Monsoons are one of the most active members of the global climate system,and the land-sea thermal contrast is the main reason for the establishment and continuity of monsoons.At present,the CMIP5 climate coupling model,which has made significant improvements in the physical process,carbon cycle,etc.,is an important tool by which to carry out research regarding climate variability and change.The characteristics of the land-sea thermal contrast in the 20th century and how the CMIP5 modes have simulated it are problems which are worthy of study.In this study,observations from NCEP/NCAR and ERA-40 are used to contrast and analyze the features of summer land-sea thermal contrast over the Asian monsoon region,and to define the land-sea thermal contrast index.The results indicate that the temperature over land decreased,while that over sea increased,and the contrast tended to decrease.On this basis,the model simulations given by the 20 climate models of historical simulations from Phase 5 of the Coupled Model Inter-comparison Project(CMIP5) are used for comparison with observation analysis.Both the best and worst models in the RCP4.5 scenario are selected for forecasting.The main results of the study are as follows:(1)The observation data show that the average temperature of the upper-middle troposphere had a remarkable trend over the Asian monsoon region from 1955 to 2005,namely the temperature over land decreased and that over sea increased.According to the selected area,we define a land index,sea index and land-sea thermal contrast index.These present a large rate of inter-annual change;the land-sea thermal contrast index reduced;and the land-sea thermal contrast index was able to reflect the features of the difference between strong and weak monsoons.The NCEP is consistent with the ERA-40,yet the NCEP has an obvious linear trend.(2)The CMIP5 models have a better simulation performance regarding the climate condition of the zonal-wind field and height field in 500 hPa and 200 hPa,yet a poor simulation performance on the temperature field.The models could not perform satisfactorily in the simulation of the index inter-annual change.The simulations of interdecadal change are that the sea index increased and land-sea thermal contrast index decreased,while the land index increased.Combined with statistical computing,it may be concluded that GFDL-ESM2G,MPI-ESM-LR and MPI-ESM-MR had better simulation performances,while BCC-CSM1-1-m,MRI-CGCM3 and MIROC5 performed poorly.The models’ capability to simulate land-sea thermal contrast can reflect the ability of simulating the summer monsoon.(3)In the situation of RCP4.5,the simulation temperature of the upper-middle troposphere over the Asian monsoon region observed by the CMIP5 models indicates that it will increase throughout the 21th century.The degree of temperature increase over sea is greater than over land in the simulation data,with both the land and sea indexes increasing,and the land-sea thermal contrast index decreasing.The poorer models have slightly higher simulation data for the future temperature field than the stronger models,especially over sea,and in addition the poorer models cannot simulate the decreasing trend of land-sea thermal contrast.