Drought Variability In The Asian Summer Monsoon Margin Of China As Inferred From Tree Rings

Temperature variations during 2011-2020 CE caused by anthropogenic forcing have yet exceeded the natural variability of the past 6500 years,and it may lead to more frequent and intense drought extremes around the world.However,the status of current drought variations remains unclear,it is,therefore,necessary to use climate proxies to infer past drought variability.Due to their annual resolution and accurate dating,tree rings have played an important role in the understanding of drought variability over the past millennia.The Asian summer monsoon margin of China is a region with a special geographical position,which is seen as ecological fragility and climate sensitivity.Tree growth is particularly sensitive to moisture variations in the region,so,we carefully studied drought variations based on three tree ring sites at a latitude gradient.The major conclusions are as follows:(1)The response of tree growth was analyzed by three tree ring sites of Chinese pine(Pinus tabuliformis Carr.)at a latitude gradient in the Asian summer monsoon margin of China.The results of correlation analysis indicated that tree growth correlated well with self-calibrated Palmer Drought Severity Index(sc PDSI);the results of the Vaganov-Shashkin(VS)model indicated that temperature significantly affects starting and end dates of tree-ring formation,and soil moisture plays an important role in the radial growth of trees during the main growing season.Considering the results of correlation analysis and VS-model,moisture in the early growing season significantly limits stem growth for trees in the study area.At site M01,moving correlations of tree growth with temperature showed a decreasing trend,while the correlations with precipitation increased under global warming.And there was a divergence between tree growth and sc PDSI.Further analysis shows that the unprecedented temperature rise has changed the relationship between tree growth and climate.(2)The average April-June sc PDSI was reconstructed by a 500-year tree-ringwidth chronology of Chinese pine on the eastern edge of the Chinese Loess Plateau(ECLP).The reconstruction indicated the driest and wettest years were 1759 CE and1649 CE,respectively,the two most prolonged drought intervals occurred in 1892-1943 CE and 1737-1775 CE,while the two most prolonged pluvial intervals occurred in1950-1992 CE and 1645-1682 CE over the past 500 years.The reconstruction captured the recent drying since the 1960 s,and we found that the duration and intensity of recent drought events did not exceed the natural change range of the climate system over the last half-millennium.The spatial correlation with sea surface temperature(SST)indicated that the drought variations on the ECLP might be modulated by the North Atlantic SST tripolar pattern.(3)The temporal and spatial characteristics of drought variations in the study area are analyzed by using three regional chronologies in the Asian summer monsoon margin of China.There has been a drying trend in both three regions since the 1950 s as the results of the mean and moving variance for the chronologies showed.But the drought series showed a wetting trend at site JFN,while there is a continued drying at site JFS and the drought series at site JFN has been in the mean state recently.Besides,the fluctuation of drought series in three regions has decreased since the middle of the 20 th century.The analysis of synchronous dry and wet events in the study area showed,synchronous droughts generally last for a long time and are severe,while wet events are characterized by short time and high frequency.In terms of differences,the dry and wet events at site JFN are characterized by long duration and high intensity,while the events are characterized by short duration and low intensity at site JFS.The composite analysis between the SST and the extreme synchronous drought(wet)events showed that the El Ni?o(La Ni?a)events in the previous winter can make the surface temperature abnormal from winter to spring,which further drives more serious synchronous drought(wet) events.