Since typhoons cause severe damage in Southeast Asia including Japan, it is important to clarify the relationship between typhoons and atmospheric-oceanic phenomena that change convective activity in the tropical western Pacific, where typhoons are generated. The El Niño/La Niña and the Indian Ocean Dipole Mode (IOD) events have been pointed ut to affect the location, number, path, and lifetime of typhoons. However, there are few studies on the intensity of typhoons when either the El Niño/La Niña or the IOD occurs, or both occur simultaneously. Therefore, the purpose of this study is to clarify how the environments of the areas through which the typhoon passes change with the El Niño/La Niña and the IOD, and how the typhoon intensity changes accordingly.

During the El Niño (La Niña) events, the western North Pacific is a less (more) favorable environment for typhoon development than neutral years, and strong typhoons tend to develop after passing through such an environment for a long (short) period of time. In addition, typhoons that generated in the southeast of the western North Pacific during the El Niño event tend to develop in a short period of time by passing through the more favorable environment such as higher sea surface temperatures. The ratio of strong typhoons increased.

During the positive IOD events, the western North Pacific is more favorable for the development of typhoons in the southeast and less favorable for the development of typhoons in the southwest than in neutral years. Therefore, some of the strong typhoons take longer to develop while others do not. On the other hand, during the negative IOD events, the western North Pacific is more favorable to the development of typhoons than in neutral years. Therefore, strong typhoons are limited to those that developed over a long period of time, and it is considered that the negative IOD events have a significant effect of slowing down the development of typhoons. There is no difference in the ratio of the number of typhoons by category between the positive and negative IOD events. During the simultaneous occurrence of the El Niño and positive IOD events (La Niña and negative IOD events), the western North Pacific is a less (more) favorable environment for typhoon development than neutral years, and strong typhoons tend to develop after passing through such an environment for a long (short) period of time. In addition, typhoons that generated in the southeast of the western North Pacific during the simultaneous occurrence of the El Niño and positive IOD events tend to develop in a short period of time by passing through the more favorable environment such as higher sea surface temperatures. It is also considered that some of the typhoons developed over a longer period of time due to the effect of the La Niña and negative IOD events at the same time, just as the effect of the negative IOD event is to slow down the development of typhoons. The ratio of moderate to strong typhoons was increased (decreased) during the simultaneous occurrence of El Niño and positive IOD events (La Niña and negative IOD events).