Abstract. The main advancements of the Beijing Climate Center (BCC) climate system model from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to phase 6 (CMIP6) are presented, in terms of physical parameterizations and model performance. BCC-CSM1.1 and BCC-CSM1.1m are the two models involved in CMIP5, whereas BCC-CSM2-MR, BCC-CSM2-HR, and BCC-ESM1.0 are the three models configured for CMIP6. Historical simulations from 1851 to 2014 from BCC-CSM2-MR (CMIP6) and from 1851 to 2005 from BCC-CSM1.1m (CMIP5) are used for models assessment. The evaluation matrices include the following: (a) the energy budget at top-of-atmosphere; (b) surface air temperature, precipitation, and atmospheric circulation for the global and East Asia regions; (c) the sea surface temperature (SST) in the tropical Pacific; (d) sea-ice extent and thickness and Atlantic Meridional Overturning Circulation (AMOC); and (e) climate variations at different timescales, such as the global warming trend in the 20th century, the stratospheric quasi-biennial oscillation (QBO), the Madden–Julian Oscillation (MJO), and the diurnal cycle of precipitation. Compared with BCC-CSM1.1m, BCC-CSM2-MR shows significant improvements in many aspects including the tropospheric air temperature and circulation at global and regional scales in East Asia and climate variability at different timescales, such as the QBO, the MJO, the diurnal cycle of precipitation, interannual variations of SST in the equatorial Pacific, and the long-term trend of surface air temperature.

The changes in summer rainfall over the Tarim Basin, China, and the underlying mechanisms have been investigated using the observed rainfall data at 34 stations and the NCEP–NCAR reanalysis data during the period of 1961–2007. Results show that the summer rainfall over the Tarim Basin, which exhibits a significant increasing trend during the last half century, is closely related to the summer middle and upper tropospheric cooling over central Asia. Mechanism analysis indicates that the middle and upper tropospheric cooling over central Asia results in a location farther south of the subtropical westerly jet over western and central Asia with anomalous southerly wind at lower levels and ascending motion prevailing over the Tarim Basin. Such anomalies in the atmospheric circulations provide favorable conditions for the enhanced summer rainfall over the Tarim Basin. Further analysis suggests that the weakened South Asian summer monsoon (SASM) could be potentially responsible for the middle and upper tropospheric cooling over central Asia. This is largely through the atmospheric responses to the diabatic heating effect of the SASM. A weakened SASM can result in an anomalous cyclone in the middle and upper troposphere over central Asia. The western part of the anomalous cyclone produces more cold air advection, which leads to the cooling. This study suggests indirect but important effects of the SASM on the summer rainfall over the Tarim Basin.

[1] Based on 14 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), uncertainty on the simulated summer precipitation over Eastern China is analyzed by investigating the intercomparison between individual model and multimodel ensemble (MME). Generally, MME has the ability in reproducing summer precipitation over Eastern China. However, large model spread exists among models in both climatology and interannual variation. The possible reason for the large model spread lies in the uncertainties on simulating large-scale circulations, e.g., East Asian subtropical westerly jet, western Pacific subtropical high, and East Asian summer monsoon. To investigate uncertainties in different regions, Eastern China is divided to four subregions: South China (SC), Yangtze-Huaihe River Basin (YHRB), North China (NC), and Northeast China (NEC). The annual cycle of regional mean precipitation from 14 CMIP5 models indicates that the model spread approaches maximum in early summer over SC and YHRB and in middle summer over NC and NEC. Uncertainties generally decrease from south to north, with the most sensitive region of SC. For different-class precipitation, the uncertainties of 14 models are small in relatively weak rain, but large in heavy and nonrainfall for all the four regions. We propose two possible reasons for the large uncertainties: different partitioning of stratiform/ convective precipitation and horizontal resolutions.

Abstract. Main progresses of Beijing Climate Center (BCC) climate system model from the phase five of the Coupled Model Intercomparison Project (CMIP5) to its phase six (CMIP6) are presented, in terms of physical parameterizations and models performance. BCC-CSM1.1 and BCC-CSM1.1m are the two models involved in CMIP5, and BCC-CSM2-MR, BCC-CSM2-HR, and BCC-ESM1.0 are the three models configured for CMIP6. Historical simulations from 1851 to 2014 from BCC-CSM2-MR (CMIP6) and from 1851 to 2005 from BCC-CSM1.1m (CMIP5) are used for models assessment. The evaluation matrices include (a) energy budget at top of the atmosphere, (b) surface air temperature, precipitation, and atmospheric circulation for global and East Asia regions, (c) sea ice extent and thickness and Atlantic Meridional Overturning Circulation (AMOC), and (d) climate variations at different time scales such as global warming trend in the 20th century, stratospheric quasi-biennial oscillation (QBO), Madden-Julian Oscillation (MJO) and diurnal cycle of precipitation. Compared to BCC CMIP5 models, BCC CMIP6 models show significant improvements in many aspects including: tropospheric air temperature and circulation at global and regional scale in East Asia, climate variability at different time scales such as QBO, MJO, diurnal cycle of precipitation, and long-term trend of surface air temperature.

Based on the outputs of historical and future representative concentration pathway (RCP) experiments produced by 28 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), future changes in climatic mean, interannual standard deviation (ISD), and long-term trends of the annual precipitation over central Asia (CA) have been estimated. Under different emission scenarios during the twenty-first century, the climatic mean and ISD (long-term trends) of the annual precipitation over CA projected by the five best models’ ensemble mean show very similar (quite different) spatial patterns to those in the twentieth century. Relatively stronger increasing rates (over 3 mm decade−1 in RCP2.6 and over 6 mm decade−1 in RCP4.5 and RCP8.5) are located over northern CA and the northeastern Tibetan Plateau. Compared to the situations in the twentieth century, the climatic mean, ISD, and long-term trends of the projected annual precipitation over most of CA under different emission scenarios exhibit robust increasing changes during the twenty-first century. The projected increasing changes in the climatic mean (ISD) of the CA annual mean range from 10% to 35% (10%–90%) under different emission scenarios with relatively large increases over Xinjiang, China (northern CA and Xinjiang). The increasing trends of the annual precipitation over most of CA are projected to intensify with relatively large increases (over 3–9 mm decade−1) located over northern CA, the Tian Shan Mountains, and northern Tibet during the twenty-first century. In addition, the intensities of the increasing changes in the climatic mean, ISD, and trends of CA annual precipitation are intensified with the emissions increased correspondingly. Further analyses of the possible mechanisms related to the projected changes in precipitation indicate that the increases of the annual precipitation over CA in the twenty-first century are mainly attributed to the enhanced precipitable water that results from strengthened water vapor transport and surface evaporation.