Results
We first examine the ability of WACCM4 to reproduce the observed annual cycles of tropical stratospheric water vapor. Latitudinal variations in seasonality of lower stratospheric water vapour are very similar between WACCM4 simulations and observational datasets from SWOOSH, with pattern correlation coefficient being above 0.9. The magnitude of the stratospheric water vapour produced by WACCM4 is about 0.5ppmv smaller than that observed, which may due to . In the southern subtropics, there is only week seasonality in the stratospheric water vapour, with slightly higher values around November. In contrast, there is large seasonality in the northern tropics with a large increase in June to August, where the spreading of the contours indicates transport of high water vapour values from the midlatitudes into the tropics during boreal summer. In addition, the annual maximum in the sourthern tropics occurs 2 months later than in that in the northern tropics. This structure is coherent with the subtropical ozone structure shown in \citet{Stolarski_2014}, implying that processes determining the climatological seasonal cycle of the tropical stratospheric water vapor are likely same as those for ozone, which was shown as a combination of the seasonal variation of the Brewer-Dobson circultion and the seasonal variation of tropical and midlatitude mixing.