Processes influencing lower stratospheric water vapour in monsoon anticyclones: insights from Lagrangian modeling

Abstract. We investigate the influence of different chemical and physical processes on the water vapour distribution in the lower stratosphere (LS), in particular in the Asian and North-American monsoon anticyclones (AMA and NAMA, respectively). Specifically, we analyze effects of large-scale temperatures, methane oxidation, ice microphysics, and small-scale atmospheric mixing processes in model experiments with the chemistry transport model CLaMS. All these processes hydrate the LS, in particular over the Asian Monsoon. While ice microphysics has the largest global moistening impact, it is small-scale mixing which dominates the specific signature in the AMA. In particular, the small-scale mixing parameterization strongly contributes to the seasonal and intra-seasonal variability of water vapour in that region and including it in the model simulations results in a significantly improved agreement with observations. Although none of our experiments reproduces the spatial pattern of the NAMA seen in MLS observations, they all exhibit a realistic annual cycle and intra-seasonal variability, which are mainly controlled by temperatures. We further analyse the sensitivity of these results to the domain-filling trajectory set-up used in the five model experiments, here-called Lagrangian Trajectory Filling (LTF). Compared with MLS observations and with a multiyear reference simulation using the standard version of CLaMS, we find that LTF schemes result in a drier global LS and drier water vapour signal over the monsoon regions. Besides, the intra-seasonal variability of water vapour in the AMA is less correlated with MLS during June--August. We relate these results to the fact that the LTF schemes produce a low density of air parcels in the moistest areas of the AMA.