K All-Large-scale Aspects of the " Summer Monsoon '

in South and East Asia

By H. Flohn

Deutscher Wetterdienst, Frankfurt/Main, Deutschland(Manuscript received 3 June 1957)

Reprinted from the Journal of the Meteor ological Societyof Japan, the 75 th Anniversary Volume

Published by the Meteor ological Society of Japan, TokyoNovember, 1957

551.553.21 : 551.589.5

Large-scale Aspects of the " Summer Monsoon "

in South and E äst Asia

By H. Flohn

Deutscher Wetterdienst, Frankfurt/Main, Deutschland(Manuscript received 3 June 1957)

1. Introduction

In classical text-books of climatology theannual course of weather and climate islargely described by means of monthlyaverages and frequencies. In some areas ofJapan this leads to considerable smoothingof the two dominant rain-periods found inearly and late summer, separated by a dryseason starting near mid-July. AlthoughJ. J. Rein has mentioned this threefoldstructure of summer weather in Japan, äsearly äs 1876, and although the late T.Okada has thoroughly described the Baiu-Season (15) with its westerly disturbances,the fiction was widely accepted of a singlerain period during the whole summer con-nected with a south-easterly '' monsoonflow" from the Pacific. From the geo-graphical viewpoint, climate, landscape andeconomy of Southern and Eastern Asia(" Monsoon Asia") were not infrequentlyconsidered äs dominated by the summerlyinflow of maritime moist air with rain.However, since 1930 the aerological observa-tions have revealed a far more differentiatedpicture of the upper air currents. This iswell-known among meteorologists of EasternAsia, even if large gaps in the availableaerological informations still are to be filled.The purpose of this contribution is to drawattention to some peculiar large-scale tele-connections, i. e. space- and time- correla-tions connected with the occurrence of theearly summer rainy season in Eastern andSouthern Asia.

2. Weather sinß^ilarities and their telecon-nections

Starting from investigations of the micro-structure of the average annual course ofweather during the year on the basis ofdaily frequencies—known äs " singularitiesof weather " —, the author has found somecorrelations between distant areas. As anexample (6), the frequency of anticyclonicspells in the eastern USA and in CentralEurope during late September has a maxi-mum between 26 and 30 September, whichis well-known in folklore (Indian Summer,Altweibersommer), coinciding with a peakof anticyclonic weather at Osaka. In an-other investigation(7)two complete examplesof such a micro-structure are given forShanghai (1920-39) and Osaka (1883-1926)—the latter with daily averages of tempera-ture, pressure, precipitation (intensity andamount) and duration of sunshine, andwith daily frequencies of precipitation,cloudy and clear days. It is not intendedhere to discuss the large number of inves-tigations of these singularities; the availableliterature up to 1952 is summarized in (9),and Japanese contributions are mentionedby Takahashi (20). Wahl (22) has con-tributed much to a more general understand-ing of their aspects in the atmosphericcirculation. Comparing the advancement ofthe frontal zone (" Pacific Polar Front " =PPF) over Eastern Asia towards the north,with the rapid shift of the IntertropicalConvergence Zone (ITC), that causes the" onset of summer monsoon" over India,

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Large-scale Aspects of the " Summer Monsoon " in South and East Asia.

and with the most pronounced peak offrontal rains in Central Europe, we observethe surprising fact, that these events occur,in the average, in the second decade of

June (9).Under undisturbed conditions, the time of

the onset of monsoon is remarkably constant.As a result of ship-records from 30 yearswe obtain (Fig. l, lower half) the frequencyof precipitation observations for an areaalong the route Aden—Colombo, in theArabian Sea (8-12°N, 62-68°E). In this

12-16°N

A M 1 J J A S ' O'N D

15 -

1o -

5 -

J8-12°N

62-68°Bn=39o88

J F M A M

Fig. l Annual course of precipitation freQuencyin two areas over the Indian Ocean, 10-day-averages (after punch-card decks fromthe Seewetteramt, Hamburg).

representative sample an isolated maximumoccurs in the first decade of June; this peakis considered äs one of the most pronounced" singularities" of weather. Secondarypeaks in November and December representthe retreat of the ITC. In the upper halfof Fig. l an even more pronounced precipita-tion peak is found in mid-July in the Gulfof Bengal, but the number n of availableobservations is far smaller, and consequently

also the statistical significance. For furtherinvestigations of this type, a fixation of thedates of start and end of large-scale (area-averaged) rain-periods, representing Baiuand similar seasons for individual years inEastern Asia, seems to be highly desirable.

Yin (23) and Riehl (17) have consideredthe large-scale and disrupt shifting of theplanetary jet-stream from North India tothe northern edge of the Tibetan Plateau.This relationship has also been investigatedrecently in China (14).

In 1954 Sutcliffe and Bannon (19) demon-strated a relationship between the shift of200 mb-winds over Arabia from W to E, thetransition from polar to tropical tropopauseover the Iraq and the onset of the IndianMonsoon at the Malabar Coast. RecentlySuda and Asakura (18) have shown thatthe onset of the Indian summer monsooncoincides rather strongly with the start ofthe Baiu-season in Japan. However, it seemsnecessary to investigate this in more detail,considering individual stations or areas.

In Central Europe, the most pronouncedfrequency peaks of precipitation and coolingare caused by blocking anticyclones near0-10° W and subsequent upper troughs orcold pools in the area 10-20°E. Accordingto the Catalogue of European Large-ScaleWeather Patterns (13), the two highestfrequency peaks of large-scale weather typeswith northerly flow at Central Europe-cf.(13), Table I, N (GT)-occur at June 11and May 31 with frequencies of 37 % and33% respectively of all cases (1881-1951).A rather similar annual trend has beendemonstrated for blocking highs in thesector 20°W— 10°E (2). Their frequencyexceeds 25% of all cases only in the periodMay 25—June 20. This Situation togetherwith its teleconnections over Southern andEastern Asia form s part of a hemisphericcirculation-pattern, which is obviously cor-related with the thermodynamic and kine-matic influence of the elevated mountainsof Central Asia,

75 th Anniversary Volume of the Journal of the Meteoroiogical Society of Japan

3. The role of the Tibetan Plateau

Yin (23) and Riehl (17) have shown, howthe advance of the Indian Summer Mon-soon is connected with the disintegration ofthe planetary jet-stream in subtropicällatitudes and with the nearly contempora-neous formation of a new jet at 40-45°N,on the northern flank of the Central AsiaticMountains. These are dominated by thevast and elevated Tibetan Highlands wherethe average altitude exceeds 4500 m for anarea of at least 1.7-106 kms2.

It is easily understood that during sum-mer and in subtropicäl latitudes the TibetanHighland with its rolling or rather flatsteppe or barren land acts äs an elevatedheat source. All explorers from the Schla-gintweits (near 1855) and Sven Hedin(near 1900) to recent times—describe at thisseason the high frequency of cumulonimbus,of severe squalls and showers with sleet,hau or graupel during daytime, This isconfirmed by the observations of the ItalianExpedition to the Caracorum (1), on whichthorough hourly meteorological observations(including pilot bälloons and pyrhelio-meters!) were made during the summer of1914 at the plateau Depsang (35.3 °N, 78.0°E,5362 m). Under these conditions it maybe assumed that in summer near noon theaverage lapse rate of the atmosphere inthe ground layer (up to about 1000-1500 mabove surface) will be practically dry-adiabatic and/or, above the condensationlevel, near moist-adiabatic, while in thelowest 100-200 m it may be slightly süper-adiabatic. In addition it is allowable toassume at noon that the direction of surfacewinds deviates only slightly (about 20°)from the representative wind 500-1000 mabove the surface. This is also confirmedby the unique pilot-balloon data of Depsang.

Now we are able to use all availablemeteorological observations from expeditionsäs a first approximation to compute theheight and temperature of the 500 mb-level,

Fig. 2 Temperatures at the 500 mb-level, Julyand August, over the Central AsiaticHighlands.

Fig. 3 500 mb-contours, July and Augusta =aerological datab =fixed stationsc = non-stationary expedition observationsd =resultant pilot balloon windse = direction of medium cloudsf = surface noon wind direction.

the freezing level etc(ll). Thishas been done

with great care, especially by excludingseries where the thermometer may beaffected by radiation errors. When sotreated, the result, together with the sur-face winds near noon, reveals the existenceof a thermal anticyclonic cell in the mid-

troposphere above the Highlands, slightlydisplaced towards east. Here the tempera-tures at the 500 mb-level (Fig. 2) areslightly above freezing, presumably +1 or-f-2°C, which is confirmed by the aerologicalstations of North India (about -2°C) toge-

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Large-scale Aspects of the "Summer Monsoon " in South and East Asia.

ther with easterly winds in the north.Then the mid-tropospheric temperaturesare, during summer, higher than elsewhereat the same level in the atmosphere, causedby the peculiar (but numerically unknown)radiation balance of subtropical high nioun-tains. The southeasterly upper winds at500 mb over Burma and Assam, the easterlyflow at the southern edge of the Himalaya,together with cloud directions from NWover Eastern Tibet and from WSW in theCaracorum, confirm the existence of athermal mid-tropospheric anticyclone (Fig.3) in an area where up to 1956 no aerologicalinformation was available. Travelling uppertroughs only occasionally are able to sweepaway for a short period the warm air abovethe Highland; then westerly winds areobserved near 300 mb over North India witha resulting " break of the monsoon" (8).

In the author's opinion, the mechanismof the large-scale shifting of the troposphericwinds over India (8) can be most easilyunderstood if we take into account theseasonal warming of the Tibetan Highland,acting like a switch. This warming gra-dually weakens the westerly subtropicaljet over North India, then reverses themeridional gradient of pressure and tem-perature up to more than 20 kms and createsa new jet north of Tibet (40-45°). As atypical example of a summer cross-sectionalong 75°W the average conditions for July1956 (Fig. 4) may be given, where theSW-Monsoon over India äs a part of theContinental equatorial westerlies reaches the6 km-level, separated by the subtropicaleasterlies from the extratropical westerlieswith a jet core near 45°N and a secondarymaximum near the northern coast of Siberia.

The formation of a thermal anticycloneover Central and Eastern Tibet, occurringnormally about 15-30 days before solstice,causes also a shifting of the orographicallyinfluenced upper troughs. The broad troughnear 90°E (Gulf of Bengal), described byRiehl (17) and Ramaswamy (16), and pre-

W

•W

sx

Fig. 4 Average meridional cross-section of tem-perature and zonal component of wind inthe troposphere along 75°E, July 1956(drawn by H. Oeckel).

formed by the Southern fange of the Hima-layas, vanishes, and the Eastern Asiatictrough in the lee of the mountains js sharp-ened. The convergence of relatively coolnorth-westerly flow and warm moist airfrom the SW over Western China, describedby Thompson (21), strengthens the oro-graphically fixed frontal zone at 30-35°Nand plays an important role for the Baiu-Season. Upstream, a new upper troughtends to form near 68°E, also preformedin the low-land of Turkestan between Hin-dukush and Pamir-Tienshan. A comparableSituation of troughs and ridges can fre-quently be observed over North America,in summertime, on both flanks of thewestern mountains and highlands between105° and 122°W. *>

So the large-scale teleconnections between*> In Arizona, Bryson and Lowry demonstrated

(3) a sudden change from NW flow to SW flow nearJune 15. But the development of " monsoonal "rains at the beginning of July seems to be nearlyhomologous to that over Bengal, since it is con-nected with a shift to ESE-winds at 500 mb, andto SSE-winds at 800 mbs, indicating a transport ofwarm moist air from the Gulf of Mexico to thisdesert area.

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75 th Anniversary Volume of the Journal of the Meteorological Society of Japan

the switching of the wind distribution overthe whole area from the Iraq to the southerncoas't of China, and the onset of summ errains over Southern and Eastern Asia—äsproduced by the northward movement ofthe ITC and the PPF respectively—are con-sidered äs result of the seasonal warming ofair over'the Tibetan Highlands. It seemsnot quite clear, however, how the con-temporaiieous occurrence of blocking anti-cyclones and the Splitting of the planetaryjet in the European and Alaskan sector maybe linked with these events. But such aProblem in a hemispheric - scale generallymust be studied'in relationship to the inten-sity of the zonal circulation, äs' proposedby Wahl (22).

The hypothesis of an active role of theseasonal warming of the Tibetan Highlandsis confirmed by some statistical studies onthe onset of the Indian Summer Moiisoonaccording to the data given in (5).

1) If June is substantially warmer (colder)than normal at Leh (34.1°N, 77. 6°E, 3514m)—temperature deviations distributed into 3equal classes —the monsoon Starts at thesouthern part of the peninsula (Travancore-Cochin) at May 26 (30), near 18-20° Latitudeat June 5 (10), at Delhi (28.5° Lat.) atJune 30 (Jury 6). The last date is correlatedwith the temperature at' Leh in July withr =—0.41. The same value was--found be-tween the onset of monsoon at 20°N (Kolaba)and the temperature at Leh in June.

2) The time-relationship of the develop-ment of the monsoon from S to N is ratherweak and. insignificant. The correlationbetween the beginning of the dry season atW Java (6-8 °S) and the monsoon onset near17°N is only +0.24; that between the lattertime and the onset at New Delhi is evennegative, namely —0.17.

In addition it should be stated, that theonset of monsoon at Delhi occurs at June28 in years when the hemispherical zonalIndex (500 mb) in June is above normal,while in years with normal or low zonal

index this date averages July 4.

4. General considerations

The persistence of an anticyclonic subtropicalcell over Tibet, in July and August, mustbe taken into account when describing thedistribution of atmospheric currents andconvergence zones over Southern and EasternAsia. At the 700 mb- and the 500 mb-level(10), a zone with easterly flow separatesthe extratropical westerlies from the. equa-torial westerlies. The so-called BengalBranch of the Indian Summer Monsoonforms part of these subtropical easterlies,and is separated by a strong but fluctuatingconvergence (ITC) from the Arabian Branch,äs a part of the equatorial westerlies ex-tending from the Atlantic off West Africaover Africa and S-Asia to the Marianas inthe Pacific. Having this in mind, it seemsnot advisable to consider the PPF over Ea-stern Asia—being a convergence and frontalzone within the extratropical westerlies—äs a continuation of the ITC over India (10),äs frequently done even in well-known text-books and papers. Due to the strong zonaldifferences of surface pressure the planetarybelt of subtropical easterlies is lifted fromthe surface to approximately 700 mbs intwo regions: in the area 40-60 °E, wherewinds from NW (" shamal") are predomi-nating, and in the area 100-120°E, whereapparently an indistinct weak average flowfrom S or SW carries moist " equatorial air "towards the frontal zone. But since theeasterly flow still exists in upper levels, itshould be preferred not to confound twoconvergence zones of quite different meaning,which are normally separated by a largebelt of easterlies together with the sub-tropical divergence.

In a recent investigation, Flohn andOeckel (11) have demonstrated the existenceof a net ivater vapour transport from W to Eduring July 1952 over Korea and the mainland of Japan, äs assumed earlier (7). OverOkinawa and Southern China (Hongkong)

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Large-scale Aspects of the "Summer Monsoon" in South and East Asia.

tOO°£ 110

o soo looo tsoo xuogcnf'sec

Fig. 5 Average transport of water vapour between1000 and 700 nib (arrows) and between700 and 400 mb (dashed arrows), July1952 (drawn by H. Deckel).

the water vapour flux is concentrated inlower layers (mostly below 700 mbs) anddirected roughly from SW (Fig. 5). Thisresult contrasts strongly with former text-book views, but is well in accordance withsimilar findings over the east coast of NorthAmerica. Thus the hydrological cycle be-tween ocean and continents is rather com-plex; in humid regions the annual actualevapotranspiration over land is only slightlysmaller than the oceanic evaporation, andexport of moisture from continent to oceanis far from being excluded.

The original meaning of the term"monsoon"is nothing eise than a seasonal shifting ofdominating winds. From the view-point ofour present knowledge of the relations be-tween precipitation, vertical component andconvergence of wind, and the predominantflow in the layer of low and medium clouds,it seems not advisable to include in this

term the annual course of precipitation andcloudiness. This was frequently used inclimatological text-books, following Woeikof,Hann and other classical authors. Inde-pendently, Chromow (4) and the author(7, 8) have demonstrated, that the physicalcause of large-scale monsoons is not in eachcase and necessarily the differential heatingof continents and oceans, but—especially intropical latitudes—the seasonal shifting ofplanetary wind belts. Over the centralPacific, the ITC remains tbroughout theyear between 0° and 10°N. But in conti-nental sections—like Africa—the ITC travelsfrom 18°N in July to 15°S in January, andover India it reaches even 30°N, due to theexcessive warming of the elevated parts ofthe Asiatic continent. If the distance be-tween ITC and equator exceeds a criticalvalue near 10° Lat, an area of unstationaryquasi-geostrophic westeiiies is formed inthat zone. In such sections large seasonalvariations of the planetary pressure and windbelts with latitude—of course in accordancewith their cellular structure—are occurringand cause, for a given Station, monsoonalwind shiftings. Thus it seems justifled toassume (8) that on a homogeneous land-covered earth the seasonal travel of plane-tary wind belts reaches its highest values,resulting in a series of zonal strips withmonsoonal wind variations. Physically,this hypothetical mechanism is produced bythe thermal reaction of Continental surfaceto the seasonal variations of the radiationbalance. In contrast to this, on a homo-geneous water-covered earth the planetarywind belts should remain nearly constantat all seasons, äs in the central Pacific.

I am indebted to my collaborator Dipl.Met. H. Oeckel for his careful assistance instatistical calculations and drawings.

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75 th Anniversary Volume of the Journal of the Meteorological Society of Japan

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, 1955: WMO Tech. Notes 9, WMO-No. 38, TP 13. —9) Floto, H. 1954: Verbreitung undUrsache der Witterungsregelfälle. In: Witterung und Klima in Mitteleuropa, 2.-Aufl. Forsch, deutsch.Landeskde. 78, 168-184- —10) Flohn, H. 1955: Der indische Sommermonsun-als Glied der plane-tarischen Zirkulation der Atmosphäre. Ber. deutsch. Wetterdienst 22, 134-139. —11) Flohn, H. 1955:Bemerkungen zur Klimatologie von Hochasien, (in: Abh. Ak. Wiss. Lit. Mainz, in press), cf. alsoArch. Meteor. Geophys. Bioklim. B 6, 193-206. —12) Flohn, H. and H. Oeckf/,1956: Water vapourflux during the summer rains over Japan and Korea. Geophys. Mag. 27, 527-532. —13) Hess, P.H. Brezowsky, 1952: Katalog der Großwetterlagen Europas. Ber. deutsch. Wetterdienst US-Zone 33.—14) Koo, Gh.. Gh. 1955: A Tibeti-fennsik dinamikus hatäsa a keletazsiai l£gkörzesre. Id'jaras 59,204-211. —15) Okada, T. 1910: The Bai-U or rainy season in Japan. Bull. cent. meteor. Obs. Japanl, g. —16) Ramaswamy, K. 1956: The subtropical jet stream and its role in the development oflarge-scale con^ection. Tellus 8, 26-60. —17) Riehl, H. 1954 : Tropical Meteorology. London. —18) Suda, K. and T. Asakura, 1955 : A study on the unusual " Baiu " season in 1954 by means ofnorthern hemisphere upper air mean charts. /. meteor. Soc. Japan 2, 33, 233-244. —19) Sutcliffe,R. C. and J. K. Bannon, 1954: Seasonal changes in upper-air conditions in the Mediterranean-MiddleEast Area. Scient. Proc. Internat. Ass. Meteor. X. Gen. Ass. Rome 322-334. —20) TakahasM, K. 1955:Dynamical climatology (in Japanese). Tokyo. —21) Thompson, B.W. 1951: An essay on the generalcirculation of the atmosphere over South-East Asia and the West Pacific. Quart J. r. meteor. Soc.77, 569-597- —22) Wahl, E.W. 1953: Singularities and the general circulation. /, Meteor. 10, 42-45, 129-141. —23) Yin, M. T. 1949: A synoptic-aerologic study of the onset of the summer monsoonover India and Burma. /. Meteor. 6, 394-400.

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