Contributions of parameterized gravity waves and resolved equatorial waves to the QBO period in a future climate of CESM2

Contributions of the resolved waves and parameterized gravity waves to changes in the quasi-biennial oscillation (QBO) in a future simulation (2015-2100) under the SSP370 scenario are investigated using the Community Earth System Model 2 (CESM2) with enhanced vertical resolution and are compared with those from four CESM2 historical simulations (1979-2014). The maximum QBO amplitude of the future simulation is 26.0 m s-1, which is slightly less than that of the historical simulations (27.4-29.3 m s-1). However, the QBO period in the future simulation is much shorter: 21.6 months in the early-future (2015-2050) and 12 months in the late-future (2065-2100) period, than in the historical simulations (23.5-30.9 months). The shortened QBO period in the future is primarily due to increases in both resolved wave forcing and parameterized gravity wave drag (GWD) in the stratosphere, with a more significant contribution by the GWD. As convective activity becomes stronger in the future simulation, the momentum flux of parameterized convective gravity waves at the cloud top increases, resulting in stronger GWD in the stratosphere. The increases in the magnitude of westward GWD dominate those of eastward GWD in the stratosphere. This is due to a significant increase in westward momentum flux in the troposphere, especially during the descending easterly QBO, and enhanced westerlies in the lowermost stratosphere, which introduces a westward anomaly. For the resolved waves, Kelvin wave forcing is a key contributor to increased eastward forcing in the future simulation, with relatively minor contributions by other equatorial planetary waves.