Understanding Boreal Summer UTLS Water Vapor Variations in Monsoon Regions: A Lagrangian Perspective

Water vapor in the Upper Troposphere and Lower Stratosphere (UTLS) plays a crucial role in climate feedback by influencing radiation, chemistry, and atmospheric dynamics. The amount of water vapor entering the stratosphere is sensitive to cold point temperatures (CPT), making Northern Hemisphere summer monsoons more favorable for transporting water vapor into the stratosphere. This study uses a Lagrangian method to reconstruct water vapor over the Asian (ASM) and North American (NAM) monsoons, investigating their contributions to stratospheric water vapor. The Lagrangian method tracks air parcels and identifies the coldest temperature along each trajectory, contrasting with local methods that rely on vertical temperature profiles. The reconstructed water vapor fields are validated against satellite observations from SAGE III/ISS and NASA’s Aura MLS. SAGE III/ISS shows stronger moisture enhancements than MLS, but both datasets reveal similar water vapor anomalies within the ASM and NAM anticyclones. Although the Lagrangian method is dry-biased compared to observations, it effectively reconstructs UTLS water vapor (correlation coefficient ~0.75), capturing moist anomalies in the ASM but performing less well in the NAM. Our analysis shows that, rather than local conditions, large-scale cold point tropopause temperatures in the vicinity of the monsoons primarily drive the moisture anomalies, with NAM water vapor significantly influenced by long-range transport from South Asia. Some convection-related processes, such as east-west shifts within the ASM, are not fully captured due to unresolved temperature variability in ERA5 and missing ice microphysics. Despite biases and computational challenges, the Lagrangian method provides valuable insights into UTLS water vapor transport.