Sources, Pathways and Timescales for the Troposphere to Stratosphere Transport over Asian Monsnoon Regions in Boreal Summer

Based on a Lagrangian dispersion model FLEXPART driven by the NCEP/NCAR global forecast system reanalysis data, the authors study the pathways and timescale of troposphere to stratosphere transport (TST) over the Asian monsoon regions during the summer of 2005 using the air parcels back trajectories, as well as the surface sources, defined in terms of the locations where each trajectory last left the atmospheric planetary boundary layer. The following conclusions are drawn in the paper: (1) The boundary layer sources are most located (approximately four fifths) vertically above two areas, one is the south of the Tibetan Plateau, the Indian Ocean, and the Bay of Bengal, and the other is the tropical western Pacific regions. These two areas correspond to those regions previously identified as sources for active deep convection. Even though the two regions make the same contribution to transportation from the PBL to the base of tropopause layer (about 16 km), the transport from the tropopause layer to the stratosphere with a much smaller fraction (about 10%) arriving at the 20-22 km height is dominated by the former (approximately 75%). These analysis results emphasize the importance of Asian monsoon regions in the TST, with the Tibetan Plateau, the Indian Ocean, and the Bay of Bengal being the dominant source of the stratospheric “overworld air.(2) The analysis of trajectories reveals that the air parcels arrive at and cross the tropopause mainly over the south of Tibetan Plateau and its adjacent areas (25°N-35°N, 90°E-110°E). These pathways represent a regional pattern and are largely co-controlled by the South Asia high circulation in the upper troposphere, the northern hemispheric subtropical jet and the equatorial easterly jet. Then about at 150 hPa, the most air parcels are transported westward, the rest entered the South Asian monsoon regions.(3) The results also reveal that there are two main processes in the TST. One is a fast transport process related to the convection uplift which can bring the boundary layer mass into the stratosphere in 1-2 days and make a contribution about 10%-30% of the overall mass transport. The other is a slow transport process relatively which may be related to the large-scale vertical transport due to atmospheric radiative heating in the upper troposphere. Overall, a part of transport from the boundary layer to the lower stratosphere is so rapid that this can represent an important route by which very short-lived substances, emitted at the surface, can influence the lower stratospheric ozone and other tracer budget.