Using ten climate models in the Paleoclimate Modelling Intercomparison Project Phase Ⅲ (PMIP3),we investigate possible variations of large-scale environmental factors important to tropical cyclone (TC) genesis over the North Atlantic in the last millennium.The genesis factors include potential intensity,vertical wind shear,midtropospheric relative humidity,and absolute vorticity.Based on the ensemble mean of the PMIP3 models,potential intensity that predicts the theoretical maximum of TC intensity is broadly enhanced over the North Atlantic in the Medieval Warm Period (MWP,950～ 1200A.D.)relative to the Little Ice Age (LIA,1600～ 1850A.D.),especially at the mid-latitude.Vertical wind shear,generally hampering TC genesis and intensification,is mainly reduced over a zonal band between 10°N and 30°N in the MWP.Relative humidity at 600hPa that measures midtropospheric moist content is broadly increased over the eastern tropical North Atlantic,where the low-level absolute vorticity is also increased.We then calculate a genesis potential index (GPI) as a way to summarize environmental changes to overall favorability of TC genesis.It is found that conditions are generally favorable for TC genesis over the North Atlantic in the MWP relative to the LIA,especially over the main develop region and the northwestern North Atlantic.Higher potential intensity,smaller vertical wind shear,larger relative humidity,and stronger absolute vorticity point to the increased GPI over the main develop region,while the larger GPI over the northwestern North Atlantic is mainly attributed to increased potential intensity and reduced vertical wind shear.Meanwhile,the steering flow,estimated as the mean wind between 850hPa and 300hPa,exhibits an anomalous easterly in the MWP relative to the LIA,which generally benefits TC landfall over the North America.The model results are broadly consistent with geological evidence in terms of TC climatology on centennial timescale.Additionally,sensitivity experiments based on the MPI-M model indicate that both increased solar irradiation and reduced volcano eruptions contribute to a higher GPI in the MWP relative to the LIA.%本文利用古气候模式比较计划第三阶段(PMIP3) 10个耦合模式结果研究了过去千年北大西洋热带气旋生成潜势的空间特征.对于影响热带气旋生成的大尺度环境场而言,模拟结果表明中世纪暖期(950～ 1200A.D.)热带气旋潜在强度相对于小冰期(1600～ 1850A.D.)在北大西洋普遍增强,尤其是中纬度地区.中世纪暖期垂直风切变在10°～30°N之间的纬度带显著减小,而对流层中层相对湿度和低层绝对涡度在北大西洋东部增大.基于热带气旋生成潜势指数,中世纪暖期大尺度环境条件有利于热带气旋在北大西洋的生成和发展.同时,中世纪暖期热带气旋高空引导气流为东风异常,有利于热带气旋登陆北美地区.在百年尺度热带气旋气候态的角度,模拟结果与地质证据基本一致.此外,敏感性试验结果表明太阳辐射增加和火山活动减少对中世纪暖期北大西洋热带气旋生成潜势增加均有作用.
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