The variation in tropical cyclone genesis over the western North Pacific during the El Niño summers

In climatology, the tropical cyclones become active and shift northward over the western North Pacific (WNP) around pentad 41 (20–24 July), corresponding to the WNP summer monsoon onset, which is a phenomenon characterized by the abrupt northeastward extension of strong convection. This study investigates the change in tropical cyclone genesis during El Niño summers, which have been confirmed to coincide with a delayed onset of the summer monsoon. The results show that the spring-up of tropical cyclones is delayed for the El Niño summers. Specifically, there are negative tropical cyclone genesis frequency and genesis potential index anomalies over the region (15°–30°N, 110°–160°E) during pentads 41–44. Accordingly, the average number and genesis latitude of tropical cyclones decreases significantly. The diagnosis of the genesis potential index further reveals that the key factors affecting the tropical cyclone genesis lie in the mid-tropospheric relative humidity and vertical wind shear.