Convective Gravity Waves and the Madden-Julian-Oscillation: A numerical study

Weather systems in the tropical Indian Ocean region are prominently influenced by the Madden-Julian-Oscillation (MJO) (Madden and Julian, 1972). Once an MJO active cycle is established, it can drive surface weather for several months, forcing heavy rainfall and droughts on the Indian Subcontinent. Although understanding of the MJO has improved over the last decade, MJO still considerably degrades forecasting skill, particularly in the Asian Monsoon region (Kim et al. 2014). This is especially true for seasonal prediction. The interaction of gravity waves (GW) from convection during MJO active phases is one of the various sources of uncertainties in MJO modeling. We developed a coupled model of convective gravity wave (CGW) forcing and propagation to evaluate the entire life-cycle of GWs from their convective excitation to their dissipation in the upper stratosphere / lower mesosphere region. CGW forcing at source level was calculated using the Song & Chun (2005) model. GW trajectories were calculated using GROGRAT (Marks & Eckermann 1995). Simulations were performed for all respective MJO phases for MJO cycles during a 30 years period using MERRA and CFSR data for the full spectrum of CGWs. Our results show a clear connection between 200 hPa winds and the direction of zonal momentum flux during MJO phases of increased convection (phases 1-4) in the Indian Ocean. This prominent momentum flux feature changes with the progress of each MJO cycle. Zonal means of momentum flux and gravity wave drag (GWD) also show this tautology. Furthermore, we estimated the impact of GWD forcing in the tropical upper troposphere to stratosphere region in relation to the CGW source spectra at launch altitude. We compare these results with MJO composites of momentum flux at cloud top level and various altitudes as a function of MJO phase. Finally, we validate our findings with observations from satellite measurements.