The Climate Variability In The East Asian Summer Monsoon Boundary Zone And Its Westerlies-Monsoon Synergy During The Mid-holocene And Present Day

The spatial fluctuations of the East Asian summer monsoon(EASM)northern boundary on interannual and interdecadal scales caused by the changes of the EASM strength are named the monsoon boundary zone(MBZ).The MBZ is a transition zone between the monsoon and mid-latitude westerly circulation system.It is also a typical farming-pastoral ecotone,is sensitive to climate change and prone to frequent natural disasters.Previous studies mainly understand the climate change in the MBZ from the perspective of the EASM,the impact of the mid-latitude westerlies on the MBZ lacks attention.The synergy of the EASM and westerly over the MBZ makes it is crucial to understand the combined effect of the two system rather than their individual influence.Therefore,in this thesis I discussed the climate change characteristics of the MBZ and the related mechanism by considering the synergy between the EASM and westerly.Combined the paleoclimate proxy data and climate model simulation,I further investigated the mechanisms of the climate change in the MBZ during the mid-Holocene.The main conclusions are summarized as follows:1.A new EASM northern boundary index was defined,called the Climatological Northern Boundary Index(CNBI),which has significant implications for the climate,ecology and geography and is also readily applicable to paleoclimatic research.The CNBI is defined as summer(May-September)2 mm day-1 isoline.The MBZ indicated by the CNBI has a good relationship with the spatial distribution of modern land cover types,the transitional climate zone and the potential natural vegetation types in China.The locations of the MBZ also correspond well to the locations of major shifts in wind direction.These results suggest that the CNBI has a clear physical significance for the climate,ecology and geography.The MBZ covers the central part of Gansu,the northern part of Ningxia,the eastern part of Inner Mongolia and the northeastern region in China.It extends northward to the border between China and Mongolia and retreats southward to Shandong-central Henan.The more northward location of the EASM northern boundary,corresponds to the enhanced summer monsoon and westerly winds,as well as the northward shifting the cyclone over Northeast China,and vice versa2.The synergy between the westerlies and EASM leads to the northward shift of the EASM northern boundary and the increased precipitation in the MBZ.The meeting of the mid-latitude westerlies and summer monsoon associates with the cold and warm/humid air collide over the MBZ,which increases atmospheric instability that favors the ascending motion and precipitation in the MBZ.The westerlies-monsoon synergy also could contribute to the westerlies-monsoon interaction,via inducing a local cyclonic anomaly in the MBZ.This cyclonic anomaly further intensifies the westerlies and monsoon flow through a dynamical amplification,favors the precipitation in the MBZ.The interannual variability of westerlies is largely modulated by the mid-latitude Silk Road pattern(SRP)and the meridional displacement of the Asian westerly jet(JMD),and the EASM variability is mainly modulated by El Nino-Southern Oscillation(ENSO).Therefore,we defined a nonlinear westerlies-monsoon interaction index(WMII)using the summer westerly index and EASM index and weight by referring to the correlation coefficients between the two factors and MBZ precipitation.The WMII could depict the main features of the synergistic effects of the westerlies and EASM and the westerlies-monsoon synergy on the precipitation fluctuations in the MBZ3.The MBZ and the Mongolian Plateau(MP)dominated by the westerly circulation reveal the consistent variability in summer precipitation on interannual and interdecadal timescales.The mid-latitude wave train over Eurasia,with positive(negative)geopotential height anomalies over the North Atlantic and Arid Central Asia(ACA)and negative(positive)geopotential height anomalies over Europe and the MP,is the key factor responsible for the consistency of precipitation variability in the MP and the MBZ.The positive anomalies over the North Atlantic and AC A and negative anomalies over Europe and the MP would enhance the transport of westerly and monsoon moisture to the MP and North and Northeast China.They could also strengthen the Northeast Asian low,enhance the EASM,and trigger the anomalous ascending motion over the MP which promotes precipitation in the MP and the MBZ4.During the mid-Holocene,the orbital forcing could cause significant northwestward shift of the EASM northern boundary,and the vegetation feedback could lead to further northwestward shift of the EASM northern boundary.The maximum northward extension of the EASM northern boundary is approximately 213 km due to the orbital forcing,and approximately 90 km due to the vegetation feedback The enhanced EASM,as well as the weakened and northward shift of East Asian westerly jet(EAWJ)due to the orbital forcing and vegetation feedback are the main factors that dominate the northwestward shift of the EASM northern boundary.The decreased meridional air temperature differences(MTD)in the mid-latitude due to the orbital forcing result in the northward movement of the EAWJ.The expansion of grassland and the decrease of forest in Northeast Asia led to the increase of the MTD in Northeast Asia,which lead to further northward movements of the EAWJ and result in an earlier transition of the Meiyu stage and a longer summer stage.Meanwhile,the weakening of the EAWJ could enhance the upper-level divergence over the North and Northwest China,providing the dynamic condition for the increased precipitation in the MBZ.This northward movement of the EAWJ due to the vegetation feedback could terminate the Meiyu stage earlier and lengthen the Midsummer stage.All these processes associated with the enhanced EASM moisture transportation,eventually contribute to the increased precipitation in the MBZ and northwestward of the EASM northern boundary.