Quantifying the sources of increasing stratospheric water vapour concentrations in the 21^st century

Abstract. According to satellite measurements from multiple instruments, water vapour (H₂O) concentrations, in most regions of the stratosphere, have been increasing at a statistically significant rate of ∼1–5 % dec⁻¹ since the early 2000s. Previous studies have estimated stratospheric H₂O trends, but none have simultaneously quantified the contributions from the main sources: temperature variations in the tropical tropopause region, changes in the Brewer-Dobson circulation, and changes in methane (CH₄) concentrations and its oxidation. Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) measurements are used to estimate altitude/latitude-dependent stratospheric H₂O trends from 2004–2021 due to these sources. Results indicate that rising temperatures in the tropical tropopause region play a significant role in the increases, accounting for ∼1–4 % dec⁻¹ in the tropical lower-mid stratosphere, as well as in the mid-latitudes below ∼20 km. By regressing to ACE-FTS N₂O concentrations, it is found that in the lower-middle stratosphere, general circulation changes have led to both significant H₂O increases and decreases on the order of 1–2 % dec⁻¹ depending on altitude/latitude region. Making use of measured and modelled CH₄ concentrations, the increase in H₂O due to CH₄ oxidation is calculated to be ∼1–2 % dec⁻¹ above ∼30 km in the Northern Hemisphere and throughout the stratosphere in the Southern Hemisphere. After accounting for these sources, there are still regions of the midlatitude lower-mid stratosphere that exhibit significant residual H₂O trends increasing at 1–2 % dec⁻¹. Results are discussed that indicate these unaccounted for increases could potentially be explained by increases in upper tropospheric molecular hydrogen.