Introduction

Stratospheric water vapour (SWV) is an important greenhouse gas, contributing significantly to global climate change by a strong SWV feedback at +0.3 W/(m2·K) \cite{Dessler2013}. Sensitivity studies with climate models demonstrated that even small changes in lower stratospheric water vapor can lead to notable changes of radiative forcing and the temperature at the surface (Solomon et al., 2010). Understanding its controlling factors and underlying physics is thus critically important for the predictions of global climate variability and change. 
The amount of water vapor in the stratosphere is governed by two major processes. One is the oxidation of methane, which is the only important chemical source of water vapor in the stratosphere (ADD). The other is the transport of water vapor to the tropical lower stratosphere through the tropical tropopause layer where the lowest temperature (the so-called cold point temperature) determines the dehydration process (Randel et al. 1998, 2004; Oman et al. 2008; Fueglistaler et al. 2005, 2009; Rosenlof and Reid 2008; Schoeberl and Dessler 2011).