Characteristics and mechanisms of global monsoon changes during the 4.2 ka BP event

<p>The 4.2 ka BP event is one of the most severe centennial-scale cold and megadrought events during the Holocene, with serious influences on the collapse of the Akkadian Empire, and many other old civilizations. Based on 173 proxy records across the global monsoon regions and isotope-enabled Community Earth System Model last millennium ensemble (CESM-iLME), a group of precipitation <sup>18</sup>O reanalysis for the period 2 ka-5 ka BP were reconstructed through the hybrid gain analog offline ensemble Kalman filter (HG-AOEnKF) with the fast implementation framework of Last Millennium Reanalysis (LMR) paleoclimate data assimilation. The time series showed that the 4.2 ka BP event in all northern Hemisphere monsoon regions exhibits as monsoon weakening, but are not synchronous. Spatially, consistent with the proxy data, during the 4.2 ka BP event, the East Asian summer monsoon shows wet anomalies in the south and drought anomalies in the north, and the North African monsoon, South Asian summer monsoon, and North American monsoon show unified drought anomalies. While, all three southern Hemisphere monsoon, i.e., South American monsoon, Australian monsoon, South African monsoon, show wet anomalies during the 4.2 ka BP event. Based on model simulations, it is found that the potential mechanism behind the 4.2ka BP event is the multi-century-scale fluctuations in SSTs across the North Atlantic and AMOC strength, superimposed on the steady decline in SSTs and AMOC led by long-term changes of orbital forcing.</p>