Future Changes in Synoptic-Scale Conditions Causing Widespread Heavy Precipitation Events over Japan

To identify and characterize the synoptic-scale precipitation systems causing widespread heavy precipitation events over Japan and to evaluate their possible future changes, annual maximum of area-averaged daily and 5-day accumulated precipitation for 720 years were analyzed for both historical and 4 K warming climates using a large ensemble dataset with 5 km horizontal resolution. According to statistical cluster analysis, the approach of tropical cyclones is the primary factor causing widespread heavy precipitation events in both the historical and 4 K warming experiments, although the Baiu front and migratory extratropical cyclones also contribute to event occurrence. The frequency of tropical-cyclone-associated events is lower in the 4 K warming climate compared with the historical experiment because the occurrence frequency of tropical cyclones is lower over the western North Pacific. The decrease in frequency of tropical-cyclone-associated events leads to a relative increase in the frequency of events associated with other precipitation systems (i.e., the Baiu front and migratory extratropical cyclones) under the warming climate. The anomalous moisture supply in the 4 K warming experiment causes the widespread heavy precipitation derived from the Baiu front and migratory extratropical cyclones to intensify to reach a magnitude comparable to that of historical-climate tropical-cyclone-associated events.