Investigating the abrupt change of tropical cyclone (TC) activity in the Western North Pacific by using different TC genesis indices

Abstract

The tropical cyclone (TC) genesis indices are used to investigate the abrupt change of TC genesis frequency (TCGF) in the Western North Pacific (WNP) from 1979 to 2014 and to discuss the factors leading to the abrupt change. This study shows that TCGF had an abrupt decrease in 1997, but the popular TC genesis indices used in recent studies do not reflect this feature. An appropriate TC genesis index must consider both thermal and dynamic effects to reflect the TCGF abrupt change in the WNP. We modify the dynamic and thermal effects of the existing indices and propose a new TC genesis index, MoχGPI, which is consistent with the observational abrupt change of TC activity regardless of spatial distribution or time‐series patterns. A budget analysis of MoχGPI is conducted to discuss the individual contribution of large‐scale environmental parameters to the TCGF abrupt change. The results show that the change in relative vorticity (including contributions not only from the environmental relative vorticity, but also a part from the relative vorticity by TCs themself) is the dominant effect leading to the TCGF abrupt change in 1997, followed by the vertical wind shear effect.

Changes of local tropical cyclone activity over the South China Sea under global warming in high‐resolution atmospheric model projections

“…The “genesis potential index” (GPI) proposed by Emanuel and Nolan (2004) (hereafter, ENGPI) is a popular metric used to examine the environmental factors responsible for TC activity change associated with climate variability of various timescales, including MJO (e.g., Camargo et al ., 2009; Zhao et al ., 2019), ENSO (e.g., Camargo et al ., 2007; Tsuboi and Takemi, 2014) and global warming (e.g., Murakami et al ., 2011; Tsou et al ., 2016). However, some recent studies noted that the performance of the original ENGPI, as well as its several revised versions, in representing the long‐term TC variability is highly basin‐dependent, with a much poorer performance in the WNP compared to the North Atlantic (Yu et al ., 2018; Hsiao et al ., 2020). Therefore, a modification of ENGPI is required before it can be applied to the SCS.…”