BCC_CSM对北极海冰的模拟:CMIP5和CMIP6历史试验比较

本文利用北京气候中心气候系统模式(BCC_CSM)在最近两个耦合模式比较计划(CMIP5和CMIP6)的历史试验模拟结果，对北极海冰范围和冰厚的模拟性能进行了比较，结果表明:(1) CMIP6改善了CMIP5模拟海冰范围季节变化过大的问题，总体上更接近观测结果；(2)两个CMIP试验阶段中BCC_CSM模拟的海冰厚度都偏小，但CMIP6试验对夏季海冰厚度过薄问题有所改进。通过对影响海冰生消过程的冰面和冰底热收支的分析，我们探讨了上述模拟偏差以及CMIP6模拟结果改善的成因。分析表明，8?9月海洋热通量、向下短波辐射和反照率对模拟结果的误差影响较大，CMIP6试验在这些方面有较大改善；而12月至翌年2月，CMIP5模拟的北极海冰范围偏大主要是海洋热通量偏低所导致，CMIP6模拟的海洋热通量较CMIP5大，但北大西洋表层海流的改善才是巴芬湾附近海冰外缘线位置改善的主要原因。CMIP试验模拟的夏季海冰厚度偏薄主要是因为6?8月海洋热通量和冰面热收支都偏大，而CMIP6试验模拟的夏季海冰厚度有所改善主要是由于海洋热通量和净短波辐射的改善。海冰模拟结果的改善与CMIP6海冰模块和大气模块参数化的改进有直接和间接的关系，通过改变短波辐射、冰面反照率和海洋热通量，使BCC_CSM模式对北极海冰的模拟性能也得到有效提高。

Comparison of simulation results of the Arctic sea ice by BCC_CSM: CMIP5 and CMIP6 historical experiments

In this paper, the simulation performance of the sea ice concentration, ice extent/area, and ice thickness of the Arctic is compared based on the results of the climate system model of Beijing Climate Center (BCC_CSM) in the recent two coupling model comparison programs (CMIP5 and CMIP6). The results show that, compared with the results in the CMIP5, the CMIP6 historical experiment recently released has different degrees of improvement in the Arctic sea ice extent seasonal cycle and sea ice thickness, which are shown as follows: (1) In the CMIP6 experiment, the simulated sea ice extent in summer is larger, while the sea ice extent in winter is smaller, which is generally closer to the observation result than the CMIP5. (2) The sea ice thickness simulated by BCC_CSM in the two CMIP is relatively small, but the CMIP6 experiment slightly improves the problem of excessively thin sea ice thickness. Through the analysis of the variables (such as SST, air temperature and radiation flux, turbulent heat flux, ocean heat flux) in different spheres of the climate system affecting the sea ice process, we discuss the causes of the above simulation errors and the improvement of CMIP6 simulation results. The analysis shows that the Arctic sea ice extent simulated by BCC_CSM model in CMIP5 from August to September is small, which is mainly caused by greater ocean heat flux, larger downward short-wave radiation and smaller albedo, while CMIP6 experiment has a great improvement in these aspects. From December to February, the Arctic sea ice extent simulated by CMIP5 is relatively large mainly because of the low ocean heat flux, and the ocean heat flux simulated by CMIP6 is larger than that of CMIP5. However, the improvement of the north Atlantic regional ocean surface current is the main reason for the improvement of the sea ice extent in this region, especially the sea ice edge in the Baffin Bay. The thinner summer sea ice thickness simulated by CMIP5 is mainly due to the larger ocean heat flux and ice surface heat budget from June to August, while the improvement of sea ice thickness simulated by CMIP6 is mainly due to the improvement of ocean heat flux and net short-wave radiation. The improvement of sea ice simulation has direct and indirect relationship with CMIP6 sea ice module and atmospheric module parameterization. By changing short-wave radiation, ice surface albedo and ocean heat flux, the performance of BCC_CSM mode on Arctic sea ice has been effectively improved.