Tropical cyclone (TC) is one of the most intense weather systems in the world. A TC is called a typhoon when its wind speed exceeds 34 knots in the Western North Pacific (WNP). The WNP basin (0º-30ºN, 100ºE-180º) exhibits a high level of TC activity and is the most active region globally. About 26 TCs enter the WNP basin each year, leading to huge loss of life and property (Zhang et al., 2009, Wang et al., 2021). Many studies point out that an increasing pattern in TC-induced damages over the past several decades (Knutson et al., 2010; Mendelsohn et al., 2012; Peduzzi et al., 2012). A better understanding of how the behavior of TCs may change (such as their frequency) has scientific and socio-economic values. Under the background of global warming, whether the number of TCs is increasing has attracted much attention.
Great efforts have been put in improving our understanding of changes in TC activity (Zhao and Wang, 2019; Liu et al., 2019, Yamaguchi et al., 2020). Numerous studies have shown the favorable large-scale conditions for TC formation (Bister and Emanuel, 1997; Ritchie and Holland, 1997; Molinari et al., 2000; Nolan et al., 2007; Vu et al., 2020; Walsh et al., 2020). Several general conducive conditions are: (a) warm sea surface temperature (SST); (b) weak vertical wind shear; (c) high relative humidity; and (d) large vorticity. Understanding the environmental conditions that cause changes in TC genesis is very important for practical applications (such as TC forecast).
TCs show significant variability at multiple time scales, due to various climate forcing (Wang and Chan 2002; Vecchi and Soden 2007; Kim et al. 2008; Zhan et al. 2012; Li and Zhou 2013; Lin and Chan 2015; Wang et al. 2015; Mei and Xie 2016). The Madden–Julian Oscillation (MJO) with 30–90 day period can regulate WNP TC activity (Li and Zhou, 2013; Liu et al., 2021; Wang et al., 2018; Zhao and Wang, 2019). In the WNP basin, the active phase of the MJO enhances the local convection, which in turn leads to the increase in TC activity in the basin. Quasi-biweekly oscillation (QBWO) is another intra-seasonal mode, but with a more localized effect, that also affects WNP TC activity (Li and Zhou 2013). The El Niño-Southern Oscillation (ENSO) is known as the main phenomenon occurring at inter-annual time scales (Trenberth and Caron 2000; Timmermann et al. 2018). ENSO has significant impacts on WNP TCs (Kim et al., 2011). Zhao and Wang (2016, 2019) examine a stronger inter-annual relationship between ENSO and TCs in the WNP during the boreal summer from 1998 to 2015. The correlation is 0.60 at 95% confidence level. At decadal time scales, many factors affecting the TC frequency have already been suggested. The researchers mainly focus on the following factors: (a) the strengthened western North Pacific subtropical high (Liu and Chan, 2020); (b) the anomalous westward WNP SST gradient (Choi et al., 2015); (c) phase shifts of the Pacific Decadal Oscillation (PDO) (Zhao and Wang, 2016); (d) the Inter-decadal Pacific Oscillation (He et al., 2015; Zhao et al., 2018), and also (e) the Atlantic Multi-decadal Oscillation (AMO) (Zhang et al., 2020; Zhang et al., 2017). The potential mechanisms of WNP TC decadal changes remain to be studied.
Most studies suggest that the frequency of WNP TCs decreased from 1979 to 2010 (Tu et al., 2009; Yokoi and Takayabu, 2013; Hsu et al., 2014). Few studies have examined the change of TC activity from 2010 to 2020. Most of the previous analyses on TCs focus on the first or the second half of the year, or a certain period of the year, and the number of TCs throughout the year is less covered in previous studies. With global warming and the emergence of various extreme weather phenomena, how did the TC frequency per year change during 2010–2020 compared to the pre-2010 decades? And what caused those changes? To our knowledge, these issues have not been explained in previous studies. Here we investigate the possible reasons associated with the changes in the WNP TC frequency during 2010–2020 based on observations and reanalysis data.
The aim of this paper is to explore the change of the WNP TC frequency, and the corresponding large-scale atmospheric changes and oceanic SST changes. The rest of this paper is organized as follows. Section 2 describes the data used in this study. The TC increase is illustrated in Sect. 3. In Sect. 4, we explore the possible influences of large-scale atmospheric environment and ocean conditions associated with TC increase. The main interrelated discussion and conclusions are presented in Sect. 5.