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

Abstract

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.

Tropical cyclones over the western north Pacific since the mid-nineteenth century

“…This indicates that recent TC landfall locations in Japan tend to shift northeastward in Japan. It may be influenced by atmospheric circulation conditions over the WNP during 1977-2019 (Liu and Chan 2019). However, the increasing tendency during 1977-2019 seems that it includes within a fluctuation in the 100-year time scale variability from our 143 years data analysis.…”

Is the tropical cyclone surge in Shanghai more sensitive to landfall location or intensity change?

“…Murakami et al (2020) showed a clear poleward shift of TC frequency along the east China coast for 1980-2018. Liu and Chan (2019) found that the mean landfall intensity is increasing at the east coast of China due to more frequent landfalls by intense typhoons.…”

Interdecadal variability of tropical cyclone genesis frequency in western North Pacific and South Pacific ocean basins

“…Klotzbach and Landsea (2015) found a flattening out of the increasing trend for the very intense TCs (V max ⩾ 113 kt) reported by Webster et al (2005), after extending the dataset of Webster et al (2005) by ten years, but the increasing tendency for both intense and very intense TCs is still unchanged. In a recent study by Liu and Chan (2019), the frequencies of TC genesis and of very intense TC occurrence were certainly shown to be not necessarily correlated.…”

“…However, this does not mean that we can draw the same conclusion with confidence in a particular ocean basin. For example, an increasing trend in the North Atlantic (NA) ocean basin in recent decades (Mann andEmanuel 2006, Bruyère et al 2012) and an abrupt decrease in the western North Pacific (WNP) ocean basin near the end of the 20th century (Liu and Chan 2013, 2019, He et al 2015, Hu et al 2018, Zhao et al 2018b have been reported. Why does the TC genesis frequency have a particular trend in these ocean basins?…”