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A Contrasting Study of the Rainfall Anomalies between Central Tibet and Central C. C. Chang1 India during the Summer Institute of Atmospheric Physics Monsoon Season of 1979 Academia Sinica, Beijing Abstract ratio thus computed is classified into four categories: Based on a comparison of rainfall anomalies between central India Weak monsoon day (W): 0 < r < 0.5 and central Tibet in July and August 1979, a negative correlation be- Normal monsoon day (N): 0.5 < r < 1.5 tween them is found. When an active monsoon prevailed over cen- Strong monsoon day (S): 1.5 < r < 4.0 tral India, a break monsoon occurred over central Tibet, and vice versa. The large-scale circulation conditions for an active Indian Vigorous monsoon day (V): r > 4.0 monsoon are characterized by the presence of a large area of nega- tive height departures over the Indian Peninsula and large areas of Thus, we have a uniform and consistent standard of classi- positive height departures over central Tibet. On the other hand, the fication for the monsoon rainfalls on both sides of the circulation conditions responsible for a break monsoon in India Himalayas. are characterized by frequent wave-trough activity over Tibet and the regions to the west of Tibet, and by a dominating high-pressure area over the Indian Peninsula. 2. Comparison of the rainfall anomalies between cen- tral India and central Tibet 1. Methods of analysis Figure 1 shows time series of the rainfall ratio of central India The rainfall data were taken from the Indian Daily Weather (r7) and central Tibet (rc) for July and August 1979. Some sa- News, the Records of the Central Meteorological Bureau of lient points can be noted from this figure. The longest break China, and other conventional data sources. We have chosen monsoon over India occurred during the period from 15 Au- central India as the region representative of the Indian mon- gust to the end of the season, on about 20 September. Ac- soon regime because this area is more sensitive to monsoon cording to private communications by meteorologists in the anomalies than the rest of the peninsula, as pointed out by India Meteorological Department, it is believed that this Hamilton (1977). On the other hand, the central part of the break monsoon of 1979, lasting for 37 days, set a record for Qinghai-Xizang (Tibet) Plateau has been selected for com- the last 100 years, breaking the long-standing record that parison with central India, mainly because of the availability took place during the period from 29 July to 18 August 1906. of rainfall data at 12 surface stations there, including the The sudden change in regime from an active monsoon to a Lhasa station. break monsoon around 13 August should be noted. One The definition of the summer monsoon was treated by Yeh would fail to describe properly the characteristic features of et al. (1979) and will not be deliberated here. the monsoon rainfall if only the monthly rainfall data were The rainfall ratio used for the present comparisons is de- used because, as in the present case, positive and negative fined as anomalies may cancel each other. During the same period, however, the rainfall anomalies r = R/R over central Tibet were almost opposite, resulting in a nor- where R is the daily rainfall and R is the daily mean rainfall mal or somewhat stronger-than-normal monsoon rainfall amount averaged over a long period. The monsoon rainfall during August 1979. The active monsoon in central India occurred during the period from 28 July to 11 August, whereas over central Tibet 'Dr. C. C. Chang is professor emeritus of the Catholic University a break monsoon was predominant during the same period. of America in Washington, D.C. For many years he served as chair- Likewise, in July two break monsoons over India occurred man of the meteorology department of that university, and as during 1-7 July and 23-25 July, and approximately corre- member representative of Catholic University in the University Cor- sponded to two periods of active monsoon over central poration for Atmospheric Research (UCAR). He is now on an ex- tended research leave to the People's Republic of China as a visiting Tibet. professor. (Comments by E. R. Reiter.) The above-mentioned evidence seems to suggest that there 20 Vol. 62, No. 1, January 1981 Unauthenticated | Downloaded 10/01/21 01:26 AM UTC Bulletin American Meteorological Society 21 FIG. 1. Time series of daily rainfall ratio (labeled on left side of diagram) over central India (r/) and central Tibet (rc) for July and August 1979. Categories of the monsoon are indicated on the right side of the diagram. FIG. 4. 500 mb height departures from normal at 1200 GMT, 5 August 1979. 3. Relationship between rainfall anomalies in two con- trasting regions and the large-scale circulation conditions at 500 mb Figure 2 shows the 500 mb analysis at 0000 GMT, 20 August 1979, which was selected as being typical for the large-scale circulation patterns prevailing during the period of break monsoon over central India from 15 August to 20 Sep- tember. Although the circulation features during this period varied more or less from one day to the next, Fig. 2 neverthe- less can be considered as representative of the basic features FIG. 2. 500 mb map at 0000 GMT, 20 August 1979. Units of the large-scale circulation. A strong and stable blocking in 10 gpm. high was located to the west'of the Ural Mountains. Wave troughs moved over Tibet within the north-northwesterly airflow ahead of the high-pressure ridge, causing rainfall events whenever they met with the moist and warm air from northern India and from the Bay of Bengal. Under such cir- cumstances the stations over central Tibet received normal or above-normal rainfall amounts. The cold air traveling along the leading edge of the blocking high was also advected across Iran and into the west coast region of the Indian Pen- insula. This persistent, southward intrusion of cold air may have been one of the principal causes for the large-scale fail- ure of the Indian monsoon (Joseph, 1978). In addition, it can be seen that a high-pressure area dominated most of the In- dian Peninsula, a condition that can suppress the growth of convection there. At the same time the Iranian subtropical high was pushed southward so that the northern Arabian Sea was dominated by this high, thus weakening the northern branch of the Somalia jet. It is generally accepted that the weakening of the Somalia jet is associated with deficits in the monsoon rainfall over the Indian Peninsula. Similar circula- tion features also are evident from the map of 500 mb height FIG. 3. 500 mb height departures from normal at 1200 GMT, 22 August 1979. Units in 10 gpm. departures (Fig. 3). Over central India positive height depar- tures (AH > 70 gpm) prevailed, and as a consequence the rainfall ratios were extremely low = 0.06). On the other hand, a large area of negative height departure covered cen- may be a negative correlation between the monsoon rainfall tral Tibet, where above-normal rainfall amounts were re- anomalies over the midsection of India and those over cen- corded. tral Tibet, a fact indicated by Gao You-xi and his associates (private communication). Figure 4 shows the 500 mb height departures from normal Unauthenticated | Downloaded 10/01/21 01:26 AM UTC 22 Vol. 62, No. 1, January 1981 break monsoon over central India during early July 1979. The patterns of height departures are in many ways similar to those in Fig. 3. We have also analyzed the circulation situa- tion for the relatively short break monsoon of 20-25 July, and similar results have been obtained (figure not shown). 4. Conclusion In summary, we can reach the conclusion that two different large-scale circulation patterns are responsible for the con- trasting regimes of monsoon rainfall in the two regions stud- ied here, central India and central Tibet. Acknowledgments. I appreciated the use of data from the Central Meteorological Bureau in China and the discussions with many FIG. 5. 500 mb height departures from normal at 1200 GMT, scientific staff members of that bureau. Thanks are due to Profs. T.- 2 July 1979. C. Yeh and S.-Y. Tao for their full support and helpful suggestions. Thanks also go to Mrs. W. W. Wang for conducting a synoptic study. at 1200 GMT, 5 August 1979. This map was selected as repre- sentative of the circulation patterns prevailing during the ac- tive monsoon period covering the 15 days from 28 July to 11 References August. It is interesting to note that the patterns of 500 mb height departures over central India and central Tibet in Fig. Hamilton, M. G., 1977: Some aspects of break and active monsoon 4 are almost completely opposite to those shown in Fig. 3, over southern Asia during summer. Tellus, 29, 335-344. Joseph, P. V., 1978: Subtropical westerlies in relation to large-scale i.e., an area of negative height departures now replaces the failure of Indian monsoon. Indian J. Meteorol. Hydrol. Geophys., area of the positive height departure over the midsection of 29, 412-418. India, whereas positive height departures now dominate over Yeh, T. C., Y. X. Gao, M. C. Tang, S. W. Lo, C. B. Shen, D. Y. Gao, central Tibet. As a result, a break monsoon prevailed over Z. S. Song, Y. F. Qian, F. M. Yuan, G. Q. Li, Y. H. Ding, Z. T. central Tibet with a rainfall ratio as low as 0.23 on 7 Chen, M.

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