The relationship between warming of the Indian Ocean basin and tropical cyclones (TCs) in the Western North Pacific (WNP) was investigated and compared with that between El Niño-Southern Oscillation (ENSO) and TCs in the WNP. Abnormal warming of the Indian Ocean basin is correlated well with the total TC frequency (especially for weak TCs) in the WNP. Warming of the Indian Ocean basin occurs during the abnormal anticyclone in the WNP, which suppresses convection and TC generation, and vice versa. ENSO affects mainly the frequency of intense TCs in the WNP. In an El Niño development year, the monsoon trough deepens and stretches eastward, causing TCs to be borne eastward. Because TCs have longer lifetimes over the ocean, the frequency of intense TCs increases. In a La Niña development year, the monsoon trough is shallow, causing TCs to be borne westward, decreasing the frequency of intense TCs. The relationship between ENSO and the frequency of intense TCs also exhibits a decadal variation. The correlation coefficient of the ENSO index and TC frequency was 0.532 and 0.687 in 1950-1969 and 1990-2009, respectively, whereas it was only 0.081 in 1970-1989.