Modelling Study On The Variation And Mechanisms Of The East Asian Summer Monsoon Northern Boundary Across Different Interglacial Periods From The Middle Pleistocene To The Future

The range of the northern boundary of the East Asian summer monsoon（EASM）,which moves with the strength of the monsoon,is called the EASM transition zone.The northern region of the EASM transition zone is located between arid and semi-arid regions,and the southern region is located between semi-arid and semi-humid regions,which is a climatic transition zone from arid to humid climate,and also a vulnerable ecological zone and an ecological transition zone that is highly sensitive to climate change.Therefore,understanding and predicting the future changes in the northern boundary of the EASM and its mechanisms is an important prerequisite for the rational planning and formulation of sustainable development strategies for socio-economy in arid and semi-arid regions.Since we are currently in an interglacial period that will last even longer,studying the climate changes during past interglacial periods can help us understand the natural patterns of climate system changes and the process of environmental evolution under natural forcing warming,providing important references and bases for studying and predicting future climate change.We conducted simulations using the Earth System Model COSMOS（ECHAM5/JSBACH/MPIOM）to explore the characteristics and physical mechanisms of the changes in the northern boundary of the EASM during 11 interglacial periods since the Middle Pleistocene.Furthermore,based on the simulation results of different orbital parameters and mountain uplift sensitivity experiments,the response of the northern boundary of the EASM to different forcings is further analyzed.In addition,this study also examines the impact of greenhouse gas increases on the EASM northern boundary by using a total of 115 datasets from 23 climate models under historical and 4 future scenarios in the Coupled Models Intercomparison Project Phase 6（CMIP6）.The response characteristics and mechanisms of the northern boundary of the EASM to orbital forcing,topography,and different levels of greenhouse gas forcing were systematically analyzed.The following conclusions were mainly drawn:（1）Since the Middle Pleistocene,the earth has experienced 11 interglacial periods,the climate of the EASM region and its northern boundary are influenced by both orbital parameters and greenhouse gases.During 9 interglacial periods of MIS 1,MIS 5.5,MIS 7.3,MIS 7.5,MIS 9.3,MIS 11.3,MIS 13.1,MIS 15.1,and MIS17,the surface temperature in most regions of mid-to high-latitude regions of China increased compared to the pre-industrial period,while during MIS 15.5 and MIS 19.3,the surface temperature decreased compared to the pre-industrial period.In terms of precipitation,the northern region of China experienced a decrease in precipitation only during MIS 15.5,and an increase in precipitation during the other interglacial periods.Except for MIS 15.5 and MIS 19.3,the northern boundary of the EASM move towards the northwest during the other nine interglacial periods.During these 9 interglacial periods,the combined effect of the three orbital parameters caused an increase in temperature in the high-latitude regions of the Northern Hemisphere,enhancing the land-sea thermal contrast and strengthening the western Pacific subtropical high（WPSH）,which moves northward and increased precipitation in northern China,causing the northern boundary of the EASM to move towards the northwest,with the largest amplitude occurring during MIS 5.5,which was approximately 154.6 km towards the northwest.During MIS 15.5,with CO₂ concentration lower than of the pre-industrial period,and the precession was similar to that of the pre-industrial period,but the eccentricity was about three times that of the pre-industrial period,the Northern Hemisphere received less solar radiation in summer,weakening the land-sea thermal contrast and causing the EASM to weaken and its northern boundary to retreat southward by about 298.5 km.During MIS 19.3,the combined effect of lower CO₂concentration and similar orbital parameters resulted in little change in land-sea thermal contrast compared to the pre-industrial period,resulting in little change in the intensity of the EASM and the position of its northern boundary.（2）The impact of orbital parameters on the EASM northern boundary follows the order of precession>eccentricity>obliquity.In general,precession is the main controlling factor for the movement of the northern boundary of the EASM.During the summer in the Northern Hemisphere,the northern boundary of the EASM moves northwestward in the perihelion（low precession）compared to the aphelion（high precession）.Orbital parameters regulate the changes of EASM and its northern boundary by adjusting the amount of solar radiation received in the Northern Hemisphere.Among them,the variation in precession plays a dominant role in regulating the EASM compared to the obliquity and eccentricity.As precession decreases and the summer in the Northern Hemisphere approaches the perihelion,the high-latitude regions in the Northern Hemisphere receive more solar radiation,which strengthens the seasonal contrast and increases the land-sea thermal contrast.As a result,the WPSH strengthens and expands northward,and the East Asian subtropical westerly jet（EASWJ）weakens and moves northward.These changes collectively cause an increase in precipitation of the EASM in northern China and a northwestward shift of its northern boundary.The influence of the obliquity and eccentricity on the EASM depends on the level of precession.In the scenario of the summer in the Northern Hemisphere in the perihelion（low precession）,the variation of the obliquity has little impact on the EASM and its northern boundary.However,in the scenario of the summer in the Northern Hemisphere in the aphelion（high precession）,as the obliquity increases,the land-sea thermal contrast increases,the WPSH strengthens and expands northward,and the EASWJ weakens and moves northward.These changes collectively strengthen the EASM and cause its northern boundary to shift northwestward.During the scenario of the summer in the Northern Hemisphere in the perihelion（low precession）,the larger the eccentricity,the warmer the summer in the Northern Hemisphere,and the greater the land-sea thermal contrast.This strengthens the WPSH and causes it to expand northward,and weakens and moves the EASWJ northward,collectively strengthening the EASM and causing its northern boundary to shift northwestward.The opposite is true in the scenario of the summer in the Northern Hemisphere in the aphelion（high precession）.（3）The uplift of the Mongolian Plateau,the Tibetan Plateau,and the Mongolian-Tibetan Plateau enhances the EASM and promotes the northward advance of the northern boundary of the EASM.The uplift of small terrain in the monsoon channel hinders the strengthening of the EASM in the scenarios where only the Mongolian Plateau uplifted.The uplift of the Mongolian Plateau will turn it into a heat source during the summer,and the heat effect of the plateau will cause the surface air to expand,forming a low-pressure area on the plateau surface,which weakens the mid-latitude westerlies and strengthens the EASM,pushing a northwestward migration of the EASM northern boundary.However,in the scenario of the uplift of the Mongolian Plateau,the uplift of small terrain in the monsoon channel has a certain hindering effect on the EASM.The uplift of the Tibetan Plateau similarly causes the surface air on the plateau to expand due to heating,acting like a giant"pump",attracting surrounding air to flow towards the plateau,thus enhancing the EASM and pushing its northern boundary shift to northwestward.On the other hand,the uplift of the Tibetan Plateau strengthens the WPSH,further enhancing the EASM.In the scenario of the Tibetan Plateau uplift,the uplift of small terrain in the monsoon channel has a certain enhancive effect on the EASM.The uplift of the Mongolian-Tibetan Plateau integrates the mechanisms of the enhanced EASM under separate uplift scenarios of the Mongolian Plateau and the Tibetan Plateau.In this scenario,the uplift of small terrain within the monsoon channel also has a strengthening effect on the EASM,thus the northern boundary of the EASM shifts to northwest.（4）A northwestward migration of the EASM northern boundary during the near-term（2020-2060）and late-term（2061-2099）of the twenty-first century across various Shared Socioeconomic Pathways（SSPs）.The northern boundary of the EASM is projected to move northwestward by 23-28 km and 74-76 km in the near-term and late-term,respectively,under SSP1-2.6,2-4.5,and 3-7.0,and by approximately 44km and 107 km in the near-term and late-term,respectively,under SSP5-8.5.The warming of the East Asian subcontinent surface more than the ocean during the twenty-first century under various SSPs leads to an increased contrast in near-surface temperature and sea-level-pressure in summer between the East Asian subcontinent and surrounding oceans.As a result,the intensified land-sea thermal contrast reinforces the EASM meridional circulation,leading to increased moisture transport from the Indian Ocean into northern China.Furthermore,a poleward migration and weakening of the EASWJ also favors an increase in precipitation and eventually causes a northwestward migration of the EASM northern boundary.The results suggest that the arid and semiarid ecotone will become wetter,which could significantly improve the eco-environment in the future.