Stratospheric 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 time-scales. First, we evaluate two SWV reanalysis products and show that SWV is better represented in a new-generation reanalysis product, ERA5, than in its predecessor, ERA-Interim. In particular, it is shown that SWV in ERA5 is highly consistent with observational data obtained from the SPARC Data Initiative Multi-Instrument 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 lower-stratospheric water vapour is closely linked to the tropical Quasi-Biennial Oscillation (QBO). When westerlies occupy the middle stratosphere and easterlies the lower stratosphere, a decrease is observed in lower-stratospheric water vapour due to a colder tropical tropopause and a QBO-induced enhanced residual circulation. On decadal time-scales, 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 sea-surface 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.