A climatology of the tropopause transition layer over the Asian summer monsoon derived from COSMIC radio occultation

The Asian summer monsoon (ASM) system is a significant pathway for lifting lower tropospheric water vapor and pollution into the global stratosphere, yet the structure of the overlying tropopause transition layer over the ASM region, hereafter referred to as the ASM-region tropopause transition layer (ATTL), remains insufficiently quantified. Here, 20 years (2006–2025) of high-vertical-resolution GNSS radio occultation observations from COSMIC-1/2 are used to derive a climatology of the ATTL, defined thermodynamically as the layer between the level of minimum stability (LMS) and the cold point tropopause (CPT). Unlike earlier tropopause climatologies that primarily mapped the LRT (lapse rate tropopause) or the CPT alone, this framework identifies where a physically meaningful transition layer can be defined. The results show that the ASM region has the highest seasonal-mean tropopause altitude, exceeding 16.5 km, and that the LMS reaches nearly 13 km, consistent with recent balloon-borne estimates (∼12.5 km). The physically meaningful ATTL over the convectively active monsoon domain south of 30°N is generally 3–5 km thick. These findings provide a physically constrained, observation-based characterization of the ATTL and clarify its significance for stratosphere–troposphere exchange in the ASM region.