Abstract
Understanding the dynamics between the East Asian summer (EASM) and winter monsoon (EAWM) is needed to predict their variability under future global warming scenarios. Here, we investigate the relationship between EASM and EAWM as well as the mechanisms driving their variability during the last 10,000 years by stacking marine and terrestrial (non-speleothem) proxy records from the East Asian realm. This provides a regional and proxy independent signal for both monsoonal systems. The respective signal was subsequently analysed using a linear regression model. We find that the phase relationship between EASM and EAWM is not time-constant and significantly depends on orbital configuration changes. In addition, changes in the Atlantic Meridional Overturning circulation, Arctic sea-ice coverage, El Niño-Southern Oscillation and Sun Spot numbers contributed to millennial scale changes in the EASM and EAWM during the Holocene. We also argue that the bulk signal of monsoonal activity captured by the stacked non-speleothem proxy records supports the previously argued bias of speleothem climatic archives to moisture source changes and/or seasonality.
Highlights
The East Asian monsoon system (EAM), which includes both the summer and winter Asian monsoons (EASM and EAWM), affects the hydroclimate variability of large parts of South-East Asia (Fig. 1A and B)[1,2]
During the Early Holocene to Mid-Holocene both East Asian summer (EASM) and EAWM show synchronous periods of weakening (~ 9 kyrs), strengthening (~ 6–8 kyrs) and again weakening (~ 4 kyrs). This synchronous behaviour seemingly changes after ~ 4 kyrs as both monsoonal systems diverge: while the EAWM is characterized by a steady ~ 3 kyr increase in strength from the mid-Holocene towards the modern era, the EASM shows no long-term trend during the late Holocene but instead another strong reduction at ~ 0.6 ka, which is not mimicked by the EAWM
This change in relationship between EASM and EAWM is clearly depicted in the temporal evolution of the correlation coefficient between both stacks (Fig. 2D) which demonstrates that both monsoonal systems were strongly correlated during the Early to Mid-Holocene and subsequently disconnected with experiencing partially opposing trends during the Late Holocene
Summary
The East Asian monsoon system (EAM), which includes both the summer and winter Asian monsoons (EASM and EAWM), affects the hydroclimate variability of large parts of South-East Asia (Fig. 1A and B)[1,2]. During late spring to summer, the large thermal contrast between the warm Asian continent and the adjacent colder oceans leads to moist-laden air from the Indic and Pacific Ocean to be traversed into East Asia This initiates EASM precipitation from southern China to the Korean Peninsula and Japan[3]. The relationship between AMOC and EASM has gathered substantial interest over the previous decade, as the 8.2 kyr cooling event, probably caused by a meltwater outburst that slowed the AMOC18,19, is proposed to have triggered a strong centennial-long reduction of EASM In this scenario, the reduction of the AMOC would enforce a decreased northward meridional heat transport which leads to cooling over North America and the formation of an anomalous high-pressure c ell[20].
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