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Understanding regional hydro‐climatic extreme responses to CO2 pathways is fundamental to climate change mitigation and adaptation. This study evaluates responses of extreme precipitation frequency (R30mm: days with precipitation ≥30 mm) over East Asia to idealized CO2 forcing using the Community Earth System Model (CESM1). Under a symmetric increase (ramp‐up, +1% per year until quadrupling level) and decrease (ramp‐down, about −1% per year back to the present level) of CO2 concentrations, East Asian R30mm shows an asymmetric response with higher frequency during the ramp‐down period. This hysteresis behavior is found to be due to a northwestward propagating wave response to the El Niño‐like warming, which induces a three‐cell (positive‐negative‐positive) R30mm difference pattern from central equatorial Pacific to East Asia. Monthly analysis further reveals that this asymmetry has a seasonal locking, occurring during July–September only, constrained by the background precipitation climatology over the sub‐tropical western Pacific. Plain Language Summary: Considering tremendous damages caused by regional‐scale hydro‐climatic extremes under global warming, understanding their responses to CO2 forcing is fundamentally important but in‐depth analyses have been limited. This study provides a first examination of the responses of extreme precipitation over East Asia to CO2 forcing using an ensemble simulation of the coupled climate model Community Earth System Model version. When increasing CO2 concentration at a 1%‐per‐year rate from the present level to quadrupling level and then decreasing it symmetrically, extreme precipitation frequency is found to remain higher during the ramp‐down period. This hysteresis behavior occurs strongly during July to September, implying that East Asia can experience more frequent heavy precipitation events even after CO2 concentration is recovered. Composite analysis reveals that this irreversible response originates from the El Niño‐like warming pattern, which induces a tri‐pole atmospheric wave pattern from central equatorial Pacific to East Asia. We also find that the seasonal locking of this asymmetric response is related to the background precipitation state over the sub‐tropical western Pacific. Key Points: Heavy rainfall frequency over East Asia shows an asymmetric response to CO2 forcing, remaining higher during the ramp‐down periodThe hysteresis behavior is caused by a northwestward propagating wave response to the El Niño‐like equatorial Pacific warmingThis asymmetry is evident during July–September only, constrained by background precipitation state over the sub‐tropical western Pacific [ABSTRACT FROM AUTHOR]

Convective extreme El Niño (CEE) events, characterized by strong convective events in the eastern Pacific, are known to have a direct link to anomalous climate conditions worldwide, and it has been reported that CEE will occur more frequently under greenhouse warming. Here, using a set of CO 2 ramp-up and ramp-down ensemble experiments, we show that frequency and maximum intensity of CEE events increase further in the ramp-down period from the ramp-up period. These changes in CEE are associated with the southward shift of the intertropical convergence zone and intensified nonlinear rainfall response to sea surface temperature change in the ramp-down period. The increasing frequency of CEE has substantial impacts on regional abnormal events and contributed considerably to regional mean climate changes to the CO 2 forcings.