Exploratory analysis of extremely low tropical cyclone activity during the late‐season of 2010 and 1998 over the western North Pacific and the South China Sea

Abstract

This study attempts to understand why the tropical cyclone (TC) frequency over the western North Pacific and the South China Sea was so low in 2010 and 1998 even though a strong La Niña signal occurred in both years. We found that the TC frequency during the late-season (October to December), not in the peak season (July to September), makes 2010 a record low year; the next lowest year is 1998. Specifically, four TCs were observed over the South China Sea (SCS) in the late-season of 1998, but no TCs occurred over the SCS in the same season during 2010. The genesis potential index is used to help diagnose changes in environmental conditions for TC genesis frequency. Results indicate that the decreased low-level vorticity makes the largest contribution to the decreased TC formation over the SCS. The second largest contribution comes from the enhanced vertical wind shear, with relatively small contributions from the negative anomaly in potential intensity and reduction in midlevel relative humidity. These observational results are consistent with numerical simulations using a state of the art model from the Meteorological Research Institute (MRI-AGCM 3.2 Model). Numerical experiments show that the unfavorable conditions for sharply decreased TC formation during the late-season over the SCS in 2010 mainly results from the sea surface temperature anomaly over the western North Pacific basin. This effect is partly offset by the sea surface temperature anomaly in the South Indian Ocean and Northern Indian Ocean basins.

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

“…For instance, the 1-min average wind speed, 10-min average wind speed, and 2-min average wind speed are adopted by JTWC, JMA, and CMA, respectively. Hence some aspects of TC activity are found to be not consistent and even contradictory (e.g., Song et al 2010;Kamahori et al 2006;Wu and Zhao 2012;Landsea and Franklin 2013;Zhao et al 2014a).…”

“…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.…”

Climatological analysis of passage-type tropical cyclones from the Western North Pacific into the South China Sea

“…During the November -December season weakening northeasterly flows act to weaken vertical wind shear giving rise to greater TC formation in the SCS (Lin and Lee 2011). TC formation in the SCS exhibits evident interannual and interdecadal variations (e.g., Wang et al 2012Wang et al , 2014Yan et al 2012;Zhao et al 2014).…”

“…Past studies related to TC activity in the SCS focused mainly on TC formation and/or total TC activity (e.g., Ho et al 2004;Wang et al 2007;Huang and Guan 2012;Zhao et al 2014). The specific characteristics of TC passage from the WNP into the SCS, so far, have not been comprehensively analyzed.…”

Changing relationship between La Niña and tropical cyclone landfalling activity in South China (La Niña and TC landfalling activity in South China)

“…Thus, the northward shift in genesis locations appears to be the major factor responsible for the below‐normal landfalling activity in 2007. The TC activity over the WNP is extremely inactive in 2010 (Zhao et al, ) with only 13 TCs between May and November, the lowest since 1960 (Figure (c)). With fewer TCs, the number of TC landfalls in most parts of East Asia (including South China) is lower (Figure (b)).…”