ENSO发展年与衰减年夏季环境要素对热带气旋生成频数变化的贡献

CONTRIBUTION OF ENVIRONMENTAL FACTORS TO THE CHANGE OF TROPICAL CYCLONE FREQUENCY IN THE SUMMER OF ENSO DEVELOPING AND DECAYING YEARS

利用中国气象局上海台风研究所整编的热带气旋(Tropical Cyclone,TC)最佳路径数据集和欧洲中期天气预报中心的ERA再分析资料,分析了El Niño-South Oscillation(ENSO)发展年与衰减年西北太平洋(Western North Pacific,WNP)夏季(6—8月)总TC生成频数(Tropical Cyclone Genesis Frequency,TCGF)及其区域性特征,通过潜在生成指数(Genesis Potential Index,GPI)定量诊断各环境要素对TCGF变化的贡献。结果表明,西北太平洋TCGF总数异常在ENSO各位相并不显著,但其东南象限和西部的TCGF异常存在明显差异。在ENSO各位相,GPI异常的空间分布与TCGF异常的空间型相似。同一区域,各环境要素对TCGF异常的贡献不同,反映了ENSO不同位相影响TC生成变化的机理存在差异。WNP东南部(SEWNP)是对ENSO较敏感的区域,El Niño发展年,中东太平洋异常增暖激发的Rossby波西传导致SEWNP受异常正涡度环流控制,涡度对TCGF增加的贡献最大;El Niño衰减年,西北太平洋出现低层异常反气旋,其东侧异常东北气流将湿度相对较低的水汽输送至SEWNP,相对湿度降低导致TCGF显著减少。La Niña发展年,绝对涡度减小和垂直风切变增加对TCGF减少都有影响。WNP西部仅在La Niña衰减年出现TCGF显著负异常,低层绝对涡度减小的贡献最大,因为季风槽减弱,抑制了南海附近的TC生成。

This paper is to study the characteristics of tropical cyclone genesis frequency(TCGF)and Genesis Potential Index(GPI)and to diagnose the contribution of environmental factors to the change of GPI over the western North Pacific(WNP)during the summer of ENSO developing and decaying years,based on the data from the CMA and the ECMWF.Although there exists no significant change in the total TCGF in the WNP,the TCGF shows differences in southeastern WNP(SEWNP)and western WNP(WWNP).The TCGF pattern is similar to the GPI pattern in all phases and the SEWNP is sensitive to ENSO.Various contributions of environmental factors indicate that there are different reasons for TCGF change in all four phases.During El Niño developing years,the TCGF increases abnormally in this area because of the increasing absolute vorticity and the decreasing vertical wind shear(VWS).In this phase,Rossby waves are triggered by El Niño and thus there is an anomalous cyclone and easterly monsoon trough in the SEWNP.During El Niño decaying years,WNPAC-induced anomalous northeast wind delivers climatological dry air to SEWNP,which is the main factor for the decrease of TCGF.In La Niña developing years,absolute vorticity and VWS are main factors.In western WNP,there is only a significantly negative phase of TCGF during La Niña decaying years due to the diminishing absolute vorticity.