Changes in stratospheric water vapor have important radiative and chemical impacts on climate. Here, we use Aura Microwave Limb Sounder (MLS) satellite observations and five reanalyses to examine covariations of water vapor, ozone, and carbon monoxide (CO) with the dynamical and thermodynamic environment of the tropopause layer (147–68 hPa) above the Asian summer monsoon (ASM). All reanalyses 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. Applying principal component analysis to both vertical and horizontal variations of water vapor, we identify three leading modes of deseasonalized variability. The leading mode consists of regional-scale moist or dry anomalies. Cold-point tropopause temperature anomalies are in phase (positive/wet) with lower stratosphere water vapor in the west but out of phase (negative/wet) in the east, where the largest anomalies are located. The moist phase of this mode is associated with systematically deeper convection through much of the ASM domain and anomalous ascent along the northern edge of the anticyclone. The second mode features dry or moist anomalies centered near the tropopause coupled with opposing anomalies above and below. This mode is linked to large anomalies in cold point temperature, ozone, and CO that extend across the entire southern part of the monsoon domain and into the northwest. Warm anomalies near the cold point cause negative anomalies in radiative heating that, when coupled with shifts in convective activity, explain the vertical structure of this mode. The third mode features a horizontal dipole oriented east-to-west, with a deep moist anomaly centered over the southeastern Tibetan Plateau coupled with dry anomalies in the west and vice versa. This mode is associated with coherent east–west shifts in both the ASM anticyclone and anomalous convective activity, and features similar east–west dipoles in ozone and CO. In vertical structure, the first mode peaks in the lower stratosphere (~68 hPa), the second mode around the tropopause (~100 hPa), and the third mode near the level of zero net radiative heating (~147 hPa).
 

stratospheric water vapor,reanalysis intercomparison