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A typical Madden–Julian Oscillation (MJO) generates a large region of enhanced rainfall over the equatorial Indian Ocean that moves slowly eastward into the western Pacific. Tropical cyclones often form on the poleward edges of the MJO moist-convective envelope, frequently impacting both southeast Asia and northern Australia, and on occasion Eastern Africa. This paper addresses the question of whether these MJO-induced tropical cyclones will become more numerous in the future as the oceans warm. The Lagrangian Atmosphere Model (LAM), which has been carefully tuned to simulate realistic MJO circulations, is used to study the sensitivity of MJO modulation of tropical cyclogenesis (TCG) to global warming. A control simulation for the current climate is compared with a simulation with enhanced radiative forcing consistent with that for the latter part of the 21st century under Shared Socioeconomic Pathway (SSP) 585. The LAM control run reproduces the observed MJO modulation of TCG, with about 70 percent more storms forming than monthly climatology predicts within the MJO’s convective envelope. The LAM SSP585 run suggests that TCG enhancement within the convective envelope could reach 170 percent of the background value under a high greenhouse gas emissions scenario, owing to a strengthening of Kelvin and Rossby wave components of the MJO’s circulation. 

The Madden–Julian Oscillation (MJO) is a planetary-scale weather system that creates a 30–60 day oscillation in zonal winds and precipitation in the tropics. Its envelope of enhanced rainfall forms over the Indian Ocean and moves slowly eastward before dissipating near the Date Line. The MJO modulates tropical cyclone (TC) genesis, intensity, and landfall in the Indian, Pacific, and Atlantic Oceans. This study examines the mechanisms by which the MJO alters TC genesis. In particular, MJO circulations are partitioned into Kelvin and Rossby waves for each of the developing, mature, and dissipating stages of the convective envelope, and locations of TC genesis are related to these circulations. Throughout the MJO’s convective life cycle, TC genesis is inhibited to the east of the convective envelope, and enhanced just west of the convective envelope. The inhibition of TC genesis to the east of the MJO is largely due to vertical motion associated with the Kelvin wave circulation, as is the enhancement of TC genesis just west of the MJO during the developing stage. During the mature and dissipating stages, the MJO’s Rossby gyres intensify, creating regions of low-level vorticity, favoring TC genesis to its west. Over the 36-year period considered here, the MJO modulation of TC genesis increases due to the intensification of the MJO’s Kelvin wave circulation. 

The Madden–Julian Oscillation (MJO) is a large-scale tropical weather system that generates heavy rainfall over the equatorial Indian and western Pacific Oceans on a 40–50 day cycle. Its circulation propagates eastward around the entire world and impacts tropical cyclone genesis, monsoon onset, and mid-latitude flooding. This study examines the mechanism of the MJO in the Lagrangian atmospheric model (LAM), which has been shown to simulate the MJO accurately, and which predicts that MJO circulations will intensify as oceans warm. The LAM MJO’s first baroclinic circulation is projected onto a Kelvin wave leaving a residual that closely resembles a Rossby wave. The contribution of each wave type to moisture and moist enthalpy budgets is assessed. While the vertical advection of moisture by the Kelvin wave accounts for most of the MJO’s precipitation, this wave also exports a large amount of dry static energy, so that in total, it reduces the column integrated moist enthalpy during periods of heavy precipitation. In contrast, the Rossby wave’s horizontal circulation builds up moisture prior to the most intense convection, and its surface wind perturbations enhance evaporation near the center of MJO convection. Surface fluxes associated with the Kelvin wave help to maintain its circulation outside of the MJO’s convectively active region.