Decrease of tropical cyclone genesis frequency in the western North Pacific since 1960s
Introduction
Tropical cyclones (TCs) are severe weather phenomena that can affect human life and economy in coastal regions. How the TC activity changes in the past decades and towards the future, in particular in the western North Pacific (WNP), is a hotly debated topic (Wang and Chan, 2002, Webster et al., 2006, Emanuel, 2005, Landsea et al., 2006, Li et al., 2010, Murakami et al., 2013). It is well known that tropical cyclone genesis depends on several environmental factors Gray, 1968, Gray, 1979, Emanuel and Nolan, 2004) such as SST, humidity in the lower and middle troposphere, conditional instability, vertical wind shear and vorticity. Particularly in the WNP, atmospheric circulation parameters play a more important role in interannual and interdecadal timescale due to high mean SST and moisture conditions compared to other basins (Chan, 2000, Matsuura et al., 2003, Ho et al., 2005, Fu et al., 2012).
It has been argued that despite of favorable environmental conditions in WNP, the timing of individual TC formation depends on triggering of synoptic scale disturbances (Li, 2012). Most of TCs in the WNP formed within three types of synoptic disturbances such as Rossby wave trains caused by TC energy dispersion (Holland, 1997, Li et al., 2003, Li and Fu, 2006), synoptic wave trains (Lau and Lau, 1990, Li, 2006) and easterly waves (Tam and Li, 2006, Fu et al., 2007). It was pointed out that the decrease of TC genesis frequency in the WNP under global warming was primarily caused by the decrease of synoptic scale variability in situ (Li et al., 2010), which was caused by the decrease of local background easterly shear and low-level convergence (Wang and Xie, 1996, Li, 2006, Sooraj et al., 2009).
The background mean state in the tropical Pacific experienced an interdecadal change. A positive phase of the Interdecadal Pacific Oscillation (IPO) is characterized by a warming in the equatorial Pacific and a cooling in mid-latitude Pacific (Mantua et al., 1997, Salinger et al., 2001). Such a large-scale background SST change can affect the environment conditions in the WNP (Trenberth and Hurrell, 1994, Trenberth, 2010, Miller et al., 1994, Hong et al., 2014) and associated TC activity (Camargo et al., 2008, Wood and Ritchie, 2013, Wang and Liu, 2015). It has been shown that the phase of the IPO has two obvious shifts since 1960 (Hartmann and Wendler, 2005, Lyon et al., 2013). One happened around 1976/77 and another around 1998/99. How did TC activity change in the recent years?
The objective of the current study is to investigate TC activity change in the recent years and the cause of it. Given a number of environmental parameters, we would like to reveal the essential environmental parameter responsible for the interdecadal change of TC genesis frequency in the WNP. The remaining part of the paper is organized as followings. In Section 2, the data and analysis method are first introduced. Section 3 is devoted to illustrate the essential environmental factors responsible for the decrease of TC frequency throughout the three periods. Physical mechanisms responsible for the change of the key environmental parameters are discussed in Section 4. Finally, a summary and discussion are given in Section 5.
Section snippets
Data and method
The datasets used in the study consist of: 1) TC best-track data from International Best Track Archive for Climate Stewardship (IBTrACS) (Knapp et al., 2010), 2) SST data from the Hadley Center Global Sea Ice and Sea Surface Temperature dataset (HadISST) (Rayner et al., 2003), and 3) monthly mean fields of atmospheric variables from National Centers for Environmental Prediction (NECP)-National Center for Atmosphere Research (NCAR) reanalysis (Kalnay et al., 1996). All the analyses are conducted
Environmental factors responsible for steady decrease of TC genesis frequency
Fig. 1 shows the time evolution of total number of WNP TCs in JJASO from 1960 to 2014. It is interesting to note that the TC genesis frequency decreases steadily from ID1 (1960–1976) to ID2 (1977–1998) and from ID2 to ID3 (1999–2014). The average TC numbers for the three periods are 33, 25 and 21 respectively. The decrease of the TC genesis number for the three periods is statistically significant, passing a 95% confidence level.
In order to investigate the cause of the step-by-step decrease, we
Cause of the interdecadal change of vertical wind shear
To understand the cause of the change of the background vertical wind shear, the differences of zonal wind and the vertical velocity averaged over 0–20°N between ID2 and ID1 and between ID3 to ID2 are shown (Fig. 6). The difference map shows pronounced anomalous upward motion in the WNP and downward motion in the central Pacific. From ID1 to ID2, rainfall is enhanced over 120E. Strong westerly wind anomalies appears in the upper troposphere in the WNP. All these features imply an enhanced
Conclusion and discussion
In this study, the interdecadal change of TC genesis number in the WNP since 1960 is investigated. According to the phase change of the IPO, the analysis period of 1960–2014 is divided into three sub-periods: 1960–1976 represented as ID1, 1977–1998 as ID2, and 1999–2014 as ID3. It was found that TC genesis frequency shows a significant step-by-step decrease from ID1 to ID3. The analysis of environmental parameters shows that most of the parameters such as SST, vertical velocity, divergence,
Acknowledgements
This work is jointly supported by China National Key R&D Program2017YFA0603802 and 2015CB453201, NSFC grants 41630423, 41475084, 41575043 and 41375095, NSF grant AGS-1565653, Jiangsu projectsBK20150062 and R2014SCT001, and the priority academic program development of Jiangsu Higher Education institutions (PAPD). This is SOEST contribution number 10285, IPRC contribution number 1299 and ESMC number 197.
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