The reported decreasing trend of the annual tropical cyclone (TC) landfalls in Southern China and increasing trend in Southeastern China in recent decades are confirmed to be an abrupt shift occurred at the end of the 20th century, based on a statistical analysis. The opposite trends in the two adjacent regions are often considered as a result of tropical cyclone landfalls in southern China being deflected northward. However, it is demonstrated in this study that they are phenomenally independent. In fact, the abrupt decrease of TC landfalls in Southern China occurs due to an abrupt decrease of the westward events in the post-peak season (October-December), as the consequence of a significant decrease of the TC genesis frequency in the southeastern part of the western North Pacific (WNP) ocean basin. On the other hand, the abrupt increase of TC landfalls in Southeastern China occurs due to an abrupt increase of the northwest events in the peak season (July-September), as the consequence of a statistically westward shift of the TC genesis. The relevant variations of the TC genesis are shown to be mainly caused by the decreased relative vorticity and the increased vertical wind shear, which, however, are intrinsically related to the accelerated zonal atmospheric circulation driven by a La Niña-like sea surface warming pattern over WNP developed after the end of 20th century.

The known trends of poleward migration for the tropical cyclone (TC) genesis in both hemispheres are discussed from different perspectives. It is shown that the poleward migration rate of the annually averaged latitude of TC genesis in the Northern Hemisphere is significantly affected by the regional variations of TC number in recent decades, especially an increase in the North Atlantic Ocean and a decrease in the western North Pacific Ocean. The poleward migration rates of TC genesis in the two hemispheres get closer when the effect of the regional TC number variation is excluded. The poleward migration of TC genesis without the effect of regional TC number variation is found to have a good correlation with the poleward shift of the edges of the tropics in both hemispheres. A decreasing trend of the cyclonic vorticity in the lower-troposphere over the tropical ocean regions is also identified in both hemispheres, which leads to a poleward shift of the equatorward boundary for TC genesis. The poleward migration of TC genesis after the effect of regional TC number variation is excluded and can thus be considered as a result of the tropical expansion. It is shown that the genesis of TCs with a different intensity has a different migration rate. When excluding the effect of the regional TC number variation, the poleward migration of TCs with a different intensity has a similar trend in both hemispheres. The tropical storms and intense typhoons have significant poleward migration trends, while the weak typhoons behave differently.

Variability of tropical cyclone (TC) genesis frequency in the western North Pacific and South Pacific ocean basins in the interdecadal scale is studied. It is demonstrated that the TC genesis frequency in these ocean basins experienced an abrupt decrease near the end of the 20th century. The decreased occurrence of TC genesis in the two recent decades is mainly located in the low-latitude regions at the eastern side of the two ocean basins. It is also shown that a significant part of the decreased TC genesis occurred during October to December, i.e. the post-peak season in the western North Pacific ocean basin and the pre-peak season in the South Pacific ocean basin. The interdecadal trend of variation in the TC genesis frequency in these adjacent two ocean basins seems to be mainly due to a common mechanism, i.e. the variation in the atmospheric vorticity. In contrast to the decrease in the total TC genesis, the intense typhoon occurrence frequency experienced an interdecadal increase during the same period. This trend for intense typhoons is particularly clear in the western North Pacific ocean basin. The zonal distribution of the increased number in the intense typhoon occurrence shows a similar pattern to the increased value of the sea surface temperature. It is then suggested that the variation in the intense typhoon occurrence frequency in the western North Pacific ocean basin is related to a change in the La Niña-like sea surface warming pattern.

The establishment of a tropical cyclone (TC) trajectory model that can represent the basic physics and is practically advantageous considering both accuracy and computational cost is essential to the climatological studies of various global TC activities. In this study, a simple deterministic model is proposed based on a newly developed semiempirical formula for the beta drift under known conditions of the environmental steering flow. To verify the proposed model, all historical TC tracks in the western North Pacific and the North Atlantic Ocean basins during the period 1979–2018 are simulated and statistically compared with the relevant results derived from observed data. The proposed model is shown to well capture the spatial distribution patterns of the TC occurrence frequency in the two ocean basins. Prevailing TC tracks as well as the latitudinal distribution of the landfall TC number in the western North Pacific Ocean basin are also shown to agree better with the results derived from observed data, as compared to the existing models that took different strategies to include the effect of the beta drift. It is then concluded that the present model is advantageous in terms of not only the accuracy but also the capacity to accommodate the varying climate. It is thus believed that the proposed TC trajectory model has the potential to be used for assessing possible impacts of climate change on tropical cyclone activities.

Long-term variations in the translation speed of tropical cyclones (TCs) in the South China Sea (SCS) are examined based on five TC datasets from different institutions. TC translation speed during the TC peak season in the SCS shows an evident rhythm of interdecadal change throughout 1977–2020. This interdecadal change in TC translation speed in the SCS can be well reproduced by a newly developed trajectory model. The model results indicate that the interdecadal change in TC translation speed is primarily due to an interdecadal change in the steering flow in the SCS. Such an interdecadal change in the steering flow is closely related to an east-west shift of the subtropical high in the western North Pacific (WNP) ocean basin, which may be driven by the zonal sea surface temperature (SST) gradient between the North Indian (NI) and WNP ocean basins. A new index of the zonal SST gradient is proposed, which is shown to be effective for indicating the interdecadal change in east-west shift of subtropical high, and thus, the TC translation speed in the SCS.