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

Stratospheric water vapor is a powerful greenhouse gas, and it can directly affect the radiative balance and temperature structure of the stratosphere. Although previous studies have investigated the water vapor variability associated with the quasi-biennial oscillation (QBO), the seasonal differences 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 QBO (represented by the 30 hPa QBO index) exerts the greatest influence on 100 hPa water vapor at a lag of six months. During northern summer, the peak amplitude of 100 hPa water vapor under different QBO phases in tropical regions reaches ±0.12 ppm at a six-month lag, while in winter it reaches ±0.2 ppm. The dehydration effect by cold temperature in the lower stratosphere is also more effective in boreal winter than in summer. The intensity of the QBO-related secondary circulation is stronger in the boreal winter than in summer, which not only influences the cold point tropopause temperature in tropical regions but also drives the transport of stratospheric water vapor. The mean vertical transport term via the QBO-related residual circulation is the leading factor controlling the water vapor distribution in the tropical lower stratosphere. Although the CMIP6 models simulated the lagged effect of the 30 hPa QBO on lower stratospheric water vapor, they tend to underestimate the water vapor amplitude, and the seasonal contrast is underrepresented in most models.