A Comparative Study Of The Summer Monsoon On The East And West Sides Of The Pacific Ocean Recorded By Stalagmites During The Late Holocene

It is well known that climate change and its indirect changes in the earth's surface system are closely related to social development.Therefore,the prediction of future climate change and climate abrupt change events have become a common global problem.The study of paleoclimate can help us to understand the law of climate evolution on a longer time scale.Monsoon climate is one of the major climate types in the middle and low latitudes,affecting a wide range of regions and large populations.As an important period of the evolution of human civilization,the climate change during late Holocene has been reconstructed by using different geological carriers.However,the studies of each monsoon area are concentrated in a single monsoon area,and the overall comparison of each monsoon is insufficient,ignoring the common influence of the Pacific Ocean on the monsoon on both sides.In this paper,the climate change in southwestern China was reconstructed from 3.1 ka to 0.6 ka BP by Shijiangjun cave stalagmite with a resolution of 3.5 years,which was accurately dated,and compared with other records in the Asian monsoon region.The changes of Asian summer monsoon and summer monsoons on both sides of the Pacific Ocean is dicussed during the late Holocene in this paper.Moreover,Exploring the impact of the tropical Pacific on the summer monsoon on both sides will helpful to understand the low latitude signal of the driving mechanism of climate change.The main conclusions of this study are as follows:?1?SJJ7?¹⁸O records show that the Indian summer monsoon weakened during 2.8ka BP?2930²808 BP?,2.4 ka BP?2452²349 BP?,1.9 ka BP?1959¹869 BP?,1.5 ka BP?1587¹494 BP?,1.2 ka BP?1285¹162 BP?and 1.0 ka BP?1022⁹18?and wasstrengthened during the Medieval Warm Period,indicating that the Indian summer monsoon fluctuates on hundred-year scale during 3108⁶93 BP.Overall,the Indiansummer monsoon is weakening during this period.This may response to theweakening of late Holocene solar radiation on the orbital scale.In addition,there is little record in the warm period of Sui and Tang Dynasties,and SJJ7 has a clear record of this warm period.?2?Compared with other records,SJJ7 accurately recorded 2.8 ka BP and 1.5 ka BP events,same with the East Asian monsoon records,which responded to the millennial-scale climate change in the high latitudes of the Northern Hemisphere.2.8 ka BP occurred between 2930 and 2808 BP,lasted 122 years in SJJ7.It suggest that the slowdown of the North Atlantic meridional circulation would lead to a weakening trend of the Asian summer monsoon.However,the internal variations of the Indian summer monsoon and the East Asian summer monsoon are different in 2.8 ka BP and this inter-decadal difference may be caused by the Pacific ENSO or PDO activities.In the positive phase of PDO?El Ni?o event?,the sea surface temperature in the tropical Middle East Pacific increased,the sea surface temperature in the Northwest Pacific negative anomaly,the western Pacific subtropical high strengthened and westward extension,and the rain belt moved southward to the Yangtze River basin,resulting in more precipitation in the Yangtze River basin than in Northern and southern China.?3?The?¹⁸O records in northern Australia and Asian summer monsoon stalagmites showed an same phase change,which was anti-phase phase with that of the Indonesia stalagmites between 2.8 ka BP and 1.5 ka BP.This may be related to the contraction and expansion of the tropical rain belt,which results in the consistent change of precipitation in the southern part of the Australian monsoon and the Asian monsoon region,and the contrary change of precipitation in the central part of the Australian monsoon region.However,during the 2.8 ka BP,1.5 ka BP and the Little Ice Age,the stalagmite?¹⁸O value is Increased,and the Australian summer monsoon strengthened in the Medieval Warm Period.This in-phase change may be related to ENSO activity.?4?During the 2.8 ka BP,1.5 ka BP and the Little Ice Age,the North American summer monsoon weakened.In addition,the Asian monsoon recorded 2.4 ka BP,1.9 ka BP,1.2 ka B and 1.0 ka BP.Reflected in different records.This indicates that the North American summer monsoon and the Asian summer monsoon change in phase on the 100-year scale.The weak monsoon event of 1.0 ka BP may be related to the decline of Maya civilization,and again indicates that climate change is closely related to social development.?5?During the period of 2.8¹.5 ka BP,the South American summer monsoon weakened,and strengthened during the Medieval Warm Period,which showed a trend of in-phase change with the Asian summer monsoon.The South American summer monsoon did not increase in the Bond 1 and 2 events,differentiating from the early-Middle Holocene.But during the Little Ice Age,the South American summer monsoon increased.?6?The records of the Holocene show that the summer monsoon on both sides of the Pacific Ocean in the late Holocene tended to weaken and then increase.The high-resolution,accurate,and continuous records of each monsoon region were further selected for comparison,indicating that each summer monsoon weakened during the period of 2.8¹.5 ka BP,and began to strengthen after 1.5 ka BP.This was corresponding to a significant increase in ENSO activities in the late Holocene,and showed an opposite trend to ENSO.A large number of studies have shown that ENSO has an impact on summer monsoon precipitation on both sides of the Pacific Ocean.In the El Ni?o state,the surface warm water in the IPWP?Indonesia Pacific Warm Pool?region moves eastward or the warm pool is narrowed,and the low-level cold seawater surge increases,resulting in a decrease in the SST of IPWP and a weakening of the tropical intraseasonal oscillation?Madden-Julian Oscillation,MJO?.The rising branch of the Walker circulation moves eastward,weakening the Asian summer monsoon and the Indian-Australian summer monsoon.In the state of El Ni?o,ITCZ is closer to the equator,the thermodynamic difference between the tropical ocean and the continent is reduced,and precipitation is reduced in the south-central part of the North American monsoon region.At the same time,the equatorial westerly winds in the Atlantic Ocean have increased significantly,the subtropical jets have increased,and the subtropical cyclones across the equator have suppressed the convection of the entire Amazonian and Atlantic tropical convergence belts,resulting in a weak South American summer monsoon.In the positive phase of PDO?El Ni?o event?,the sea surface temperature in the tropical Middle East Pacific increased,the sea surface temperature in the Northwest Pacific negative anomaly,the subtropical high in the western Pacific strengthened its westward extension,and the rain belt moved southward to the Yangtze River basin,resulting in more precipitation in the Yangtze River basin than in Northern and southern China.