Spatio-temporal Characteristic And Influences Of Arid-humid Variations In Summer Climate During 1960-2010 In The Asian Monsoon Margin Of Northern China

Arid-humid condition of climate,an important indicator of regional water-heat balance,plays a significant role in the hydrologic and energy cycle.Asian monsoon margin of northern China(AMNC)is the key region of recent researches,with a strong response and record to regional climate change and global environmental change.However,the main influences of arid-humid variations in summer climate in AMNC,and how the arid-humid variations in summer climate in AMNC response to the intensity changes of the East Asian summer monsoon(EASM)still need further research.Identify the above issues can deepen the understanding of the mechanism of climate change in AMNC and is essential for understand the intrinsic connection between regional and global climate change and their driving mechanism.In this study,daily meteorological data of 114 stations in AMNC during summer(June to August),1960-2010 were used to calculate accumulated temperature during period of daily mean temperature?10°C,potential evapotranspiration,and the aridity index K to analyze the spatio-temporal characteristic of climatic elements and aridhumid variations in AMNC,and the rotated empirical orthogonal function(REOF)was used to extract dominant spatial and temporal fields and divide study area into different regions based on arid-humid variations in summer climate.Then,arid and humid years were defined according to the changes in anomaly value of time coefficient corresponding to each region.After that,this study used monthly reanalysis data to calculate the East Asian summer monsoon index(EASMI),the westerly wind index(WWI),and the north boundary index of East Asian Summer Monsoon(NBEASMI),and backward-water vapor trajectories,water vapor flux and divergence in arid and humid years to analyze the main influences of arid-humid variations in summer climate in each region of AMNC and then discuss how the precipitation and arid-humid variations in summer climate in AMNC response to the intensity changes of the East Asian summer monsoon.The results show as follows:(1)The aridity index K in AMNC significantly decreased during 1960-1977 and1987-1995,increased during 1977-1987 and 1995-2010.Based on the first six space fields extracted by the REOF,the AMNC was divided into six subregions,which were mainly located in the northeast,mid-east,central,south,southwest,and northwest part of the AMNC.Moreover,there are significant regional differences in aridity index K in AMNC.(2)The EASM significantly weakened during 1960-1968 and 1998-2010 and strengthened during 1968-1976 and 1976-1998;the westerly wind significantly weakened during 1960-1969 and strengthened during 1969-1982 and 1982-2010.(3)There are significant regional differences in backward-water vapor trajectories,water vapor flux and divergence in AMNC.The proportion of water vapor trajectories during arid and humid years in each subregion showed differences,possibly indicating the changes in water vapor trajectories can affect summer precipitation and arid-humid variations.Moreover,water vapor basically converged over the mid-east,northeast,and south region in both arid and humid years;water vapor basically diverged over the southwest,and northwest region in both humid and arid years although significant water vapor transport existed;water vapor basically converged over most part of the central region although water vapor diverged over small part of the central region.Through the analysis of trend and regional comparison of the above results,the following conclusions are reached:(1)The summer climate in AMNC significantly became humid during 1960-1977 and 1987-1995,arid during 1977-1987 and 1995-2010.There are significant regional differences in arid-humid variations in summer climate in AMNC: the summer climate became humid in central,southwest,and northwest part in AMNC and arid in mid-east,northeast,and south part in AMNC before the 1970s;except for the south part in AMNC,the summer climate in the other five subregions became arid after the mid-1990s;the summer climate in the central and southwest part in AMNC similarly became humid before the mid-1990 s and arid after the mid-1990s;the mid-east and northeast part in AMNC similarly showed a trend of arid-humid-arid although changes in the former lag behind changes in the latter.(2)Precipitation significantly influences the interannual-interdecadal changes in arid-humid variations in summer climate in AMNC.However,the water vapor sources of precipitation differ in AMNC: the southwest and northwest region were mainly affected by the precipitation recycling process associated with regional evapotranspiration;the south and northeast part were mainly affected by water vapor transported by the EASM;the mid-east and central region were mainly affected by water vapor transported by the EASM and the westerly wind.(3)Response of arid-humid variations in summer climate in AMNC to the intensity changes in the EASM is most significant in the northeast region while weak in the southwest and northwest region,and arid-humid variations in summer climate in mideast,central,and south region partly response to the intensity changes in the EASM.The reasons for these differences are as follows: Northeast China Cold Vortex,a stable and unique weather system with long duration,can constitute a very favorable circulation background for summer precipitation over the northeast region.Development of blocking high and the East Asian jet stream over the mid-east and central region can influence the effect of EASM on precipitation,for these atmospheric circulations can affect the northward advancing process when the EASM intensifies,and then affect the influence of the EASM on precipitation.Since the south region locates in the southernmost part of AMNC,when the EASM intensity enhances and reaches the northernmost margin,the influence of the EASM there is not as significant as the northeast region,and when the EASM intensity weakens and reaches a southerly margin,the influence of the EASM there is more significant than other northerly regions.In the southwest and northwest region,water transport is ‘dry' and they mostly have downward flow,meanwhile,they have strong evapotranspiration and their potential evapotranspiration are much larger than actual precipitation,thus,their arid-humid variations in summer climate are significantly dependent on the precipitation recycling process associated with regional evapotranspiration.