Mechanistic evaluation of reanalysis composition and circulation in the Asian monsoon tropopause layer

Changes in stratospheric water vapor and other constituents have important radiative and chemical impacts on climate. Here, we use Aura Microwave Limb Sounder (MLS) satellite observations and five meteorological and composition-focused reanalyses to examine covariations of water vapor, ozone, and carbon monoxide (CO) within the dynamical and thermodynamic environment of the tropopause layer (147–68 hPa) above the Asian summer monsoon (ASM). All reanalyses capture the thermal environment near the tropopause well and largely capture the climatological distributions and seasonal cycles of water vapor, along with the seasonal ‘ozone valley’ and convective enhancement of CO above the monsoon. The primary balance is between advective hydration and cold trap dehydration near the cold point; however, data assimilation effects are of the same order as the leading balance and therefore cannot be neglected. Applying principal component analysis to both vertical and horizontal variations of water vapor, we identify three leading modes of deseasonalized variability. The first mode, which consists of regional-scale moist or dry anomalies on the interannual scale, is decomposed into a linear trend over 2005–2021 and detrended interannual variability. The spatial pattern and sign of the linear trend in tropopause-layer water vapor over this period differ between Aura MLS and the reanalyses despite a consistent increasing trend. Signatures of interannual variability are otherwise largely consistent, except for ozone in the Japanese Reanalysis for Three-Quarters of a Century (JRA-3Q), which assimilates only total column ozone. Detrended interannual variability in water vapor can be attributed mainly to the pre-monsoon influence of the quasi-biennial oscillation. The second mode features dry or moist anomalies centered in the northeastern and southwestern quadrants of the anticyclone coupled with weaker opposing anomalies in the southeast, while the third mode features a horizontal dipole oriented east-to-west. The second and third modes vary on subseasonal scales and often occur in quadrature, representing the propagation of quasi-biweekly waves across the monsoon domain. The overall consistency between Aura MLS and reanalysis-derived modes of variability in UTLS water vapor in this region is a promising sign that atmospheric reanalyses are increasingly able to capture the processes controlling water vapor near the tropopause.