Role of Circumnavigating signal and Mesoscale Waves in the Initiation and Propagation of the October DYNAMO MJO event in a cloud-permitting model

We investigates the roles of two factors in the initiation and propagation of October MJO events observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the year 2011 (CINDY)/Dynamics of the MJO (DYNAMO) field campaign: (1) circumnavigating signals and (2) mesoscale gravity waves. A series of cloud-permitting sensitivity experiments are performed at 9km horizontal grid spacing by applying a time-space filter (20-96 days and 0-9 wavenumbers) that removes MJO signals either from initial condition (NoMJOinIC), or lateral boundary condition (NoMJOinBC), or from both initial and boundary conditions (NoMJOinICBC). These simulations are compared with the control simulation at the same resolution without any filtering. The control simulation realistically captures the initiation of MJO around 16th Oct at ~60E and shows the slow eastward propagation of MJO event in surface precipitation, zonal wind, humidity and large-scale vertical motion. The NoMJOinIC experiment also captures the realistic representation of MJO initiation and propagation, but the other two experiments (NoMJOinBC and NoMJOinICBC) fail to depict the event. It is found that the NoMJOinIC experiment successfully simulates the onset of lower level convergence to the east of the convection center due to Kelvin wave responses along with warmer lower to middle troposphere and warm SST anomaly that favors the development of boundary-layer moisture convergence to the east of the convection center. Both the NoMJOinBC and NoMJOinICBC experiments are unable to capture the above mechanism, resulting in failure of portraying the eastward MJO propagation. This suggests that MJO initiation and propagation are more controlled by the globally circumnavigating signals rather than the local dynamics for the October DYNAMO MJO event. More detail reasoning of failure of MJO on NoMJOinBC and NoMJOinICBC experiments are currently being investigated based on the dynamics and variability of westward-propagating equatorial diurnal and 2-day waves. Initial analyses lead us to hypothesize that strong precipitation diurnal cycles triggered over the maritime continents propagate westward in the form of westward inertia-gravity waves over the eastern Indian Ocean, possibly influencing the MJO initiation in both the CNTL and NoMJOinIC experiment, but more detail investigation is underway.