We employ the Whole Atmosphere Community Climate Model, version 4 (WACCM4) (Marsh et al. 2013), which is the atmospheric component of the coupled climate system model Community Earth System Model (CESM) (Garcia et al. 2007). The standard version has 66 vertical levels extending from the ground to 4.5 × 10⁻⁶ hPa (160 km geometric altitude), with a vertical resolution of 1.1–1.4 km in the tropical tropopause layer and the lower stratosphere (< 30 km). All simulations use a horizontal resolution of 1.9× 2.5 (latitude × longitude) and do not include interactive chemistry (Garcia et al. 2007). Fixed greenhouse gas (GHG) values used in the model radiation scheme are based on emissions scenario A2 of the Intergovernmental Panel on Climate Change (IPCC) (WMO, 2003) over the period 1980–2015. And the prescribed ozone forcing, with a 12-month seasonal cycle averaged over the period 1980–2015 from CMIP5 ensemble mean ozone output, is used in our simulations. The Quasi Biennial Oscillation (QBO) forcing time series is determined using a 28-month fixed cycle.

A group of model integrations are used to isolate the impact of IPWP Niño and IPWP Niña on the SWV. Briefly, we examined three 30-year time-slice simulations forced by repeating annual cycles of SSTs that represent IPWP Niño and IPWP Niña. Composite SST anomalies for IPWP Niño and IPWP Niña (see Figs. 1c, and 1d in Zhou et al. 2017) are used to force the simulations. The key point is that these model integrations provide many samples of the atmospheric response to identical SSTa and are long enough to achieve statistical robustness (Garfinkel et al. 2013b).