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 has been performed with the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the effect of disturbances associated with intraseasonal oscillation on the cyclogenesis. From the analysis of observational data, it is found that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with OLR and precipitation from the equator to 20N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the Bay of Bengal and then resided over the Maritime continent. The northward migration of zonal wind, OLR, and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with the northward migration and with the timing that active convective region associated with the MJO resides over east side of the Maritime continent. The incipient disturbances as the factor of the initiation of cyclone Nargis formation are formed during the westerly wind burst passing through the Bay of Bengal after the monsoon onset, and developed to TCs in the ensemble simulation. However, for an ensemble member with not-simulated northward migration as monsoon onset, any TC does not formed in the Bay of Bengal. It is also found that the effect of the easterlies across the north of the Malay Peninsula is important for TC formation in our simulation.