AbstractStratospheric water vapour (SWV), in spite of its low concentration in the stratosphere as compared to the troposphere, contributes significantly to the surface energy budget and can have an influence on the surface climate. This study investigates the dynamical processes that determine SWV on interannual to decadal timeâscales. First, we evaluate two SWV reanalysis products and show that SWV is better represented in a newâgeneration reanalysis product, ERA5, than in its predecessor, ERAâInterim. In particular, it is shown that SWV in ERA5 is highly consistent with observational data obtained from the SPARC Data Initiative MultiâInstrument Mean (SDI MIM). Second, we investigate the variability of tropical SWV and its relationship to dynamical stratospheric variables. The analyses show that the interannual variability in the tropical lowerâstratospheric water vapour is closely linked to the tropical QuasiâBiennial Oscillation (QBO). When westerlies occupy the middle stratosphere and easterlies the lower stratosphere, a decrease is observed in lowerâstratospheric water vapour due to a colder tropical tropopause and a QBOâinduced enhanced residual circulation. On decadal timeâscales, the composite analysis of the boreal winter in two typical periods shows that less SWV is related to a warm anomaly in the North Atlantic seaâsurface temperature, which leads to stronger upward propagation of planetary wave activity at high latitudes, a weaker polar vortex and an enhanced residual circulation. The opposite occurs during periods with higher concentrations of SWV.