Upper tropospheric moistening during the Asian summer monsoon in a changing climate

This study investigates the projected changes in the upper troposphere and lower stratosphere (UTLS) water vapor over the Asian summer monsoon (ASM) region based on satellite records, numerical simulations using variable-resolution global climate model focused over south Asia (HIST-natural and anthropogenic forcing in the historical period, and FUT-following RCP4.5 in future), and Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. The simulations generally reproduced the seasonal cycle in the UTLS water vapor and regional water vapor maximum. With progressive warming in future, excessive upper tropospheric moistening is noted over the ASM region in far-future (2070-2095) climate against the HIST climate (1980-2005) with water vapor mixing ratio increasing to similar to 7.5 ppmv relative to similar to 5 ppmv noted in the HIST. It is further noted that projected changes in water vapor are linked to anomalous warming (similar to 1-4 K) in the upper tropospheric layers juxtaposed with zonally elongated ASM anticyclone and enhanced water vapor flux divergence by amplifications in rotational winds. Further, the simulations indicate robust increase in ASM upper tropospheric water vapor as compared to those at mid- and lower- troposphere in accordance with the Clausius-Clapeyron temperature dependence of moisture response to warming and amplified troposphere warming with altitude. A simple comparison between the ASM and the entire globe indicates that upper tropospheric water vapor-temperature relationship has a similar response, however, the projected variability in temperature and moisture is significantly larger (about twice) over the ASM region highlighting strong regional influence. Nonetheless, the projections indicate that ASM is a potential regional source in modulating UTLS water vapor budget in a warming climate.