Characteristics Of Tropical Cyclone Changes In Southeast China Recorded By Tree-rings

Tropical cyclones are one of the most devastating natural disasters.Accompanied by ocean storm surges,mountain torrents,and strong winds,tropical cyclones have caused widespread damage to the natural and social environment along the coast of China.A further understanding of the frequency and intensity of tropical cyclones will contribute to the government’s public safety planning.Chinese scholars have conducted a large amount of in-depth research on the characteristics and mechanisms of contemporary tropical cyclones.However,there are still disputes about how tropical cyclones respond to global warming and air-ocean factors,as well as their contributions to human activities and natural variability to tropical cyclones.This is largely due to the fact that contemporary tropical cyclone records only cover a segment of warming since the Little Ice Age,and it is urgent to carry out historical reconstruction of tropical cyclones based on natural proxies.Among a lot of natural proxies,tree-rings have become one of the most widely used indicators in high-resolution climate reconstruction in the past due to their advantages such as accurate dating,high temporal resolution,and extensive and dense spatial indicators.Tree-rings have also proven to be effective means of recording tropical cyclones,for foreign researchers have successfully reconstructed the history of tropical cyclones in the United States,South Korea,and other regions based on late-wood stable isotope and forest disturbance sequences from tree-rings.However,the research on tropical cyclone reconstruction based on tree-ring data has not yet been published in China.Unlike the phenomenon of rain and heat synchronization in general monsoon regions,Fujian Province and surrounding areas in China are common dry in summer,and tree-rings are sensitive to summer precipitation.The proportion of tropical cyclone precipitation during summer in this region is relatively high（50%-80%）.Therefore,the tropical cyclone signal in tree-rings is stronger.At the same time,there are a relatively wide range of middle-aged and elderly trees in this region,making it an ideal area for conducting tree-ring-tropical cyclone reconstruction.This study was based on the radial growth and stable oxygen isotopes of trees in 4 study areas in southeastern China,combined with tree-ring growth monitoring and the higher resolution intra-annual tree-ring oxygen isotopes,to explore the tree growth response to tropical cyclones in the region,and to reconstruct the history of tropical cyclones in southeastern China over the past 223 years.The main contents and conclusions of this study are as follows:（1）This study was based on the high-resolution tree-rings intra-annualδ¹⁸O(10δ¹⁸Os per ring)of Cryptomeria japonica var.sinensis Miquel（a typical subtropical tree species in China）and tropical cyclone precipitation from 1965 to 2017.The potential of trees to record tropical cyclones in Fuzhou was evaluated.The results show that the correlation between tree-ringδ¹⁸O of the 7th and 8th segments of the 10segments of Cryptomeria japonica var.Sinensis Miquel for a year and tropical cyclone precipitation is generally negative,and these two segments are effective for recording tropical cyclones in the study area.After performing high pass filtering between tree-ringδ¹⁸O and the tropical cyclone precipitation in the segment most sensitive to tropical cyclone precipitation,the negative correlation between the two time series has been further improved（r=-0.55,p<0.001）.This indicates that in the southeast region of China,the strong tropical cyclone precipitation signals are recorded in the tree-ringδ¹⁸O of the second segment after the dividing line between early-and late-wood.（2）The early-and late-woodδ¹⁸O in tree-rings is widely used to reflect precipitation signals from different growing seasons of trees.This study used ARMA model to establish the early-and late-woodδ¹⁸O residual（residual=observation value-prediction value）chronologies of Pinus massoniana in Longyan region from1804 to 2015,combined with tropical cyclone records in historical data records,to develop tree-ring late-woodδ¹⁸O-tropical cyclone research.The results show that the≤0 late-woodδ¹⁸O residual value corresponds to a strong tropical cyclone precipitation event（tropical cyclone precipitation>300 mm）,indicating an unusually light oxygen isotope composition.This indicates that tree-ring late-woodδ¹⁸O residual value can effectively record tropical cyclone precipitation signals of study area.（3）We explored the potential that the growth release recorded by three tree species in the Gu Mountain plot in Fuzhou reconstruct tropical cyclone every five years.The results show that the sensitivity to tropical cyclone interference events varies among tree species.During 1920 to 2013,there is a significant positive correlation between the moderate growth release recorded by the Keteleeria fortunei and the tropical cyclone wind speed every five years（p<0.01）,which explains 49.9%of the variance of the regional tropical cyclone wind speed from 1954 to 2013.The reconstruction results have a relatively high synchronous variability with the disaster events caused by tropical cyclone precipitation and strong winds recorded in historical data,indicating that the reconstruction is relatively reliable.With the latitude poleward shift of the maximum intensity of tropical cyclones in the northwest Pacific,the wind speed of tropical cyclones affecting southeast China has also significantly decreased since 1990.In addition,the reconstructed tropical cyclone wind speed is modulated by the ENSO in the Pacific Ocean.A higher sea surface temperature in the Pacific can maintain high humidity conditions in the lower atmosphere,and make local convection active,which is conducive to the formation of tropical cyclones.（4）Using the growth release data recorded by Tsuga longibracteata in the Tianbaoyan plot in Yong’an area,the maximum wind speed variation history of the regional tropical cyclone during every five years from 1786 to 2009 is reconstructed.The reconstruction generates 11 abnormally high and 7 abnormally low periods of tropical cyclone maximum wind speed in the past 223 years.The comparison with the reconstruction work and historical data of other tropical cyclones in the vicinity further verifies the reliability of the reconstruction of this study.The reconstructed tropical cyclone maximum wind speed had a significant downward trend since 1990,which is related to the poleward shift of the maximum intensity for the tropical cyclone in the northwest Pacific Ocean.In addition,in the La Ni?a year,the intensity of tropical cyclones affecting the study area is relatively strong,while in the El Ni?o year,the opposite happens.