Role of Warming over the Tibetan Plateau in Early Onset of the Summer Monsoon over the Bay of Bengal

In this study we examine the mechanisms of the onset of the Southeast Asian monsoon (SEAM) over the Bay of Bengal and the South China Sea in terms of thermal contrast between the Tibetan Plateau and surrounding ocean based upon 5-day mean ECMWF circulation field data (1980-89) and 5-day mean GMS equivalent black body temperature (T BB ) data. The early onset of the SEAM is recognizable at Pentad 28 (May 16-20) with accelerated low-level monsoon westerlies followed by second enhancement of the monsoon activities in early June. The warming over the Tibetan Plateau from spring to summer is found in the 200 500 hPa thickness data oil about 15-day intervals. Of importance is the observational evidence that the warming phase over tle Tibetan Plateau around mid-May is concurrent with the early onset of the SEAM. Thus, the thermal contrast between the Tibetan Plateau and the adjacent ocean is likely to induce the acceleration and eastward extension of the low-level monsoon flow, causing the abrupt commencement of the SEAM including onset, of the South China Sea monsoon (SCSM). This relationship between low-level wind over the key region (10°-20°N, 80°-120°E) and 200-500 hPa thickness over the Tibetan Plateau is also confirmed based on the correlation analysis in the interannual variabilities. An influence for the mid-latitude atmosphere, stationary Rossby waves are generated over the South China Sea and propagate in a northeastward direction toward Japan because of the cyclonic vorticity and the tropical heat source associated with the onset of the SCSM. As a result of this wave propagation, a high pressure anomaly appears over Japan, which is consistent with a singularity of clear skies around Japan in mid-May (Kawamura and Tian, 1992).

[1]  T. N. Krishnamurti,et al.  On the Onset Vortex of the Summer Monsoon , 1981 .

[2]  T. Nakazawa,et al.  Transition from the Southern to Northern Hemisphere Summer Monsoon , 1985 .

[3]  T. Nakazawa.  Seasonal Phase Lock of Intraseasonal Variation during the Asian Summer Monsoon , 1992 .

[4]  Jun Matsumoto,et al.  Summer Monsoon over the Asian Continent and Western North Pacific , 1994 .

[5]  M. Yen,et al.  The Water Vapor Transport Associated with the 30 50 Day Oscillation over the Asian Monsoon Regions during 1979 Summer , 1988 .

[6]  T. Nitta.  Observational Study of Heat Sources Over the Eastern Tibetan Plateau during the Summer Monsoon , 1983 .

[7]  T. Yasunari,et al.  Maturing process of the summer monsoon over the western North Pacific : A coupled ocean/atmosphere system , 1996 .

[8]  K. Lau,et al.  Seasonal Variation, Abrupt Transition, and Intraseasonal Variability Associated with the Asian Summer Monsoon in the GLA GCM , 1996 .

[9]  T. Yasunari,et al.  Abrupt Seasonal Change of Large-Scale Convective Activity over the Western Pacific in the Northern Summer , 1995 .

[10]  T. Murakami,et al.  Relationship among Seasonal Cycles, Low-Frequency Oscillations, and Transient Disturbances as Revealed from Outgoing Longwave Radiation Data , 1986 .

[11]  M. Yanai,et al.  The Large-Scale Circulation and Heat Sources over the Tibetan Plateau and Surrounding Areas during the Early Summer of 1979. Part II: Heat and Moisture Budgets , 1984 .

[12]  Tsing-Chang Chen,et al.  Some Effects of the Intraseasonal Oscillation on the Equatorial Waves over the Western Tropical Pacific–South China Sea Region during the Northern Summer , 1996 .

[13]  N. Hirasawa,et al.  Abrupt change in the characteristics of the cloud zone in subtropical east Asia around the middle of May , 1995 .

[14]  C. Fu,et al.  The relationship between Tibet-tropical ocean thermal contrast and interannual variability of Indian Monsoon rainfall , 1985 .

[15]  Bin Wang,et al.  Northern Hemisphere Summer Monsoon Singularities and Climatological Intraseasonal Oscillation , 1997 .

[16]  Tsing-Chang Chen,et al.  Diurnal Variation of Outgoing Longwave Radiation in the Vicinity of the South China Sea: Effect of Intraseasonal Oscillation , 1995 .

[17]  Tsing-Chang Chen,et al.  An Observational Study of the South China Sea Monsoon during the 1979 Summer: Onset and Life Cycle , 1995 .

[18]  Minoru Tanaka.  Intraseasonal Oscillation and the Onset and Retreat Dates of the Summer Monsoon over East, Southeast , 1992 .

[19]  J. Matsumoto.  The Seasonal Changes in Asian and Australian Monsoon Regions , 1992 .

[20]  T. Nitta.  Long-Term Variations of Cloud Amount in the Western Pacific Region , 1986 .

[21]  K. Lau,et al.  Seasonal and intraseasonal climatology of summer monsoon rainfall over East Asia , 1988 .

[22]  J. Matsumoto.  Seasonal transition of summer rainy season over indochina and adjacent monsoon region , 1997 .

[23]  J. Chan,et al.  An observational study on the onset of the summer monsoon over South China around Hong Kong , 1997 .

[24]  You-xi Gao,et al.  Some Aspects of Recent Research on the Qinghai-Xizang Plateau Meteorology , 1981 .

[25]  M. Yanai,et al.  The onset and interannual variability of the Asian summer monsoon in relation to land-sea thermal contrast , 1996 .

[26]  T. Nitta.  Convective Activities in the Tropical Western Pacific and Their Impact on the Northern Hemisphere Su , 1987 .

[27]  M. Murakami.  Analysis of the Deep Convective Activity Over the Western Pacific and Southeast Asia , 1983 .

[28]  Chih-Pei Chang,et al.  Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea , 1995 .

[29]  Chengfeng Li,et al.  Seasonal Heating of the Tibetan Plateau and Its Effects on the Evolution of the Asian Summer Monsoon , 1992 .

[30]  Haiyan He,et al.  Onset of the Asian Summer Monsoon in 1979 and the Effect of the Tibetan Plateau , 1987 .