Drought in the Asian summer monsoon region is linked to a weakened inter-hemispheric temperature gradient

The welfare of the large human population of East Asia is closely linked to East Asian monsoon precipitation, which underpins regional ecosystems, economies, and the development of human civilizations. Historical examples include Neolithic population migrations across the Hu Line in response to precipitation changes and links between civilization collapses and extreme drought events. Hence, understanding the hydrological evolution of the East Asian monsoon region—especially the spatiotemporal pattern and origins of droughts—is crucial for ecosystem maintenance and socioeconomic planning. Previous research has addressed hydrological evolution across multiple timescales, identifying bipolar and tri-polar patterns in the middle Holocene, similarities between hydrological modes during the Little Ice Age (LIA) and the last deglaciation, and the Qinling Mountains as a boundary between northern and southern hydrological regimes during the LIA. However, it remains unclear whether antiphase hydrological modes existed in the early Holocene, and even the early Holocene hydrological evolution of northern China is debated—whether the wettest interval occurred in the middle Holocene or the early Holocene. Some lake records (e.g., Diaojiaohaizi, Daihai) indicate prolonged early Holocene dryness in northern China, whereas monsoon intensity on orbital timescales tracks 65°N insolation. Proposed mechanisms for extreme hydrological events include local geomorphic and hydrological changes (e.g., groundwater sapping in Hunshandake Sandy Land), and high-frequency ASM variability modulated by phenomena such as the PDO and ENSO. These foci impose temporal and spatial limitations, creating uncertainty about the broader climate system’s role in regional droughts. Closed lakes primarily balance precipitation inputs against evaporation and runoff; alpine closed lakes can serve as rain gauges, though runoff and evapotranspiration can bias precipitation estimates. Former shoreline elevations, when well dated, allow quantitative paleo-lake level reconstructions, yet such records are often fragmentary. Recently, %OH-GDGTs and %Cren biomarkers have been validated as paleo-lake-level proxies, enabling higher-resolution reconstructions. In this study, sedimentary GDGT (%OH-GDGTs) and compound-specific δ2H records are used to reconstruct Daihai Lake levels (northern China) since the last deglaciation. Combined with regional records, these reconstructions are used to characterize hydrological evolution and abrupt drought events in northern China and to infer broader ASM evolution and its drivers.

Studies have documented hydrological evolution across the ASM region at various timescales, identifying: (1) bipolar and tri-polar hydrological patterns in the middle Holocene; (2) analogous hydrological modes during the LIA and the last deglaciation; and (3) the Qinling Mountains as a persistent boundary between northern and southern hydrological regimes during the LIA. Debate persists regarding early Holocene conditions in northern China: pollen-based studies and loess paleosols often support a middle Holocene wetness maximum, whereas other lake records (e.g., Diaojiaohaizi, Daihai) indicate early Holocene aridity in northern China. On orbital scales, ASM intensity broadly follows 65°N insolation. Mechanisms invoked for extreme hydrological events include geomorphic-hydrologic feedbacks (e.g., groundwater sapping driving irreversible drought and cultural impacts) and precipitation anomalies associated with high-frequency ASM variability modulated by PDO/ENSO. Lake shoreline archives can quantify paleo-lake levels but are often discontinuous, and lake water balance can be influenced by runoff and evapotranspiration. Newly validated biomarkers (%OH-GDGTs, %Cren) offer quantitative, high-resolution lake-level reconstructions that can help resolve spatial-temporal patterns and test mechanisms.

Study site: Daihai Lake (40°29′–40°37′N, 112°33′–112°46′E; 1218 m a.s.l.) is a hydrologically closed lake in arid–semiarid northern China (Inner Mongolia), mainly recharged by precipitation and six rivers. Mean annual temperature is 5.9 °C; precipitation is 406.4 mm (≈80% in summer); evaporation exceeds 1100 mm. Maximum modern water depth is ~7 m. Sampling: In October 2020, a continuous 1235-cm sediment core (DH20B; 40.57858°N, 112.672831°E) was retrieved from 6 m water depth using a UWITEC piston corer. The core was sub-sampled at 1-cm resolution and freeze-dried for lipid analysis. Chronology: Sixteen AMS 14C dates were obtained (4 plant remains, 8 bulk sediment, 4 pollen concentrates). Plant remains and bulk sediment were dated at Beta Analytic; pollen concentrates at Lanzhou University. An age–depth model was built using Bacon in R with the IntCal20 calibration curve; the basal age was extrapolated. Proxy rationale and calibration: Group I.1a aquatic Thaumarchaeota produce OH-GDGTs in subsurface waters; %OH-GDGTs respond to water depth, enabling quantitative paleo-lake-level reconstruction. Soil Group I.1b Thaumarchaeota contribute Cren and Cren′, potentially biasing source apportionment of these compounds in sediments; however, OH-GDGTs production is in-lake, with OH-GDGTs detected in surface sediments but not soils. A calibration relating %OH-GDGTs to water depth was developed by combining dated Daihai shoreline data with %OH-GDGTs from corresponding core intervals, showing a positive correlation. Note: calibration lacks shallow-water controls (<25 m), potentially biasing reconstructions at low lake levels. Lipid extraction and GDGT analysis: 211 core samples, 5 soil samples, and 4 surface sediment samples were extracted (DCM:methanol 9:1, ultrasonic, N2-dried), fractionated via silica gel columns (hexane, DCM, methanol), and the polar fraction (GDGTs) analyzed by UPLC-APCI-MS/MS (Agilent 1290 II + 646B). Separation used two Hypersil Gold Silica columns (150 mm × 2.1 mm × 1.9 µm) at 40 °C; flow 0.3 mL/min; 10 µL injection; solvent program from 84% n-hexane/16% ethyl acetate to 100% B for OH-GDGTs separation, with SIM monitoring of characteristic m/z values. Analyses conducted at Lanzhou University. Compound-specific hydrogen isotopes: δ2H determined using Trace GC–Delta V Advantage IRMS (China University of Geosciences, Wuhan). Injector at 280 °C; oven: 50 °C (1 min), 10 °C/min to 210 °C (2 min hold), 4 °C/min to 300 °C, then 10 °C/min to 310 °C. Internal standard: squalane (δ2H = −167‰). Results reported relative to VSMOW. Data availability: New Daihai hydrologic reconstruction data available at https://doi.org/10.6084/m9.figshare.25107689; other datasets sourced from published records (precipitation, shoreline, cave δ18O, ENSO variability, tropical Pacific SSTs, hemispheric temperature differences, etc.).

- A high-resolution Daihai Lake record since the last deglaciation, using %OH-GDGTs calibrated to water depth and compound-specific δ2H, reveals hydrological evolution at the northern EASM margin. A strong positive correlation between %OH-GDGTs and water depth supports quantitative lake-level reconstructions (with noted shallow-water limitations). - Regional synthesis shows: (1) during the early Holocene, both northern and southern China were generally wet, although two significant early Holocene droughts occurred in northern China under the influence of Northern Hemisphere ice sheets; (2) a dipolar pattern of “southern drought–northern floods” developed in the middle Holocene; and (3) mid- to late-Holocene saw synchronous extreme droughts across the ASM region despite the dipole. - Seven well-defined drought events across the ASM region were identified at approximately: 11.7–11.3, 10.6–10.2, 9.6–9.0, 7.6–7.3, 7–6, 4.7–4.2, and 3.9–3.4 cal kyr BP, characterized by decreased precipitation, reduced humidity, lake salinization, and falling water levels. - Early Holocene high lake levels in northern China likely reflect both monsoon precipitation and non-monsoon recharge (e.g., snow/ice meltwater), consistent with depleted δ18O of lake water during the early–middle Holocene and shoreline evidence of highstands. - Freshwater forcing and Northern Hemisphere cryospheric influence during deglaciation/early Holocene impeded northward advance of ASM precipitation and contributed to early Holocene droughts in northern China by weakening Atlantic overturning and monsoon rainfall. - In the middle to late Holocene, changes in the inter-hemispheric temperature gradient (ΔT_N–S) drove southward ITCZ shifts and strengthened Hadley circulation, suppressing EASM penetration into interior East Asia; concurrently, enhanced zonal SST gradients in the tropical Pacific strengthened the Walker circulation and shifted the WPSH westward, promoting broad ASM-region drought conditions. - Sea level rise to near-modern levels by mid-Holocene limited the role of changing land area/water vapor availability; thus, latitudinal (ΔT_N–S/ITCZ/Hadley) and zonal (ΔT_W–E/Walker/WPSH) controls best explain the synchronous extreme droughts in the mid–late Holocene.

The study addresses the long-standing question of how large-scale climate dynamics generated abrupt hydrological declines in the ASM region. Findings show that while early Holocene conditions were broadly wet in both northern and southern China, northern China experienced distinct droughts driven by cryosphere–ocean interactions. Deglacial meltwater pulses likely weakened North Atlantic overturning and ASM precipitation, creating a mismatch between monsoon circulation and precipitation extent that limited the northward reach of the rainbelt. As the Holocene progressed and sea level stabilized, mid–late Holocene drought synchrony across the ASM region is best explained by changes in the inter-hemispheric temperature gradient and resulting southward ITCZ shifts, intensified Hadley circulation (enhancing low-level northeasterlies and blocking northward monsoon penetration), and strengthened Walker circulation associated with increased tropical Pacific zonal SST gradients and westward WPSH displacement. These mechanisms together suppressed monsoon rainfall across East Asia, yielding regionally coherent droughts despite an overall dipolar hydrological pattern. The integration of Daihai lake-level reconstructions with regional records (pollen-based precipitation, lake shorelines, peat biomarkers, cave δ18O, ENSO variability, tropical Pacific SSTs, hemispheric temperature contrasts) provides a coherent framework linking observed drought timing with large-scale ocean–atmosphere dynamics. The results underscore the sensitivity of the EASM to inter-hemispheric thermal gradients and tropical Pacific SST gradients, highlighting pathways through which future inter-hemispheric and tropical changes could induce widespread East Asian drought.

This work provides a quantitative, high-resolution reconstruction of lake-level changes at Daihai Lake since the last deglaciation using %OH-GDGTs and δ2H, and synthesizes multi-proxy regional evidence to identify seven synchronous drought events across the ASM region during the Holocene. It clarifies that early Holocene northern China droughts were linked to cryosphere-driven freshwater forcing, while mid–late Holocene extreme droughts were governed by a weakened inter-hemispheric temperature gradient (southward ITCZ shifts, strengthened Hadley circulation) and enhanced tropical Pacific zonal SST gradients (stronger Walker circulation and westward WPSH). These mechanisms explain the emergence of a mid-Holocene dipolar hydrological pattern and the occurrence of widespread extreme droughts thereafter. Future research should: (1) improve %OH-GDGTs–depth calibrations with shallow-water data to reduce low-level biases; (2) extend similar high-resolution reconstructions to additional lakes across East and South Asia for spatial validation; (3) develop coupled model–proxy experiments to quantify the relative roles of ΔT_N–S, ITCZ shifts, and ΔT_W–E in driving ASM hydroclimate; and (4) refine chronologies of sea-ice and meltwater forcing to better resolve early Holocene drought mechanisms.

- The %OH-GDGTs–water-depth calibration lacks data for shallow depths (<25 m), potentially biasing reconstructions at low lake levels. - Contributions of soil-derived Cren and Cren′ (from terrestrial Group I.1b Thaumarchaeota) may complicate source apportionment of some GDGTs in sediments, although OH-GDGTs are produced in-lake. - Closed-basin lake level reflects integrated precipitation–evaporation–runoff balance; surface runoff and evapotranspiration can bias precipitation inference. - Shoreline records used for regional synthesis can be fragmentary due to hydrodynamic perturbations or absent shoreline formation, potentially limiting spatial completeness. - Some regional droughts (e.g., around 7–6 cal kyr BP) may exhibit spatial discontinuities in lake shoreline expression, complicating characterization of event extent and severity.