Analysis of Summer Monsoon Fluctuations Over India

February 1976 M. Murakami 15

Analysis of Summer Monsoon Fluctuations over India

By Masato Murakami

Meteorological Research Institute, Tokyo (Manuscript received 26 August 1975, in revised form 17 November 1975)

Abstract

Temporal fluctuations appearing in the summer monsoon over India are investigated by using the data of 1962. By the method of spectrum analysis, it is revealed that two major periodicities exist, at least, in the temporal fluctuation of the monsoon. One is the oscillation around 5 days period and the other is around 15 days pericd. The oscillation around 5 days period appears mainly in the range from the north Bay of Bengal through the monsoon trough region in northern India. The structure of the disturbance which causes this periodicity is examined by the method of cross spectrum analysis. The results show that the disturbance is a westward-moving one and its longitudinal wavelength is about 30*. This disturbance seems to represent the so-called monsoon low. The vertical structure of these monsoon lows indicates that their cyclonic circulation is prevailing in the lower troposphere and the axis of the trough slightly tilts westward. Moreover, the monsoon low is accompanied with a distinct warm core in the upper troposphere. In the lower levels, the amplitude of the temperature is small and the disturbance is neither warm- nor cold cored. It is also shown that the monsoon low has a steering level at the height around 500 mb level. The oscillation around 15 days period is revealed to be in connection with the active/weak cycle of monsoon. The intense wind fluctuations associated with this cycle appear both in the upper and the lower troposphere, being in phase with each other. At the stage of active monsoon, it is revealed that the area with cyclonic circulation is formed over the Bay of Bengal. This cyclonic circulation is accompanied with cold temperature anomalies in the lower troposphere and warm anomalies in the upper levels. Besides, it is also shown that the depth of the moist layer over the Bay increases at this stage. These situations strongly suggest that the active monsoon condition over India is characterized by the enhanced convective activity over the Bay. Nearly opposite situations occur at the stage of weak monsoon and the anticyclonic circulation is formed over the Bay. The latitudinal shift of monsoon trough can be explained by superposing these two circulations on the distribution of the mean flow. The transition between the active and the weak stage is also investigated by applying the time-composite technique to the time series of surface pressure anomalies. At the stage of active monsoon, a large low pressure area is formed over the Bay of Bengal and the high pressure anomalies appear at the weak stage. By examining the time sequences of this transition, it is shown that a pair of high- and low pressure anomalies rotate clockwise over the wide area including the Bay of Bengal, Tibetan Plateau, whole Indian subcontinent and Indo-China. As for the nature of this rotation, it is suggested that the north-south standing oscillation between Tibetan Plateau and the Bay of Bengal together with the east-west one between Indo-China and the Indian subcontinent can cause the clockwise rotation mentioned above. It is also discussed that these oscillations seem of reflect the temporal variations of the intensity of the mean meridional- and the mean zonal circulation cells respectively.

the north Indian Ocean has a great significance 1. Introduction because of its associated copious rain. Many For the climate of the Indian subcontinent and works have been done about the climatology Southeast Asia, summer monsoon blowing over and the seasonal march of this monsoon (see 16 Journal of the Meteorological Society of Japan Vol. 54, No. 1

Ramage, 1971 for example). Over the Indian mer monsoon. He further discussed the cor- subcontinent, it has been revealed that a belt of respondence of each peak with the known dis- low pressure area is formed along the southern turbance, but the structural consistency with periphery of Himalayas during the summer mon- each other does not seem to be fully examined soon season. This belt is called "monsoon in his study. trough" and it is known that disturbances such In this study, we intend to reveal the charac- as monsoon lows and depressions travel west- teristic period of monsoon fluctuations and to ward through this region (Koteswaram, 1962). investigate what kind of system causes each A major amount of rain in Gangetic Plane is periodicity. The data are rearranged from Indian known to be produced by these disturbances. Daily Weather Report 1962 and the upper air Numerous synoptic case studies were made by observation stations used in this study are shown the Indian meteorologist and recently by Krish- in Fig. 1. The period June through August is namurti et al. (1975). They made an analysis selected as the summer monsoon season and of the intense monsoon depression traveling the year 1962 is expected to be the one when westward through the northern India. In their the nearly normal monsoon condition occurs. analysis, it is shown that the depression is ac- 2. Mean structure of the summer monsoon companied with a well defined warm- and cold cored structure in the upper and the lower Prior to the analysis of the temporal varia- troposphere respectively. Upward motion was tion, the examination of the mean basic state of revealed to occur in the west of the trough. the summer monsoon over India seems to be On the other hand, it is also known that there beneficial. In Fig. 2, the latitude-height section exists a long term fluctuation of the monsoon of the mean zonal wind is shown. They are circulation with period around 15 days in the derived from the data of the stations located upper and the lower troposphere (Murakami, roughly along 80*E meridian. In the south of 1972: Krishnamurti et al., 1973). This fluctua- 20*N, monsoon westerly blows in the lower tion seems to be related to the active/ weak troposphere and a strong easterly exists in the cycle of monsoon. Findlater (1969) found that the speed of low level Somali jet along the Mean Zonal Wind (m/sec ) eastern coast of Africa also fluctuates in close association with the amount of rainfall in the western coast of India. Recently, Keshavamurty (1973) made a statis- tical study of the time series data of the wind over India by the method of spectrum analysis. His analysis shows that there exist some dominant spectral peaks in the wind variation during sum-

Fig. 2 Latitude-height section of the mean zonal wind for the period June through August 1962. Fig. 1 Observation stations over In- The section is roughly along dia used in this study. the 80*E meridian. February 1976 M. Murakami 17

Mean Temperature (*c )

Fig. 3 Same as Fig. 2 except for (a) time-mean temperature and (b) its deviation from latitudinal average.

upper troposphere. Between the upper and the one over the southern India, there is a remark- lower winds, considerable vertical shear exists able thermal gradient through the middle and the in the middle troposphere. The monsoon trough upper troposphere. In the lower troposphere, is located in the latitudinal belt near 25*N. This latitudinal thermal contrast is weak and the region corresponds to the northern end of the gradient rapidly disappears in the tropopause. It SW monsoon in the lower troposphere. Though is also a matter of interest that the concentration the weak westerly remains in the figure after the Calcutta process of time average, easterly wind often reaches to the surface in this region during the summer monsoon. The vertical shear of zonal wind is weak over this area and the horizontal shear, being cyclonic in the lower and anticyclonic in the upper troposphere, becomes prevailing. Fig. 3a shows the latitude-height section of the mean temperature roughly along 80*E meridian. Over the Indian subcontinent, the tropopause lies near 100 mb level with the minimum temperature about -75*C. In the troposphere, it is clear that the temperature increases with increasing latitude during the summer monsoon season. In order to see this latitudinal contrast clearly, the deviation of the mean temperature from its latitudinal average at each level is shown in Fig. 3b. In this figure, relatively warm Fig. 4 Mean vertical profile of poten- core exists in the upper troposphere over the tial temperature (*), equivalent southern periphery of Himalayas, which seems potential temperature (*e) and to be connected with the warm air over Tibetan the specific humidity (q) at Plateau. Between this warm core and the cold Calcutta. 18 Journal of the Meteorological Society of Japan Vol. 54, No. 1 of the thermal gradient appears to occur over entrainment. the monsoon trough region around 25*N. 3. Periodicities in the temporal variation Fig. 4 represents the mean vertical stratification at Calcutta in terms of the potential tem- In order to see whether there exist some perature, equivalent potential temperature, and periodicities in the variation of monsoon flow, the specific humidity. This station is located in the the method of spectrum analysis is applied to eastern end of monsoon trough and faces the head the data of all the stations over India. Fig. 5 Bay of Bengal on the south. As will be discussed shows the results at Calcutta in terms of the in the following section, this station is also im- vertical distribution of power spectral densities portant in examining the cyclogenesis of the of the zontal and the meridional winds with monsoon disturbances. Over this region, the their mean values. In this figure, remarkable mean dry static stability in the troposphere is spectral peaks appear in the period range 10- to about 2 * 10-2 km-1. The vertical profile of the 15 days and 4- to 5 days apart from the power specific humidity indicates that there exists a due to the long term variations. The oscillation thick moist layer in the lowermost atmophere around 15 days period seems to be prominent in with the maximum value of 21 g/kg. This fact the zonal wind component, the power of which is reflected in the profile of equivalent potential shifts to the somewhat longer period in the upper temperature so that the conditionally unstable troposphere. The spectral peaks in the period layer with positive dry static stability and nega- 4- to 5 days appear both in the zonal and the tive moist stability appears below 600 mb level meridional wind equally. These peaks, especially at Calcutta. These features including the lower that of the meridional wind, exist in the lower moist layer and the conditionally unstable troposphere below 5 km level. stratification resemble with the results obtained The spectral peaks in the period range around by Jordan (1958). His results are often re- 15 days and 4- to 5 days appear in the data of ferred to as a typical stratification of the tropical other stations as well. The horizontal distribu- atmosphere near the ITCZ. In both regions, air tions of the variances due to these two period parcel lifted up from the surface is likely to ranges are exhibited in Fig. 6. The distribution penetrate through 200 mb level in case of no shown in Fig. 6a indicates that the amplitude

Calcutta

Fig. 5 Vertical distributions of the power spectra of the zonal (left) and the meridional (right) wind components and their mean values. Units of the power spectral densities are (m/sec)2 day and the values more than 100 are shaded. February 1976 M. Murakami 19

of the oscillation around 5 days period is large days period detected in this section is associated in the range from the north Bay of Bengal with these monsoon lows. On the other hand, through the monsoon trough region in the lower oscillation around 10- to 15 days period seems troposphere. As the oscillation around 10- to 15 to be in relation to the active/ weak cycle of days period dominates mainly in the zonal wind monsoon flow as discussed by Murakami (1972) component, Fig. 6b shows the distribution of and Krishnamurti et al. (1973). In the follow- variances of this zonal wind. In this figure, the ing section, we investigate the structure of the situation resembles with that in Fig. 6a, but the disturbances which bring the oscillations of these range of maximum variance is more confined in two period ranges and examine the correspond- the monsoon trough region. Though the figures ence mentioned above. are not shown in this paper, the most obvious contrast between the two period range lies in 4. Structure of the disturbances the fact that the large amplitude appears only 4.1 Period o f monsoon lows (*5 days) in the lower troposphere in case of the oscillation around 5 days period. As for the oscillation In order to see the three dimensional struc- around 15 days period, the range of maximum ture, the method of cross spectrum analysis is variance appears also in the upper troposphere applied to the disturbance around 5 days period. over the same region as in Fig. 6b. One can At first, horizontal propagatability is investigated easily anticipate this fact from the vertical using the data of the stations located in the distribution of power spectral densities shown range of large variances shown in Fig. 6a pre- in Fig. 5, too. viously. Fig. 7 represents the relation between In the head Bay of Bengal and the mon- the phase difference of disturbances and the soon trough region, it has been known that longitudinal difference of stations in the period the synoptic-scale disturbances travel westward range 4.0 through 6.0 days. Values are estimated through this area quasi-periodically in every 4- from the data of the meridional wind component to 5 days. These disturbances are called "mon- and averaged in the lower troposphere 0.3 soon lows" distinguished from the monsoon de- through 5.4 km level. The values with coherence pressions which occur in the frequency about less than 0.6 are excluded from the process of 1.8 per month (Keshavamurty, 1973). So, it is estimation. After that, the regression line is likely to suppose that the oscillation around 5 drawn subjectively in the diagram. In this

V'2 4.0- 6.0 days U'2 8.57-15.0 days

Fig. 6 Horizontal distribution of the variances at 3 km level produced by (a) the meridional wind in the period range 4.0 through 6.0 days and (b) the zonal wind in the period range 8.57 through 15.0 days. Units are m2/sec2. 20 Journal of the Meteorological Society of Japan Vol. 54, No. 1

days period, we can estimate that its longitudinal phase speed is about 6*/day. Moreover, we can also estimate that the horizontal wavelength of the disturbance is about 30* in longitude from the inclination of the line. This fact is consistent with our knowledge that the monsoon lows are the synoptic-scale disturbances traveling westward from the head Bay of Bengal through the monsoon trough region. The results of the inter-level cross spectrum analysis are shown in Fig. 8 in terms of the vertical distribution of variances, coherences and the phase differences relative to the 3 km level. This analysis is made with the time-series data of Calcutta where the amplitude of the disturb- Fig. 7 Relation between the phase ances is large as shown in Fig. 6a previously. difference of the wind fluctua- In Fig. 8, it is shown that the variances of the tion (**) and the longitudinal meridional wind component decrease with height difference (**) of the stations above the 5 km level. The coherences relative in the period range 4.0 to the 3 km level are also small in the upper through 6.0 days. levels. It is remarkable that the distribution of the phase differences in time shows the systematic diagram, it is clearly seen that the values are phase increase with height. Considering this well approximated by the line and the phase with the result that the disturbance propagates difference increases with increasing longitude.It westward, it can be concluded that the axis of indicates that the disturbances in this period the disturbance tilts slightly westward with range propagate westward in the lower trop- height. osphere. From the inclination of the regression The spectral peak around 5 days period also line and from the fact that this diagram shows appears in the temporal variation of thermal the propagatability of the disturbance around 5 variables. Fig. 9 shows the vertical distributions

V' 4.0-6.0 DAYS

Fig. 8 Vertical distributions of the variance (left), the phase differences (middle) and the coherences (right) of the meridional wind in the period range 4.0 through 6.0 days. February 1976 M. Murakami 21

4.0 --6.0 DAYS Calcutta the upper levels and the lowermost layer i about 1/4 cycle. Variances of the specific humidity are nearly constant below the 3 kn level. However, the phase distribution indicate that the axis of the specific humidity tilts east ward in the lower troposphere. In the discussions mentioned above, vertical structure of each variable is examined by the method of inter-level cross spectrum analysis At the final stage of this subsection, we apply the method of inter-variable cross spectrum analysis and investigate the whole vertical struc- Fig. 9 Same as Fig. 8 except for the ture of the monsoon lows. In this process, the phase (left) and the variance mean structure of monsoon lows are recon- (right) of the temperature (T') strutted by estimating the amplitude from the and the specific humidity (q'). variance of each variable and the relative phase relationship from the result of cross spectrum analysis. The results are summarized in Fig 10. This figure shows the mean vertical structure of the monsoon low passing over Calcutta for one cycle. Originally, phase relationship relative to the surface low (on the order of 1 mb in magnitude) is estimated from the phase dif-. ference in time. In this figure, the phase difference is transformed into the longitudinal difference by considering the fact that the disturbance propagates westward and its longitudinal wavelength is 30*. The vertical trough line is determined as the line where the meridional wind changes from the northerly to the southerly. From the structure of the meridional wind, it is seen that the cyclonic circulation of the monsoon low is prominent in the lower troposphere below the 500 mb level. The trough line coincides with the surface low at the lowermost layer and tilts slightly westward with height as discussed in Fig. 8. However, the associated anomaly of the temperature is large in the upper troposphere. In these upper levels, the area of Fig. 10 Mean vertical structure of warm anomaly nearly coincides with the trough . monsoon lows at Calcutta. It indicates that the monsoon low has a warm core in the upper troposphere. This upper warm of the variances of the temperature and the core is consistent with the fact that the in- specific humidity together with their phase dif- tensity of the cyclonic circulation decreases in ferences relative to the 7.2 km and 3 km level the upper levels. In the lower levels near the respectively. As for the temperature fluctuation, surface, the anomaly is small and the disturb- large variances appear in the upper troposphere ance is neither warm cored nor cold cored. The in contrast with the wind fluctuations . Vertical area of moist anomaly appears east of the trough distribution of the phase differences shows that in the lower troposphere. This area coincides the phase of temperature oscillation increases well with the southerly wind. Relatively dry rather rapidly with height in the lower trop- anomaly takes place with the northerly wind . osphere. It suggests that the axis of the tem- These configurations strongly suggest that the perature deviation has a large westward tilt in northward eddy transport of the water vapor the lower levels. The phase difference between occurs near the head Bay of Bengal in asso- 22 Journal of the Meteorological Society of Japan Vol. 54, No. 1

ciation with the monsoon lows. It might possibly The westward phase speed of the monsoon play an important role in the transportation lows was estimated at about 5 m/ sec previous- of the water vapor across the monsoon trough ly. Comparing this speed with the vertical profile from the Bay of Bengal toward the Tibetan of the mean zonal wind at Calcutta, it is con- Plateau as well as the transport by the mean cluded that this disturbance has a steering level meridional flow. at the height around 500 mb level. From the The well-known "monsoon depressions" are theoretical point of view, this fact seems to be more intense than the monsoon lows and they one of the essential features which must be occur not in every 4- to 5 days but about once considered in the cyclogenesis of the monsoon in two weeks. Recently, Krishnamurti et al. lows.

(1975) made a case study of the vertical structure 4.2 Active/ weak cycle (*15 days) of a monsoon depression. Comparing their results with the discussions mentioned above, we Unfortunately, the method of cross spectrum can note that the monsoon lows resemble with analysis does not well succeed in the study of the monsoon depressions in many respects. These the fluctuation around 10- to 15 days period. two kinds of disturbances are both in the range This difficulty seems to be caused mainly by two of synoptic scale and propagate westward with reasons. One is that the ratio of the period nearly the same speed. Cyclonic circulations of (*15 days) to the total record length (*90 days) both disturbances are prominent in the lower is large for the decisive discussions of cross troposphere and the both have distinct warm spectra. The other is that the periodicity of cores in the upper troposphere. They also show this fluctuation is not so stationary. The period that the monsoon depression has a cold core in of fluctuation varies in the range from 10 days the lower troposphere, but the monsoon low through 20 days as shown immediately after discussed in this study is neither warm- nor cold this discussion. So, in this subsection, the analysis cored in the lower level. Considering these re- is made by using a time-composite technique. semblances and some differences, it is likely to In Fig. 11, a vertical time section is exhibited suppose that the monsoon depression can be for the deviations of the zonal and the meri- intensified from the monsoon low under some dional wind from their mean values at Calcutta. favorable conditions which are not known yet. The time series are processed by the low-pass Its cold core in the lower troposphere is likely filter so that the variations with period less than to be formed in this developing stage. 10 days are suppressed. In this figure, it is

Calcutta

Fig. 11 Vertical-time section of deviations of the zonal (upper) and the meridional (lower) wind from their mean values at Calcutta. Time series are processed by the low-pass filter and the negative values are shaded. February 1976 M. Murakami 23

amount of rainfall in the western coast of India can be regarded as an index of the monsoon westerly blowing against this coast. In his work, it is discussed in relation to the Somali jet along the eastern coast of Africa. In this study, however, it is compared with the zonal flow at Calcutta near the head Bay of Bengal. In Fig. 12, it is quite obvious that the total rainfall in the western coast and the zonal flow near the head Bay of Bengal vary out of phase with each other. It indicates that when the monsoon westerly becomes intensified in the western coast, the easterly wind anomaly becomes strong in Fig. 12 Time series of vertically the head Bay of Bengal. The time when the average tonal wind at Cal- easterly wind anomaly is most intensified at cutta (upper) and the total Calcutta can be regarded as a stage of active 5-day rainfall of 4 stations monsoon. On the contrary, the maximum wester- in western India (lower) ly wind anomaly at Calcutta corresponds to the drawn after Findlater (1969). stage of weak monsoon. So, in the next step, clearly seen that the variation with 10- to 20 the times of the maximum easterly and the days period is prominent in the zonal wind. westerly wind anomalies at Calcutta are picked Moreover, this variation also appears in the up. Then, the low-pass filtered data at these upper troposphere, being in phase with that at times are composited in order to see the whole lower levels. This fact is in remarkable contrast situations of the active and the weak monsoons. with the variations associated with the monsoon Fig. 13a, b show the composite map of lows. anomalies of the wind, temperature and specific Fig. 12 shows the above-mentioned variation humidity at 850 mb level at the stage of maxi- of the zonal wind at Calcutta averaged vertical- mum easterly at Calcutta. This stage corresponds ly from surface through 500 mb level and the to that of active monsoon as discussed above. variation of the total overlapped 5-day rainfall It is remarkable in Fig. 13a that large easterly of 4 stations in western India. The variation of wind anomalies appear along the monsoon rainfall is drawn after Findlater (1969)'s analysis trough region in northern India. Westerly wind for the same period of the year. This total anomalies take place in the western coast and

Fig. 13 Time composite map of anomalies of (a) the temperature (solid line:*C), the wind (arrows) and (b) the specific humidity at 850 mb level at the stage of active monsoon. 24 Journal of the Meteorological Society of Japan Vol. 54, No. 1 the southern part of India. These wind anom- data, it can be said that the area with cyclonic alies indicates that the monsoon westerly across shear extends from the lower levels up to the the Arabian Sea is strengthened along its course 300 mb level at least over the Bay of Bengal over the western coast and the southern part and the central part of India. On the con- of India. This situation is in agreement with trary, the distribution of temperature anomalies the increase of rainfall in these regions. It is is nearly out of phase with that at lower levels. also noteworthy that a large area with cyclonic Temperature increases over the head Bay of wind shear is formed in the northern Bay of Bengal in contrast with the decrease in the Bengal. Distribution of temperature anomalies lower levels and the cold anomaly appears over shows that the cold anomaly in the lower trop- northwestern India which belongs to the area osphere occurs in coincidence with the area of of warm anomaly through lower levels. cyclonic shear in the Bay. Relative increase of In the above discussions, the situation over the temperature occurs in the range from north- the north Bay of Bengal can be summarized as western through northern India near the Hima- the cooling of the lower atmosphere, the increase layas. Fig. 13b shows that the moistening of of the depth of moist layer and the warming the lower atmosphere also occurs in the Bay of of the upper atmosphere together with the cy- Bengal. Considering that the layer with large clonic wind shear. These facts strongly sug- specific humidity exists near the sea surface gest that the convective activity is enhanced over throughout this season over the Bay, Fig. 13b this area. Though the data of direct observa- indicates that the depth of moist layer increases tions of the convective activity over the Bay at the stage of active monsoon. Relatively dry are not available in this study, it seems to be anomaly takes place in the area ranging from an important aspect of the stage of active mon- northwestern through northern India. soon. Furthermore, the lower cold core and Fig. 14 exhibits the composite map of the the upper warm core over the Bay imply that wind and temperature anomalies at 300 mb level the cyclonic circulation over this area is most in the upper troposphere for the stage of active intensified in the mid-tropospheric levels; the monsoon. It is apparent that the easterly wind disturbance like as the mid-tropospheric cyclone anomaly over the monsoon trough region and exists over the Bay at the stage of active mon- the westerly wind anomaly over southern India soon. also appears in this upper level. It is note- As easily anticipated, an opposite situation worthy that the mean upper easterly flow ex- occurs at the stage of weak monsoon. Figs. isting over the southern India is diminished at 15a, b represent the composite map of the the stage of active monsoon. Though the situa- anomalies of wind, temperature and specific tion in the uppermost troposphere cannot be humidity at 850 mb level at this stage. The well examined due to the lack of observation intense westerly wind anomalies appear over the monsoon trough region and the westerly is diminished (easterly wind anomaly) in southern India. These anomalies suggest that the axis of the low-level monsoon westerly across the Arabian Sea is shifted northward, directing toward the northern India at this stage. The area with anticyclonic wind shear appears over the Bay of Bengal. These aspects agree well with the fact reported by Ramamurthy (1972). In his study, it is mentioned that the winds, normal- ly being easterlies at 25-28*N during the active monsoon period, are replaced by the westerlies in the "break" monsoon period. It should be noted that the term "break" used by him and many others does not show any mode with periodicity. In this study, however, it is revealed that the similar situations appear repeatedly as Fig. 14 Same as Fig. 13a except an one stage of the active/ weak cycle with period for 300 mb level. around 15 days. Moreover, by considering with February 1976 M. Murakami 25 the distribution of the mean zonal wind in Fig. anticyclonic wind shear also appears clearly 2, the wind anomalies exhibited in Fig. 13a and over the Bay of Bengal and the central part Fig. 15a can explain the latitudinal shift of the of India. The increase of the easterlies over monsoon trough from the viewpoint of stream- southern India agrees with the discussion made line analysis. Thiruvengadathan (1972) discussed by Dixit and Jones (1965). They mentioned that the more southerly position of the monsoon that the typical break monsoon situation can be trough occurs during active monsoon conditions characterized by the extension of the easterly and the more northerly position occurs during regime over the Bay of Bengal and southeast weak monsoon. His discussion is just in agree- India. They also showed that these easterlies ment with the situations suggested by Figs. 13a replace the normal strong westerlies in the lower and 15a. troposphere for more marked breaks. As for The distribution of temperature anomalies in the distribution of temperature anomalies at 300 Fig. 15a shows that the lower tropospheric tem- mb level, it is shown that the cold anomalies perature rises over the head Bay of Bengal and appear over the Bay and the western coast of the range from central through southern part of India in contrast with the warm anomalies over India. Cold anomalies take place in the range these regions in the lower troposphere. Over from northern India through the foothills of the Bay of Bengal, the warm anomaly in the Himalayas. In Fig. 15b, it is shown that the lower troposphere and the cold one in the upper dry anomalies occur in the lower troposphere levels imply that the anticyclonic circulation over over the Bay. This indicates that the depth of this area is most intensified in the mid-trop- the lower moist layer over the Bay is reduced ospheric levels. The existence of this mid-trop- at this stage. The moistening of the lower ospheric anticyclone is also described in the troposphere occurs mainly in the southern tip case study of Dixit and Jones (1965) as an one of the Indian peninsula and the northern part of the characteristic features of the break mon- of India near Himalayas. This distribution is soon condition. Besides, the convective activity in agreement with our knowledge that the rain over the same area is likely to be suppressed falls mainly in the foothills of Himalayas and during this stage. the southern tip of India during the period of In the above discussions, two extreme stages weak monsoon. corresponding to the active and the weak mon- The composite map of the upper troposphere soons are investigated. In the final step of this at the stage of weak monsoon is exhibited in subsection, we intend to show the whole active/ Fig. 16. It is apparent that the westerly wind weak cycle in terms of the variation of sur- anomaly over the monsoon trough region and face pressure anomalies. The time series of the easterly wind anomaly over southern India surface pressure anomalies from its mean value also take place at 300 mb level. The area with are low-pass filtered in order to suppress the

Fig. 15 Same as Figs. 13a, b except for the stage of weak monsoon. 26 Journal of the Meteorological Society of Japan Vol. 54, No. 1 variations with period less than 10 days. Then, toward the Himalayas and probably further the data are categorized into the eight phases north toward the Tibetan Plateau in the lower denoted by Phase 1 through Phase 8. Phase 1 troposphere. It seems to be one of the dis- and 5 denote the time when the maximum tinguished features during the active monsoon easterly- and the maximum westerly wind period. Besides, it should be noted that the anomaly occur at Calcutta, respectively. These longitudinal pressure gradient also exists at this Phase 1 and 5 correspond to the stage of stage between the Bay of Bengal and the Arabian active and weak monsoons as discussed pre- Sea. viously. Phase 3 and 7 denote the time when The composite maps of Phase 2, 3, 4 are the zonal wind anomaly at Calcutta is zero. shown in Figs. 17b, c, d respectively. They That is, the easterly wind anomaly changes into represent the transition from the stage of active the westerly wind anomaly through Phase 3 and monsoon to the stage of weak monsoon. In the westerly becomes easterly through Phase 7. these figures, it is shown that the low pressure Phase 2, 4, 6 and 8 occupy the intermediate area in the Bay of Bengal first moves westward times. Since the period of this active/ weak cycle to the central India and then, it moves north- is about 15 days, the time interval between each ward toward the Himalayas. Keeping pace with phases is about 2 days. The composite maps this, the area of positive pressure anomalies of the surface pressure at Phase 1 through 8 moves westward from Burma into the Bay. The are exhibited in Figs. 17a through 17h respec- wind anomaly at Calcutta also veers with time tively. from the easterly to the westerly in this transi- Fig. 17a shows the surface pressure anomalies tion though the figures are not shown in this in Phase 1 corresponding to the active monsoon. paper. This fact can be easily anticipated by Large area with lowering surface pressure ap- the inspection of pressure anomalies shown in pears over the range from the Bay of Bengal Figs. 17b through 17d. They indicate that the through the central part of India. It can be direction of the pressure gradient near Calcutta seen that this low pressure system is consistent turns clockwise with time through the transition. with the cyclonic circulation of the wind in the After these sequences, the latitudinal pressure lower troposphere shown in Fig. 13a. A re- gradient over the monsoon trough region is remarkable latitudinal pressure gradient appears versed as well as the reversal of the wind over the monsoon trough region in northern anomalies. India. This agrees with the intense easterly Fig. 17e shows the composite map at Phase 5 wind anomalies over this area in the manner of corresponding to the stage of weak monsoon. geostrophic balance. This pressure gradient over It is clearly seen that the large high pressure the monsoon trough region shows the increase area is formed centering over the Bay of Bengal. from the low pressure area in the Bay of Bengal Though it is reported that the surface heat low is formed over the Indian subcontinent during the severe '`break" monsoon (Ramamurthy, 1972), this study shows that it is not always associated with the moderate active/weak cycle. The area of positive pressure anomalies extends to the central India and an distinct latitudinal pressure gradient appears over the monsoon trough region. We can see that this gradient consists of two kinds of anomalies with opposite sign. In the south of the monsoon trough region , the pressure increases, while it decreases toward the Himalayas in the north. This situation can also explain the northward shift of monsoon trough during the time of break monsoon studied by Raghavan (1973) in terms of the sea-level pressure. In addition, the longitudinal gradient appears over the Indian subcontinent at this stage, Fig. 16 Same as Fig. 14 except for too, decreasing toward the Arabian Sea. the stage of weak monsoon. Phase 6, 7, 8 shown respectively in Figs. 17f, February 1976 M. Murakami 27 g, h, represent the rivial of the active monsoon India during Phase 6 through Phase 8. Keep- over India. The high pressure area centering ing pace with this, the area of negative pressure over the Bay of Bengal in Phase 5 moves west- anomalies moves westward from Burma into the ward and then, northward to the northwest Bay of Bengal. Through these sequences, it can

Fig. 17 Continued to page 28. 28 Journal of the Meteorological Society of Japan Vol. 54, No. 1

Fig. 17 Composite map of the surface pressure anomalies in (a) Phase 1 through (h) Phase 8. be seen that the direction of the pressure between the above-mentioned regions. It is a gradient over the monsoon trough region turns well-known fact that the rainfall activity near clockwise with time. The veering from the Himalayas is enhanced during the weak mon- westerly to the easterly also appears in the soon period when the convective activity over change of wind anomalies near the head Bay the Bay of Bengal seems to be suppressed. The of Bengal, though the figures are not shown in opposite situation appears to occur at the stage this paper. This clockwise rotation of the sys- of active monsoon. The existence of a mean tem can be also found in the study made by zonal circulation cell is also suggested by Das Rahmatullah (1952). In his study, he classified (1962), using 10-layer, quasi-geostrophic model the variation of the monsoon flow into five and Mooley (1970) reported the fact that stages and showed that the axis of the monsoon droughts over northern India are usually asso- trough gradually rotates clockwise through these ciated with an excess of rainfall over Indo- stages. However, since his study is based on the China. It should be noted that the east-west raw data, he does not seem to succeed well in standing oscillation coupled with the north-south detecting the spatial disturbance which causes one with some phase lag can cause the clock- this clockwise rotation. wise rotation of the system mentioned in this Through the sequences shown in Figs. 17a section. through 17h, it is remarkable that the spatial 5. Summary and remarks disturbance associated with the active/weak cycle is not the one which propagates merely In this study, we examine the characteristic in the longitudinal and/or latitudinal direction. features appearing in the summer monsoon It seems that a pair of high- and low pressures fluctuations over India. It is revealed that the rotate clockwise around the area near the head two major periodicities appear in the temporal Bay of Bengal. The rotating area seems to variation of the monsoon. One is the oscillation cover the wide range including the Bay of around 5 days period and the other is around Bengal, whole Indian subcontinent, Arabian Sea, 15 days period. The oscillation around 5 days Tibetan Plateau and Indo-China. Though the period prevails mainly in the range from the nature and the mechanism of this rotation are north Bay of Bengal through the monsoon not fully revealed yet, it is likely that the stand- trough region located roughly along 25*N. ing oscillation between the Tibetan Plateau and Analysis of the wind variation shows that this the Bay of Bengal plays an important role in oscillation is confined in the lower troposphere close connection with the standing oscillation below the 500 mb level. Furthermore, by the between the Indian subcontinent and Indo- method of horizontal cross spectrum analysis, it China. These standing oscillations can be thought is revealed that the disturbance associated with to reflect the temporal change of intensities of this oscillation moves westward through the mon- the mean meridional- and the zonal circulations soon trough region with the phase speed of February 1976 M. Murakami 29 about 6*/day. The longitudinal wavelength of Bay also increases at this stage. The distribution the disturbance is shown to be about 30*, of temperature anomalies shows that the cy- indicating that the disturbance is classified into clonic circulation over the Bay accompanies a the synoptic-scale ones. These properties well cold core in the lower troposphere and a warm coincide with the features of the disturbances core in the upper troposphere. All these situa- often referred to as "monsoon lows" by the tions mentioned above strongly suggest that the Indian meteorologist. convective activity over the Bay of Bengal is The obtained vertical structure of these mon- enhanced at the stage of active monsoon over soon lows shows that their cyclonic circulation India. Moreover, the configuration of the lower prevails mainly in the lower troposphere below cold- and the upper warm core indicates that the 500 mb level. The axis of the trough tilts this cyclonic circulation is most intensified in slightly westward through these levels. However, the middle troposphere. That is, the existence the amplitude of the temperature variation is of the mid-tropospheric cyclone over the Bay large in the upper-levels above 500 mb and it is appears as an one of the characteristic features shown that the monsoon lows have a well- during the active monsoon period. defined warm core in the upper troposphere. In Nearly opposite situation occurs at the stage the lower troposphere, its amplitude is small and of weak monsoon. Though the detailed discus- the the disturbance is neither warm- nor cold sions are not repeated here, we can well ex- cored in these levels. Relative moistening of the plain the latitudinal shift of the monsoon trough lower atmosphere occurs in the east of the and the variation of the upper easterly flow trough in coincidence with the southerly wind. consistently by comparing the two situations of This situation suggests that the northward eddy the active and the weak stages. Over the Bay transport of the water vapor from the Bay of of Bengal, a mid-tropospheric anticyclone ap- Bengal occurs in association with the monsoon pears at this stage as reported by Dixit and lows. From the theoretical point of view, it Jones (1965) in the case of break monsoon. should be also noted that the monsoon low has Their study and many other case studies were a steering level at the height around the 500 mb made mainly about the "break" conditions when level. the mean flow pattern is completely destroyed Comparing the above-mentioned aspects of and the anomalies can be easily detected by monsoon lows with those of the monsoon de- using the raw data. Besides, usually the term "break" does not convey the meaning of the pression studied by Krishnamurti et al. (1975), it is revealed that both disturbances resemble quasi-periodic mode. However, this study reveals with each other in many respects. This re- that the similar situation occurs repeatedly as a semblance seems to suggest that the monsoon moderate active/weak cycle around 15 days depression might develop from the monsoon period. Moreover, the variation of the convec- low under some favorable conditions being un- tive activity over the Bay of Bengal seems to play known yet. If it is really the case, the question an important role for this cycle. when and how the monsoon low develops into The transition between the active and the the depression still remains as a challenging weak stage is also investigated by using the time problem. sequences of the surface pressure anomalies. At As for the oscillation around 15 days period, the stage of active monsoon, a low pressure area the time series data in the monsoon trough is formed over the Bay of Bengal, being con- region and the western coast of India show that sistent with the cyclonic circulation over this the easterly wind becomes maximum in the area obtained by the analysis of wind fluctua- monsoon trough region at the time when the tions. The latitudinal contrast is remarkable be- monsoon westerly and its associated rainfall is tween the low pressure anomalies over the Bay active in the western coast. On the contrary, and the high pressure anomalies near the westerly wind is strengthened in the monsoon Tibetan Plateau. As the time proceeds, the low trough region at the time of weak monsoon. pressure area over the Bay moves westward into The composite map at the stage of active mon- the central India and then, moves northward soon reveals that a large area of cyclonic cir- toward the Himalayas. Keeping pace with this, culation is formed over the Bay of Bengal a high pressure area moves westward from throughout the lower and the upper troposphere. Burma into the Bay. Thus, at the stage of weak The depth of the lower moist layer over the monsoon, an area of high pressure anomalies 30 Journal of the Meteorological Society of Japan Vol. 54, No. 1

is formed over the Bay with the anticyclonic reported by Miller and Keshavamurty (1968) circulation of wind anomalies. Throughout this is not discussed due to the lack of available transition, the direction of pressure gradient over data. Moreover, we have some evidence sug- the monsoon trough region as well as the direc- gesting the existence of a westward moving tion of wind anomalies turns clockwise with disturbance around 5 days period in the upper time and is reversed at the stage of weak mon- easterly flow over southern India. This disturb- soon. ance might correspond to the one discussed by The rivival of active monsoon occurs in a Koteswaram and George (1958) and by Kotes- similar way. The high pressure area over the waram and Rao (1963), but the density and Bay first moves westward and then, northward the extent of the observations of the upper toward the Himalayas. Keeping pace with this, troposphere seem to be still insufficient to in- a low pressure area moves westward from vestigate its detailed nature. The forthcoming Burma into the Bay of Bengal. These sequences MONEX (Monsoon Experiment) is expected to reveal that the spatial disturbance associated give a valuable opportunity to evolve these in- with the active/ weak cycle is not the one which vestigations. propagates merely in the longitudinal and/or latitudinal direction. It seems that a pair of low Acknowledgements and high pressure area rotate clockwise through The author wishes to express his thanks to this cycle over the region including the Bay Prof. K. Gambo for his valuable encouragement of Bengal, whole Indian subcontinent, Indo- throughout the work. The author is also in- China and the Tibetan Plateau. As for the debted to Dr. M. Iida for his kind help in nature of this rotation, it is plausible that the collecting the data. Thanks are extended to north-south standing oscillation between the Mrs. C. Yata for drafting all the figures and Tibetan Plateau and the Bay of Bengal plays to Mrs. R. Maruyama for her assistance in the an important role in connection with the east-west rearrangement of data. standing oscillation between the Indian subcontinent and Indo-China. These oscillations are References likely to reflect the temporal change of the Das, P. K., 1962: Mean vertical motion and non- intensities of the mean meridional- and the zonal adiabatic heat sources over India during the circulation cells respectively. The east-west oscil- monsoon. Tellus, 14, 212-220. lation coupled with the north-south one with Dixit, C.M., and D. R. Jones, 1965: '`A kinetic phase lag can cause the above-mentioned clock- and dynamical study of active and weak mon- wise rotation. soon conditions during June and July 1964." However, to examine the above hypothesis, Rep. Int. Meteor. Center, Bombay. we need the network expanded over wider area Findlater, J., 1969: A major low-level air current including the Bay of Bengal, Tibetan Plateau, near the Indian Ocean during the northern Indo-China, Arabian Sea and the western Pacific, summer. Quart. J. Roy. Meteor. Soc., 95, 362- south Indian Ocean, if possible. This network 380. Jordan, C. L., 1958: Mean soudings for the West will also provide useful informations which Indies area. J. Meteor., 15, 91-97. enable us to answer the question how the Keshavamurty, R. N., 1973: Power spectra of large- moderate active/weak cycle burst into the "break" conditions scale disturbances of the Indian southwest . From the viewpoint of re- monsoon. Indian J. Meteor. Geophys., 24, 117- gional experiment, the observation in the Bay 124. of Bengal seems to be important. As discussed Koteswaram, P. and C. A. George, 1958: On the previously, the active monsoon condition over formation of monsoon depressions in the Bay India seems to be characterized by the en- of Bengal. Indian J. Meteor. Geophys., 9, 9- hanced convective activity over the Bay. Besides, 22. this region is undoubtedly significant for the 1962: "Movement of tropical storms over study of the energetics and the cyclogenesis of the Indian Ocean." Proc. Inter-regional Seminar on Tropical Cyclones, Tokyo. monsoon lows and depressions. and N. S. Bhaskara Rao, 1963: Formation Finally, it must be said that not the all dis- and structure of Indian summer monsoon de- turbances occuring in the summer monsoon over pression. Austral. Meteor. Mag., 41, 62-75. India are discussed in this study. For example, Krishnamurti, T. N., S. M. Daggupaty, J. Fein, M. a mid-tropospheric cyclone over the Arabian Sea Kanamitsu and J. D. Lee, 1973: Tibetan high February 1976 M. Murakami 31

and upper tropospheric tropical circulations Sci., 29, 827-836 during northern summer. Bull. Amer. Meteor. Raghavan, K., 1973: Break-monsoon over India. Soc., 54, 1234-1249. Mon. Wea. Rev., 101, 33-43. M. Kanamitsu, R. Godbole, C. B. Chang, Rahmatullah, M., 1952: Synoptic aspects of the F. Carr and J. H. Chow, 1975: Study of a monsoon circulation and rainfall over Indo- monsoon depression (I), Synoptic structure. J. Pakistan. J. Meteor., 9, 176-179. Meteor. Soc. Japan, 53, 227-240. Ramage, C., 1971: Monsoon Meteorology. Academic Miller, F. R., and R. N. Keshavamurty, 1968: "Struc- Press, New York, 296 pp. ture of an Arabian Sea summer monsoon Ramamurthy, K., 1972: Case study of a heat wave system." Int. Indian Ocean Expedition, Meteor. during July 1966. Indian J. Met. Geophys., 23, Monogr., 1. 165-172. Molley, D. A., 1970: "Ph.D. Thesis" Poona Univer- Thiruvengadathan, A., 1972: Synoptic situations as- sity. sociated with spells of strong and weak mon- Murakami, T., 1972: Equatorial troposperic waves soon over Konkan. Indian J. Met. Geophys., induced by diabatic heat sources. J. Atmos. 23, 207-210.

インドにおける夏のモンスーンの変動に関する解析

村上勝人気象研究所

インドの夏のモンスーンにあらわれる時間変動に関する解析を,1962年の資料を用いて行った。スペクトル解析の手法により,この時間変動には二つの主な周期性があることがわかる。一つは約5日を中心とする周期であり,他の一つは約15日を中心とするものである。 5日周期の変動はベンガル湾北部からインド北部にかけてあらわれる。クロス・スペクトル解析により,この変

動をもたらす擾乱は西進し,その東西波長は経度にして約30度であることがわかる。これらの特性は同地域で従来 monsoon low と呼ばれている擾乱の性質と一致する。垂直構造の解析から,このmonsoon low に伴なう風の低気圧性循環は対流圏下層で卓越し,トラフの軸はやや西に傾いていることが示される。さらに,この擾乱は上部対流圏に顕著なwarm coreを伴ない,対流圏中層にsteering leve1 をもつこともわかった。このmonsoon low といわゆるmonsoon depression との関連についても論ずる。約15日周期の変動はモンスーン自体の強弱の周期と関連しており,これにともなう風の変動は対流圏の上下両層を通じてあらわれる。強いモンスーンの時,インド南部では西風が強まるが,北部では東風が強化され,ベンガル湾を中心として広範囲な低気圧性循環の場が形成される。この循環は下層に低温域,上層に高温域を伴ない,同時にベンガル湾上の湿潤層の厚さも増大していることが示される。これらの状況はベンガル湾上での活発な対流活動を示唆するものであり,このことは強いモンスーンを特徴づける重要な要素であると思われる。弱いモンスーンの時にはほぼ反対の状況が実現する。 time-composite の手法を用いて,モンスーンの強弱に伴なう地上気圧の変動の調査も行った。強いモンスーンの時には,ベンガル湾を中心として気圧は降下し,弱いモンスーンの時には気圧上昇がみられる。この間の遷移を調べると,気圧の上昇域と,下降域とが対をなしてベンガル湾,チベット高原,インド亜大陸およびインドシナ地域を含む広い範囲上で時計まわりの回転をしていることがわかった。この回転の機構に関しては,ベンガル湾とチベット高原との間の南北循環の変動,およびインドシナ地域とインド亜大陸間の東西循環の変動が重要な寄与をなしているようである。