Figure 2 shows the seasonal evolution of stratospheric water vapor anomalies at 80 hPa during IPWP Niño/Niña. The seasonal variations are evident, with a winter transition from the dry phase to wet phase during IPWP Niño, and vice versa for IPWP Niña. This sign change is in agreement with the results in Garfinkel et al. (2013), which also shown an phase transition induced by ENSO. The reason for this sign change will be discussed in following dynamical analysis. In addition, differences between the NT and ST seasonal evolution is clearly shown. The drying effect of IPWP Niño have larger absolute values in fall, and the moistening effect of IPWP Niña have the largest hemispheric differences in winter. For IPWP Niño, the peak values of drying effect is between 0.2 ppmv to 0.3 ppmv in NT winter, being ~0.15 ppmv larger than that in ST. For IPWP Niña, the peak values in NT fall is ~0.1 ppmv larger than that in ST.