Seasonality and variability of stratospheric water vapour

Stratospheric water vapour (SWV) plays a critical role in the climate system by modulating the radiation budget and influencing the stratospheric chemistry. Studying changes of SWV on global scale is helpful for our understanding of climate change. This thesis aims to gain an improved understanding of the stratospheric processes and dynamic mechanisms that determine the seasonality and variability of SWV. Water vapour is characterized by its compound, which leaves an isotopic fingerprint in relevant atmospheric and hydrologic processes. The thesis starts with analyzing the global features of three stable water isotopes (SWIs) in the stratosphere by using satellite retrievals from Odin/SMR. The spatial pattern of SWI indicates clear effects of methane oxidation in the upper stratosphere, dehydration at the tropopause and stratospheric transport via the Brewer-Dobson circulation (BDC). In addition to the tropical tape recorder in the lower stratosphere, a pronounced downward propagation of the seasonal signal from the upper to the lower stratosphere is observed in high-latitudes. These observed features are further compared to model outputs to identify possible causes of model deficiencies in reproducing the distribution of SWV. The downward propagation signal of zonal wind has been demonstrated in the high-latitude stratosphere in spring seasonal transition in the Southern Hemisphere, but not in the Northern Hemisphere. This inter-hemispheric difference is due to the stronger stratospheric planetary wave activity in austral spring than in boreal spring. With strong wave activity in spring, the transition is inclined to occur first at the stratopause followed by a downward propagation to the lower stratosphere. In particular, the stronger the upward propagation of planetary waves in high-latitudes in spring the earlier the stratospheric seasonal transition. The new generation reanalysis ERA5 represents climatological distribution and seasonal cycle of SWV better than its predecessor ERA-Interim by assimilating more satellite observations. The variability of SWV in ERA5 is highly consistent with SDI MIM observation. The interannual variability of water vapour in the lower stratosphere is found to be closely linked to the tropical Quasi-Biennial Oscillation (QBO) and QBO-induced residual circulation. On decadal scale, the deficit of SWV in boreal winter is associated with a warm sea surface temperature (SST) anomaly in the North Atlantic, which leads to stronger upward propagation of planetary waves, resulting in a warmer pole in the lower stratosphere, colder tropical tropopause and stronger BDC, hence less water vapour enters the stratosphere through the tropopause and the anomaly extends to the entire stratosphere. Sensitivity experiments for a CO2 doubling scenario are performed with the model WACCM to investigate the SWV response to climate change. The response of SWV is dominated by the warm SST, which is induced by CO2 doubling in a coupled ocean-atmosphere system. The enhanced SST leads to a moist troposphere and warmer tropical and subtropical tropopause, resulting in more water vapour entering the stratosphere from below. A large increase of SWV in the lower stratosphere, in turn, affects stratospheric temperature. It results in a warming in the tropical and subtropical lower stratosphere, offsetting the cooling caused by CO2 doubling in general.