Interaction Between Stratospheric Kelvin Waves and Gravity Waves in the Easterly QBO Phase

A general circulation model is used to study the interaction between parameterized gravity waves (GWs) and large‐scale Kelvin waves in the tropical stratosphere. The simulation shows that Kelvin waves with substantial amplitudes (∼10 m s−1) can significantly affect the distribution of GW drag by modulating the local shear. Furthermore, this effect is localized to regions above strong convective organizations that generate large‐amplitude GWs, so that at a given altitude it occurs selectively in a certain phase of Kelvin waves. Accordingly, this effect also contributes to the zonal‐mean GW drag, which is large in the middle stratosphere during the phase transition of the quasi‐biennial oscillation (QBO). Furthermore, we detect an enhancement of Kelvin‐wave momentum flux due to GW drag modulated by Kelvin waves. The result implies an importance of GW dynamics coupled to Kelvin waves in the QBO progression. Plain Language Summary The variability of the tropical atmosphere at altitudes of about 18–40 km is dominated by a large‐amplitude long‐term oscillation of wind, the quasi‐biennial oscillation, which has a broad impact on the climate and seasonal forecasting. This oscillation is known to be driven by various types of atmospheric waves with multiple spatial scales. Using a numerical model, this study reports a process of interaction between those waves on different scales, which has not been illuminated before. The result implies a potential importance of this process in the progression of the quasi‐biennial oscillation. Proper model representations of these multiscale waves and tropical convection are required to simulate this process. Key Points Kelvin waves affect the longitudinal and vertical distribution of parameterized gravity‐wave drag in the stratosphere significantly This effect can make contribution to the zonal mean of gravity‐wave drag, thereby affecting the quasi‐biennial oscillation progression Gravity‐wave drag modulated by Kelvin waves also alters the Kelvin‐wave momentum flux in the middle stratosphere