Changes in the Southern Hemisphere westerlies during last interglacial marine isotope stage 5e and its linkage to the Asian summer monsoon

Considerable ambiguity remains over the triggers for the abrupt climate deterioration that terminated the Last Interglacial Marine Isotope Stage 5e (MIS5e) period. A prevailing explanation is a sudden reduction in Atlantic Meridional Overturning Circulation (AMOC) in response to the reduced boreal summer insolation. Nonetheless, this opinion is challenged by the early weakening of the Asian summer monsoon (ASM) documented in the Chinese speleothem oxygen isotope (δ¹⁸O) ratio records. Here we propose a new mechanism involving a northerly shift of the Southern Hemisphere mid-latitude westerlies (SHW), based on saturation magnetization (M_s) and stable carbon (δ¹³C) and oxygen isotopes from a speleothem in northern Tasmania. The M_s record shows high-frequent peaks at 122.8–119.7 ka B.P., while δ¹⁸O and δ¹³C ratios first increase significantly (122.8–122.2 ka B.P.) and then decrease (122.2–119.7 ka B.P.). These M_s peaks suggest a northward shift of SHW at 122.8–119.7 ka B.P., which precedes the earliest ASM weakening for MIS5e demise and coincides with the reduced Agulhas leakage. This indicates that when SHW reaches a northerly position, the reduced Agulhas leakage leads to an anomalous warming in the southern Indian Ocean, further weakening the cross-equatorial flow. Such a process would aggravate the ASM weakening caused by the decline in boreal summer insolation and initiate an early MIS5e demise.