Northern Hemisphere Sudden Stratospheric Warming and Its Downward Impact in Four Chinese CMIP6 Models

Abstract

Using the World Meteorological Organization definition and a threshold-based classification technique, simulations of vortex displacement and split sudden stratospheric warmings (SSWs) are evaluated for four Chinese models (BCC-CSM2- MR, FGOALS-f3-L, FGOALS-g3, and NESM3) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) with the Japanese 55-year reanalysis (JRA-55) as a baseline. Compared with six or seven SSWs in a decade in JRA-55, three models underestimate the SSW frequency by ~50%, while NESM3 doubles the SSW frequency. SSWs mainly appear in midwinter in JRA-55, but one-month climate drift is simulated in the models. The composite of splits is stronger than displacements in both the reanalysis and most models due to the longer pulse of positive eddy heat flux before onset of split SSWs. A wavenumber-1-like temperature anomaly pattern (cold Eurasia, warm North America) before onset of displacement SSWs is simulated, but cold anomalies are mainly confined to North America after displacement SSWs. Although the lower tropospheric temperature also displays a wavenumber-1-like pattern before split SSWs, most parts of Eurasia and North America are covered by cold anomalies after split SSWs in JRA-55. The models have different degrees of fidelity for the temperature anomaly pattern before split SSWs, but the wavenumber-2-like temperature anomaly pattern is well simulated after split SSWs. The center of the negative height anomalies in the Pacific sector before SSWs is sensitive to the SSW type in both JRA-55 and the models. A negative North Atlantic Oscillation is simulated after both types of SSWs in the models, although it is only observed for split SSWs.

摘要

依照世界气象组织的平流层爆发性增温(SSW)定义方法,本文系统评估了四个参与国际第六次耦合模式比较计划的中国模式(即BCC-CSM2-MR、FGOALS-f3-L、FGOALS-g3和NESM3)对SSW及其对流层影响的模拟技巧。基于平流层极涡几何型态的参数阈值分类技术,可以将SSW分为极涡偏移型和极涡分裂型。研究结果表明,再分析资料中SSW平均每10年爆发6–7次;相比之下,三个中国CMIP6模式(BCC-CSM2-MR、FGOALS-f3-L和FGOALS-g3)远远低估了SSW的爆发频率,只能模拟出半数事件,而NESM3模拟的SSW频率相比再分析资料增加了一倍。再分析资料中观测的SSW主要出现在一二月,四个模式模拟的SSW均偏晚一个月,主要发生在二三月。与再分析结果基本一致的是,多数模式模拟的分裂型SSW爆发之前,正涡旋热通量异常积累时间比偏移型SSW更久,因此分裂型SSW的平均强度相比偏移型SSW强。模式可以成功再现极涡偏移型SSW爆发之前的1波型温度异常分布(欧亚大陆冷异常,北美洲大陆暖异常),偏移型SSW爆发之后的冷异常主要集中在北美的有限区域。在分裂型SSW爆发之前也在近地面观测到1波型温度异常分布,分裂型SSW爆发之后近地面温度异常为2波主导。四个模式对分裂型SSW爆发之前的1波异常温度分布具有不同的模拟能力,但对分裂型SSW爆发之后的2波型温度异常分布普遍具有较好的再现能力。再分析资料和模式模拟都表明,北太平洋的对流层低压异常中心位置因SSW类型不同而异。此外,再分析资料只在分裂型SSW爆发之后出现北大西洋涛动负位相;而模式模拟的负位相北大西洋涛动在偏移型和分裂型SSW爆发之后均出现。