Research On Dryness And Wetness Changes Reconstruction And Extreme Drought And Flood Disaster Events In Northern Shanxi And Shaanxi Based On Tree-ring Width

Due to the impact of global warming,extreme drought and flood events are frequent and human production and life are increasingly affected by climate extremes.Drought and flood are common phenomena in the world climate system.In recent years,large-scale severe disasters have been observed all over the world,and the high social and economic damage caused by them has led the world to pay attention to extreme drought and flood events.The northern part of Shanxi and Shaanxi Province,located in the middle transition zone between the semi-humid and semi-arid regions,is one of the most water-scarce regions in China and is more sensitive to climate change.Therefore,in order to better understand regional climate change under global climate change and to assess the intensity and frequency of extreme climate events,and thus to more rationally address the impacts of climate extremes and make predictions,it is particularly important to obtain past long-term climate history changes in the region.In this paper,we reconstructed the Palmer Drought Severity Index(PDSI)series from January to August of the year for 384 years in the study area by using tree annual rings as proxy information,combined with instrumental meteorological data,using a linear regression model and a random forest model;analyzed the response of tree annual rings to climate change in the area,clarified the limiting factors of tree radial growth,explored the interannual variability characteristics and intrinsic driving factors of the two reconstructed series We also explored the interannual variability and intrinsic drivers of the two reconstructed series,and verified the occurrence of historical extreme drought and flood events in the study area using historical literature.The main conclusions are as follows:(1)The tree ring width index was significantly correlated with temperature from October of the previous year to October of the current year,and temperature was significantly correlated with tree-wheel width index in December of the previous year(r=0.30,P<0.05)and June of the current year(r=-0.33,P<0.05).Precipitation in February(r=0.38,P<0.05)and March(r=0.31,P<0.05)of the current year was significantly and positively correlated with the tree-wheel width index.the PDSI index was significantly and positively correlated from October of the previous year to October of the current year(P<0.05).The highest correlation between PDSI index and width index was found in the seasonal window combination from January to August of the current year(r=0.68,P<0.01).(2)The Palmer Drought Index(PDSI)series was reconstructed using tree ring width information for the period 1635-2018 in northern Jin-Shaanxi,extending the historical period of drought data in the region.the R~2 of variance explained by the regression equation of the PDSI index series was 43.45%.(3)The PDSI index series experienced six wet periods and six dry periods during 384 years,of which 1648-1675,1700-1713,1733-1811,1871-1894,1939-1948 and 1953-1976 were wet periods;1638-1647,1676-1699,1714-1732,1831-1870,1895-1938,and 1977-2010 were dry periods.Extreme wet years occurred in68 years,accounting for 17.70%of the reconstructed series,and extreme dry years occurred in 86 years,accounting for 22.39%of the reconstructed series.The multi-window spectral analysis revealed the existence of 144-,25-,9-,and 2-6-year quasi-periodic variability characteristics of the PDSI index series.(4)By comparing the reconstructed sequences with historical documentary data,it was found that both extreme drought years and extreme wet years have corresponding historical events recorded in the areas near the sampling sites,and the PDSI index reconstructed sequences also recorded widespread drought events that occurred in northern China from the 1920s to the 1930s.The long-term extreme drought in the late Ming Dynasty posed a great threat to the output of food crops.Civilians were unable to maintain their normal food needs.Famine,epidemic disease and drought led to the rise of peasant uprisings.These events showed the connection between the reconstruction and the process of civilization.(5)The PDSI index series were found to be relatively consistent in terms of trends from the 1880s to the 1990s and from the 1920s to the 1930s by comparison with other reconstructed series.Spatial correlation analysis with sea surface temperature and water vapor flux analysis found that the study area was mainly influenced by large-scale scale sea-air interactions,showing significant correlation with the Pacific Decadal Oscillation(PDO).