Evolution And Prediction Of Nonlinear Trend Of Dry/Wet Conditions In China

The evolution of dry/wet conditions not only affects global and local hydrological cycles and ecological environment changes,but also serves as an important basis for indicating agricultural production and socio-economic development.As a vulnerable and sensitive area in response to global climate change,warming rate in China has been much higher than the global average in the past few decades.The changes in the spatial pattern of precipitation and increases in potential evapotranspiration caused by longterm warming directly affect the spatiotemporal variation of dry/wet conditions.At the same time,the vast and diverse landforms have resulting in significant differences in the characteristics of dry/wet conditions in different regions in China.Therefore,focusing on the analysis of the changes in the dry/wet conditions and conducting research on its response mechanism to global warming in China has high indicative significance and reference value for formulating relevant strategic policies.Research has shown that since the 1960s,the western China has mainly shown a wetting trend,while eastern China has mainly shown a drying trend.Further mechanism analysis indicates that the synergistic effect of westerly circulation and monsoon circulation on the inter-decadal scale is the main reason affecting the evolution of dry/wet conditions in China.However,previous studies are mostly based on linear prior trends,resulting in conclusions which are highly influenced by the starting point of the time series,and it is difficult to further display the evolution rate of dry/wet conditions.In addition,for the evolution of the dry/wet conditions,it is not only necessary to pay attention to the current changes and impact mechanisms,but also to discuss whether this trend can continue to be maintained under global warming in the future,providing reliable scientific basis for the long-term development decisions in China.Based on this,firstly,we combine the ensemble empirical mode decomposition method to extract the long-term nonlinear trend component of aridity index in China,analyze the temporal and spatial evolution characteristics,and reveal its nonlinear evolution rate of this trend component.Secondly,using various atmospheric circulation indices,reveal the synergistic mechanism of atmospheric circulation on the long-term nonlinear trend of dry/wet conditions in eastern and western China.Finally,based on the results of the model evaluation,select models with high simulation skills,and apply the model bias correction method to estimate the differences in the response of dry/wet conditions under different Shared Socioeconomic Pathway scenarios in the future.The main conclusions are as follows:(1)Revealed a new feature of asymmetric changes in the drying and wetting trends in eastern and western China.The study on the long-term nonlinear trend component of aridity index from 1901 to 2020 found that the long-term trend of aridity index mainly exhibits four nonlinear characteristics,monotonic increase,monotonic decrease,first increase and then decrease,and first decrease and then increase.Most areas in eastern China have experienced a first increase and then a decrease in the degree of wetness,and some areas have shown a drying trend in the past decade.On the contrary,in western China,most regions have experienced an increase first and then a decrease in the degree of dryness,while some regions have shown a wetting trend in the past decade.This indicates that the eastern and western China are undergoing asymmetric changes in dry/wet conditions.(2)Clarified the synergistic mechanism of circulation systems such as the westerly jet,the South Asian High,and the Western Pacific Subtropical High on the changes of dry/wet conditions.The analysis of the key factors affecting the long-term trend of dry/wet conditions in China shows that precipitation is a significant factor affecting the changes in dry/wet conditions.Further studies of physical mechanism have found that the long-term trend of precipitation are mainly influenced by the long-term trend of the westerly circulation,followed by the long-term trend of the South Asian monsoon circulation.Compared to before the transition of the dry/wet conditions in China(1901-1940),the high-altitude jet significantly strengthened after the stable transition of the dry/wet conditions(1971-2010),with the weakened South Asian High,which enhanced the upward motion over the western China,increased the vapor flux within the region,and increased the precipitation,further leading to a wetting trend.The eastern region is significantly affected by the change of the South Asian high,with both high and low altitude in a divergent state.The vertical movement within the region weakens,and the vapor flux decreases,thereby affecting precipitation changes and further leading to a drying trend.(3)Evaluated the main influencing factors for the differences in response to drying and wetting trends in China under different Shared Socioeconomic Pathway scenarios.By evaluating the simulation skills of models on the long-term nonlinear trends of dry/wet conditions in China,it was found that the temporal consistency between model simulations and observation data is better than spatial consistency,while the results of model bias correction based on observation constraints are significantly better than the results before correction in the historical period.The model estimation results indicate that as of the end of this century,under the lower emission scenario,most areas in the eastern China showed a drying trend,while most areas in the western part showed a wetting trend,which was mainly influenced by long-term changes in precipitation;Under high emission scenarios,areas with a drying trend in China are mainly distributed in South China and Northeast China,while areas with a wetting trend are mainly distributed in Southwest China,Qinghai Province and surrounding areas.This trend is mainly influenced by long-term changes in potential evapotranspiration.