| Summary: | Previous studies have shown that background oceanic and atmospheric environments can influence not only the formation but also the intensity of tropical cyclones. Typhoon Soudelor in August 2015 is notable in that it underwent two rapid intensifications as the storm passed over the Philippine Sea where the 26 °C isotherm (<i>Z</i><sub>26</sub>) was deeper than 100 m and warm eddies abounded. At the same time, prior to the storm’s arrival, an anomalous upper-level anticyclone developed south of Japan and created a weakened vertical wind shear (<i>V<sub>s</sub></i>) environment that extended into the Philippine Sea. This study examines how the rapid intensification of Typhoon Soudelor may be related to the observed variations of <i>Z</i><sub>26</sub>, <i>V<sub>s</sub></i> and other environmental fields as the storm crossed over them. A regression analysis indicates that the contribution to Soudelor’s intensity variation from <i>V<sub>s</sub></i> is the largest (62%), followed by <i>Z</i><sub>26</sub> (27%) and others. Further analyses using composites then indicate that the weak vertical wind shear produced by the aforementioned anomalous anticyclone is a robust feature in the western North Pacific during the developing summer of strong El Ninos with Oceanic Nino Index (ONI) > 1.5.
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