Impacts of the Indo-Pacific Warm Pool on Lower Stratospheric Water Vapor: Seasonality and Hemispheric Contrasts

Using time-slice experiments performed with version 4 of the Whole Atmosphere Community Climate Model (WACCM4) and satellite observations, we investigate hemispheric differences in, and the seasonal dependence of, water vapor transport into the stratosphere in response to Indo-Pacific warm pool (IPWP) sea-surface temperature (SST) variations. Specifically, the amplitude of the lower stratospheric water vapor (SWV) response to a warmer (cooler) IPWP (i.e., IPWP Nino [Nina]) occurs mainly in boreal winter when the IPWP SST forcing is identical for different seasons, and the response in the northern tropics (NT) is greater than that in the southern tropics (ST). The seasonality in and hemispheric contrasts of the SWV response to IPWP Nino and IPWP Nina are traced to the tropical cold point temperature, including its zonally symmetric pattern associated with tropical upwelling and its zonally asymmetric features that manifest as the morphology of the coldest point region. Together with the seasonal variations in topical upwelling response, the morphology of the coldest point region, which is affected by equatorial planetary waves and the summer monsoon, determines the seasonality and NT-ST contrasts in the SWV response to IPWP SST forcing. Among these factors, the importance of the climatological displacement of convective activity from the equator to NT associated with the summer monsoon is highlighted.