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Analysis of Summer Monsoon Fluctuations Over India

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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 spec- trum 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 clock- wise 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 peri- od 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 anti- cyclonic in the upper troposphere, becomes pre- vailing. Fig. 3a shows the latitude-height section of the mean temperature roughly along 80*E meri- dian. Over the Indian subcontinent, the tropo- pause 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 stratifica- tion 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.
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