Spatiotemporal Variation Of Spring Indices(SI) In China And Its Influence Mechanism In Recent Decades

In the context of global warming,the response of vegetation to climate change has become a primary issue for humans' living environment.Therefore,this study focuses on the changes in onset of spring based on the variations of spring vegetation under climate change in recent decades.In this study,the daily maximum and minimum temperature data of the Chinese meteorological sites are used to calculate the spring indices,including the first leaf index,the first bloom index,and the last freeze index.These spring indices represent the arrival date of early spring,the coming date of late spring,and the date of the last freeze event in spring,respectively.The average spring index is calculated by averaging the indexes of early spring and late spring to illustrate the coming of overall spring.The damage index,the differences between the leaf index and the last freeze index,is applies to indicate the degree of damage to the vegetation caused by the last freeze event.We apply trend analysis,correlation analysis,Empirical Orthogonal Function(EOF),and regression analysis to analyze the basic features and corresponding physical mechanism of the above indices.This study can make up the lack of spring indices research in China.The main conclusions are as follows:(1)The spatial distributions of the annual average first leaf index,first bloom index,spring average index,and the last freeze index indicate that the spring arrival date of China is significant latitudinal dependence,and the spring came later/earlier in high/low latitude.The onset of spring also become later with altitude.In addition,the changes of spring indices,except the damage index,all exhibits advance trend with high spatial consistency over the recent 60 years.The average advance trend is 1.5 to 4.5/10 years.The response of damage index is regional dependence.(2)There are two main EOF spatiotemporal distribution modes of spring index in China.The first mode represents a national unified response,and the main controlling factor is the Atlantic Multidecadal Oscillation(AMO)calculated by analyzing its PC1.AMO controlled the spring index by influencing the tropospheric temperature in the Eurasian continent,especially in East Asia.The spring index shows an advance in nationwide due to the phase change of AMO since the 1990 s.(3)The second EOF mode of spring index shows a north-south dipole pattern,and the associated large-scale mechanism is the North Atlantic Oscillation(NAO)calculated by analyzing its PC2,between which the relationship is positive.ANO mainly controls the atmospheric circulation in Eurasia to form two pressure centers in the Mongolian high-pressure area and the Southern China,which causes that phenological-climate response displays the opposite form in north-south China.At the positive phase of NAO,the Mongolian high-pressure will strength,and then affects the warm advection in northern China and the cold advection in southern China,which lead to the advance of spring index in the former and the delay in the latter.