Reprinted from "Monsoons of the worid",Pages 75 to 88

Recent investigations on the mechanism of the"Summer Monsoon" of Southern and Eastern Asia

H. FLÖHN

Deutscher Wetterdienst, Frankfurterstr•

ABSTRACT. Based on all available climatological data, the Formation of a quasi-persistent anticyclonic cell over the Tibetan Plateau, at 500 mb during summer, is demonstrated,caused by the excessive heating of the middle troposphere. The rapid advance of theIntertropical Convergence Zone towards north, described äs "burst of monsoon", is relatedto a reversal of the meridional gradicnts of temperature and pressure, a fact which could alsobe revealed from statistical studies.

On the basis of about 130,000 punched maritime observations from two meridionalstrips of the Indian Ocean, near Long. 65° and 85°E and north of Lat. 6 (10)° S, theaverage pattern of wind, cloudiness and precipitation is investigated ; 10-day averages areused to demonstrate the "singularities" of the annual course. The transport of water vapourover India and the coasts of Eastern Asia is derived from aerological data.

After some earlier contributions to the knowledge of the monsoonphenomena of Southern and Eastern Asia (Flöhn 1950, 1953a) from theaerological point of view, more recent investigations have been carried outsince 1953, from which only some preliminary results are published (Flöhn1955, 1957). It was planned to study from the view point of dynamicclimatology :

(1) the representative annual course of wind and weather over Indiaand the adjacent seas,

(2) the time-relationships of the monsoon rains to other summer rainperiods over Europe and Eastern Asia,

(3) the role of the Tibetan Plateau in the atmospheric circulation äs amechanical obstacle and an elevated heat source, together with its influence,

(4) the large-scale variations of upper wind Systems accompanying themonsoon period over India, and

(5) the large-scale transport of water vapour precipitated during thesummer rains of Southern and Eastern Asia.

The following lines intend to summarize the results now availablewith special stress to the Indian Summer Monsoon.

1. After some investigations of the so-called "singularities of weather",on the base of average daily frequencies of precipitation, cloudiness and otherdata (Flöhn 1943, 1950, cf. also Ramakrishnan 1953), a large-scale time-rela-tionship betvveen the summer rainy periods of India, Eastern Asia and Europehad been conceived äs a working-hypothesis, which should be checked on thebase of representative data. Since the maritime observations available inpunched card form at the Seewetteramt Hamburg of the German WeatherService cover quite homogeneously a period of 30 years, a thorough study ofthese should reveal some basic facts of the annual course of wind and weather

76 H. FLOHN

Fig. 1. Grid-areas for climatological investigations at theIndian Ocean along 65° E and 85° E

at the Indian Ocean, not disturbed by orographic influences. This investigationwas also planned to check the results already gained by Meinardus (1893) from36,000 sailing-ship observations (Flöhn 1953a) during the period 1885-1890with more than 130,000 more recent observations.

Since the data are to a large extent restricted to the main shipping routesbetween Aden, Bombay, Colombo, Calcutta and Singapore, their space distri-bution along the meridians of 65°E and 85°E is far from heilig uniform.Nevertheless, the data had been evaluated in grid-areas of 2° latitude and 6°longitude (Fig. 1), from the coastal regions near Karachi and Calcutta to Lat.6° and 10°S respectively.

The annual course of wind along these two meridional Strips confirmsour knowledge based on several maritime climatological atlases. North ofLat. 2°N we observe (Fig. 2) the marked shift from the northeast mousoonin northern winter—which should be described äs northeast trade, i. e., äspart of a planetary wind System—to the southwest monsoou in sumrner.However, it seems remarkable that the transition from the southeast tradeof the Southern hemisphere to the southwest monsoon normally takesplace at Lat. 2-3° S, and that, in wiuter, the equivalent transition from thenortheast trade to the northwest monsoon over Indonesia can beobserved at Lat. 2-3°N. Both facts had been pointed out by Meinardus (1893),but äs early äs 1686 Halley had described the first one in bis map (cf. Fig. 2,page 68 in this volume). However, they are still contradictory to usual textbook schemes (Flöhn 1953a). They reveal the existence of a narrow annual

SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA 77

Südwintlkomponente a [in m/sec jArabisches Meer Golf fo/j Bengalen

Westwindkomponente u (in nt/sec)Arabitcties Meer Ootfvon Bengalen

Fig. 2. South and West component of resultant surface winds (m sec -l)along 65° E (Arabian Sea) and 85° E (Bay of Bengal), 30 years

belt of westerlies at the Indian Ocean near the equator, confirmed also byaerological data from Addu Atoll, Padang, Singapore and some mountainstations at Sumatra and Celebes (Flöhn 1950, 1951). These equatorial wester-hes had been firstly described by Meinardus (1893) and then rediscovered äs aplanetary feature by Fletcher (1945). They extend from the vvestern coast ofAfrica to the Marianas in the Southwest Pacific äs well äs between aboutLong. 120°W and the Western Coast of South America äs shown in a prelimi-nary map (Fig. 3, Flöhn 1951).

Other featuresof interest are the marked contrast between the cloudyand rainy southwest monsoon äs compared with the dry and clear north-east trade (Figs. 4 and 5). A dynamical explanation of this relationshiphas been discussed elsewhere (See page 100) ; the equatorial spurs of thesoutheast trade are comparatively wet and cloudy, possibly due to the altitudeof the trade wind Inversion (2000-2500 m in the Madagascar region) and a

tenfdency for convergence in the same area. But in general, the rainy and

78 H. FLOHN

Fig. 3. Preliminary map of the area of Equatorial Westerlies in any layerbelow 3 km for northeru and soutbern smnmer

Due to lacking data, the extension of westerlies pver South America is liypothetical,and a connection between the westerlies a t Central Africa and those at the Indian Oceanbetween 30° and 60° E seems to be not supported by evidence

Niederschlagshäufigktit (in %)Arabisches Alter Golf WH Bengalen

Fig. 4- Average rsin frequency (% of all ship-observations) along 65°E and 85°E

Bewölkung (in Zehnteln )Arabisches Mter Golf wn Sengalen

F,v. 5. Average eloudiness (in tenths) along 65°E und SS°E

SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA 79

12-16°H

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

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

Fig. 6. Average rain frequency (%) at hvo areas of theIndiao Oeean, 30 years, for each decade of the year

cloudy season covers a shorter period äs the southwest monsoon, with exception of the rains of the retreating monsoon in November and December, overthe Bay of Bengal, which arc related.to the occurrence of frictional convergenceat the western flank of individual cyclonic disturbances c/. Krislina Rao, p. 89).As a working hypothesis, this discrepancy niay be due to the shallowness of thewesterly current before the onset of the monsoon rains and of the heat lowcirculation over the Indian Peninsula.

Over the Arabian Sea, the frequency of rains is fairly large during earlysuinmer but small during the "retreat of the monsoon" in autumn. On theother hand, over the Bay of Benga! the rain frequency is high during the retreat-ing monsoon, but surprisingly small during the "advance of the monsoon"in June. Thus we observe 011 both sides of the Indian Peninsula a maikedcontrast between a rainy season in early summer at the westerncoast and the Arabian Sea, compared with an autumn rainy season at the Bayof Bengal äs well äs over the eastern coast and large areas of the interior ofthe Deccan Plateau. From this evidence it seerns not possible to ascribe thiscontrast only to orographical effects but the author feels not competent togive a complete and physically sound explanation.

80 H. FLÖHN

The most surprising result (lower portion of Fig. 6) could be revealed onthe base of a large sample of data in a selected area of the Arabian Sea (Lat.8-12°N, Long. 62-68°E), evaluated in decadal averages. Here an isolated rainmaximura occurs in the first decade of June (.~ 18% compared with o17% inthe adjacent decades) ; this siginficant peak is to be considered äs the mostpronounced "singularity" of weather. Secondary peaks in November andDecember represent the retreat of the monsoon. An even more pronouncedmaximum (upper portion of Fig. 6) is observed in mid-July in the Bay ofBengal (Lat. 12-16°N), but is derived from only 4695 observations and, conse-quently, of lesser statistical significance. Individual and average data of theonset of the monsoon at different areas of the Western Ghats had been givenby Ramdas and collaborators (1954) and for Delhi by Bhullar (1952).

2. Recently, Sutcliffe and Bannen (1954) demonstrated, for the years1948 to 1953, a time-relationship between the shift of 200-mb winds from westto east over Aden and Bahrein, the disrupt transition from polar to tropicaltropopause over Habbaniyah and the onset of the monsoon at the Malabarcoast. Independently Suda and Asakura (1955) obtained for the period1930-1944, a similar coincidence between the first appearance of Baiu rains(together with anticyclonic conditions over the Okhotsk Sea) and the onset ofthe southwest monsoon over India. These coincidences merit further investiga-tions, on the base of more detailed and complete observations (Staff MembersAcademia Sinica 1957).

During the same time of the year, at the first half of June, the frequencyof blocking anticyclones between Iceland, Scandinavia and the British Isles(producing northerly flow over Central Europe together with cold rains)reaches its annual maximum (June 11). After that date, the frequency ofblocking anticyclones in the European Sector drops substantially during thesecond half of June, July and August (Hess and Brezowsky 1952). It deservesinterest that Bryson and Lowry (1956) demonstrated a sudden change fromnorthwest to southwest flow over Arizona, south of the elevated highlands ofthe "Great Basin", near June 15, half a month before the development of"monsoonal" rains produced by warm and moist air frorn the Gulf of Mexicoover this arid area. A new hemispheric atlas of 5-day averages of surfacepressure for a 20-year period, prepared by E. Wahl and R. A. Bryson, willpresent more details of such large-scale time- and space-correlations.

Obviously the rapid advance of the Indian monsoon near mid-June iscorrelated with the frequent occurrences of blocking highs on the western andthe eastern coasts of the Eurasian Contiuent and homologous features in otherparts of the atmospheric circulation. This advance coincides also with theformatioii of an orographically preformed quasipersistent trough of theextratropical westerlies near Long. 68°E, on the western fringes of the CentralAsiatic Highlands, äs demonstrated by Yin (1949), and with a sharpening ofthe annual quasi-stationary trough just off their eastern fringes nearLong. 110°E.

Obviously the advancing summer season creates, during the first half ofJune, a number of sudden changes in the general atmospheric circulation,which are mutually correlated. At this time, shortly before northern summersolstice, the temperature differences between land and sea are comparativelylarge, but in addition to this the heating effect of the high plateaus in aridsubtropical zoues amounts to its rnaximurn, thus producing a large-scaleshifting of the planetary wind belts together with their climatic long-wavepattern.

SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA 81

3. It has been demonstrated by Yin (1949) and Riehl (1954), that the"burst" of the Indian Summer Monsoon is connected with the disintegrationof the subtropical jet streara over northern India and with the formation of anew jet at 40-45°N, at the northern edge of the Central Asiatic highlands andmountain chains which we may shortly define äs Tibetan Plateau. This"Plateau", where the average altitude exceeds 4500 m over an area of at least1.7xl06 km2, acts firstly äs a mechanical obstacle to the atmospheric currents,enforcing the westerly planetary jet to concentrate its core either to the south(October—May) or to the north of it (mainly July—August), while in June andSeptember disrupt (and sometimes multiple) transitions between both patternsare observed (Figs. 7 and 8). As described by Thompson (1951) and others,an orographically fixed convergence zone is situated on the eastern fringe ofthe highlands, where during all seasons air masses with different life historyare enforced to converge and to create the entrance or the "root area" of thePacific Polar Front, sharpened by the orographically preformed climatic troughof Eastern Asia.

While the Tibetan Plateau has been described earlier äs a cold sourceduring winter (Ramage 1952), the predominance ofeasterly flow north of thevery warm ascents of northern India during July and August leads to thehypothesis of a warm anticyclone above the Tibetan Plateau*. From thoroughevaluations of all available informations, the author could demonstrate theexistence of extremely high temperatures in the free atmosphere near the500-mb level, where the 0°C level exceeds not only 500 mb, but reaches with6.1 to 6.2 km (Fig. 9), its greatest altitude on the globe (Flöhn 1953 b.) Themiddle (and upper) troposphere above the Tibetan Plateau is at least 6-8°warmer than the equatorial atmosphere over southern India, in accordancewith the strong, upward increasing and extremely persistent easterly flow overIndia above 7 km (Venkiteshwaran 1950, Koteswaram 1956).

Tndirect aerological data above Tibet had been derived from a thermo-dynamical Interpretation of carefully selected surface noon observations duringa number of expeditions. As described by all explorers since the Schlagint-weits (at about 1855), the summer lapse rate during day time must be nearadiabatic and frequent cumulonimbus clouds with gusts and showers ofhail ,eraupel and sleet confirm a moist adiabatic lapse rate above the condensationlevel. After the hourlv observations and double-theodolite pilot balloon dataobtained by an Ttalian expedition 1914 CAlessandri and Ginori 1931), atDepsang-Plateau(5362 m) east of the Karakorum, the winds at 1000-1500 mabove Station are verv closely reoresented by the surface wind observations atnoon and afternoon (12-18 hr), due to the high vertical exchange of momentum.

From those data it was possible to verify formation of permanent warmanticyclone, described at first by the author (Flöhn 1950, p. 38f) with an anti-cyclonic cell at 225 mb (Flöhn 1950, Fig. 24) with its centre near Lat. 29°N,Long. 95°E and somewhat weaker at 500 mb (Flöhn 1950, Fig. 23), recentlyconfirmed by Chinese meteorologists (Staff members, Academia Sinica 1957, Fig.2b) demonstrating a 500 mb anticyclone near Lat. 28°N, Long. 96°E on the baseof aerological ascents at Lhasa and other Tibetan stations. According to theabove mentioned indirect data, the author derived in 1953 (Fig. 10), a 500 mb

This effect of an elevation of the heat source in mountain regions has been demons-trated (Flöhn 1950 in Bei-, dtsch. Wetterdienstes US Zone, 31, pp. 18-19, Fig. 2) with a zonaltemperature cross-section along Lat. 32°N. The altitude of the isotherm peak near Long.90°E (in summer) is novv, forthe Tibetan Plateau itself, to be increased by additional400-500 m.

82 H. FLOH N

So m rn e rT~7

|, ~ -Fig. 7. The effect of the Tibetan Plateau to the mid-tropospheric ffow, July —August

Winter"-£•£-"• ->,i,"

^l"sJ-—

Fig. 8. Tlie effect of the Tibetan Plateau to the mid-tropospheric flow, November-March

Figs. 7 & S. Arrows^streamlines at 6-10 km ; dashed arrows=flow at 2-3 km (Fig. 1),1-1.5 km (Fig. 8) ; circles=upper troughs ; PPF=Pacific Polar Front

SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA 83

Fig. 9. Altitmie oi 0°C-levcl above Tibet (uuit 100 gpm) —FloÜn 1953b, 1955b

Fig. 10. Approxtmate 500-mb eonfoiirs (gpm; above Tibe! (derived 1953, unpublished)Figs. 9 & 10, a aeroiogical stations ; b~fixed surface stations ;

c expedition surface noon observations ; d =pilot balloon datu ;e=direetion of clouds near 500 mb ; t'=surface noon winds

H. FLOHN

1Q°N 20* 30

10'N 20' 30-

Fig. 11. Mean meridional cross-section of temperatiire (abovc) and geosfrophiezonal wind coniponcnt (below) along 75°E, July 1956

map for July and August, with a centre near Lat. 30°N, Long. 96°E (the positionLat. 33°N, Long. 90°E, given in Flöhn 1953 b, p. 273 proved later äs amisinterpretation). The small discrepancy between this position situatedsoutheast of the centre of the Tibetan Plateau may be related to the persistentrelease of latent heat above the mountains of Assam, Upper Burma andsouthwest China, with its abundant rain in the vicinity of the average summe rposition of the intertropical convergence zone (ITC). This cell, situated abovethe central and eastern part of the Tibetan Plateau, links between the largewarm anticyclone over North Africa and the Near East and a similar cell overthe subtropical Pacific, in the average separated by quasi-persistent climatictroughs near Long. 68°E and 110°E (Flöhn 1955a, Fig. 3). This anticyclonic cell,äs produced by the heating of the middle troposphere from the elevated surfaceof the Tibetan Plateau, can be swept away for some days by a travelling coldtrough of the extratropical westerlies, but it quickly recrcates in its rear. Thishas been demoustrated by short period occurrences of westerly wiuds at8-12 km above northern India (frequently connccted with "breaks" of themonsoon) and by strong aperiodic fluctuations of 24 hours averagedtemperatures at Depsang.

The existence of maximum temperatures just above the Tibetan Plateauhas been confirmed by radiosonde ascents rnade by the USSR at Khorog,during the summer of 1956, 500 mb temperatures above 0°C*. A meridional

* These observations at Khorog (Lat. 37.5°N, Long. 71.5°E, 2800 m) are shortlymentioned also by V. A. Bugaew (Ac. Sc. USSR, Abstracts Rep. Int. Ass. Meteor., XI. Gen.ASS, IUGG, Toronto 1957).

SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA 85

eross-section from Ceylon to the North Pole demonstrates this teroperaturemaximum near 37°N (Fig. 11).

Summarizing the aerological results, we obtain several maps with theaverage geostrophic flow near the surface, at 700 mb and at 500 mb (Flöhn1955a). These maps may demonstrate the existence of a practically conti-nuous easterly flow above 700 mb äs a part of the planetary tropicaleasterlies, dividing the equatorial westerlies south of the ITC from theextratropical westerlies. Due to the large zonal pressure gradiert t below700 mb we observe over Iran and Iraq a dominant flow from northerly direc-tions ("wind of 120 days"), over Southern China (Staflf Members, Acad. Sinica1957, Figs. 4 and 5) a dominant southerly flow, both obscuring partly the exist-ence of the planetary easterly wind belt above 700 mb. From these aerologicalresults, the following facts should be stressed —

(1) The existence of all planetary wind belts during the summer monsoonover the Asiatic Continent, slightly displaced in a northwarddirection, and

(2) The marked difference between the tropospheric flow over Southernand Eastern Asia during the summer rainy period.

Discussing these facts it seerns not justified (Flöhn 1955a) to merge theITC over India—which should be continued partly in a diffuse form, crossingUpper Burma, Hainau and Luzon to its position near Lat. 10 N at the CentralPacific—with the Pacific Polar Front within the westerlies over China andJapan, which roots in the orographically fixed convergence zoue at the easternfringe of the Tibetan Plateau (Thompson 1951).

4. The predominant effect of this cell is the rcversal of the normaltemperature and pressure gradient in the layers between 600 and 300 rnb, corre-lated with the advance and vertical extension of the equatorial westerlies below500 mb and the formation of a deep, strong and very persistent easterly ilowabove 500 mb, concentrated in a tropical easterly jet above 150 mb (Kotes-waram 1956). This reversal, produced by the seasonal warming ot the TibetanPlateau (Flöhn 1955a) acts like a switch for the atmospheric circulation over thesouthern half of Asia, the largest of all continents. 1t produces nearly simulta-neously (in most years in the flrst half of June) the following

(a) The sudden swing of the westerly jet from south of the Himalayaa tothe north of the Tibetan Plateau, and similarly over Eastern Asia,

(b) The migration of tbe tropical easterlies towards northern India—known under the misleading name "ßay of Bengal Brauch of theMonsoon"—together with their extension to the stratosphere and theformation of an easterly jet in Lat. 15-20°N near the tropopause,

(c) The advancement of the ITC to northern India together with a rapidextension of the equatorial westerlies ("burst of the monsoon"),

(d) The formation of a climatic trough near Long. 68°E correlated wiiha predominance of westnorthwesterly flow over Central and LasternEurope and with the (erroneously; so-called "Luropean SummerMonsoon",

(e) The sharpening of the climatic trough near Long. 110°£ causing theBaiu-rains of Japan, Central and Northern China.

86 H. FLÖHN

TABLE l

Lalitude Leh Warm Cold

10-12°N Travancore-Cochin May 26 May 30

17-20°N Ratnagiri—Colaba June 5 June 10

28.5°N Delhi June 30 July 6

This hypothesis of an active role of the seasonal war min g of the TibetanPlateau can be confirmed by some statistical studies on the onset (or establish-ment) of the summer monsoon over India.

(a) If we use the climatic records of Leh, in a tirst approximation, äsrepresentative tor the climate of Tibet and distribute the deviations i'romnormal June temperature int o 3 equal classes (warm, normal, cold), we obtainthe average date (Table 1) l'or the establishrnent of the monsoon rainsover India.

The (contemporaneous) correlation between the onset oi monsoon atColaba and the June temperature oi Leh is-0.41, the same äs for Delhi withthe July temperature of Leh. Fora conciusive compaiison the Leh lempera-tures—or prei'erably those öl a more representative Station—should be evaluat-ed using decade, weekly or pentade averagcs instead of monthly values. Tuenit seems advisable to sludy such corielations carefully, haviug in mind apossible application to medium ränge forecasting, since theie exists a markedpersistency of the temperature auornalies of Leh, duriug May and June.

(b) In the Damodar catchment (Prarnanik and Rao 1953) we find asimilar correlatiou between the Start of the monsoon rains in early June andLeh temperature ; when Leh is cold in June, the rain frequency oi the area inthe hrst decade of June, is 1.5 days, compared with 4.9 days with warmtemperatures at Leh. (For the first half of June, the relevant figures are 3.5and 8.2 days, for the monthly amount of rain 134 and 255 nun, respectively).

(c) In contrast to (a) the time relationship of the monsoon establish-ment liom south to north was louud to be surpnsirigly weak and insigm-ticant. The correlation between the Start of the dry season at West Java(Lat. 6-8"S) and the monsoon onset near Lat. 17°N is only +0.42, betweenthat tinie and the onset at Delhi even negative. From this evidence, thehypothesis of an active displacemeut of the mousoon convergence (ITC) fromsouth to north is not supported.

(d) The large scale relationships between subtropical and middle lati-tudes can be demonstrated by the fact, that in years with a high hemisphericalzonal index (Lat. 35-55"IN) in June the onset of monsoon at Delhi occurs, inthe average, at June 28, but in years with iow index only at July 4. However,no similar relationship could be found in latitudes south of 25°N.

5. The geographer's concept of "Monsoon Asia" is by and large derivedfrom the idea, that the summer rains—which are to be considered äs the mostpowerful natural phenornenon in these over populated areas with theiradmirable rieh and prominent cultures—are produced by a large scale inflowof moist maritime air to the continent. This concept cannot be supported byaerological evidence in Eastern Asia, since at least north of Lat. 30°N theprevailing winds during (and before) the summer rainy period are off-shore

SUMMER MONSOÖN OF SOUTHERN AND EASTERN ASIA 87

westerlies. In addition to this it could be shown (Flöhn and Oeckel 1956)that the water vapour tramport in the troposphere of these regions originatesfrom west and only south of about Lat. 30°N from southwest (over Okinawaand Hongkong, presumably over the whole Southern China and Formosa), andthat this water vapour transport is even more persistent than the winds at thesame levels. This result contrasts remarkably with the usual text book scheine,but agrees well with similar findings over the East Coast of America (Bentonand Estoque 1954). Here the inflow of moist air into the continent arisesmostly from the Gulf of Mexico.

Over Eastern Asia the source of water vapour precipitated during thesummer rains is np to now not exactly known, but can be found by somepainstaking calculations based on the Chinese aerological network. OverIndia and Thailand some recent calculations (Oeckel, äs yet unpublished)demonstrate that the moisture flux deviates not very much from the averagewind pattern ; during July it is directed from westsouthwest over the Deccanand over Bangkok, and from eastsoutheast only north of the ConvergenceZone near the foot-hills of the Himalayas. The question. how and whenmoisture is transported across the mountain ranges between India and Chinaremains at present unanswered, but is left to future computations. In thisway we may check the hypothesis of Albrecht (1951), that the bulk of themonsoon rains originates from the evaporation area of the southeast trade inthe Indian Ocean between Australia and Madagascar.

Regardiug the total water balance in the vast areas of Monsoon Asia,we should not forget that internal medium-and small-scale circulations areof/great importance, due to the unexpected high evaporation ofwooded orirrigated crop land, which according to all available data (Albrecht 1940, 1947,1951) is not much less than that of the ocean, in some cases possibly evenhigher. From such considerations we may easily understand, that on theeastern coast of the continents we observe, on the average, a large export ofmoisture from land to sea.

6. Summarizing our investigations and all available evidence, wecome to the result, that the seasonal warming of the elevated heat source ofthe Tibetan Plateau and the following reversal of the meridional temperatureand pressure gradients south of Lat. 35°N produces the far-reaching switchof the planetary wind belts over Southern and Eastern Asia, together with the"burst of the monsoon''over the Indian sub-continent. The essential contri-bution of low level disturbances travelling mostly from eastsoutheast towestnorthwest to the monsoon rains has been widely studied by Indian nieteo-rologists. The complex interaction of these "monsoon lows" äs a sort ofeasterly waves, with extratropical westerly troughs in the upper troposphereis considered to be responsible for the pulsatory character of the monsoonrains (Pisharoty and Desai 1956)*. From this point of view a few specialquestions may be formulated for future investigations—

(a) Can the retreat of the Indian Monsoon and the accompanyingshifting of the planetary wind belts during early autumn be considered äs a

*The frequency of such coincidences—with their strong influence on the Himalayanprecipitation—seems to decrease substantially from the region near Long. 90°E towardswest. This is refiected in the distribution of rainfall and, consequently, Vegetation, äsclearly demonstrated by the beautiful and fairly detailed Vegetation "maps l : 2 Mill.published recently by U. Schweinfurth : Die horizontale und vertikale Verbreitung derVegetation im Himalaja (Bonner Geogr. Abhandl. Heft 20, 373 S, 1957).

88 H. FLOHN

reflected image of the advance ? What physical effects are responsible for thelarge scale discrepancy in the precipitation pattern '?

(b) What dynamical causes contribute to the low intensity of monsoonrains over Rajasthan, Sind and Pimjab—äs well äs over Arabia—comparedwith the northeastern part of India, Burma and East Pakistan ?

(c) What effects are responsible for the early onset of the summer rainyseason over Assam, Burma and partly Bengal ? (The results of Ramaswamy1956, for winter and spring season suggest a similar relationship of convectiverains to the climatic trough near Long. 90°E also for May and early June).

Tn addition to this, a complete survey of the heat and radiation balancea t a repräsentative point of the Tibetan Plateau, seems to be highly desirable.

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1957 Ibid., 3l, Pt. l.1956 Tellus, 8, pp. 26-60.

1954 Indian J. Met. Geophys., 5, 4, p. 307.

1957 Tellus, 9, pp. 432-446.1955 /. met. Soc. Japan, 33, 6, pp. 233-244.1954 Proc. Int. Ass. Met.,X Gen. Ass.,

Rome, pp. 322-334.1951 Quart. J. R. met. Soc., 77, pp. 569-597 .1950 Mem. India met. Dep., 28, Pt. 2.1949 /. Met., 6, p. 393.