The East Asian monsoon (EASM) plays a vital role in the region's ecology, economy, and human civilizations. Changes in monsoon precipitation have profoundly impacted human migration and the rise and fall of societies. Previous studies have explored EASM hydrological evolution, identifying bipolar and tripolar patterns, but inconsistencies remain, particularly regarding the early Holocene in northern China. Some studies suggest a wet early Holocene, while others point to a dry period. This research seeks to resolve this controversy and understand the mechanisms driving extreme hydrological events in the EASM region, particularly focusing on the role of the inter-hemispheric temperature gradient and its influence on the ITCZ and Hadley Circulation. The research uses high-resolution paleohydrological data from Daihai Lake in northern China, combined with other regional records, to reconstruct the hydrological evolution and identify the causes of abrupt drought events.

Existing literature presents a complex picture of EASM hydrological evolution. Studies have documented bipolar and tri-polar patterns of hydrological evolution in the middle Holocene, along with similarities between the Little Ice Age and the last deglaciation. The Qinling Mountains have been identified as a boundary between northern and southern hydrological regimes during the Little Ice Age. However, the existence of antiphase hydrological modes during the early Holocene remains unclear, and even the hydrological evolution of northern China during the early Holocene is debated. While some studies suggest the wettest period in northern China was during the middle Holocene, others argue it occurred in the early Holocene. Previous research has also examined various mechanisms for extreme hydrological events, including groundwater depletion and high-frequency variability of the ASM modulated by the Pacific Interdecadal Oscillation and ENSO. However, the influence of the climate system on regional drought events remains uncertain due to temporal and spatial limitations in previous studies.

This study employed a multi-proxy approach using sedimentary records from Daihai Lake in northern China to reconstruct high-resolution paleolake levels since the last deglaciation. Specifically, the researchers utilized sedimentary GDGTs (%OH-GDGTs) and δ²H as paleo-lake-level proxies. The %OH-GDGTs, produced in situ by aquatic Thaumarchaeota, were validated as a reliable indicator of water depth through a calibration function developed by combining previous lake shoreline data with %OH-GDGTs data from dated core sediments. A 1235-cm-long continuous sediment core (DH20B) was collected from Daihai Lake and subsampled at 1-cm intervals. Chronological control was established using AMS ¹⁴C dating of plant remains, bulk sediment, and pollen concentrates, with an age-depth model constructed using the Bacon program. Lipid extraction and GDGTs analysis were performed using UPLC-APCI-MS/MS, while compound-specific hydrogen isotopes (δ²H) analysis was conducted using a Trace GC coupled with a Delta V advantage isotope ratio mass spectrometer. The data from Daihai Lake was integrated with other regional paleoclimate records to characterize the hydrological evolution of northern China and the broader EASM region, with a specific focus on identifying and understanding abrupt drought events.

The study revealed an inverse hydrological pattern between northern and southern China during the last deglaciation, the middle Holocene, and the Little Ice Age (southern floods/droughts – northern droughts/floods). Northern China showed evidence for a wetness maximum in the middle Holocene supported by pollen data and increased paleosol development. High lake levels were observed in several lakes during the early to middle Holocene, suggesting a wet period. However, the study indicates that the early Holocene high lake levels in Daihai Lake might be attributed to non-monsoon precipitation recharge rather than solely monsoon intensification. In southern China, the hydrological pattern is opposite to that in northern China, except during the early Holocene. At least seven well-defined drought events were identified: 11.7–11.3 cal kyr BP, 10.6–10.2 cal kyr BP, 9.6–9 cal kyr BP, 7.6–7.3 cal kyr BP, 7–6 cal kyr BP, 4.7–4.2 cal kyr BP, and 3.9–3.4 cal kyr BP. These events were almost synchronous throughout the EASM region, marked by low precipitation, humidity, and lake salinization. The early Holocene drought events in northern China appear to be independently driven by freshwater forcing, possibly linked to ice sheet meltwater impacting thermohaline circulation and ASM northward extension. The study further suggests that extreme drought events during the middle and late Holocene are linked to a weakened inter-hemispheric temperature gradient, leading to a southward shift of the ITCZ and suppression of the ASM by the Hadley circulation. Intensification of the Walker circulation and westward shift of the WPSH, driven by changes in the zonal SST gradient in the tropical Pacific, also contributed to these droughts.

The findings of this study offer new insights into the mechanisms driving extreme drought events in the EASM region. The results strongly suggest that changes in the inter-hemispheric temperature gradient, influencing the ITCZ and Hadley circulation, are critical in explaining the synchronous drought events during the middle and late Holocene. The influence of the ITCZ, coupled with sea-level variations, helps explain inconsistencies in some past climate records. While ice-sheet forcing played a role in early Holocene droughts, it's insufficient to explain the more extreme events of the middle and late Holocene. The study emphasizes the importance of considering both latitudinal and zonal SST gradients in the tropical Pacific to fully understand the complexities of EASM hydrological variability. The study highlights the crucial interplay between large-scale atmospheric circulation patterns and regional hydrological responses.

This study provides a comprehensive, high-resolution reconstruction of EASM hydrological evolution, clarifying discrepancies regarding the early Holocene in northern China and identifying the crucial role of the inter-hemispheric temperature gradient in regulating extreme droughts. The findings demonstrate the interconnectedness of large-scale climate patterns and regional hydrological changes, offering valuable insights for understanding future climate change impacts on the EASM region. Future research could focus on refining regional-scale climate models to better capture the dynamics of the ITCZ and Hadley Circulation, and further exploring the interactions between these large-scale patterns and regional hydrological systems.

The study's primary limitation is the reliance on a single lake (Daihai Lake) for high-resolution data in northern China. While this lake provides valuable insights, expanding the analysis to include more sites across the region would enhance the robustness and spatial coverage of the findings. Additionally, while the study successfully establishes a link between inter-hemispheric temperature gradients and drought events, further investigation is needed to quantify the precise relationships and thresholds for these events. Finally, the study focuses on past climatic events; the direct applicability of these findings to the prediction of future droughts needs further research.