Interdecadal variation of frequencies of tropical cyclones, intense typhoons and their ratio over the western North Pacific

This study explores whether a higher frequency of tropical cyclone (TC) occurrence would lead to more intense typhoons (ITYs) over the western North Pacific (WNP) during 1975–2016. The proportion of ITYs shows an upward trend, with a low- (1975–2001) and a high-proportion period (2002–2016). There is an interdecadal variation in TC and ITY frequencies within the period 1975–2001, with three subperiods (1975–1986, 1987–1997 and 1998–2001) identified. Both the annual frequencies of TCs and ITYs are below normal during 1975–1986 and 1998–2001 but above normal during 1987–1997. A comparison between the periods 1975–1986 and 2002–2016 (similar TC frequency but different ITY frequency) shows a significant increase in ITY occurrence to the north and west of the major development region during 2002–2016, which is mainly due to the longer duration of intensification or higher intensification rate so that the TCs can develop into ITYs before making landfall or moving to higher latitudes. Comparing the periods 1987–1997 and 2002–2016 (similar ITY frequency but different TC frequency) shows that the decrease in TC frequency during 2002–2016 is mainly related to a reduction in TC genesis in the southeastern part of the WNP. The west-northwestward shift in genesis positions during 2002–2016 leads to an increasing frequency of ITYs towards Japan, the Korean Peninsula and the east coast of China. The stronger vertical wind shear and lower moist static energy (MSE) (weaker vertical wind shear and higher MSE) in the southeastern and northwestern parts of the WNP are related to the lower (higher) annual frequencies of both TCs and ITYs during 1975–1986 (1987–1997). The higher tropical cyclone heat potential in the recent decade indicates an expansion of the area favourable for intensification, which is another factor responsible for the higher proportion of ITYs during 2002–2016.