The evolution of tropospheric temperature field and its relationship with the onset of Asian summer monsoon

The mean state and year-to-year variations of the tropospheric temperature fields are analyzed in light of their relationship with the establishment of the East Asian summer monsoon (EASM), and the Indian monsoon (INM). Primary data for the analysis include the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis from 1982 to 1996 and the European Center for Medium-Range Weather Forecast (ECMWF) Reanalysis (ERA) from 1979 to 1993. The analysis reveals that, in most of the analyzed years, the meridional temperature gradient in the upper troposphere reverses at, or before the onset time of the summer monsoon in both the EASM region and the INM region. On the average, the reversal time of the meridional temperature gradient in the EASM region (INM region) is concurrent with (one pentad earlier than) the onset time of the summer monsoon. A budget analysis shows that the strong horizontal warm advection and the diabatic heating are the main contributors to the upper tropospheric warming against the strong adiabatic cooling during the pre-onset period over the EASM region. Over the INM region, however, strong adiabatic heating due to subsidence is the major warming process against diabatic cooling, and the strong horizontal cold advection related to the persistent northwestlies to the southwestern periphery of the Tibetan Plateau. The results show that the seasonal shift of the South Asian High in the upper troposphere, and the establishment of the EASM and the INM are closely related to the seasonal warming, which results in the reversal of the meridional temperature gradient in the upper troposphere over the two monsoon regions.