| The gale winds and heavy rains accompanying tropical cyclones（TCs）can cause enormous damage to the lives and property of island and coastal residents.The severity of the damage caused by TCs is largely determined by their moving speed,with slower-moving TCs causing more severe damage due to their longer duration.Previous research has mostly focused on changes in TC translation speed on a global or basin scale,whereas TC disasters generally occur after landfall.The western North Pacific is the most active basin for TC activity in the world,but the changes in TC translation speed after landfall in this region are not yet clear.In this study,the TC best track data from the Shanghai Typhoon Institute of the China Meteorological Administration was used to explore the characteristics of changes in the translation speed of TCs that made landfall in East Asia from 1979 to 2019,with a focus on the mechanism for changes in the translation speed of TCs that made landfall in China during the boreal summer.The main conclusions of the study are as follows:It was found that there is a decreasing trend in the translation speed of TCs that make landfall in China during July and August from 1979 to 2019,while the translation speed of TCs that make landfall in Guangdong and Hainan in September increases.Overall,the translation speed of TCs that make landfall during June to August in China decreases significantly,and from July to October,except for TCs that make landfall in Guangdong and Hainan,where the translation speed slightly increases.The changes in the translation speed of TCs that make landfall in Japan,the Korean Peninsula,and the Indo-China Peninsula show distinct regional characteristics,with a decrease in the translation speed of TCs that make landfall in the southwestern part of Japan,the southern and northern parts of the Indo-China Peninsula and the Korean Peninsula,and an increase in the translation speed of TCs that make landfall in the northeastern Japan and the central and southern regions of the Indo-China Peninsula.In addition,there is a significant upward trend in the translation speed of TCs that make landfall in the Philippines during August to October.Using observational data and numerical experiments,the mechanism of the decrease in translation speed of TCs that make landfall in China in the boreal summer was further studied.It was found that the translation speed of TCs that made landfall in China has decreased by1.2 m s-1over the past 40 years,mainly due to the decadal deceleration of TC translation speed in the late 1990s,which is a result of the Pacific Decadal Oscillation（PDO）phase transition and Indian Ocean warming.The average translation speed of TCs decreased from6.1 m s-1per year from 1979 to 1997 to 5.3 m s-1per year from 1998 to 2019,with a decrease of about 13%.Further research found that the deceleration of TCs is mainly due to changes in the steering flow.Numerical experiments show that the deceleration of the steering flow is a result of the combined effects of the PDO phase transition and Indian Ocean warming.On the one hand,the transition of the PDO to a cold phase in the late 1990s led to the strengthening of Walker circulation and convective heating in the marine continent,generating an anomalous cyclonic circulation in low level.The weakened northeast steering flow to the northwest of the cyclonic circulation decelerated the steering flow in southern China,leading to the decrease in TC translation speed.On the other hand,SST warming in the Indian Ocean induces an anticyclonic circulation,which accelerates the steering flow in southern China and partially counteracts the effect of PDO,leading to the decrease of TC translation speed in China over the past 41 years.This result reveals the synergistic impacts of external and natural variabilities in climate system on the changes in TC translation speed and has important implications for understanding the future changes in TC moving speed. |
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