Seasonality of the Quasi-biennial Oscillation signal in water vapor in the tropical stratosphere

Abstract. Stratospheric water vapor is an important greenhouse gas, which affects the radiation balance and temperature structure of the stratosphere and troposphere. Although previous studies have investigated the water vapor variability associated with the quasi-biennial oscillation (QBO), the seasonal difference in the water vapor QBO are still not well understood. Using the ERA5 reanalysis and SWOOSH observations, this study compares the stratospheric water vapor distribution in northern winter and summer under different QBO phases. The water vapor and zonal winds are positively correlated in the mid-to-lower stratosphere; however this relationship weakens in the northern summer. The mean vertical transport term via the QBO related residual circulation is the leading factor controlling the water vapor distribution in most of the stratosphere. This dynamic transport of water vapor in the lower stratosphere by the residual circulation is larger in boreal winter than in summer. Further, the dehydration effect by cold temperature in the lower stratosphere is also more effective in boreal winter than in summer. Tropical deep convection exhibits opposite variations for a given QBO phase in boreal winter versus summer especially over the Indo-Pacific Oceans. This further enhances the temperature difference between the QBO easterly and westerly phases in winter and reduces the temperature contrast in summer. It is still a challenge for models to reproduce the water vapor QBO: CMIP6 models tend to underestimate the water vapor QBO amplitude, and the seasonal contrast in the water vapor QBO between boreal winter and summer is underrepresented in most models.