An Abrupt Decrease in the Late-Season Typhoon Activity over the Western North Pacific*

Abstract

In 1995 an abrupt shift in the late-season (October-December) typhoon activity over the western North Pacific (WNP) is detected by a Bayesian changepoint analysis. Interestingly, a similar change also occurs in the late-season sea surface temperature series over the western Pacific, eastern North Pacific, and portions of the Indian Ocean. All of the counts, lifespans, and accumulated cyclone energy of the late-season typhoons during the 1995-2011 epoch decreased significantly, compared with typhoons that occurred during the 1979-94 epoch. The negative vorticity anomaly is found to be the leading contributor to the genesis potential index (GPI) decrease over the southeastern sector of the WNP during 1995-2011. To elucidate the origin of the epochal change in the dynamic environmental conditions, a suite of sensitivity experiments is conducted based on the latest version of the Japan Meteorological Research Institute atmospheric general circulation model (MRI AGCM). The ensemble simulations suggest that the recent change to a La Niña-like state induces an unfavorable dynamic condition for typhoon genesis over the southeastern WNP. Warming in the Indian Ocean, however, contributes insignificantly to the circulation anomaly related to typhoon genesis over the southeastern WNP. The frequency of typhoon occurrence reveals a basinwide decrease over the WNP in the recent epoch, except for a small increase near Taiwan. An empirical statistical analysis shows that the basinwide decrease in the frequency of the typhoon occurrence is primarily attributed to a decrease in typhoon genesis, while the change in track is of less importance.

Perspective on the northwestward shift of autumn tropical cyclogenesis locations over the western North Pacific from shifting ENSO

“…1 of Hsu et al 2014 or He et al 2015). However, the percentage of TC intensification into typhoon during the late TC season (i.e., autumn) is higher than that in the peak TC season (i.e., summer) (Hsu et al 2014), especially in September (He et al 2017). For example, many super-typhoons such as Tip (October 1979), Maemi (September 2003), Megi (October 2010), Haiyan (early-November 2013), and Bopha (late-November 2012) were all generated during the late TC season.…”

“…Because the warming due to greenhouse gases will not only lead to more frequent extreme ENSO (Cai et al 2014(Cai et al , 2015a and ENSO diversity (e.g., Hu et al 2016a, b;Chen et al 2015), but also fewer-but-stronger TCs (e.g., Knutson et al 2010) since the negative contributions from the reduced TC frequency and its genesis location-related duration tend to offset the positive contribution from the increasing TC intensity (Lin and Chan 2015). In other words, changes in TCLs and ENSO in the context of global warming may primarily contribute to the change in the frequency of intense typhoons (e.g., Wu and Wang 2008;Wang et al 2013, Hsu et al 2014. That is, TCL shift often affects TC intensity via changing the durations over the ocean (i.e., TC has more or less time and distance over the WNP warm pool for developing) (e.g., Wang and Chan 2002;Camargo and Sobel 2005;Wu and Wang 2008;Zhao et al 2011Zhao et al , 2014bWang et al 2013;Zhao and Wu 2014).…”

“…Meanwhile, the western North Pacific (WNP) tropical cyclones (TCs) experience an active to less-active transition (see Fig. 1 of Hsu et al 2014 or He et al 2015). However, the percentage of TC intensification into typhoon during the late TC season (i.e., autumn) is higher than that in the peak TC season (i.e., summer) (Hsu et al 2014), especially in September (He et al 2017).…”

“…It also should be noted that the mean latitude of WNP tropical cyclogenesis locations (TCLs) was clearly further to the north in summer than in autumn (not shown), mainly due to the distinctly different seasonal climatic backgrounds (e.g., warm pool, monsoon flows, etc.). Thus, we emphasize that it is important to separately study the WNP TC activity during autumn (e.g., Hsu et al 2014;Zhao et al 2014a;Zhao and Wang 2016;Choi et al 2017) and that in summer.…”

Pacific island tropical cyclones are more frequent and globally relevant, yet less studied

“…Hsu et al (2014) discussed the recent changes in the number of Western Pacific catastrophic TCs that have occurred in the late season. Shifts in seasonal occurrence may be yet another mechanism through which future global change factors modify the destructive potential of TCs.…”