Ensemble Simulation of Cyclone Nargis by a Global Cloud-System-Resolving Model-Modulation of Cyclogenesis by the Madden-Julian Oscillation

An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the effect on cyclogenesis of disturbances associated with intraseasonal oscillations. An analysis of observational data reveals that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with active cloud areas and precipitation from the equator to 20 degrees N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the bay and then resided over the Maritime continent. The northward migration of low-level zonal wind, outgoing longwave radiation (OLR), and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with northward migration and with a timing such that the active convective region associated with the MJO resides over the east side of the Maritime continent. The incipient disturbances that contributed to the initiation of cyclone Nargis formed during the period when the westerly wind burst passed through the Bay of Bengal after the monsoon onset and developed to TCs in the ensemble simulation. However, for an ensemble member for which northward migration as a monsoon onset is not simulated, no TC is formed in the Bay of Bengal. It is also found that the effect of the easterlies across the northern part of the Malay Peninsula is important for TC formation in our simulation.