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郝赛, 毛江玉. 西北太平洋与南海热带气旋活动季节变化的差异及可能原因[J]. 气候与环境研究, 2015, 20(4): 380-392. DOI: 10.3878/j.issn.1006-9585.2014.14063
引用本文: 郝赛, 毛江玉. 西北太平洋与南海热带气旋活动季节变化的差异及可能原因[J]. 气候与环境研究, 2015, 20(4): 380-392. DOI: 10.3878/j.issn.1006-9585.2014.14063
HAO Sai, MAO Jiangyu. Differences of the Seasonal Variations of Tropical Cyclone Activities over the South China Sea and the Western North Pacific and the Possible Causes[J]. Climatic and Environmental Research, 2015, 20(4): 380-392. DOI: 10.3878/j.issn.1006-9585.2014.14063
Citation: HAO Sai, MAO Jiangyu. Differences of the Seasonal Variations of Tropical Cyclone Activities over the South China Sea and the Western North Pacific and the Possible Causes[J]. Climatic and Environmental Research, 2015, 20(4): 380-392. DOI: 10.3878/j.issn.1006-9585.2014.14063

西北太平洋与南海热带气旋活动季节变化的差异及可能原因

Differences of the Seasonal Variations of Tropical Cyclone Activities over the South China Sea and the Western North Pacific and the Possible Causes

  • 摘要: 利用1945~2011年美国联合台风预警中心(JTWC)西北太平洋热带气旋资料,研究了南海(5°N~25°N,110°E~120°E)与西北太平洋(5°N~25°N,120°E~180°)热带气旋生成位置、生成频数、强度和持续时间的季节变化差异及其成因。从热带气旋路径穿越经度带频数的角度,探讨了ENSO对气旋活动年际变化的影响。结果表明,南海热带气旋活动显著地受季风调控。在南海冬季风作用下,1~4月热带气旋生成于10°N以南且频数较少、强度较弱,这主要是低层气旋式相对涡度和弱东风切变区偏南造成的。相反,受夏季风影响,6~9月是热带气旋生成最多、最频繁的季节,大都生成于南海北部17°N附近。在5月(10月)的季节转换期,生成位置大幅度北进(南撤)且生成频数显著增加(减少),取决于风速垂直切变及中层的相对湿度的急剧转变。11、12月两海域热带气旋生成于10°N以南主要归因于其上空中层大气相对湿度较北部偏大。在西北太平洋,热带气旋生成的季节变化没有南海显著,只在7月有一次明显的变化,7~10月是热带气旋活动的“盛期”。在强度上,西北太平洋大部分区域全年均为弱东风切变,因此热带气旋以台风为主且持续时间长;但南海多为热带风暴。ENSO事件使得不同季节热带气旋生成区域和气旋路径地理位置发生显著变化。在El Niño事件期间,穿越南海所在经度带路径频数为负距平,而西北太平洋经度带为正距平;在La Niña事件期间,情况相反。

     

    Abstract: The characteristics and differences of seasonal changes of the tropical cyclone (TC) activities in terms of genesis location, genesis frequency, intensity, and duration are analyzed over the South China Sea (SCS) (5°N-25°N, 110°E-120°E) and the western North Pacific (WNP) (5°N-25°N, 120°E-180°) using the TC datasets for the period 1945-2011 from the Joint Typhoon Warning Center (JTWC). Further examined are the impacts of ENSO events on the interannual variability of TC activities over the two areas from the perspective of TC longitude crossings. The results show that the TC genesis in the SCS is modulated significantly by the SCS monsoon, with a small number of weak TCs forming only south of 10°N during months of January to April when the SCS winter monsoon prevails, which results from favorable dynamical conditions confined to lower latitudes in terms of low-level cyclonic vorticity and weak vertical wind shear. In contrast, the period from June to September is the SCS TC season during which TCs generate most frequently under the impact of the SCS summer monsoon season, with majority of cyclogenesis forming around 17°N. Abrupt northward (southward) shift of TC genesis position accompanied by the apparent increase (decrease) in cyclogenesis frequency takes place in the seasonal transition of May (October), depending on the onset (retreat) of the SCS summer monsoon in relation to drastic changes of the weak vertical wind shear and mid-layer relative humidity higher than 40% in the SCS. With the higher relative humidity concentrating over southern SCS in the mid-layer, the TC genesis positions again return back south of 10°N during November-December. However, the TC genesis in the WNP exhibits a gradually seasonal variation with an evident change only appearing in July. Note that the period July-October is the dominant typhoon season in the WNP. Because weaker vertical easterly wind shear is present over the NWP in almost all months of the year, most TCs generated in this region can reach a stronger intensity and develop into typhoons. But over the SCS, the TC intensity is weaker than that over the NWP, with most TCs evolving into only tropical storms. ENSO events exert a significant impact on changes in regions favorable for TC genesis during different seasons and consequently on geographical variations in TC tracks. During an El Niño event for both developing phase and decaying phase, the departure from climatology of monthly TC longitude crossings is negative anomaly in the SCS and positive anomaly in the WNP. Opposite situation occurs during a La Niña event.

     

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