Observed Interannual Relationship between ITCZ Position and Tropical Cyclone Frequency

Abstract

There are no well accepted mechanisms that can explain the annual frequency of tropical cyclones (TCs) both globally and in individual ocean basins. Recent studies using idealized models showed that the climatological frequency of TC genesis (TCG) is proportional to the Coriolis parameter associated with the intertropical convergence zone (ITCZ) position. In this study, we investigate the effect of the ITCZ position on TCG on the interannual time scale using observations over 1979-2020. Our results show that the TCG frequency is significantly correlated with the ITCZ position in the North Atlantic (NA) and Western North Pacific (WNP), with more TCG events in years when the ITCZ is further poleward. The ITCZ-TCG relationship in NA is dominated by TCG events in the tropics (0-20°N), while the relationship in WNP is due to TCs formed in the east sector (140-180°E). We further confirmed that the ENSO has little effect on the ITCZ-TCG relationship despite it can affects the ITCZ position and TCG frequency separately. In NA and WNP, a poleward shift of ITCZ is significantly associated with large-scale environment changes favoring TCG in the Main Development Region (MDR), However, the basin-wide TCG frequency has a weak relationship with the ITCZ in other ocean basins. We showed that a poleward ITCZ in the Eastern North Pacific and South Pacific favors TCG on the poleward flank of the MDR, whilst it suppresses TCG on the equatorward flank, leading to insignificant change in the basin-wide TCG frequency. In the South Indian Ocean, the ITCZ position has weak effect on TCG frequency due to the mixed influences of environmental conditions.

The Relationship between the Typhoons Affecting South China and the Pacific Decadal Oscillation

“…These conditions are not conducive to the intensification of SCTCs, and they lead to a smaller proportion of SCTYs (Figure 6b). Slightly north of the tropical convergence zone, typhoons in the northwest Pacific occur significantly more frequently than normal [18], and their intensity is closely related to the vertical wind shear [37]. There is a significant positive correlation between the frequency of typhoons and the injection intensity at the approach to the tropical easterly jet in the western Pacific [38].…”

“…The PDO's cold (warm) phase is characterized by the presence (absence) of a steeper thermocline in the northwest Pacific, which hinders the eastward propagation of warm water and leads to an increase (decrease) in the number of rapidly intensifying typhoons. For example, at the beginning of the PDO's cold phase in 2006, the intensity of the typhoons that made landfall or severely affected Guangdong increased significantly [18][19][20]. Most of the above studies focused on the impact of the PDO on the tropical cyclones in the northwest Pacific; however, South China possesses a unique terrain and landforms, and the tropical cyclones affecting South China also possess unique characteristics, though we still lack a comprehensive understanding of the changes in the proportion of typhoons (including typhoons, severe typhoons, and super typhoons) affecting South China in terms of their total number and their connections to ocean-atmosphere anomalies in different PDO phases.…”

Long-range prediction of the tropical cyclone frequency landfalling in China using thermocline temperature anomalies at different longitudes

“…These statistical methods consistently utilize lagged relationships between the observed TC activity and slowly evolving climate modes to provide the seasonal forecasts of TC activity (Wu et al, 2005). Climate modes that influence the TC activity, include the El Niño-Southern Oscillation (ENSO) (Chan et al, 1998;, western Pacific subtropical high (WPSH, , quasi-biennial oscillation , North Pacific Oscillation (Wang et al, 2007;Chen et al, 2015;Zhang et al, 2018), Antarctic Oscillation (Ho et al, 2005;Wang et al, 2007), Hadley circulation (Zhou and Cui, 2008;, and the intertropical convergence zone (ITCZ) (Liao et al, 2023).…”