Role of Warming over the Tibetan Plateau in Early Onset of the Summer Monsoon over the Bay of Bengal and the South China Sea

Abstract

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 200-500hPa thickness data on about 15-day intervals. Of importance is the observational evidence that the warming phase over the 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 200-500hPa 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).