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Break in the Indian Summer Monsoon By R. RAMANADHAM, P. VISWESWARA RAO a n d J. K. PATNAIK1)

Summary - The changes in circulation patterns over Eurasia during break monsoon condition over India are studied in comparison to the active monsoon condition. Break monsoon condition seems to set in over the India Sub-Continent in association with eastward movement of middle latitude westerly trough at 500 mb, having large amplitude extending into west Pakistan and northern India. Simultaneously the subtropical anticyclonic ridge over Arabia protrudes into central and Peninsular India. The high latitude blocking high over East Siberia retrogrades and the East Asiatic trough deepens and moves eastwards. The west Pacific subtropical ridge recedes eastward from the China continent. During this period the monsoon trough shifts, from its normal position over Gangetic plains, northwards to the foot of the Himalayas and the monsoon westerlies in the lower troposphere extends right upto the rim of the Tibetan Plateau. The sub-tropical ridge line in the upper troposphere shifts northwards during break and lies approximately above the lower monsoon trough. This seems to provide an effective process of removing ascending air in the lower monsoon trough causing exceptionally heavy rainfall over Assam and along the foot of the Himalayas.

1. I n t r o d u c t i o n

The I n d i a n southwest m o n s o o n sets in over the s o u t h w e s t c o a s t o f I n d i a in the beginning o f June a n d establishes over the whole c o u n t r y b y a b o u t the m i d d l e o f July. It holds sway p r a c t i c a l l y over the whole c o u n t r y until the beginning o f September after which the m o n s o o n begins to w i t h d r a w f r o m the n o r t h w e s t India. D u r i n g these m o n s o o n m o n t h s the c o u n t r y receives m o s t o f the a n n u a l rainfall, t h o u g h it is n o t u n i f o r m l y d i s t r i b u t e d over space a n d time in a year. F u r t h e r the m o n s o o n activity varies f r o m y e a r to year even in typical m o n t h s o f July a n d August. In some years there m a y be p e r i o d s o f i n t e r r u p t i o n s in the m o n s o o n rains over m a j o r p a r t o f the c o u n t r y lasting several days k n o w n as ' b r e a k s ' in the m o n s o o n . The m o s t n o t e w o r t h y synoptic situation associated with the I n d i a n m o n s o o n in July a n d A u g u s t i-c the m o n s o o n trough. D u r i n g these t w o m o n t h s , this t r o u g h lies steadily over the G a n g e t i c p l a i n stretching r o u g h l y f r o m 30~ 75~ to 23~ 88~ It is well k n o w n t h a t the ' b r e a k m o n s o o n ' situation is associated with the n o r t h w a r d shift o f this t r o u g h f r o m the n o r m a l p o s i t i o n to the f o o t o f the H i m a l a y a s extending r o u g h l y f r o m 32~ 75~ to 26~ 90~ D u r i n g this p e r i o d heavy rainfall occurs a l o n g a n d near the f o o t o f the H i m a l a y a s which is responsible for severe floods in A s s a m a n d Bihar. 1) Department of Meteorology and Oceanography, Andhra University, Waltair, India. 14 PAGEOPH 104 (1973/III)

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Several attempts have been made over the years to discover associations between the break and the circulation changes in the troposphere over India and the neighbourhood. According to MALURKAR [1212), the northward movement of the monsoon trough may be due to accentuation of seasonal low pressure cell in West China, presumably due to a more southerly travel of extratropical disturbances than usual for the season. He also postulated that the 'July breaks' are at least partly caused by the absence of the monsoon pulses crossing the equator from the South to the North over Indian longitudes. KOTESWARA~ [6] associated these breaks with the westward passage of low pressure areas in the equatorial easterlies in the middle troposphere across the south Bay of Bengal. Later KOTESWARAM [7,8, 9, 10] showed that the equatorial westerlies at 500 mb spread as far north as 18~ during the break and the easterly yet stream at 200 mb. occupies a more northerly position than usual. RAMAN [18] has attempted to correlate the breaks with the northward movement of the typhoons in the southwest pacific. KULKARN! [11] and MOOLEY [13] observed in the absence of a low or depression in the Bay of Bengal, a western disturbance or a westerly wave moving across the extreme north of the country causes the monsoon trough to shift to the foot of Himalayas and the consequent break in the monsoon. PARTHASARATHV [14] PISHAROTY and DESA! [16] have associated the break monsoon condition with a more southerly passage of the extra tropical disturbances along the Himalayas. Later PISHAROTV and ASNANI [17] studied the flow pattern at 500 mb. level during normal and break monsoon situations over India and the neighbourhood and observed that the trough in the westerlies is displaced to the east and the Tibetan high moved northeatwards. The anticyclonic cell over Iran extends to central parts of India and the ridge from the Pacific high recedes to southwest and a well-marked low appears over China. They concluded that the change over to the pattern of break is likely to be associated with a change in the general circulation elsewhere over the northern hemisphere as well. RAMASWAMI [20] showed that the variations in the midtroposheric flow patterns over and to the north of India during active and weak summer monsoon spells are more significant than variations in the moisture content of the air or the positions of the monsoon trough at sea level or the stream line convergence in the lower tropospheric wind field. He further showed that the breaks are associated with the extention of a large amplitude trough from middle latitudes at 500 rob. level into sub-tropical Indo-Pakistan. In a subsequent study, RAMASWAMY [21] observed during break conditions pronounced low-index circulation prevails in the middle latitude westerlies north of the of the Himalayas leading to large amplitude troughs in the westerlies protruding into Indo-Pakistan area at 500 mb level and aloft. These troughs move eastward across the Tibetan plateau contributing to upper divergence and the associated 2) Numbers in brackets refer to References, page 646.

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heavy rainfall over and near the Himalayas. In association with these eastward moving troughs the anti-cyclone over Iran and Arabia extends into northwest and central India and the northern parts of the Indian Peninsula resulting in dry weather over the greater part of these regions. He concluded that the dynamical processes associated with the movement of the sub-tropical westerly jet into Indo-Pakistan and its dynamical interaction with the tropical easterly jet play a fundamental role in the development of breaks in the Indian southwest monsoon. All the past workers emphasized that perturbations in high latitude flow patterns influence the weather over India. The purpose of the present paper is to ascertain the relationship between the changes in circulation patterns in the tropospheric circumpolar westerlies and breaks in the Indian southwest monsoon.

2. Data Utilised, Methods of Analysis and Case Selected for the Present Study The data contained in the microfilms of Northern Hemispere data tabulations supplied by the U. S. Weather Bureau are utilised in the preparation of the charts for the present study. The data for India are taken from the Indian Daily Weather Reports. Contour charts at 700 mb, 500 mb, and 200 mb, levels for 3rd July 1967 representing the active monsoon and for 9th 1967 during break monsoon condition are prepared to study the circulation patterns. Also the contours pattern at 500 mb and 200 mb levels are examined from July 3rd to 10th, 1967 to ascertain the daily variations of the positions of the pressure centres. In drawing contours over the Indian region, greater weight is given to the winds than to the radio-sonde height values on account of the errors in the radiosonde data (RAMASWAMY[t9]). In addition, vertical time sections for a number of Indian Stations and daily cross-sections of zonal winds along the meridian 75~ are also prepared and analysed. In preparing the latter, the wind data of all stations lying within 5 ~ longitudes of the 75~ meridian are utilised. The monsoon was active over greater part of the country in the first week of July 1967. The axis of the monsoon trough at sea level which was over the Gangetic plain on 6th began to shift north-wards and was running along the foot of the Himalayas on 7th and a general break in precipitation began to set in over the country. However, heavy rainfall occurred along and near the foot of the Himalayas. This situation continued until 10th and began to revive over the country with the formation of a low pressure area at the head of the Bay of Bengal due to which monsoon trough has shifted slightly southward on 1 lth.

3. Analysis of contour charts a) Active monsoon condition Fig. 1 illustrates the flow pattern over Eurasia in the middle troposphere on 3rd July, 1967 when the monsoon is active over major part of the country. A middle

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latitude trough in the westerlies lies along between 45~ and 55~ extending from 40~ to 65~ throughout the troposphere. A similar trough known as East Asiatic trough, exists in the westerlies between 130~ and 145~ extending from 30~ to 55~ A blocking high is observed at all levels over East Siberia with the centre at 70~ and 110~ In the lower troposphere an extensive low pressure area exists over the Indian subcontinent with the monsoon trough extending east-west across India and adjoining seas (not reported). At 500 mb i. e. in the middle troposphere an anticyclonic cell exists west of its normal position over Arabia and the ridge from the west Pacific high extends upto 100~ along about 25~ There is a feeble anticyclonic cell over Tibet. The subtropical ridge in the upper troposphere along 25~ lies almost over the monsoon trough at 700 mb (not reported). b) Break monsoon conditions Figs. 2, 3 and 4 illustrate the flow patterns over Eurasia in the lower, middle and

Figure 1 Contour pattern at 500 mb level on 3.7.1967

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upper troposphere on 9th July 1967, a typical break monsoon day. The middle latitude trough in the westerlies is displaced to the east with an increasing amplitude and extends into Indian latitudes. The East Asiatic trough deepened and moved eastwards. The blocking high over East Siberia showed a well marked retrograded motion. In the lower troposphere the 'Monsoon Trough' shifted to the extreme north of the country and the Tibetan Plateau is surrounded by a cyclonic circulation. The ridge associated with the Pacific high receded eastward. The disappearance of the 3120 G P M contour over the continent indicates the retreat of the Pacific high eastwards. In the upper troposphere the subtropical ridge line shifted northwards and lies above the monsoon trough along 30 ~ The subtropical high pressure belt is fractured into three closed anticyclonic cells lying over west Pakistan, China and West Pacific Ocean respectively (Figure 4). Fig. 5 shows the successive positions at the 500 mb level of the various features of

Figure 2 Contour pattern at 700 mb level on 9.7.1967

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circulation during the period 3rd to 10th. It can be seen from the Figure that the middle latitude trough in the westerlies on 3rd is along 45~176 extending from 40~176 The blocking high over East Siberia has retrograded due to which the upper portion of the above trough is broken off and moved westwards on 7th. The lower portion of the trough moved eastwards and its amplitude increased after 6th as it approached Tibet and the feeble anticyclonic cell over Tibet collapsed. The East Asiatic trough which is along 130~176 on 3rd has deepened presumably due to the retrograded motion of the East Siberian blocking high. After 6th this trough moved eastwards, however the upper portion of the trough is more or less stationary. Simultaneously the ridge associated with the west Pacific high receded eastwards. This can be observed from the fact that the 5880 G P M contour has disappeared from the China Continent. Another significant change is the extension of the high over Arabia and Iran into central and Peninsular India and adjoining Indian seas.

Figure 3 Contour pattern at 500 mb level on 9.7.1967

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4. Analysis of the cross-sections of zonal winds a) Active monsoon conditions (Fig. 6) The easterly jet stream extends between 5~ and 25~ with its core near about 16~ at 16 k m level with an intensity of 70 knots. The monsoon westerlies extend up to about 8 km level and their northward extension is up to 20~ latitude. These westerlies are of m a x i m u m intensity of 30 knots below 3 km level. Beyond the monsoon westerlies, at lower levels up to 8 km level, easterlies maintained by the deflected monsoon current are observed north of the monsoon tlough. The westerly jet stream extends between 35~ and 45~ with its core of an intensity of 80 knots near about 39~ at a height of 12 km. b) Break monsoon condition (Fig. 7) During break conditions, the core of the easterly jet seems to move slightly south-

Figure 4

Contour pattern at 200 nab level on 9.7.1967

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wards to 14~ without any change of wind speed and height. A secondary maximum of easterlies of more than 50 knots are observed between 19~ and 23~ between the levels of 13 km and 16 km. The depth of the lower monsoon westerlies over Peninsula has considerably decreased to about 5 km level and wind speeds are also decreased. Another noteworthy feature is the absence of the easterlies over northern India, instead there is a continuous belt of westerlies in the lower and middle troposphere. The westerly jet stream holds sway between 35~ and 45~ with the core of 100 knots along 40~ at a height of 12 kin. There is a pronounced horizontal wind shear between 35 ~ and 39~ and between 41 ~ and 46~ above 6 km.

Figure 5 Successive positions of trough lines and ridge lines at 500 mb level during July 3 to 10, 1967 (The trough lines are shown as and ridge lines as . . . . . . The figures at the two ends of the lines indicate respective dates. H3, Ha, Hs,...,/-/lo indicate successive positions of the blocking high)

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Discussion

From Fig. 5 it is evident the trough in the middle latitude westerlies is moving eastwards. However, due to the retrograded motion of the high latitude blocking high, the trough has broken up and the upper portion moved westwards while the lower portion moved eastwards. During periods of low index circulation when there is pronounced meridional flow with jets embedded this middle latitude trough extends to latitudes south of 40~ over Russian Turkestan (RossBY[22]) and move eastwards over the Tibetan Plateau. Similar situation seems to prevail during the break monsoon period and these displaced troughs are made to move over the Tibetan Plateau. The westerlies above the plateau are much weaker than those at the same level to the east or west of the plateau. This is due to the sheltering effect exercised by the Tibetan Plateau on the airstream flowing over and near the Plateau (ACADEMIA SINICA[1], FLOHN[5]). This pronounced weakening of the basic current is responsible for this trough to get retarded and at the same time its amplitude grows large (BERGGREN, BOLIN and ROSSBY[2], and ROSSBu Thus the middle latitude eastward moving trough extends into west Pakistan and northern India. This invading large amplitude trough generally causes the Tibetan High to shift eastward and weaken it. It may even destroy it completely at 500 mb level as in the present case. The eastward displacement of the Tibetan high together with eastward displacement of the southern end of the semi-permanent trough in the westerlies permit a more southerly passage of extratropical disturbances along the Himalayas during the break monsoon period (MALURKAR[12], PARTHASARATHY[14],PISHAROTY and D~SAI[16]). Simultaneously, the easterlies over north India associated with the Tibetan High also get weakened considera n y and may even be completely replaced by westerlies in the middle troposphere. Though the Tibetan high is absent at 500 mb level, its presence at higher levels is beyond doubt since above 9 km level the easterlies prevail and are increasing in speed along the southern periphery of this high and are concentrating into a jet stream with its coie over Peninsular India near about 14~ During normal monsoon the west pacific substropical ridge extends to China continent in the lower and middle troposphere. The position of this is closely related to the middle latitude westerly circulation (ACADEMIASINICA[1 ]). The eastward retraction of the west Pacific ridge seems to associate with the eastward movement of the East Asiatic trough. This movement of the ridge permits the passage of typhoons round this high and their consequent recurvature before striking the Chinese mainland. This explains RAMAN'S[18] observation regarding the association of break monsoon with the recurvature of the Pacific typhoon. Further the retrograded movement of the high latitude blocking high, earlier mentioned, seems to be responsible for the deepening of the East Asiatic trough. The Southwest monsoon is considered as a circulation centered round the monsoon trough over north India. During break monsoon period this monsoon trough shifts northwards and the monsoon westerlies to the south of this trough blow right upto

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the foot of the Himalayas. Hence there is partially an orographic effect in addition to the dynamic convergence at lower levels. It can be seen from Fig. 2 that in the lower troposphere westerlies over most of north India are the extension of the monsoon current from the south. The extension of the middle latitude westerly trough into the Indian latitudes made a number of previous workers believe that dry circumpolar westerlies replace the monsoon current and thus cause a break in the monsoon. They explained that the break in the peninsular rains is caused by the cutting off the monsoon current from the southern hemisphere. This belief, however, does not agree with the fact since good amount of rainfall occurs over and near the Himalayas in the zone of the westerlies. If they are dry circumpolar westerlies, rain fall should decrease and not increase. Thus it seems that the break monsoon is the case of maximum extension of the monsoon circulation in the lower troposphere with the monsoon trough shifting farther north over Tibet instead of the disruption of the monsoon caused by the incursion of circumpolar westerlies (KOTESWARAM [101). The middle latitude westerly trough which penetrated into west Pakistan and north India at 500 1-fib level (Fig. 5) is moving relatively slowly. The westerlies are considerably weaker ahead of the trough than its rear and consequently the trough is of the diffluent type (with fanning contours). So there is a pronounced upper level divergence ahead of the trough line (PETTERSSENet al. [15], RAMASWAMY[19])which induces up-draught of air from the lower levels. Further, during break monsoon, the upper tropospheric ridge line shifted northwards lying over Assam and adjoining areas and is approximately above monsoon trough thus providing an effective mode of removing ascending air (DUNN[4]). It can be seen from the figure, on July, 7 the middle latitude trough extended upto 27~ along between 75 ~ and 80~ In addition there is a trough located over Assam in the westerlies. The monsoon trough is along the rim of the Tibetan Plateau and the moist southwest monsoon current pervades these regions in the lower troposphere. Thus the low level convergence of the moist monsoon air in the monsoon trough, together with the orographic effect mentioned earlier, the high level divergence and the effective removal of the ascending air act together in causing heavy rainfall in Assam and along and near the foot of the Himalayas. In association with the eastward movement of the large amplitude trough over north India the belt of heavy rainfall along the foot of the Himalayas moves eastward. This trough merges with the minor trough over Assam on 9th. The middle latitude trough flattened out on 1 lth and simultaneously low pressure area has formed overhead of the Bay of Bengal which is responsible for the southward shift of the monsoon trough and the onset of the active monsoon conditions again over India. An other note worthy feature at 500 mb level and aloft is the southeastward extension of the anticyclonic cell over Arabia into central and peninsular India. This is due to the influence of the eastward moving large amplitude trough and its southward protrusion into northern India. This protruding ridge is responsible for the change of easterly flow over north India to westerly to northwesterly flow. This high protrudet into India south of 30~ and contribute to the dry weather conditions over that pars

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o f the country (PISHAROTY and ASNANI [t 7], RANASVAMY[21 ], DATTA and GEORGE [3]). Such an extension of the Iranian high seems to be responsible for the lowering of the height of the m o n s o o n westerlies over peninsular India as well as weakening of the m o n s o o n current (Fig. 7). In the upper troposphere the subtropical ridge line shifted northwards and is associated with a slight northward m o v e m e n t o f the westerly jet. However, a considerable increase in the intensity of jet core is conspicuous with a p r o n o u n c e d increase in the horizontal wind-shear above 6 k m level. Thus it can be seen that south of 60~ in general, the systems seem to have eastward displacement. The general rearrangement of the long wave pattern in the circumpolar westerlies influences the weather over Asia in general and India in particular. Further the wave patterns in the upper tropospheric middle latitude westerlies are m u c h more p r o n o u n c e d than those in low latitude easterlies. Hence the study of the circulation patterns in the circumpolar westerlies is of m u c h forecasting value and leads to successful forecasting of the vagaries of the Indian Southwest Monsoon. REFERENCES [1] ACADEMIASINICA,On the general circulation over Eestern Asia (H), Tellus 10 (1958), 61, 62, 68. [2] R. BERGGREN,B. BOLINand C. G. ROSSBV,An aerological study o f zonal motion, its perturbations and break-down, Tellus 1 (1949), 32-33. [3] R. K. DATTA and C. J. GEORGE, Synoptic situation associated with the unprecedent flood in Assam during the third week o f August 1962, Proceedings of Symposium of Hydrometeorology of India with special reference to flood forecasting and warning, New Delhi (1966), pp. 157162. [4] E. G. DLrNN,Atlantic Hurricanes (1960), 197. [5] H. FLOHN, Beitrgige zur Klimakunde yon Hochasien, Archiv fiJr wissenschaftliche Geographie XII, 4 (1958), 304. [6] P. KOTESWARAM,Upper level 'lows" in low latitudes in the Indian area during southwest monsoon season and "break' in the monsoon, Indian Journal of Meteorology and Geophysics 1 (1950), 162-164.

[7] P. KOTESWARAM,Sci. Rept. Dept. Met. Univ. Chicago, Project NR-082-120 (1956), 23. [8] P. KOTESWARAM,The easterly jetstream in the Tropics, Tellus 10 (1958), 43-57. [9] P. KOTESWARAM,The Asian summer monsoon and the general circulation over the tropics, Report of the Syrup. on Monsoons of the World. In. Met. Dept., New Delhi (1960), 105-110. [10] P. KOTESWARAMand N. S. BHASKARARAO, The structure o f the Asian Summer monsoon, Aust. Met. Mag. 42 (1963), 35-55 [11] S. B. KULKARNI, A study of western disturbances and breaks in the southwest monsoon, Ind. Jour. Met. Geophys. 7 (1956), 5. [12] S. L. MALURKAR,Notes on analysis o f weather o f India and neighbourhood, Memoirs India Met. Dept. 28 Pt. 4 (1950), 139-215. [13] D. A. MOOLEY, The role of western disturbance in the production o f weather over India during different seasons, Ind. Journal. Met. Geophysics. 5 (1959), 329-336. [14] K. PARTHASARATHY,Weather in relation to the floods in Bihar, Bengal and Assam during July and August, Ind. Jour. Met. Geophys. 5 (1954), 329-336. [15] S. PETTERSON,E. G. DUNN and L. L. MEANS,Report o f an experiment in forecasting o f cyclone development, J. Met. 12 (1955), 58-67. [16] P. R. PISHAROTYand B. N. DESAI, Western disturbances and Indian weather, Ind. Jour. Met. Geophys. 7, 4 (1956), 333-338 [17] P. R. PISHAROTYand B. C. ASNANI,Flow pattern over India and neighbourhood at 500 mb during

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the monsoon, Report of the Symposium on Monsoons of the World, Ind. Met. Dept. New

Delhi (1960), 112-117. [18] C. R. V. RAMAN,Breaks in Indian southwest monsoon and typhoons in the southwest Pacific, Curr. Sci. 24 (1955), 219-220. [19] C. RAMASWAMY,On the sub-tropical jet stream and its role in the development o f large scale convection, Tellus 8 (1956), 52. [20] C. RAMASWAMY,A[preliminary study o f the behaviour o f the Indian southwest monsoon in relation to the westerly jet-stream, Special Palm6n No. Geophysics. 6 (1958), 455-476. [21] C. RAMASWAMuBreaks in the lndian summer monsoon as a phenomenon o f interaction between the easterly jet stream, Tellus 14 (1962), 337-349. [22] C. G. ROSSBY, The atmosphere and the sea in motion, (Oxford University Press, New York, 1959), 1%27. (Received 22nd March; in revised form 29th May 1972)