Decoupling of East Asian and Indian summer monsoons since the middle of the Late Holocene

Abstract

Substantial hydroclimate archives suggest that the Asian Summer monsoon (ASM) is driven directly by summer hemispheric solar insolation on millennial to orbital timescales. In addition, precisely dated speleothems from the two regimes of ASM subsystems show strong coherence between East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) rainfall variability on millennial to orbital timescales. Further, Holocene stalagmites from the ASM domain exhibit a synchronous trend following the northern hemispheric summer insolation general declining trend throughout the Holocene. However, the co-evolution and synchronous variations in the two monsoon domains of ASM were often debated. In this study, the high-resolution oxygen isotope (δ18O) profiles of  two precisely dated stalagmites (hereafter Kadapa composite), collected from the Deccan Indian cave, established with 2913 oxygen isotope data and 60 U/Th ages, provide a continuous history of ISM intensity for the last three millennia i.e., the late Holocene. The Kadapa composite indicates large variability with the disappearance of the declining trend in ISM strength around the middle of the late Holocene, particularly at the end of the Roman Warm Period (RWP). The preliminary analysis using multi-proxy data in and around the subcontinent suggests an abrupt hydroclimate shift at the end of RWP, i.e., ~1.7-1.8 kyr BP and high-resolution stalagmite data from both monsoon regimes reveal decoupling of EASM and ISM since the end of RWP. It is also noted that this hydroclimate shift is not limited to the ASM domain but rather observed from major hydroclimate zones around the globe. However, this hydroclimate shift and decoupling could remain overlooked. This monsoon hydroclimate shift was rapid and took a few decades to a century worldwide and was a departure from the solar insolation-driven drying trend, continuing since the mid-to-late Holocene. Furthermore, the analyses reveal a dramatic increase in positive Indian Ocean Dipole events, ENSO variability, and intensification of El Niño and La Niña events around 1.7-2.0 kyr BP. Combined with different marine and terrestrial records, we suggest that the tropical Indian and Pacific ocean-atmospheric circulation could initialize a mean monsoon climate shift in the middle of the Late Holocene. Though the solar forcing remains one of the factors of orbital to millennial-scale ISM rainfall variation, tropical Indo-Pacific ocean-atmospheric interactions may dominate and dictate the ISM rainfall variability on sub-millennial time scales during weakened Solar Insolation. Additional high-resolution multiproxy paleoclimate and simulations are needed to understand such monsoon climate shifts, to mitigate the future abrupt hydroclimate shifts under the current global warming trend.