The East Asian summer monsoon(EASM) is a distinctive component of the Asian climate system and critically influences the economy and society of the region. To understand the ability of AGCMs in capturing the major features of EASM, 10 models that participated in Coupled Model Intercomparison Project/Atmospheric Model Intercomparison Project(CMIP5/AMIP), which used observational SST and sea ice to drive AGCMs during the period 1979–2008, were evaluated by comparing with observations and AMIP II simulations. The results indicated that the multi-model ensemble(MME) of CMIP5/AMIP captures the main characteristics of precipitation and monsoon circulation, and shows the best skill in EASM simulation, better than the AMIP II MME. As for the Meiyu/Changma/Baiyu rainbelt, the intensity of rainfall is underestimated in all the models. The biases are caused by a weak western Pacific subtropical high(WPSH) and accompanying eastward southwesterly winds in group I models, and by a too strong and west-extended WPSH as well as westerly winds in group II models. Considerable systematic errors exist in the simulated seasonal migration of rainfall, and the notable northward jumps and rainfall persistence remain a challenge for all the models. However, the CMIP5/AMIP MME is skillful in simulating the western North Pacific monsoon index(WNPMI).
The trends and fluctuations of observed and CMIP5-simulated yearly mean surface air temperature over China were analyzed. In general, the historical simulations replicate the observed increase of temperature, but the multi-model ensemble(MME) mean does not accurately reproduce the drastic interannual fluctuations. The correlation coefficient of the MME mean with the observations over all runs and all models was 0.77, which was larger than the largest value(0.65) from any single model ensemble. The results showed that winter temperatures are increasing at a higher rate than summer temperatures, and that winter temperatures exhibit stronger interannual variations. It was also found that the models underestimate the differences between winter and summer rates. The ensemble empirical mode decomposition technique was used to obtain six intrinsic mode functions(IMFs) for the modeled temperature and observations. The periods of the first two IMFs of the MME mean were 3.2 and 7.2, which represented the cycle of 2–7-yr oscillations. The periods of the third and fourth IMFs were 14.7 and 35.2, which reflected a multi-decadal oscillation of climate change. The corresponding periods of the first four IMFs were 2.69, 7.24, 16.15 and 52.5 in the observed data. The models overestimate the period of low frequency oscillation of temperature, but underestimate the period of high frequency variation. The warming rates from different representative concentration pathways(RCPs) were calculated, and the results showed that the temperature will increase by approximately 0.9℃, 2.4℃, 3.2℃ and 6.1℃ in the next century under the RCP2.6, RCP4.5, RCP6.0 and RCP8.5 scenarios, respectively.
Based on observations and 12 simulations from Coupled Model Intercomparison Project Phase 5(CMIP5) models, climatic extremes and their changes over China in the past and under the future scenarios of three Representative Concentration Pathways(RCPs) are analyzed. In observations, frost days(FD) and low-temperature threshold days(TN10P) show a decreasing trend, and summer days(SU), high-temperature threshold days(TX90P), heavy precipitation days(R20), and the contribution of heavy precipitation days(P95T) show an increasing trend. Most models are able to simulate the main characteristics of most extreme indices. In particular, the mean FD and TX90 P are reproduced the best, and the basic trends of FD, TN10 P, SU and TX90 P are represented. For the FD and SU indexes, most models show good ability in capturing the spatial differences between the mean state of the periods 1986–2005 and 1961–80; however, for other indices, the simulation abilities for spatial disparity are less satisfactory and need to be improved. Under the high emissions scenario of RCP8.5,the century-scale linear changes of the multi-model ensemble(MME) for FD, SU, TN10 P, TX90 P, R20 and P95 T are-46.9,46.0,-27.1, 175.4, and 2.9 days, and 9.9%, respectively; and the spatial change scope for each index is consistent with the emissions intensity. Due to the complexities of physical process parameterizations and the limitation of forcing data, great uncertainty still exists with respect to the simulation of climatic extremes.
