Muted millennial-scale monsoon fluctuations from Southwest China

The South Asian Summer Monsoon (SASM) plays a key role in controlling precipitation in the South Asia. Orbital and millennial SASM precipitation changes have been, respectively, linked to boreal high-latitude summer insolation and North Atlantic climate. However, causes for the heterogenous spatial SASM precipitation patterns remain debated. Here, we report SASM precipitation histories based on multiproxy evidence from the northwest South China Sea during the past 40 ka. Using fine-grained magnetic minerals as a proxy for precipitation, we find that SASM precipitation in Southwest China varied following the local insolation with muted millennial-scale fluctuations, contrasting to evolutions in other Asian monsoon regions. Transient model simulations reveal that local orbital insolation generated a tripole precipitation pattern over the East Asia by influencing atmospheric circulation during the early last deglaciation, with high orbital insolation corresponding to increased precipitation in Southwest China. Our study highlights influences of local insolation on East Asian precipitation patterns on orbital and millennial timescales.南亚夏季风（South Asian Summer Monsoon, SASM）是调控南亚地区降水过程的关键因子。轨道尺度与千年尺度的南亚夏季风降水变化，分别与北半球高纬度夏季日照及北大西洋气候存在关联。然而，南亚夏季风降水空间分布异质性的成因仍存在广泛争议。本研究基于南海西北部过去4万年的多代用指标证据，重建了南亚夏季风降水演化历史。研究以细粒磁性矿物作为降水代用指标，发现中国西南地区的南亚夏季风降水随本地日照辐射变化，且千年尺度波动较为平缓，这与亚洲季风其他区域的演化特征形成鲜明对比。瞬态模式模拟结果显示，末次冰消期早期，本地轨道日照通过影响大气环流，在东亚地区形成了三极型降水格局，其中高轨道日照对应中国西南地区降水增多。本研究揭示了本地轨道日照在轨道与千年尺度上对东亚降水格局的调控作用。