Sub-seasonal Variability in the Boundary Layer Sources for Transport into the Tropopause Layer in the Asian Monsoon Region

Abstract. The Asian summer monsoon (ASM) is associated with an upper-level anticyclone and acts as a well-recognized conduit for troposphere-to-stratosphere transport. The Lagrangian dispersion and transport model FLEXPART forced by ERA-Interim data from 2001–2013 was used to perform climatological modeling of the summer season (May–July). This study examines the properties of the air mass transport from the atmospheric boundary layer (BL) to the tropopause layer (TL), with particular focus on the sub-seasonal variability in the tracer-independent BL sources and the potential controlling mechanisms. The results show that, climatologically, the three most impactful BL source regions are northern India, the Tibetan Plateau, and the southern slope of the Himalayas. These regions are consistent with the locations of sources identified in previous studies. However, upon closer inspection, the different source regions to the BL-to-TL air mass transport are not constant in location or shape and are strongly affected by sub-seasonal variability. The contributions from the Tibetan Plateau are most significant in early May but decrease slightly in mid-May to mid-June. In contrast, the contributions from India and the southern slope of the Himalayas increase dramatically, with peak values occurring in mid-July. Empirical Orthogonal Function (EOF) analysis provides further evidence that the BL sources in the ASM region vary across a wide range of spatiotemporal scales. The sub-seasonal behavior of these BL sources is closely related to the strength of persistent deep convection activity over the northern Bay of Bengal and its neighboring areas.