Impact of Convective Gravity Waves on the Tropical Middle Atmosphere During the Madden-Julian Oscillation

The Madden-Julian oscillation (MJO) prominently influences weather systems in the tropical Indian Ocean region. Although understanding of the MJO has improved over the last decade, insufficient simulation of the MJO still considerably degrades forecasting skill of atmospheric models. In this study we investigate the interaction of gravity waves (GWs) from convection with the background atmosphere during MJO cycles between 1979 and 2010. For this, we utilize a parameterization of convective GW (CGW) forcing and propagation to evaluate the entire life cycle of GWs from their convective excitation to their dissipation in the tropical upper stratosphere region. CGW forcing at source level was calculated using the Song and Chun (2005, https://doi.org/10.1175/JAS-3363.1) parameterization. We performed simulations for all respective MJO phases and MJO cycles during a period of 32 years using Climate Forecast System Reanalysis data for the full spectrum of CGWs. Our results show a clear connection between 850 hPa winds and zonal momentum flux during all MJO phases. This prominent momentum flux feature changes with the progress of each MJO cycle. Zonal means of eastward directed momentum flux also show this correlation. Furthermore, we estimated the spectral characteristics of launched GWs and found significant GW momentum flux for horizontal wavelengths as short as 80 km at launch altitude. Eastward GW drag (GWD) consistently shows a maximum at altitudes below 40 km and prominent westward GWD at altitudes higher than 40 km. Additionally, we investigated the impact of the phase of the quasi-biennial oscillation (QBO) on our findings with GWD found stronger during the QBO easterly phase compared to the QBO westerly phase.