Long-term intensification of the East Asian Summer Monsoon (EASM) lifecycle based on observation and CMIP6

In 2018, Japan experienced successive extremes, flood and following heat wave. The East Asian summer monsoon (EASM) has lifecycle and depending on the cycle, the basic condition of rainfall and heat event is decided. Thus, to examine the variability to the basic condition which is capable to make extreme event favorable, the long-term change of the EASM lifecycle is analyzed based on observation datasets and historical simulations of the Couple Model Intercomparison Project Phase 6 (CMIP6).

 According to the observation, the active phase of EASM has intensified and the break phase becomes longer, resulting in a shorter but stronger rainy season followed by a longer dry spell. This intensification in the precipitation evolution is accompanied by increased lower tropospheric southwesterly wind and convergence of water vapor flux, suggesting a dynamical cause. The widely reported westward extension of the Western North Pacific Subtropical High associated with the warming climate is a likely driver. Some of the CMIP6 models were able to capture the climatology of the EASM lifecycle and its intensification similar to those observed, but the majority of models still did not properly simulate the EASM lifecycle.