The Decadal Variability of Tropical Cyclone Activity over the Northwest Pacific and Its Connection with Ocean-atmosphere Interaction

西北太平洋颱風活動的年代際變化及其與海氣相互作用的關係

Student thesis: Doctoral Thesis

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Author(s)

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Detail(s)

Awarding Institution
Supervisors/Advisors
  • Lin ZHANG (Supervisor)
  • Wen ZHOU (External person) (External Co-Supervisor)
Award date16 Oct 2023

Abstract

The Northwest Pacific (NWP) is the most active basin for tropical cyclone (TC) development. Directed by the easterlies in the southern flank of the West Pacific subtropical high (WPSH), TCs bring a tremendous amount of rainfall to the coastal regions of China. This thesis aims to address decadal variations in TC activity over the NWP and TC induced rainfall over China, as well as their connections to modes of climate variability.

TC activity during July to September (JAS) exhibits the strongest decadal variability. There are two inactive periods (i.e., 1973–1988 and 2003–2020) for the number of TC genesis during 1960–2020. This aligns with two abrupt expansions of the WPSH and increases in sea surface temperatures (SSTs) in the eastern Indian Ocean (EIO). Specifically, anomalously warm SSTs in the EIO strengthen the downdrafts and the WPSH over the NWP via a Kelvin wave adjustment. The intensified WPSH causes decreased low-level vorticity and suppressed convective activity, which are unfavorable conditions for TC genesis.

The associated TC rainfall also exhibits similar decadal shifts during the 1970s, 1990s, and early 2000s. The dominant factors responsible for TC rainfall variability vary from region to region. The changes in TC rainfall over South China are mainly attributed to the TC genesis effect and the accompanied TC rainfall frequency effect. On the contrary, TC rainfall in East China is primarily modulated by the TC rainfall intensity effect and the TC track effect. It is worth noting that TC rainfall events have been increasing north of the Yangtze River since the 2000s, primarily driven by a northwestward shift in TC tracks.

Motivated by previous studies, this thesis also reveals that the preceding-autumn Indian Ocean dipole (IOD) and preceding-winter El Niño-Southern Oscillation (ENSO) can trigger different TC rainfall patterns in the following summer. The preceding ENSO modulates TC rainfall over the entire coastal region over China, while the preceding IOD tends to produce a dipole pattern of summer TC rainfall, with opposite impacts north of the Yangtze River and southeastern China. The contributions of the TC rainfall intensity effect and the TC track effect are the key factors responsible for the differences under various phase combinations of IOD and ENSO.