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In the Indian monsoon realm, spatial homogeneity of hydroclimate response to glacial abrupt climate events in the North Atlantic Ocean and its underlying mechanism are contentious. Here we reconstruct sea surface temperature (SST) and seawater oxygen isotope (δ18O sw) in the central Andaman Sea, which span over the past 26,000 years, to investigate the potential driving mechanism of millennial-scale climate changes in tropical Indian Ocean. Our results demonstrate that millennial-scale SST and δ18O sw changes during the last deglaciation are associated with changes in Asian monsoon system which follows North Atlantic climate changes. In particular, during the transition from BØlling/AllerØd (B/A) to Younger Dryas (YD), the strengthening of East Asian winter monsoon (EAWM) promotes southward invasion of northern cold air masses to Indo-China Peninsula, cooling the Andaman Sea. Meanwhile, the enriching δ18O sw is a consequence of reducing local rainfall amount and Irrawaddy River inflow due to a weakening Indian summer monsoon (ISM). With aid of an isotope-enabled climate model, our results further demonstrate that the changes in EAWM and ISM during B/A-YD transition are closely coupled with variations of Atlantic meridional overturning circulation (AMOC), corroborating its pivotal role in millennial-scale changes in tropical Indian Ocean hydroclimate during the last deglaciation. • Andaman Sea characterizes abrupt changes in sea surface temperature and δ18O of seawater during the last deglaciation. • δ18O sw and precipitation are characterized by spatial homogeneity of response to abrupt climate change in North Atlantic. • Millennial-scale temperature changes in Andaman Sea show a local feature controlled by East Asian winter monsoon. [ABSTRACT FROM AUTHOR]

The ocean heat content (OHC) and associated surface stratification in tropical oceans are ideal variables to monitor global climate change. However, the variations in surface stratification of the tropical Indian Ocean at the millennial time scale have been less understood. In this context, the present study aimed at reconstructing the sea surface temperature (SST) and thermocline water temperature (TWT) using Mg/Ca of planktonic foraminifera in the sediments of the Andaman Sea, which is one of the hot centers in the Indo-Pacific warm pool (IPWP) over the past 42 ka. The study results confirm the SST changes are consistent in response to the high-latitude temperature records with strong sub-orbital and millennial variations, while the TWT patterns trend remarkably different from the millennial-scale SST and show the variations correspond to the natural summer insolation that triggers the latitudinal movements of the Intertropical Convergence Zone (ITCZ). Furthermore, salinity influences the thermal differences (ΔT=SST-TWT) by varying the vertical temperature and stratification. Analysis of available data shows that the mean northward position of ITCZ would input more freshwater into the Andaman Sea. Freshwater lids can reduce mixing by acting as effective barriers and preventing heat from transferring from the surface to the subsurface, thereby causing subsurface cooling and increased stratification. In contrast, more southward ITCZ (boreal winter monsoon) supplies less freshwater into the Andaman Sea. A highly saline surface water and air cooling promotes mixing and subsurface warming, resulting in decreased stratification. The study results reveal a millennial-scale vertical thermal seesaw synchronizing in the tropical Indian Ocean with abrupt Atlantic meridional overturning circulation (AMOC) changes. During the Bolling-Allerod (B/A) warming period, the dynamic AMOC maintains the surface water stratification by controlling the subsurface cooling processes with increased precipitation and freshwater brought by the northward ITCZ. In contrast, Younger Dryas (YD) and Heinrich events (H) are characterized by reduced summer monsoon precipitation associated with the sluggish AMOC and southward ITCZ, resulting in less stratification and subsurface warming in the Andaman Sea. The thermal proxy records adopted in the study afford robust evidence on the millennial-scale variability in surface stratification driven synchronously by AMOC and regional evaporation vs. precipitation changes in the tropical Indian Ocean, offering new insights into the impacts of the IPWP OHC on abrupt climate changes. • Millennial-scale climate variability was reconstructed by SST and TWT in the Andaman Sea. • Salinity influences the thermal differences (ΔT) by varying the vertical temperature and stratification. • Millennial scale variability of surface stratification was driven synchronously by AMOC and tropical dynamic changes. [ABSTRACT FROM AUTHOR]