| Zhou Qun,Chen Wen. 2020. Unstable relationship between spring NAO and summer tropical cyclone genesis frequency over the western North Pacific. Acta Oceanologica Sinica, 42(5):65－76 |
| Unstable relationship between spring NAO and summer tropical cyclone genesis frequency over the western North Pacific |
| 春季北大西洋涛动与夏季西北太平洋热带气旋生成频数关系的年代际变化 |
| Received:September 23, 2019 |
| DOI：10.1007/s13131-019-1509-0 |
| Key words:spring North Atlantic Oscillation summer tropical cyclone western North Pacific |
| 中文关键词: 春季北大西洋涛动 夏季热带气旋 西北太平洋 |
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| Abstract: |
| The present study reveals the fact that the relationship between the spring (April-May) North Atlantic Oscillation (NAO) and the following summer (June-September) tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) during the period of 1950-2018 was not stationary. It is shown that the relationship between the two has experienced a pronounced interdecadal shift, being weak and insignificant before yet strong and statistically significant after the early 1980s. Next we compare the spring NAO associated dynamic and thermodynamic conditions, sea surface temperature (SST) anomalies, and atmospheric circulation processes between the two subperiods of 1954-1976 and 1996-2018, so as to illucidate the possible mechanism for this interdecadal variation in the NAO-TC connection. During the latter epoch, when the spring NAO was positive, enhanced low-level vorticity, reduced vertical zonal wind shear, intensified vertical velocity and increased middle-level relative humidity were present over the WNP in the summer, which is conducive to the genesis of WNP TCs. When the spring NAO is negative, the dynamic and thermodynamic factors are disadvantageous for the summertime TC formation and development over the WNP. The results of further analysis indicate that the persistence of North Atlantic tri-pole SST anomalies from spring to the subsequent summer induced by the spring NAO plays a fundamental role in the linkage between the spring NAO and summer atmospheric circulation. During the period of 1996-2018, a remarkable eastward propagating wave-train occurred across the northern Eurasian continent, forced by the anomalous SST tri-pole in the North Atlantic. The East Asian jet flow became greatly intensified, and the deep convection in the tropics was further enhanced via the changes of the local Hadley circulation, corresponding to a positive spring NAO. During the former epoch, the spring NAO-induced tri-pole SST anomalies in the North Atlantic were non-existent, and the related atmospheric circulation anomalies were extremely weak, thereby leading to the linkage between spring NAO and WNP TC genesis frequency in the following summer being insignificant. |
| 中文摘要: |
| 利用中国气象局发布的热带气旋（TC）资料和NCEP/NCAR再分析数据集，本文考察了1950-2018年期间春季（4-5月）北大西洋涛动（NAO）与随后夏季（6-9月）西北太平洋上空TC生成频数关系的年代际变化。结果表明，春季NAO与夏季TC生成频数的关系存在明显的年代际振荡，自上世纪80年代早期以来二者呈显著的正相关关系，但在此之前二者的关系并不显著。选取两个典型时段考察春季NAO对海表温度（SST）场和大气环流场的不同影响，发现1996-2018年期间，当春季NAO为正位相，随后夏季西北太平洋上空低层涡度增强、垂直风切变减小、上升运动加强、中层相对湿度增大，这些均有利于TC的生成。进一步的分析表明，NAO引起的北大西洋三极子SST异常能够从春季一直持续到夏季，这在联系春季NAO与夏季西北太平洋上空大气环流中起到了关键作用。当春季NAO为正位相，在欧亚大陆北部出现一个与北大西洋三极子SST异常密切相关的高空纬向波列，造成下游东亚急流显著增强，通过局地经向环流的改变，引起热带西北太平洋地区的深对流活动加强。相反，1996-2018年期间，当春季NAO为负位相，在北大西洋三极子SST异常强迫的遥相关波列的作用下，夏季热带西北太平洋地区的深对流减弱，环境因子不利于夏季西北太平洋上空TC的形成和发展。然而，1954-1976年期间，春季NAO引起的北大西洋三极子SST异常分布型几乎不存在，与之相关的大气环流异常非常弱且不显著，造成春季NAO与夏季西北太平洋上空TC生成频数的关系明显减弱。 |
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