On The Major Evolution Characteristics And Associated Mechanisms Of Global Monsoon Precipitation Over The Past 20000 Years

The paleo-records revealed the long-term variation of the regional monsoon precipitation and the anti-phase relationship between the Northern Hemisphere(NH)and Southern Hemisphere(SH)monsoon precipitation on millennial timescale during the past 20 000 years.However,the external drivers and the physical mechanisms responsible for the above phenomenon are still elusive.Based on the simulation of Transient Climate Evolution over the last 21000 years(Tra CE-21ka),we used the Empirical Orthogonal Function(EOF)analysis to identify the leading EOF modes of the global monsoon precipitation in the full-forcing experiment.The results show that the first and second modes can well represent the long-term evolution of global monsoon precipitation and the millennial-scale anti-phase relationship of interhemispheric monsoon precipitation as indicated in the proxy records during the past 20000 years.Further analyses are focused on identifying the dominant external forcings and associated physical mechanisms,and the main findings are as follows:(1)The full-forcing experiment reasonably reproduces coherent variation of regional monsoon precipitation over the NH or SH and the anti-phase relationship between the NH and SH monsoon precipitation during the past 20 000 years as indicated by paleo-records.The longterm variation of global monsoon precipitation could be characterized by the EOF1 and PC1 of annual mean global monsoon precipitation,representing the dipole pattern between the AfroAsian and North American monsoon precipitation in the NH and the uniform pattern of the SH monsoon precipitation.The EOF2 and PC2 of annual mean monsoon precipitation manifest the millennial-scale oscillation over both hemispheres with a see-saw variation between the NH and SH.(2)Based on the four single-forcing sensitive experiments results,it is clear that the external forcings that play an important role in the long-term evolution of global monsoon precipitation are greenhouse gases,ice sheets,and orbital parameters.The increase in greenhouse gases concentration enhances the global monsoon precipitation by enhancing the atmospheric water vapor content,while the precipitation pattern is modulated by the NHwarmer-than-SH and El-Nino-like sea surface warming pattern,leading to an increase in AfroAsian and decrease in North American monsoon precipitation.The melting of ice sheets causes the cooling of the tropical North Atlantic.It would reduce the monsoon precipitation over North America,North Africa,and South America.The weakening of Australian and South African monsoon precipitation is associated with reduced atmospheric water vapor content and zonal wind vertical shear.In terms of orbital insolation forcing,it generates the NH-warmer-than-SH and land-warmer-than-ocean temperature patterns that favor increasing NH land monsoon precipitation in JJA,while the overall SH continents cooling at middle-low latitude and landsea thermal contrast decrease SH land monsoon precipitation in DJF.The above mechanisms lead to the strengthening of Afro-Asian monsoon precipitation and the significant decline of North American monsoon precipitation in the NH.In contrast,the decreased SH monsoon precipitation is attributed to the combined influence of orbital insolation forcing and ice sheets forcing.(3)On a millennial scale,the NH meltwater discharge is responsible for the see-saw variation of monsoon precipitation between the NH and SH.The increase of NH meltwater discharge could weaken the Atlantic Meridional Overturning Circulation(AMOC),decreasing northward across-equatorial heat transport.The consequent NH-cool and SH-warm surface temperature pattern reduces the northward water vapor transport from the SH,leading to the weakened NH Hadley circulation and southward movement of the Intertropical Convergence Zone(ITCZ).Therefore,the SH monsoon precipitation is enhanced,but the NH monsoon precipitation is weakened.The opposite physical process is still valid for the NH-increase and SH-decrease of monsoon precipitation when NH meltwater discharge is weakened.