Recent Changes in Hemispheric Asymmetry of Stratospheric Water Vapor

Stratospheric moistening after 2000, with a more pronounced trend in the Northern Hemisphere (NH), has been detected through satellite, in situ observations, and model simulations. This study aims to provide an updated analysis of the hemispheric asymmetry of stratospheric water vapor (SWV) for the period from 2000 to 2020 based on merged satellite data set and Chemical Lagrangian Model of the Stratosphere (CLaMS) simulations driven by ERA5. Over post-2000 decades, significant changes were identified in comparison to the pre-2000 period, notably a reversal in the SWV difference between the Northern and Southern Hemispheres, particularly in the midlatitudes. This shift is largely attributed to a slowdown in the deep branch of Brewer-Dobson circulation (BDC) in the Northern Hemisphere relative to the Southern Hemisphere after about year 2000. The cooling-induced dehydration is the primary driver of the significantly lower SWV values in the Antarctic stratosphere than the Arctic stratosphere. This asymmetry increased before 2000 at a rate of 0.1–0.2 ppmv per decade. After 2000, however, this difference has decreased, primarily due to ozone recovery in the Southern Hemisphere, despite the enhanced northward transport of water vapor driven by the changes in the BDC. The study emphasizes the growing need for long-term SWV monitoring in the Southern Hemisphere to better understand global water vapor dynamics.