Reprinted from "Monsoons of thePages 65 to 74

Monsoon winds and General Circulation

H. FLÖHN

Deutscher Wetterdienst, Frankfurterstr

ABSTRACT. Starting from a purely descriptive definition of monsoon winds äspredommant air currents with a marked seaspnal shift, the classical explanation of monsoonswill becompared with their geographical distribution. From a comparison ofoceanic andContinental conditions at the globe—using for example the central Pacific and the Indo-Australian Section (orAfrica)-the hypothesis will be discussed, that monsoonal wind shiftsare largely caused by the seasonal transfer of the planetary wind and pressure belts overa continental section, due to the strong response of Continental surfaces to the seasonal varia-tions of solar radiation. The zona! pattern of monsoons will be described (according to a mapof Chromow). In relation to this, Japse rate and raininess of tropica! monsoons are stronglycorrelated with the zonal (and meridional) wind components.

1. In most text books of climatology and, to a lesser extent, of meteoro-logy, the discussion of monsoon winds is based on the classical ideas of Halley(1686) and Woeikof (1874). In a short and abbreviated manner, we maysummarize the usual text book scheme äs follows —

(1) Monsoon winds are predominant air currents with a markedseasonal shift.

(2) Monsoon winds blowing from sea to land are raoist, unstable andrainy, those in the opposite direction are dry, stable and rainless.

(3) The physical cause of monsoon winds is to be found in difierentialheating of land and sea, i.e., in the different response of the earth'ssurface to solar radiation.

(4) Tropical monsoons over the summer hemisphere are to be conceivedäs trade winds of the winter hemisphere crossing the equator anddeflected by Coriolis forces.

These definitions* and basic ideas had been derived from early climato-logical studies restricted to a rather incomplete collection of surface observa-tions. So it is not surprising, that due to the increasing amount pf surfaceobservations and specially after the Start of our present aerological epoch theseideas became slowly questionable and doubtful. With exception of definition(1), which is purely descriptive, the above mentioned Statements—äs soundand well based they may appear—conflicted more and more with the observa-tions. Especially the following observed facts are incompatible with anexplanation äs given under (2) to (4).

2. At the Mediterranean coast of Egypt we observe in summerpersistent stable and rainless northerly winds ("Etesians") blowing from sea

*Here we disregard the use of the term <:monsoon" for certain large scale weatherpatterns (äs in Europe), which can be defined rather objectively (Seilkopf 1956), since we may,at best, speak of "monsoonal weather patterns" or "monsoonal impulses". So far äs theseindividual cases do not produce a seasonal wind shift according to objective climatologicaldefinitions äs proposed by Conrad (1937) or Chromow (1950, 1956), this use pf the term"monsoon" should not be encouraged, in order to avoid possible misunderstandings. Simi-larly, it seems advisable to use the term "monsoon" only for those summer rainy seasonswith a clear and unambiguous correlation with seasonally shifting tropospheric wind patterns.

66 H. FLÖHN

to land. In contrast to this, variable winds from west and south are prevailingduring winter where occasional rains are mainly produced during the advanceof a travelling upper trough, thus coinciding with surface winds from southerlydirections, i.e., off the continent.

Studies of the summer rainy season of Eastern Asia, north of Lat. 30°N,have demonstrated, that these rains are generally produced by travelling dis-turbances of the extratropical westerlies (Okada 1910, Coching-Chu 1934).In addition to this, the water vapour transport during this season is directedfrom the continent towards the ocean with a constancy exceeding that of thewinds (Flöhn and Oeckel 1956). The same fact could be revealed at theEastern Coast of the North American continent (Benton and Estoque 1954).

The striking contrast between the wet and unstable southwest monsoonand the dry and stable northeast trade on the Indian Ocean, far from coastalregions, has been found also over the equatorial Atlantic between Lat. 10°Nand 10°S Based on 366,000 punched observations at the shipping route fromEurope to South America, we observe a rain frequency of

(a) 8.1 per cent with the prevailing easterly components, 25.1 per centwith the infrequent westerlies; and

(b) 8.1 per cent with components directed towards the equator( Trades ), 12,0 per cent with those directed to the poles( Antitrades ).

Further details and explanations are published elsewhere (Flöhn 1957:

summary cf. page 100).

As a matter of fact, stability and moisture content cannot be consideredany longer äs conservative air mass properties, but are strongly correlated withthe vergence of the wind pattern and its development with time.

3. If the differential heating of land and sea by the sun's radiation isthe leading physical factor causing monsoon winds, it is surprising how fre-quently coasts of large continents are not overrun by monsoons. This isespecially true for South America, and the Easteru Coast of North America,while at the Easlern Coast of Africa, north of the equator, the monsoon windsare blowing parallel to the coast. On the other band we observe rnonsoonalwind shiftings over large oceanic areas, e.g., over the equatorial Pacific betweenLat. 0° and 10°N, from the Asiatic continent eastward to at least Long. 160°Eand then from about Long. 130°W to the coast of South America. Thegeographical distribution of monsoons äs demonstrated by Chromow (1950,revised 1956, cf. Fig. 1) shows a marked zonal pattern, only partly coincidingwith the dislribution of continents over the globe.

In addition to this, the aerological epoch has demonstrated the predomi-nance of zonal flow with travelling long waves of the Rossby type. Thepattern of zonal currents travels seasonally in the direction towards the actualsummer hemisphere. In the stratosphere above 18-20 km (near 60 mb) uptoabout 80 km we obseive a marked shift from very constant easterlies insummer season to more variable westerlies in winter, described äs a "monsoon"firstly by Whipple (1935). Since this stratospheric monsoonal shift can beobserved also above the Southern hemisphere (South Africa, New Zealand andthe Antarctic) there exists no distinct relation with the distribution of landand sea of the globe.

MONSOON WINDS AND GENERAL CIRCULAIION 67

_ _ __

S ^ EU! ^<"U l 4.MS ^^5 _"^yjc*^i c-j l ~L__ ' ' '<-.' Li" _ s r.'- _ _ ;^= ;'.]• ' •,-.,. ;• . . .;«• ~ i.", <

Fig. 1. Geographical patterli of mottsoonal winds (Chromow 1956)

4. On the Pacific and on the Atlantic we observe during the largestportion of the year—especially during northern summer—the southern tradescrossing the equator and approaching, with dominant easterly components,the Intertropical Convergence Zone which is situated persistently on theNorthern hemisphere. Since the horizontal component of Coriolis forces isvery small (/=2 ftsin <£<IO ^5 sec ~ J) tiear the equator, we find here no indica-tion of a deflection ofthose ESE winds by the Coriolis forces, at käst notbetween Lat. 0° and 5°N. On the Indian Ocean, however, we observe duringthe northern summer, that the southeast trades are detached by the southwestmonsoon at about Lat. 2-3°S, äs demonstrated already in the rriaps of Halley(1686, cf. Fig. 2), Woeikof (1874) and especially by Meinardus '(1893, cf.Fig. 3). During the northern winter, the northeast trades are detached by thenorthwest or west monsoon of the Southern hemisphere (Indonesia), also atabout Lat, 2-3°N (Meinardus 1893). Both facts are confirmed by recentinvestigations of the author together with the fact that this detachment is nota steady transition, but in most cases discontinuous. Here the classical textbook concept of a steady trans-equatorial flow, directed from the winter hemi-sphere to the summer hemisphere and deflected slovvly by the Coriolis forces,is definitely not compatible with the observed facts and should, therefore,be rejected.

II. From the above considerations it seems advisable to disregard theconnection of the te rm "monsoon" with the differential heating of land andsea, with cloudiness and precipitation patterns and also with the suspected roleof the opposite hemisphere in producing tropical monsoons. 1t seems prefer-able to use the term "monsoon" for winds in a pureiy descriptive sense, äsproposed independently by Chromow (1950) and the author (1950) andto itwestigate the physical causes of the observed seasonal wind shifts on thebasis of our recent aerological observations together with sorne dynamicalconsiderations. As a matter of convenience, we may restrict (Chromow 1950)the use ofthis term to those areas, where the resultant (or predominant)

68H. FLOHN

Fig. 2. Map of predominant winds during northern summer (Halley 1686)

N ITC 10°N

-8°

-6°

NE-PossotSW-Monsun

Äquotorigle Westwinde

v-O

SITC

o-~

Fig. 3.

^0°

2°

4°

-6

8°

•io°sNOV. DEZ JAN. F£B. MÄ'RZ APR. MAI JUNI JU1/AU6.Ci O-i ©-* O-> »-i

BESTÄNDIGKEIT s 15% -,0-50% 51-50% 51-70% »70%

Resultant surface Winds at the Indian Ocean (10°N—10°S, 86°—92°E)after the data given by W. Meinardus (1893)

Symbols for constancy (%) are shown below

<^

^'S S 'S 5 E-Passat

koNSOoisr WINDS AND GENERAL CIRCULATION 69

direction of wind shifts at least 120° with the seasons, and where the frequencyof the prevailing octant is large enough, for example, >60 (40) per cent.

Our present ideas of the general circulation of the earth's atmosphereare based mainly on the laborious investigations of Starr (1954) and Bjerknes(1955) and their numerous collaborators on the mechanism of meridional andvertical transport of momentum, internal and kinetic energy. water vapourand other quantities.

If we compute the meridional transport Ay of a quantity a (e.g., sensibleheat, angular momentum or water vapour) with the meridional component vof the wind, and denote the deviations from an average (cT, t7) over a latitudinalcircle with d, v' and the time average with . . . , we obtain

Ay = 'a v' = a v + a' v' + a v'l II III

In this well known transport equation—an elucidating and precise discus-sion has been given by Mieghem (1956)-the term I contains the Hadley trans-port with an averaged, J^-which in fact mostly results äs a small residual fromthe difference between large number of individual southerly and northerly com-ponents Term II deals with the quasi-stationary horizontal anomahes of aand v or in other words, with the anomalies produced by the distribution ofland and sea and orographical features in such a way, that the term "mon-soonal transport" may be justified, but only from a traditionai pomt of view.Term III can be described äs the Ferrel transport due to horizontal eddyexchange A similar equation governs the vertical transport Az vwh thevertical component w of the wind. Starr and Bjerknes have demonstrat-ed that the horizontal eddy exchange (äs proposed by Ferrel 1856)nröduces the largest part of these transports. IQ the tropics, the Hadleymechanism of transport by a (zonally averaged) wheel m a vertical and meri-dional" plane takes part in this mechanism, but at least the inomentum transportby horizontal eddy exchange exceeds that of the Hadley cell.

Within this planetary mechanism monsoonal wind shifts are of secondaryimportance. As a matter of fact, the planetary pressure and zonal wind beltsare seasonally displaced towards the respective summer hemisphere. But com-paring these fluctuations over continental areas, like Africa or the Indo-Australian section äs projected on a meridional plane, with those over oceanicsections e.g., the Central Pacific, we note substantial differences, especially intropical' regions. Over continental sections, the equatorial trough, defined fre-auently äs the Intertropical Convergeuce Zone (ITC), migrates from the equatorto about Lat. 18-20°N (Africa, Arabia) or even Lat. 28-30°N (India) after thenorthern summer solstice, and to about Lat. 15-18°S (South America, Africa,Australia) after the southern solstice. Over oceanic sections, however, thisseasonal migration is rather small. Over the Central Atlantic the ITC is dis-olaced from Lat. 1°N (February) to 10-U°N (August) äs demonstrated byKuhlbrodt (1942). Over the Pacific coast of Long. 160°W, this displacement

70 H. FLOHN

Qcean/c25" ,

ff Continentaldiv v precipitation prenure

div fony. KR pf>

'Fig. 4, Tropical surface cifculation pattcrns over oceanic (I)and Continental (II) sections

Right : divergence, precipitation and pressure

is described to be 18Ö lat., but this value (Gordon 1951) seerhs fär too highdue to the insufficient number of available observations. From other sourceswe may conclude that the seasonal displacement of the equatorial troughon the Central Pacific is less than 10° lat., to be compared with at least35° (including India 45°) lat. in Continental sections.

From these observational facts, mapped by Chrornow (1956), we mayconceive two different patterns of the planetary circulation of the atmosphere(Fig. 4), one idealized for a homogeneous land-covered earth, the second for acompletely oceanic globe (Flöhn 1953).

1. On a continental globe the thermally controlled pressure pattern willbe dislocated more or .less according to the displacement of the sun's zenithalPosition. Therefore, the ITC—if defined in a somewhat misleading manner ästhe surface convergence within the shallow heat lows—will travel from nearlyLat. 23°S in January to nearly 23°N in July. Due to the pressure gradientbetween ITC and the equator, a westerly current will occur In this Zone overthe summer hemisphere, with vertically unstable southwest winds (on theNorthen hemisphere) in lowest layers, deflected by surface friction. Over thewinter hemisphere stable easterly winds are predominating between the sub-tropical cells and the equator. Therefore, we ought to expect a seasonal shift-ing between east and west above the friction layer—between southwest andnortheast, near the surface, north of the equator—in that zoneof tropical mon-soons (l of Fig. 1) between equator* and the extreme position of the ITC.

*Probably due to travelling equatoriat cyclonic disturbances—äs described by Malurkarand Frolow in this volume— a Secondary Intertropical Convergence Zone (S1TC in Fig. 4,11)is formed near the equator. Here the above mentioned discontinuous wind shift—firstlydescribed by Halley (1686) in his famous map—occurs, together with convergence (supportedby the decreasing velocity of the southeast trades, Meinardus 1893) and, consequently, in-creasing rain and cloud frequency.

MONSOON WINDS AND GENERAL CIRCULATION 71

Similar shifts are to be expected in a zone near Lat. 30-35°N, where thetrades near the subtropical anticyclonic cells are prevailing in summer, replacedby the spurs of the extra-tropical westerlies and its travelling Rossby wavesduring the cold season. Examples ofthis kind can be found, among otherareas, at the Mediterranean, in Middle Asia, over the Pacific off the CalifornianCoast and off the Chilean Coast near Lat. 30°S. Under the assumption of asufficient water budget, we obtain a tendency to summer rains in the zone oftropical monsoons contrasting with winter rains in the zone of subtropical mon-$oon$ (2a and 2b of Fig. 1),

The subpolar zone of low pressure may also be dislocated in the sameway and we may expect a zone of subpolar monsoons with prevailing easter-lies in winter and westerlies in summer near Lat. 65°N. However, at presentwe find no example of this kind, since near the northern coast of Asia and(partly) North America the subpolar low cells migrate in an opposite direction,e.g., in summer towards the continents, in winter towards the oceans. Here weobserve a shallow zone of subpolar monsoons (4 of Fig. 1) due to a thermalcontrol äs described in the classical scheine under I 3.

The zone of mid-latitude monsoons äs described firstly by Chromow(1956, 3 of Fig. 1) is not to be expected on a homogeneous Continental earth.At present it is largely produced by seasonally reversing zonal pressure gra-dients between land and sea, i.e., in accordance with the classical picture (See-tion I 3), but partly situated in the interior of the continents (Middle Asia !).Due to the shallowness of the last two monsoon zones which are mostlyrestricted to the frictional boundary layer (< 1000-1500 m), they are - in contrastto (Section I 2) -generally not correlated with precipitation and cloudiness.

2. Over an oceanic globe (Fig. 4, I) the positions of the latitudinalbelts of pressure and winds are expected to be displaced seasonally not morethan 5° lat., in accordance with their present behaviour over the southernPacific, due to the very weak annual course of surface temperature, Under theassumption of a nearly Symmetrie pattern of pressure and wind on both hemis-pheres, the ITC will remain in such a distance from the equator, that the weakhorizontal component ofthe Coriolis force is too small for the formation ofan equatorial westerly belt. As we observe now over the oceanic areas of theSouthern hemisphere, the seasonal displaceinent of the subtropical anticycloniccells äs well äs that ofthe subpolar lows is not sufficient to produce monsoonalwind shifis. Only near the Southern continents we find relatively small areaswith monsoon winds, which have been described by Knoch (1930), for exampleat the coast of Chile (35-40°S), äs "Scheinmonsun" (fictitious monsoou) con-sidering that the different heating of land and sea cannot be regarded äs thetrue physical cause of this seasonal wind shift.

III. We obtain therefore the result, that monsoon winds—ofthe same kindwe observe now in tropical and subtropical latitudes—must also be expected ona completely land-covered globe, due to the strong reaction of the surface tothe seasonal Variation of solar radiation. The zonal pattern of monsoonalwinds äs demonstrated by Chromow (1950, 1956) supports strongly this hypo-thesisbased on an empirical comparison of continental and oceanic sections ofthe earth's atmosphere. But it should not be forgotten, that on coasts in azonal direction we observe, near the surface, a seasonal wind shift caused bydifferential heating ofland and sea äs prognosticated by the hypothesis ofHalley (1686). Examples of this kind are existing (4 of Fig. 1) over Northern

72H. FLOHN

Km

25

20-

N Summerw-\

N Winter

Fig. 5. Zonal wind pattern pver oceanic (left) and continenta! (right)sections during the extreme seasons

J = average position of the subtropical Jet Stream

MONSOON WINDS AND GENERAL CIRCULATION 73

Siberia äs well äs over the Canadian Archipelago (only partly shown inChromow's map, but revealed by recent data). They also exist at the NorthernCoasts of Central Europe frorn the Netherlands to Long. 22°E, here mainlybetween the months of May and November (Visser 1952), including the Cen-tral German Sea. In most cases they are shallow (less than 1000 m), äs it canbe observed'on all subpolar and mid-latitude monsoons according to Chromow'sclassification.

From this reasoning, we may revise the above mentioned text bookscheine (I, para 2 and 3—deleting para 4—) äs follows :

(2) Winds with westerly/easterly directions and a component towardsthe pole/equator have, in general, a tendency for lifting/subsidenceinstability/stability and raininess/dryness produced by Coriolis forcestogether with the spherical shape of the globe.

(3) The physical causes of monsoon winds are to be found (a) in thethermally controlled seasonal migration of the planetary pressureand wind belts in Continental sections of the globe and (b) in the(seasonally changing) differential heating of land and sea.

In the troposphere and lower stratosphere we ought to expect at a eon-tinental globe (Fig. 5) a high-reaching belt of tropical easterlies connected withthe stratospherical easterlies above the summer hemisphere, thus dividing theextratropical westerlies, at both hemispheres, with their subtropieal jet.Embedded in these tropical easterlies we obtain a strip of low-level equatorialwesterlies.

At an oceanic globe, a clash between the westerlies from both hemispheresshould prevail near the equator, thus partitioning the tropical easterlies fromthe stratospheric easterlies. Only occasionally we might expect high reachingequatorial easterlies, but low level equatorial westerlies normally should notoccur. The planetary role of the Berson westerlies (Palmer 1954) near theequator and the 20-km level is äs yet not completely known, but they hardlyseern to be related with the monsoon phenomenon.

REFERENCES

Benton, G.S. and Estoque, M.A. 1954 J. Met., 11, pp. 462-477.Bjerknes, J. and Mintz, Y. 1955 Investigatioiis oj the General Circulation

of the Atinosphere, Finalj jjKeport,AF 19 (122J-48.

Chromow S P. 1950 Isw. Wsesoj. Geogr., Obschtsch, 82,pp. 225-246.

1956 Collectioii "A.L Woeikof and present-timeclimatology", S. 84-108.

Coching-Chu 1934 /. geogr. Soc. China, l, pp. 1-27.

1934 Mein, neu. Res. Inst. Met., Nanking, 4.

Conrad, V. 1937 Met. Z, 54, pp. 313-317.

Ferrel, W. 1856 Nashville J. Med. Surg., 11.

1886 Ann. Rep. Chief Signal Officer, Washington.

74 M. FLOHN

Flohn, H.

Flohn, H. and Oeckel, H.

Gordon, A.H.

Halley, E.

Knoch, R.

Kuhlbrodt, E.

Meinardus, W.

Mieghem, J. van

Okada, T.

Palmer, C.E.

Seilkopf, H.

Starr, V.P.

Visser, S.W.

Whipple, F.J.W

Woeikof, A.

REFERENCES (contd)

1950 Bei: disch. Wetterdienstes U. S. Zorn, 18 ,

1950 Erdkunde, 4, pp. 141-162.

1953 Z. Met., 7, pp. 97-108.

1957 Beitr. Phys. Atmos., 30, pp. 18-46.

1956 Geophys. Mag., Tokyo, 27, pp. 527-532.

1951 Quart. J. R. met. Soc., 77, pp. 302-306,

1686 Phil. Trans., 16, pp. 153-168.

1930 Koppen-Geiger, Handbook Klimatologie,LG., p. 49.

1942 Forsch.-it. Erfahr Ber. ReichsaintWetterdienst, A 15.

1893 Arch. Seewarte, 13, p. 7.

1956 Beitr. Phys. Atmos., 29, pp. 55-82.

1910 Bull. cent. met. Obs., Tokyo, l, p. 5.

1954 Weather, 9, pp. 341-350.

1956 Dtsch. hydrogr. Z., 9, pp 90-102.

1954 Studies ofthe Atmospheric General Circula-tion, Final Report, AF 19-122-153.

1952 Hernel en Dampkring, 50, 61-66.1953 Geofis. pur. appl., 24, 135-148.

1935 • Quart. J.R. met. Soc., 61, pp. 285-308.

1874 Petermanns Mitt., Erganz, Heft 38.