Interannual Variability of Tropical Cyclone Scale over the western North Pacific and Related Environmental Control Mechanism

Sizes of Tropical Cyclones (TCs), which determine the extents of severe wind and precipitation, are important for both wind and rainfall forecast and mitigation of disasters. The climatological characteristics and inter-annual variability of western North Pacific (WNP) TC sizes are investigated mainly using the radius analyses for severe wind speeds of TCs in the RSMC Tokyo best track dataset with auxiliary data from the JTWC. The results indicate that the TCs with unusual sizes possess significant 2 to 4 years periods and interdecadal differences as their sizes are represented by radii of wind speed of 50 kts. Anomalous large or small size TCs tend to occur in different regions respectively. The relationships between the annual frequencies of TCs with unusual size and the environmental conditions were analyzed. It suggests that the activities of unusual TCs within two regions are closely related to ENSO events developing from the central Tropical Pacific, while those within other two regions are not. In phases of El Nino, more TCs with unusual large sizes occur in the southwest part of the WNP as well as those with unusual small sizes appear frequently in the east part of the tropical WNP; vice versa in phases of La Nina. However, the interannual variation of unusual small size TCs occurring in the South China Sea is modulated by the local air-sea-land coupled mode, that is, TBO, while that of unusual small size TCs occurring to the north of 30°N seem to have less linkage with the large scale environment, which suggests potential roles of the high frequency signals such as inner dynamics and weather scale variation. Key Words: Tropical Cyclone Size, interannual variability, ENSO, TBO