b) Mixing-ratio (fulllines) and winds (arrows) at900 mb at 9 LST, June 27,1972 (from Ninomiya, 1978)

5.11 Monsoon over Japan 149

The sequence of development of the depression is shown in Fig. 5.19(a, b, c).It is evident from Fig. 5.19 that the southerlies inject a large amount of moist

oceanic air into the frontal zone, which when lifted by the strong upward motionappears to lead to the observed heavy rainfall on 27 June 1972.

Fig. 5.19 Time series of several meteorological parameters from 21 LST, June 25, to 3 LST, June28, 1972: (a) The vertical p-velocity (ω) and the zonal wind component relative to the depression’smovement (ur); (b) Meridional component (v) of the wind, with shading indicating regions ofsoutherlies; and (c) Hourly rainfall (after Ninomiya and Yamazaki, 1979)


5.12 Monsoon over Korea

5.12.1 Historical Background

Early meteorological records of Korea were maintained by the Japan MeteorologicalAgency, since the land was under occupation of Japan. But from 1953 onward, thepeninsula became independent and was divided into North Korea and South Korea,each having its own meteorological service. The dividing line was along about 38◦Nparallel.

In 1963, the Meteorological Service of South Korea at Seoul published a detailedClimatological Atlas of Korea, based on earlier observations from 14 South Koreanstations and 11 North Korean stations up to 1960. This Atlas remains the mainsource of information regarding the climate of the peninsula.

5.12.2 Physical Features and Climate

The climate of Korea (here we mean both the Koreas) is shaped largely by itsgeographical location and physical environment.

To the north and northwest of the peninsula, the varied landforms consisting ofdeserts, mountain ranges and extensive Plains undergo large seasonal variations intemperature, being extremely cold during the winter and hot during the summer. Onthe south side, the peninsula is surrounded by ocean, i.e., the Sea of Japan to itseast, the Yellow Sea to the west and the East China Sea to the south. The narrowStrait of Korea separates it from the islands of Japan. Compared to the seasonalvariations of temperature over the land, those over the ocean are small. So, it isthe differential heating between land and ocean which plays a significant role inatmospheric circulation over Korea.

5.12.3 Winter Monsoon over Korea

During northern winter (DJF), extremely cold conditions with mean air tempera-tures often dipping to –40◦C and a powerful high pressure cell prevail over EasternSiberia and Mongolia to the north and northwest of the peninsula (see Fig. 5.2).Strong anticyclonic winds blowing around the high pressure cell (Fig. 5.3a) advectextremely cold conditions from the north and northwest, which lower air tempera-tures over the peninsula to sub-freezing levels, as shown by isolines of mean Januaryair temperatures on the basis of observations from 150 stations spread over a periodof about 20 years by McCune (1941) who divided the peninsula into 10 climaticregimes (Fig. 5.20).

5.12 Monsoon over Korea 151

Fig. 5.20 Mean January airtemperatures in Koreadelimiting 10 climaticregimes (after McCune,1941)

The sub-freezing temperatures lead to extensive snowfall over Korea betweenOctober and April when the peninsula comes under the influence of eastward-propagating baroclinic wave disturbances in the westerlies. Starting from the northin October, the snow-belt extends southward till by December it covers the wholepeninsula. It starts retreating in March and the process is completed by end of April.The mean dates of first and last dates of snowfall are given by the Climatic Atlas ofKorea, published by the Central Meteorological Office, Seoul, in 1962.

5.12.4 Summer Monsoon over Korea – Changma Season

The Meiyu–Baiu front which moves northward across China and Japan in late Juneand early July also affects the Korean Peninsula and produces heavy rain over thesouthern parts of the Peninsula, ushering in the Changma season in Korea. The meanannual precipitation (mm) of Korea, as given by the Climatological Atlas of Korea(1962), is presented in Fig. 5.21.


Fig. 5.21 Mean annualprecipitation (mm) of Korea

5.12.5 Korea’s Climatic Zones (After McCune, 1941)

McCune’s ten climatic divisions, shown in Fig. 5.20, are characterized by thefollowing:

1. An isolated mountainous region with sub-freezing temperatures during winterwhich may last for 5 months, and a short warm, moist summer;

2. The northeastern coastal belt has a cold winter with 3 months at sub-freezingtemperatures and a mild summer;

5.12 Monsoon over Korea 153

3. The northern west – a forest-covered hilly land – has a very cold dry winterwith mean January temperatures below –8◦C, and a warm wet summer;

4. The central west Korea has a mean January air temperature between –6 and– 8◦C;

5. The southern west Korea has a mean January air temperature between –3and –6◦C and an annual rainfall varying between 860 and 1370 mm;

6. The southern area which has a mean January air temperature between –3 and0◦C, has a mean annual rainfall which varies from 890 mm in the east to1500 mm in the west;

7. The southeastern coastal belt has a mild winter with mean January air temper-ature between 0 and –3◦C, and no dry months;

8. The southern coastal belt has a mild winter with mean January air tempera-ture above 0◦C and a long hot summer; rainfall exceeds 1500 mm at places,especially in the mountains;

9. Quelpart Island (Cheju Do) has a warm, moist marine climate with meanJanuary air temperatures above 4◦C and annual rainfall of about 1400 mm.

10. Dagelet Island (Ullung Do) has a mean January air temperature of 1◦C, heavywinter rainfall and annual rainfall of 1500 mm.

Cook (1964), in his survey of the climate of the Korean Peninsula, draws atten-tion to the extraordinarily long duration of sub-freezing air temperatures overNorth Korea during winter. His work largely corroborates McCune’s conclusionsregarding the climatic divisions of the peninsula and described above.

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