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. The quasi-biennial oscillation (QBO) is a phenomenon in which easterly and westerly winds alternately descend in the equatorial stratosphere roughly every 28 months. The QBO affects various atmospheric phenomena not only at the equator but in mid- and high-latitudes, such as the Madden-Julian Oscillation, the subtropical jet, and the polar vortex and associated teleconnections. There is a debate over how the QBO cycle will be altered under future climate change. A future climate modeling experiment with a version of the Community Earth System Model 2 that represents the QBO is performed, and it is found that the QBO period decreases to 12 months in the future simulation. There are two main reasons for this change: First, strengthened convection in the equatorial region leads to enhanced convective gravity wave momentum flux, resulting in stronger gravity wave drag in the stratosphere. Second, an increase in eastward forcing by equatorial Kelvin waves strengthens the overall eastward planetary wave forcing. These increases in both gravity wave drag and Kelvin wave forcing contribute to a speeding up of the QBO cycle in the future climate. Momentum budget analysis in a warming climate simulation is performed to diagnose the cause of changes in the quasi-biennial oscillation (QBO)'s period Increasing westward parameterized gravity wave drag speeds up the easterly phase of the QBO in a warming climate Increasing Kelvin wave forcing and eastward parameterized gravity wave drag speed up the westerly phase of the QBO