基于石笋记录的晚全新世太平洋东西两岸季风区气候事件对比研究

研究晚全新世季风气候演变有助于进一步认识与预测未来季风区气候变化。太平洋东西两岸是全球季风集中分布的地区,已经有大量的古气候记录发表,但是缺乏对各个季风区气候突变事件以及整体变化趋势的对比研究。针对这一问题,选取亚洲季风区、印澳季风区、北美季风区、南美季风区11个洞穴石笋δ18O和1个湖泊Ti含量,对比研究各个记录在3.5~0.5 ka B.P.期间指示的夏季风变化特征。通过对比发现四大季风区的石笋δ18O在晚全新世整体上呈现偏正趋势,指示夏季风减弱;2次重要的气候突变事件1.5 ka B.P.和2.7 ka B.P.弱夏季风事件在各个季风区内均有表现;同时也记录了一系列十年际-百年际尺度的弱夏季风事件,表明太平洋东西两岸和南北半球的夏季风都有减弱的趋势,这与先前研究认为的南北半球呈现"see-saw"模式表现出不一样的特征。晚全新世以来ENSO(El Nino-Southern Oscillation)活动的增强对太平洋东西两岸南北半球夏季风减弱具有重要影响。在El Nino事件发生时,Walker环流减弱,而且它的上升支向东移动远离西太平洋暖池,西太平洋副热带高压增强并向西移动,导致亚洲夏季风减弱。Walker环流的东移也会使得印度尼西亚-太平洋暖池(Indo-Pacific Warm Pool,简称IPWP)海温下降,热带季节内震荡减弱致使印澳夏季风减弱;此外,El Nino事件发生时,赤道东太平洋海水温度上升导致东西太平洋海水温度梯度减弱,在此状态下南美季风区低空急流(Low Level Jet,简称LLJ)减弱,导致南美夏季风减弱;同时,北美洲加勒比海低空急流增强,使得该季风区下沉气流增强,导致北美夏季风减弱。我们的研究表明,在晚全新世ENSO活动增强的状态下,太平洋东西两岸南北半球夏季风变化可能都呈现减弱趋势。

Comparison of climatic events in the Late Holocene,based on stalagmite records from monsoon regions near by the east and west Pacific

Studying the evolution of the Late Holocene monsoon climate will help to understand and predict future climate changes more accurately in the monsoon regions. In the eastern and western coasts of the Pacific Ocean where the global monsoon is concentrated, a large number of paleoclimate records have been published in these regions. However, there is still a lack of comparative and integral studies on abrupt climate events in the four monsoon regions. To address this problem, comparing the δ^18O of stalagmite records in 11 caves and Ti of 1 lake record which from four monsoon regions, namely Asian monsoon region, Indonesian-Australian monsoon region, American monsoon region, and the results show that the δ^18O values of stalagmites show a positive trend in the Late Holocene, which mean summer monsoon declined. On the other hand, weak summer monsoon events of 1.5 ka B. P. and 2.7 ka B. P. are observed in four monsoon regions. Furthermore, it also recorded a series of weak summer monsoon events on decadal to centennial timescales with decreasing trend in the east and west Pacific. However, this feature is different from the "see-saw" pattern between northern and southern hemisphere. Because of the enhanced ENSO(El Ni?o-Southern Oscillation) activity in the Late Holocene, which played an important role in the weakening of the summer monsoon between the northern and southern hemispheres of both sides of the Pacific Ocean. During El Nino events, the Walker circulation weakens and its rising limb shifts eastward away from the western Pacific warm pool. At the same time, the western North Pacific subtropical high(WPSH) strengthens and stretches westward, which resulted into the weakening of the Asian summer monsoon. Note that, the Walker circulation and its rising limb shifts eastward also led to the declining of sea surface temperature(SST) in Indo-Pacific Warm Pool(IPWP), then Indo-Australian summer monsoon weakened as Madden-Julian Oscillation(MJO) weakened. Furthermore, during El Nino events, a rising of SST in the eastern equatorial Pacific led to the weakening of the temperature gradient in the eastern and western Pacific. Consequently, the low Level Jet(LLJ) in the South American monsoon region weakens with South America summer monsoon declined. Contemporary, an enhanced LLJ(low level jet) stream over North American Caribbean, result in enhanced subsidence in this monsoon region, and it turns out that North American summer monsoon may be declined in this period. Our conclusion suggests that ENSO activity enhanced could lead to a weakening summer monsoon trend in the east and west Pacific during the Late Holocene.