Impacts of Madden-Julian Oscillation on tropical cyclone activity over the western North Pacific under global warming

The modulation of tropical cyclone (TC) activity by Madden Julian Oscillation (MJO) in the western North Pacific (WNP) is investigated based on simulation results from a pair of Hi-Resolution Atmospheric Model (HiRAM) experiments, one designed for the present-day simulation (1980-2004) and the other for the late 21st century projection (2075-2099) with the Representative Concentration Pathway (RCP) 8.5 warming scenario, at approximately 25 km spatial resolution. Major features of the observed TCs, such as the seasonal cycle, spatial distribution of TC genesis and the modulation of MJO on TC activity, are properly reproduced by HiRAM. Under global warming, HiRAM projects a marked reduction of TC genesis in all phases of MJO and a greater daily genesis rate (DGR) contrast between the enhanced (e.g., 4, 5 and 6) and suppressed (e.g., 7, 8, 1, 2 and 3) phases during the late 21st century period (2075-2099) against the present-day period (1980-2004). A modified "genesis potential index" (GPI) is proposed to elucidate the environmental factors attributing to TC activity change associated with MJO. A budget analysis of GPI clearly indicates that the suppression of TC genesis associated with MJO under global warming should result mostly from decreases in low-level relative vorticity and mid-level relative humidity. The present study highlights an example of HiRAM's application in simulating the impacts of MJO on TC activity and projecting the potential changes of MJO-TC relation under global warming over the WNP warm pool.