Understanding the Response of the Quasi‐Biennial Oscillation to Carbon Dioxide and Sea Surface Temperature Increases in Aqua‐Planet Simulations

The Quasi‐Biennial Oscillation (QBO) of the tropical stratospheric zonal winds influences tracer transport, the stratospheric polar vortex, and extra‐tropical surface variability. However, the response of the QBO to increasing greenhouse gas concentrations is still uncertain. In this study, an idealized aqua‐planet configuration of the upcoming Community Atmosphere Model version 7 (CAM7) is leveraged to understand the drivers of QBO changes in a future climate. The aqua‐planet model does not have a seasonal cycle and uses prescribed, zonally symmetric sea surface temperatures (SSTs), simplifying the attribution of QBO trends to dynamical and radiative drivers. In an idealized climate change scenario with quadrupled carbon dioxide concentrations and increased (by 4 K) SSTs, the QBO period decreases by about 10 months due to increased resolved and parameterized gravity wave drag, and the lower stratospheric QBO amplitude decreases in tandem with tropospheric expansion and strengthened tropical upwelling. Little change in the QBO amplitude and period is observed for a quadrupling of carbon dioxide alone. The QBO response in the aqua‐planet simulation mirrors that of its fully coupled counterpart, suggesting that aqua‐planets could be a useful framework for deciphering inter‐model differences in QBO projections.