Meridional oscillation of tropical cyclone activity in the western North Pacific during the past 110 years

A prominent meridional oscillation of tropical cyclone (TC) activity in the western North Pacific (WNP) during the past 110 years is identified in this paper. Based on the best-track TC data, the first three Empirical Orthogonal Functions of TC occurrence frequency generally show a northeast-southwest dipole, with the major loadings over the South China Sea (SCS) and the area near Japan. During the periods 1951-1963 and 1997-2019, TC frequency is generally lower over the SCS but higher near Japan. The opposite is true during 1969-1988. Inclusion of the historical TC data from 1910 gives two periods (1943-1963 and 1997-2019) during which the TC numbers near Japan (NJP) are higher but those over the SCS (NSCS) are lower, and two periods (1926-1942 and 1969-1988) with just the opposite pattern. The main factors responsible for this meridional oscillation include changes in the genesis positions of the TCs and their subsequent tracks associated with the interdecadal variation of the large-scale environmental flow patterns. During the periods in which the NSCS (NJP) is higher (lower), the large-scale steering flow is favorable for TCs moving to the SCS. At lower latitudes, vertical wind shear is weaker and low-level cyclonic flow is stronger, which lead to a southward shift of the genesis positions. The patterns are just the reverse during the periods in which the meridional oscillation is in the opposite phase. The oscillation is also found to be partly related to the interdecadal Pacific oscillation through changes in its low-level flow pattern.