Changes And Possible Causes Of The East Asian Summer Monsoon Under Global Warming And Low-temperature Warming Scenarios

The East Asian summer monsoon(EASM)is one of the most energetic components of the earth's climate system during boreal summer.The variability of the EASM has substantial social and economic influences.For example,anomalous EASM can cause flood and high temperature extremes.Mainly based on the historical simulations and low warming simulations [(1.5°C never-exceed(1.5deg NE),1.5°C overshoot(1.5deg OS)and 2.0°C(2.0deg NE)] of Community Earth System Model(version1.1.1,CESM1-CAM5),this study documented future changes and possible causes of the East Asian summer monsoon(EASM)in low warming scenarios.This thesis evaluated the reproducibility of CESM1-CAM5,analyzed the changes and reasonable causes of EASM and associated monsoon precipitation under three low warming scenarios in CESM1-CAM5,and compared the changes in EASM rainfall in high and low warming scenarios.Then,the thesis discussed the asymmetry and possible causes of EASM precipitation during the greenhouse gas concentration increasing and decreasing periods in three lowing warming scenarios.The main results of this study are as follows:(1)The CESM1-CAM5 model reasonably simulates the summer rainfall climatology over East Asia-Northwestern Pacific region and reproduces the patterns of correlation between EASM and precipitation.Thus,we can use CESM1-CAM5 to predict the changes in EASM intensity and the associated precipitation during twenty-first century.(2)An increase in EASM intensity and associated rainfall occurs over most parts of the East Asian region in the ensemble mean results of 1.5deg NE,1.5deg OS and 2.0deg NE scenarios during 2006-2100.In addition,the position of key rainfall region of EASM-associated precipitation in future is quite similar to that in present-day(1980-2005).There is no significant difference in the future changes in EASM intensity and EASM precipitation among the three low warming scenarios.(3)According to the moisture budget analysis,relative to the present-day climate,the main causes of the increased precipitation over East Asia in the last 30 years of the 21 st century in the 1.5deg NE,1.5deg OS and 2.0deg NE scenarios are the anomalous vertical motion,evaporation change and anomalous vertical gradient of humidity.The contributions of these three dominant terms increase sequentially under these three scenarios.However,the contributions of anomalous vertical motion,evaporation change and anomalous vertical gradient of humidity under the three scenarios display quite little differences.Over East Asia,the contributions of evaporation and vertical motion are generally larger than that of humidity over the key domain of EASM rainfall in each low warming scenario.(4)The comparison of East Asian summer monsoon precipitation between high and low warming scenarios shows that EASM precipitation under Representative Concentration Pathway(RCP)8.5 is significantly stronger than that in the three low warming scenarios at the end of the 21 st century.The analysis demonstrates that the contributions of each term of moisture budget equation in RCP8.5 scenario are larger than those in the three low warming scenarios.Under RCP8.5 scenario,the increased precipitation over East Asia should be ascribed to the anomalous vertical motion,anomalous of humidity and evaporation change.The contribution of anomalous humidity is stronger than that in the three low warming scenarios.(5)The concentration of CO2 exists the stage of ramp-down and ramp-up in the three low warming scenarios.We found that EASM precipitation exhibits asymmetries with regard to the maximum value of CO2 during the two periods.Diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that,relative to the period when CO2 ramps up,the patterns of anomalous surface ocean temperature over equatorial Pacific during CO2 ramps down are mainly ascribed to the enhancement of zonal advection and mean meridional circulation feedback over equatorial western Pacific.The patterns of temperature anomaly leads to anomalous distribution of precipitation over the western Pacific,which in turn triggers PJ-like pattern and eventually influences the asymmetry of EASM rainfall.