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.

中文翻译：

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北太平洋西部春季NAO与夏季热带气旋发生频率的不稳定关系

本研究揭示了一个事实，即春季（4月至5月）北大西洋涛动（NAO）与随后的夏季（6月至9月）热带气旋（TC）发生频率之间的关系1950年至2018年的时间不是固定的。结果表明，两者之间的关系经历了明显的年代际变化，在1980年代初之前是弱的和微不足道的，而在过去则是强势的和具有统计意义的。接下来，我们比较了春季NAO相关的动态和热力学条件，海表温度（SST）异常以及1954–1976年和1996–2018年这两个子时期之间的大气环流过程，从而阐明了这种年代际变化的可能机制。 NAO-TC连接。在后一个时期 当春季NAO为正时，夏季WNP上低层涡度增加，垂直纬向风切变减小，垂直速度加快和中层相对湿度增加，这有利于WNP TCs的产生。当春季NAO为负时，动力学和热力学因素不利于WNP夏季的TC形成和发展。进一步的分析结果表明，春季NAO引起的北大西洋三极海温异常从春季一直持续到随后的夏季，在春季NAO与夏季大气环流之间的联系中起着根本作用。在1996年至2018年期间，整个欧亚大陆发生了一次明显的向东传播的波列，由北大西洋的异常SST三极强迫。东亚急流急转直下，通过当地哈德利环流的变化进一步增强了热带地区的深对流，这与春季NAO相对应。在前一个时期，北大西洋不存在春季NAO引起的三极SST异常，并且相关的大气环流异常非常弱，从而导致春季NAO与WNP TC发生频率之间的联系夏天微不足道。