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.