A Comparison Of Dry-wet Variation Between East Asian And North American Monsoon Domains Since The Last Glacial Maximum

The climate change in East Asia and North America has a remote correlation.At different time scales,there is a certain relationship between the dry and wet changes in the two regions.Quantitative reconstruction based on paleoclimate records and inversion of the temporal and spatial changes of past drought events in the East Asian and North American monsoon regions are conducive to understanding changes in the water cycle at different time scales and provide a scientific basis for predicting future drought events.In this study,we used observation data,various paleoclimatic alternative indicators,TraCE model and PMIP3/CMIP5 model data to reconstruct and simulate the multi-time scale dry and wet changes and influencing factors in East Asia and North America since the last ice age.The main conclusions are as follows:(1)The results of inter-annual/inter-decadal studies on dry and wet changes show that: under the background of global warming,the dry and wet conditions in East Asia and North America do not respond to the changes of various climatic factors,and the temperature in East Asia and North America rises.The evaporation is increased,and the precipitation in the southwest of the East Asian monsoon region and the North American monsoon region is reduced,resulting in an increase in local drought.Affected by the increase in precipitation,the climate in northwestern East Asia is still humid when the temperature rises.The temperature increase in the eastern region of the East Asian monsoon region is not much different from other regions,but the increase in precipitation is greater,so the degree of drought is lower than in other regions of the monsoon region.This shows that when the temperature rises,the drought can only be alleviated by a greater increase in precipitation,which to a certain extent also confirms that the dry and wet changes in the East Asian monsoon region respond more significantly to temperature than to precipitation.At the same time,this paper also discusses the relationship between dry and wet changes and sea surface temperature(SST).The results show that the dry and wet changes in East Asia are positively related to most of the world's seas,and no significant related to the El Ni?o-Southern Oscillation(ENSO);the correlation between dry and wet changes in North American non-monsoon regions and ENSO events is more significant,and the correlation between dry and wet changes in monsoon regions and ENSO is weaker.(2)The results of the dry and wet changes at the millennium scale show that from Last Glacial Maximum(LGM)to the early Holocene,the summer solar radiation and greenhouse gas concentrations rose from the valley value to the peak since LGM,and the northern hemisphere continental ice sheet is the opposite.Under this forcing background,Atlantic meridional overturning circulation(AMOC)and ENSO generally show a downward trend,resulting in the overall weaker monsoon in East Asia and North America than the Holocene.There is an evolutionary trend from drought to humid.The solar radiation in winter gradually weakens,and the winter monsoon intensifies.Most of the East Asian non-monsoon region is in a relatively dry state,and the dry and wet changes are relatively gentle.The Holocene is mainly affected by solar radiation and greenhouse gas concentration,and the role of ice sheets is weak.The general trend of dry and wet changes in the monsoon regions of East Asia and North America is to gradually dry,the monsoon regions develop from wet to dry,and the non-monsoon regions are dry Exacerbated.(3)Comparing the dry and wet changes of LGM(21ka),Mid-Holocene(MH;6ka)and Preindustrial(PI;0ka),it can be seen that the dryness of monsoon and non-monsoon regions in East Asia and North America wet conditions have a reverse phase relationship.The dry and wet changes in the monsoon area mainly depend on the dry and wet conditions in summer,and the non-monsoon areas mainly depend on the dry and wet conditions in winter.The summer solar radiation of LGM is weak,the concentration of greenhouse gases is low,and the large-scale development of the northern and southern sea ice and continental ice sheets leads to a decrease in the global surface temperature.In the summer,the land cooling in the northern hemisphere is greater than that in the ocean,and the thermal difference between land and sea is reduced;In latitudes,the meridional temperature gradient increases and the monsoon weakens,so LGM becomes the driest period in the monsoon region.MH has strong solar radiation in summer,AMOC is in a high state,and ENSO is at the lowest value in the Holocene,which is conducive to enhancing the monsoon,which makes the humid monsoon climate in the monsoon area appear in the MH period.The dry and wet changes in the non-monsoon region are not synchronized in East Asia and North America.The wettest period in the East Asian non-monsoon region is the MH period,because the greenhouse gas concentration in this period is higher than that in the LGM period,resulting in an increase in SST at mid-high latitudes and an increase in westerly winds.The water vapor content makes the East Asian non-monsoon region wet.The wettest period in the North American non-monsoon region is the LGM period,mainly because the North American continental ice cover reaches the maximum during this period,and the pressure near the ice sheet is high,which promotes the westerly belt to move southward and strengthens the westerly wind,leading precipitation in the North American non-monsoon.(4)The dry and wet conditions at different time scales in the monsoon and nonmonsoon regions of East Asia and North America all have reverse phase changes,and the driving factors for dry and wet changes at different time scales are different.Under forced backgrounds such as solar radiation,greenhouse gas concentrations,and ice sheets,the dry-wet changes at the millennium scale are closely related to AMOC and ENSO.While the interannual/interdecadal scale dry and wet changes are more closely related to the interannual/interdecadal changes of precipitation,temperature and SST.