Stratosphere‐Troposphere Exchange of Water Vapor Based on Observations and Reanalyses

Abstract This study provides the first estimates of the global stratosphere‐troposphere exchange (STE) of water vapor (H 2 O) using the lowermost stratosphere mass budget approach. Observationally derived H 2 O fluxes across the isentropic surface fitted to the tropical tropopause are −1.16 ± 0.20 Pg/yr in the Northern Hemisphere extratropics, −0.94 ± 0.18 Pg/yr in the Southern Hemisphere extratropics, and 2.20 ± 0.36 Pg/yr in the tropics, resulting in a small net global flux of 0.10 ± 0.04 Pg/yr into the stratosphere. In contrast, MERRA2 and ERA5 yield global fluxes of −1.84 and −0.27 Pg/yr, respectively, suggesting a net H 2 O source in the stratospheric overworld from reanalyses. Large discrepancies in the seasonal cycle of H 2 O net fluxes at the tropopause between reanalyses and observations are also identified. These differences in the H 2 O STEs are mainly caused by biases in reanalysis H 2 O concentrations at the isentropic surface and in the lowermost stratosphere.