Covariability of dynamics and composition in the Asian monsoon tropopause layer from satellite observations and reanalysis products

Abstract

The upper-level anticyclone above the Asian summer monsoon (ASM) greatly influences variations in stratospheric water vapor, which in turn have significant effects on climate. An impending data gap underscores the need to evaluate the reliability of recent reanalysis products in this region. Here, we describe three leading modes of deseasonalized water vapor variability in the tropopause layer (147-68 hPa) above the ASM. The first mode describes regional-scale moist or dry anomalies that peak in the lower stratosphere on interannual scales. Separating this mode into linear trend and detrended components, we find that the spatial pattern and sign of the trend disagree between observations and reanalyses. These discrepancies arise from different responses in the upper troposphere despite broad agreement in the lower stratosphere. Regional water vapor budgets suggest that the stratospheric trend originates outside the monsoon region, beyond our analysis domain. Interannual variability is more consistent, and arises mainly from the pre-monsoon influence of the Quasi-Biennial Oscillation. The second mode features anomalies arcing around the northern flank of the anticyclone with weaker opposing anomalies in the southeast, while the third mode features a horizontal dipole oriented east to west. These two modes often vary in quadrature as quasi-biweekly waves propagate across the region but also vary independently when other modes of convective variability manifest in similar areas. Despite lingering questions on the linear trend, mean biases, and data assimilation effects, the consistency between observation- and reanalysis-derived variability demonstrates that reanalyses are increasingly able to capture the processes controlling water vapor near the tropopause.