Surface and Upper-Tropospheric Thermal Processes and Asian Summer Monsoon Onset

The mean state and year-to-year variations of the tropospheric temperature fields and their relationship with the establishment of the East Asian summer monsoon (EASM) and the Indian monsoon (INM) are studied using the NCEP and ECMWF reanalysis data of 15 years (1982-1996). The results show that the seasonal shift of the Tibetan High in the upper troposphere and the establishment of the EASM and the INM are closely related to the seasonal warming which causes a reversal of the meridional gradient of upper tropospheric mean temperature over the monsoon regions. On the average of 15 years, the reversal time of the temperature gradient in the EASM region (INM region) is concurrent with (one pentad earlier than) the onset time of the summer monsoon. In most years of the 15-year period, the reversal of temperature gradient coincides or precedes the onset time of the summer monsoon in both the EASM region and the INM region. The results suggest an important role of thermal processes on the establishment of the Asian monsoon. The contributors to the upper tropospheric warming over the EASM region are the strong horizontal warm advection and the diabetic heating against the adiabatic cooling due to upward motion. In the INM region, strong adiabatic heating by subsidence and the diabetic heating are major warming processes against the strong horizontal cold advection related to the persistent northwestlies to the southwestern periphery of the Tibetan Plateau.

The spatial temperature distribution near surface does not show such a gradient change like that in the upper troposphere. This raises a fundamental question about whether or not the early or late establishment of the Asian summer monsoon is directly related to the differential warming near the surface. We are performing an analysis of intraseasonal, seasonal, and interannual variability of the surface fluxes. The results will be reported in the meeting.