Lower Stratospheric Water Vapor Variations Diagnosed from Satellite Observations, Reanalysis Data, and a Chemistry–Climate Model

Stratospheric water vapor variations, which may play an important role in surface climate, have drawn extensive studies. Here, the variation in stratospheric water vapor is investigated by using data from observations of the Microwave Limb Sounder (MLS) on the Aura satellite, from the ECMWF Interim Reanalysis (ERAI), and simulations by the Whole Atmosphere Community Climate Model (WACCM). We find that the differences of annual mean stratospheric water vapor among these datasets may be partly caused by the differences in vertical transports. Using budget analysis, we find that the upward transport of water vapor at 100 hPa is mainly located over the Pacific warm pool region and South America in the equatorial tropics in boreal winter and over the southeast of the South Asian high and south of North America in boreal summer. It is found that temperature averaged over regions with upward transport is a better indicator of interannual variability of tropical mean stratospheric water vapor than the tropical mean temperature. It seems that the distributions of the seasonal cycle amplitude of lower stratospheric water vapor in the tropics can also be impacted by the vertical transport. The radiative effects of the interannual changes in water vapor in the lowermost stratosphere are underestimated by approximately 29% in both ERAI and WACCM compared to MLS, although the interannual variations of water vapor in the lowermost stratosphere are dramatically overestimated in ERAI and WACCM. The results here indicate that the radiative effect of long-term changes in water vapor in the lowermost stratosphere may be underestimated in both ERAI and WACCM simulations.