Resonant Asian Monsoon During Intermediate Conditions of the Last Deglaciation: Insights From Speleothem Records

Extensive research has explored how sweet spot conditions influence climate variability in the nonlinear Earth system. However, their effects throughout deglaciations remain unclear. Based on an annual-laminated speleothem δ¹⁸O record with unprecedented chronology precision, we first identified a two-step termination of the Asian Heinrich Period-1 at 15.11 and 14.69 ky BP (thousand years before present, where the present is 1950 CE), each marked by a centennial-scale strengthening of the Asian summer monsoon (ASM), in contrast to centennial-scale ASM weakening events observed at 17.8 and 16.09 ky BP. These transitions occurred under intermediate CO₂ and ice volume conditions, which dynamically paced the Atlantic Meridional Overturning Circulation (AMOC) fluctuations and the subsequent reoccurrence of ASM events. It highlights the role of stochastic resonance in nonlinear climate system. Additionally, these AMOC/ASM events also resonated with abrupt CO₂ risings across various pervasive modes of variability, which account for half of total CO₂ rise during last deglaciation. This substantial contribution to CO₂ increases played a key role in ice termination.