Westerly jet waviness modulates mid-latitude hydroclimate variability

Domain: earth_science
Published: 2025-11-23
Editorial outcome: major_revision

Abstract
The mid-latitude ecological environment is highly vulnerable, and increasing hydroclimate variability poses a significant threat to it. However, the mechanisms driving this variability remain unclear due to short observational records and climate model biases. Here, we present a sub-millennial resolution precipitation record spanning the past ~5.7 million years (Myr), derived from a 300.8-meter fluvio-lacustrine sediment core in mid-latitude East Asia (northern China). Our results reveal a persistent influence of the westerly jet on mid-latitude precipitation since the Pliocene, as well as a marked reduction in mid-latitude precipitation variability alongside amplified low-latitude variability after ~3 million years ago (Ma). Based on idealized sensitivity simulations, we attribute the amplified mid-latitude precipitation variability before ~3 Ma to enhanced westerly jet waviness driven by Arctic warming. Given projected increases in jet waviness under future global warming, we predict more frequent hydroclimate extremes in mid-latitude regions.

Introduction
Global mean precipitation has increased over the past century due to global warming1. However, a more critical concern is the progressively increasing precipitation variability (Fig. 1b; Supplementary Fig. 1)2–6. Amplified precipitation variability leads to frequent transitions between extreme dry and wet conditions, posing formidable threats to livelihoods and socio-economic development4—particularly in mid-latitude regions with fragile ecosystems.

Substantial research efforts in recent decades have characterized precipitation patterns across various timescales in mid-latitude regions7–16. However, few studies have specifically examined precipitation variability. Current understanding of mid-latitude precipitation variability primarily depends on modern observations and climate models outputs. Two dominant but contested mechanisms have been proposed to explain increased precipitation variability (Supplementary Fig. 1): (1) increased atmospheric moisture content associated with rising global CO2 levels3, and (2) enhanced waviness of the westerly jet (Fig. 1b)17–22. The CO2 mechanism finds stronger support in climate modeling studies1,23–25, while the jet waviness hypothesis relies more on statistical correlations from observational and reanalysis data with plausible physical mechanisms26,27. Furthermore, the relatively prominent mid-latitude precipitation variability (Supplementary Fig. 1) also underscores the importance of the westerly jet in driving this variability. Disentangling the relative contributions of jet waviness and atmospheric CO2 concentration to mid-latitude precipitation variability remains challenging. Some modeling studies even suggest that observed trends in jet waviness may reflect internal climate variability rather than a persistent signal28–31. These short observational records and climate model biases contribute substantial uncertainty to projections of mid-latitude precipitation variability26,32.

This study presents a sub-millennial resolution precipitation records spanning the past 5.7 million years (Pliocene to Pleistocene) (Supplementary Fig. 2; Supplementary Note 1), derived from a 300.8-meter sedimentary core composed primarily of fluvial and lacustrine deposits (Supplementary Fig. 3) from the Datong Basin (40°03′N, 113°26′E; 1043 m above sea level) in the Chinese loess plateau (CLP), mid-latitude East Asia (Fig. 1a; Supplementary Fig. 4; Supplementary Note 2). This region exhibits exceptional sensitivity of the westerly jet to global climate change (see below). Contrary to conventional views, our records reveal a significant intensification of precipitation commencing at ~3 Ma, coinciding with Northern Hemisphere ice sheet expansion. Crucially, we provide the evidence that Pliocene-Pleistocene precipitation was primarily controlled by a 1.2 Myr obliquity amplitude modulation (AM) cycle. This orbital signal simultaneously paced summer meridional sea surface temperature (SST) gradients in both the North Atlantic and Pacific as well as the westerly jet stream. Through idealized climate simulations combined with geological proxy integration, we demonstrate that mid-latitude precipitation was modulated by westerly jet intensity, which in turn responded to meridional summer insolation gradients and Northern Hemisphere ice volume. Our quantitative analysis of precipitation variability (see Methods), based on moving standard deviation calculations, reveals a striking contrast between epochs: mid-latitude precipitation variability was significantly amplified during the Pliocene (pre-3 Ma) compared to the Pleistocene (post-3 Ma), exhibiting an inverse relationship with low-latitude patterns. We propose that this enhanced Pliocene mid-latitude hydroclimate variability primarily resulted from increased westerly jet waviness rather than elevated atmospheric CO2 concentrations.

Results
Westerly jet influences mid-latitude precipitation at ~3 Ma
Sedimentary facies analysis of the core reveals three distinct depositional stages, documenting water-level fluctuations and hydroclimate conditions in the closed-basin system of Datong (see Methods). These stages are consistently supported by covariations in grain size, sediment color, and elemental composition (Supplementary Fig. 3). All results indicate enhanced precipitation after ~3 Ma compared to earlier periods. The Sr/Ca ratio (Fig. 2a) emerges as the most sensitive and high-resolution precipitation proxy, and was therefore selected as a representative proxy for further analysis. This wetting trend is corroborated by: (1) deep-core records from the nearby North China Plain (Fig. 2b, c)33–35, and (2) aeolian sequences from the CLP (Fig. 2d)36,37, collectively indicating a regional shift toward wetter conditions in mid-latitude East Asia after ~3 Ma. The significantly lower (higher) content of goethite (hematite) in red clay than in the overlying loess and paleosol38 also indicated increased precipitation over the CLP after ~3 Ma, for that warm and dry conditions favored the formation of hematite, while cold and wet conditions favored the formation of goethite39–41. Comparable hydroclimate changes are evident in European mid-latitude records42. Notably, Central Asia exhibited an opposite trend10,43, becoming drier after ~3 Ma (Fig. 2e), although it is also located in the mid-latitudes.

The Datong Basin’s water vapor is primarily transported by the East Asian summer monsoon (EASM) (Supplementary Fig. 1). While Tibetan Plateau uplift has influenced precipitation through intensifying EASM44, the timing remains debated for the Late Pliocene45. Nevertheless, the EASM reached its modern northern boundary as early as ~26 Ma46, suggesting plateau uplift played a minimal role in increased precipitation after ~3 Ma. The observed wetting trend after ~3 Ma was accompanied by a significant expansion of Northern Hemisphere ice volume (Fig. 2g–i)47–49 and a considerable decrease in atmospheric CO2 50. To investigate this relationship, we conducted idealized sea-ice sensitivity experiments (see Methods) and analyzed ice sheet and greenhouse gas (GHG)-forced precipitation anomalies using the TraCE-21ka (Simulation of the Transient Climate of the Last 21,000 Years) dataset51, derived from a fully coupled simulation experiment using the CCSM3 (Community Climate System Model, version 3). The resulting precipitation anomalies forced by increased sea ice exhibit a meridional tripolar pattern over East Asia (Fig. 2j), matching both modern observations (Supplementary Fig. 1) and geological records of summer precipitation changes across the ~3 Ma transition. This pattern includes: (1) drying in mid-latitude Central Asia (Fig. 2e)10,43 and low-latitude Southeast Asia (Fig. 2f)52, and (2) wetting in mid-latitude East Asia (Fig. 2a–d). These results are broadly consistent with both the ice-sheet-forced precipitation anomalies in the TraCE-21ka simulation (Supplementary Fig. 5) and a CESM1.0.4 (Community Earth System Model version 1.0.4) experiment that simulated increased sea ice under Pliocene conditions (Supplementary Fig. 6)53. In contrast, the precipitation anomalies forced by reduced GHG exhibit patterns completely opposite to those documented in the geological records across East Asia (Supplementary Fig. 5). This discrepancy effectively rules out a dominant influence of CO2 on precipitation changes during the ~3 Ma transition. The sea ice sensitivity experiments were not fully coupled, but only the atmospheric model was run (see Methods), thus, the mechanistic discussion can only focus on atmospheric processes. Although the intensification of the EASM (Fig. 2j) can transport more moisture, where this moisture ultimately precipitates is primarily controlled by convective conditions54,55. Increased precipitation after ~3 Ma over mid-latitude East Asia (Fig. 2j) was thus mainly attributed to the enhanced ascending motion of the air (Fig. 2l) caused by the strengthened westerly jet (Fig. 2k). This influence of the westerly jet on East Asia precipitation has been proved by multiple climate models54,56–58. A similar mechanism likely explains European mid-latitude wetting42, where the westerly jet dominates moisture transport22. Conversely, Central Asia’s drying reflects reduced atmospheric water content despite westerly moisture transport11,15,59,60. These findings collectively demonstrate the westerly jet’s dominant control on mid-latitude precipitation patterns at ~3 Ma.

Westerly jet influences orbital timescale mid-latitude precipitation
Spectral analysis of the Sr/Ca ratio identifies conventional Milankovitch cycles, including 100 kyr and 400 kyr eccentricity, 40 kyr obliquity, 20 kyr precession, and 10 kyr half-precession cycles (Fig. 3; Supplementary Figs. 7, 8). These orbital cycles are known to dominate East Asian precipitation dynamics on different timescales7,9,61–63. Notably, our analysis reveals two significant obliquity AM cycles (173 kyr and 1.2 Myr) that are rarely documented in Pliocene-Pleistocene East Asian precipitation records. These long-period cycles have been demonstrated to influence: (1) Triassic sea-level variations64, (2) Paleocene-Eocene precipitation patterns in mid-latitude East Asia65, and (3) Miocene-Pliocene hydroclimate in mid-latitude Central Asia66,67.

The precipitation record from the Datong core is predominantly characterized by a 1.2 Myr cycle (Fig. 3e, f, j, k; Supplementary Figs. 7, 8). Obliquity plays a fundamental role in controlling the latitudinal distribution of incoming solar radiation at the top of the atmosphere. During summer months, higher obliquity increases insolation at high latitudes while decreasing it at low latitudes68, thereby reducing the meridional insolation gradient. Crucially, the 1.2 Myr cycle dominates the AM of summer insolation gradients between 45°N and 20°N (Fig. 3c, d). Using alkenone (UK’37) as a proxy for summer sea surface temperatures (SSTs)69, our calculated summer North Pacific and Atlantic meridional SST gradients since the Pliocene similarly exhibit a dominant 1.2 Myr cycle (Fig. 3g, i, l, n). Given the well-established sensitivity of the westerly jet to meridional temperature gradients17,70–73, we propose that the 1.2 Myr precipitation cycle in the CLP is mechanistically linked to obliquity-forced variations in the summer westerly jet, since the EASM intensity, influenced by land-sea thermal contrast, is mainly modulated by precession61,74,75. Further supporting this connection, the 1.2 Myr cycle is also evident in: (1) Variations of Antarctic Circumpolar Current strength (Supplementary Fig. 9), which is modulated by the Southern Hemisphere westerly jet76, and (2) Miocene-Pliocene precipitation patterns in westerly-dominated Central Asia66,67. Notably, this 1.2 Myr climatic rhythm operates independently of Northern Hemisphere ice volume variations, as evidenced by its absence in benthic δ18O records (Supplementary Fig. 10)49.

To elucidate the relationships among obliquity, westerly jet dynamics, and precipitation patterns, we conducted fully coupled idealized obliquity sensitivity experiments (see Methods). Our simulations reveal a meridional tripolar precipitation anomaly pattern over East Asia under increased obliquity forcing (Fig. 3o). Notably, the precipitation decrease in mid-latitude East Asia appears unrelated to the EASM intensification, as evidenced by concurrent southerly wind anomalies (Fig. 3o). The most substantial westerly jet modifications occur precisely in this region (Fig. 3q), where obliquity-forced jet weakening reduces ascending air motion (Supplementary Fig. 11), ultimately decreasing precipitation. As obliquity sensitivity experiments are fully coupled, we also observed anomalies of the Western Pacific Subtropical High (WPSH) (Supplementary Fig. 12) and the Walker circulation (Supplementary Fig. 13). The increased obliquity causes the WPSH to shift eastward. The eastward-shifted WPSH is unlikely to lead to decreased precipitation in the mid-latitudes of East Asia. Therefore, changes in the WPSH are not the primary cause of precipitation changes in East Asia. Although the increased obliquity results in a reduced zonal temperature gradient in the tropical Pacific (Supplementary Fig. 14), leading to a weaker Walker Circulation (Supplementary Fig. 13), the weaker Walker Circulation only causes a reduction in near-surface wind speeds east of 160°W in the equatorial Pacific (Supplementary Fig. 14), rather than a weakening of the EASM. Thus, changes in the Walker Circulation are also not the main cause of precipitation changes. An important discrepancy emerges between our simulation results and proxy records: while increased obliquity forces reduced meridional temperature gradients and mid-latitude East Asia precipitation in our experiments (Fig. 3o, p), the 1.2 Myr band shows higher precipitation corresponding to larger SST gradients (Fig. 3). Only when the climate has a non-linear response to the obliquity, or when the proxies have a non-linear response to the climate, can the 1.2 Myr and 173 kyr cycles be recorded (Supplementary Fig. 15). We attribute this apparent contradiction to non-linear proxy responses to precipitation/lake levels77. Extremely low or high obliquity values are absent during periods of low AM of obliquity, whereas extreme values occur when the AM is high (Fig. 3a). Owing to the nonlinear response of proxies to precipitation or lake levels77, such changes are recorded only when they cross special thresholds. For example, soil magnetic parameters are insensitive to precipitation below 500 mm/year78. In mid-latitude East Asia, recorded precipitation/lake level could be attributed to extreme low-obliquity conditions during 1.2 Myr obliquity AM maxima. Within the 173 kyr and 1.2 Myr bands, precipitation variations recorded in the Datong core are in phase with those from the Lop Nur core10,67 and Aktau section66, all of which are influenced by summer precipitation in the modern climate system. In contrast, obliquity sensitivity simulation experiments suggest that their precipitation variations are anti-phase within the obliquity band, which supports our hypothesis.

Our simulations demonstrate that increased obliquity leads to increased temperatures but reduced precipitation in mid-latitude East Asia. This inverse temperature-precipitation relationship would inevitably attenuate obliquity-cycle signals in proxies sensitive to both parameters, such as soil magnetic susceptibility78. In contrast, the Sr/Ca ratio from the Datong core exhibits a pronounced 40 kyr cyclicity that dominates over the 100 kyr and 20 kyr components (Supplementary Fig. 7), despite uncertainties in phase relationships due to age-model limitations. This obliquity dominance is further supported by recent Sr/Ca records from aeolian deposits, which demonstrate persistent obliquity control on the CLP precipitation throughout the past 1.5 Myr63. These findings collectively suggest that the influence of obliquity and associated westerly jet dynamics on East Asian mid-latitude precipitation has been systematically underestimated in previous studies.

Currently, mid-latitude North America lacks long precipitation records capable of resolving hydroclimate changes across the ~3 Ma transition or obliquity AM cycles. While the North American summer monsoon contributes to moisture transport, the westerly jet has been established as the dominant control on precipitation patterns in this region79. Both modern observations and climate model simulations demonstrate the westerly jet’s significant influence on precipitation variability across all mid-latitude regions80–82. Building on this evidence, we propose that the summer westerly jet has played a fundamental role in regulating mid-latitude precipitation across multiple timescales.

Enhanced waviness of westerly jet increased mid-latitude precipitation variability
Precipitation variability, estimated using moving standard deviation (see Methods), shows a significant decrease after ~3 Ma in mid-latitude Datong Core (Fig. 4b), G3 core (Fig. 4c), Lop Nor core (Fig. 4d) records. Increased atmospheric moisture due to rising global atmospheric CO2 level3 and enhanced waviness of the westerly jet (Fig. 1d)18,20 are the two most debated forcing mechanisms that have contributed to the increased precipitation variability in mid-latitude region. Conditions after ~3 Ma featured lower atmosphere CO2 (Fig. 4h)50,83 and expanded ice volume (Fig. 4g)49,84, which climate models suggest should reduce atmospheric moisture and suppress global precipitation variability1,23–25. However, while mid-latitude East and Central Asia show dampened variability (Fig. 4b–d), low-latitude Africa and Southeast Asia exhibit amplified variability (Fig. 4e, f), indicating that CO2-driven moisture changes cannot explain the regional differences. We attribute the reduced mid-latitude precipitation variability after ~3 Ma primarily to decreased westerly jet waviness (Fig. 1d). Expanded Northern Hemisphere ice sheets85 increased meridional temperature gradients86, strengthening and straightening the westerly jet19,26,87–89. The results of our sea ice sensitivity simulations also demonstrate that increased sea ice can lead to a reduction in the waviness of the westerly jet (Supplementary Fig. 16). Modern observations confirm that jet waviness enhances mid-latitude precipitation variability (Fig. 1c, d)18,20,22,90. Conversely, low-latitude variability increases may relate to amplified global ice volume fluctuations (Fig. 4g). These findings provide critical insights for future climate projections. The Pliocene (before ~3 Ma) with slightly higher temperature and similar atmospheric CO2 concentrations to the present is considered as an analogy for near-future climate91. Therefore, we anticipate increased mid-latitude hydroclimate extremes as global warming enhances summer westerly jet waviness, supported by simulated results through climate models from the CMIP6 (Coupled Model Intercomparison Project Phase 6) under the high-emissions future scenario SSP5-8.5 (Shared Socio-economic Pathway 5- Representative Concentration Pathway 8.5) (Supplementary Note 4; Supplementary Fig. 17).

Methods
Paleomagnetic and luminescence dating
Three oriented samples with dimensions of 2×2×2 cm3 were collected from the same stratigraphic level at intervals of 30–100 cm. A total of 300 samples were subjected to progressive alternating field demagnetization (AFD) at peak fields of up to 80 mT (Supplementary Fig. 18). Additionally, 304 samples underwent hybrid demagnetization (HD), which involved sequential treatments: (1) thermal demagnetization (TD) from 80°C to 150°C, (2) progressive AFD from 5 mT to 40 mT, and (3) progressive TD from 200°C to 690°C (Supplementary Fig. 18). Progressive demagnetization was conducted using the TD-48SC thermal demagnetizer and 2G Enterprises Model 755-4K cryogenic magnetometer in a magnetically shielded room (<150 nT) at the Tongji University. Stable characteristic remanent magnetizations were successfully identified in 148 AFD samples (~50% effective rate) and 236 HD samples (~80% effective rate). The magnetic polarity sequence shows that the Datong core revealed eight normal (N1–N8) and eight reversed (R1–R8) polarity zones (Supplementary Fig. 3). The depth correlations of Brunhes/Matuyama (B/M) and Matuyama/Gauss (M/G) boundaries of the Datong core with those observed in surrounding sedimentary sections confirm the reliability of the paleomagnetic age determination (Supplementary Note 3; Supplementary Fig. 19). Six samples were collected from the top 4.6 m of the core for coarse-grained K-feldspar post-infrared infrared stimulated luminescence (pIRIR50,290) dating (Supplementary Fig. 20). Luminescence measurement was conducted using a Risø model DA-20 TL/OSL reader in the luminescence dating laboratory at the School of geography, Nanjing Normal University. Dose rate was measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and inductively coupled plasma atomic emission spectrometry (ICP-AES) at Nanjing University.

Grain-size, color, and element analysis
A total of 2610 samples were collected at 10–20 cm intervals for grain size analysis. For pretreatment, 10% H2O2 and 10% HCl were used to remove organic matter and carbonates, respectively. Subsequently, 10 ml of 0.05 mol/L sodium hexametaphosphate (NaPO3)6 was added to disperse samples, which were then treated in an ultrasonic vibrator for 10 minutes. Grain size was measured using a Malvern Mastersizer 3000 laser particle-size analyzer. Prior to measurement, gravel fractions (>2000 μm) are removed to prevent instrument damage. A total of 300 samples were taken at ~1 m intervals for color analysis. Before measuring, all samples were ground to grain sizes <74 μm (200 mesh) and pressed into a hollow ground slide. Color reflectance was measured using a portable Minolta CM-700d spectrophotometer. Both grain size and color reflectance experiments were conducted at the School of Geography, Nanjing Normal University. Element data of the Datong core were obtained by scanning at 1 cm intervals using a Geotech MSCL-XRF core scanner at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences.

Implication of grain size, color, and Sr/Ca ratio
The red coloration in fluvial-lacustrine sediments generally represents an oxidizing environment with enriched Fe3+ and low lake level, greenish-gray reflects the reductive environment with enriched Fe2+ and high lake level, and yellow reflects semi-oxidizing and semi-reducing environments with simultaneous presence of Fe2+ and Fe3+, and lower lake level than that reflected by greenish-gray, and higher lake level than that reflected by red39. Based on the color changes observed in the Datong core photographs, redness (a*), and yellowness (b*) (Supplementary Fig. 3), three distinct stages of lake levels are identified. Stage I, at the depths of ~300.8–110 m is with the lowest lake level. Stage II, at the depths of ~110–80 m is with the highest lake level. Stage III, at the depths of ~80–0 m is with a lower lake level than Stage II, higher lake level than Stage I. Fe2+ and Mn2+ remain in solution under reducing condition, as conditions become more oxidized, more Fe goes into the sediment due to the greater reducibility of Fe2+ than Mn2+. Therefore, Fe/Mn ratio can also reflect redox condition92. The highest Fe/Mn ratio occurred at the Stage I reflects the most oxidizing environment, supporting redox conditions reflected by the color. However, Fe/Mn ratio does not distinguish difference of redox conditions between Stage I and Stage III. High mean grain size and coarse fraction reflect fluvial facies with low lake level, low mean grain size and coarse fraction reflect lacustrine facies with high lake level. Therefore, mean grain size and sand fraction can reflect the lake level. Additionally, Ca and Sr have the same valence, similar radius, and similar chemical properties. In fluvial-lacustrine sediments, Ca and Sr are associated with carbonate minerals93. As the lake level deepens, the Ca and Sr content increases significantly at about 210 m (Supplementary Fig. 3), further indicating that they exist mainly in secondary carbonates. At increased salinity levels, carbonate tends to incorporate more Sr into its crystal structure94. Therefore, a higher Sr/Ca ratio reflects higher salinity levels, and thus lower lake level95. In Datong core, the higher Sr/Ca corresponds closely to higher mean grain size and sand faction. At the Stage II with the highest lake-level, mean grain size, sand fraction, and Sr/Ca ratios simultaneously reached lowest values. Besides, Sr/Ca ratio also reveals the characteristics of the stage changes revealed by core color (Supplementary Fig. 3). The removement of gravel (>2000 μm) fraction prior to grain size measurement could have resulted in that mean grain size and sand fraction not revealing the stage changes, but this does not affect the discussion of cycles in any way. Although changes in the sources of the clastic materials could influence explaining of these proxies, the provenance in the Datong Basin was found to be unchanged since 5.8 Ma96. In this study, mean grain size and sand content can reflect change cycle of lake level, while Sr/Ca ratio can reflect not only the change cycle, but also the change magnitude of lake level. In East Asia, there is a close relationship between lake level and precipitation over different timescales97,98, supported by water-and-energy balance model99. These proxies were therefore further used to reflect the precipitation over East Asia.

Age model
Luminescence measurements (Supplementary Fig. 20) indicate that the signal at depth of 3.7 m is close to saturation, while the signal at 0.9 m may have been influenced by modern processes. Eventually, only the ages at depths of 1.7 m and 2.8 m (Supplementary Table S1), together with 15 paleomagnetic age markers, were used to establish the initial age model by linear interpolation and extrapolation. Spectral analysis of mean grain size based on this initial age model revealed dominant cycles of 1.2 Myr, 400 kyr and 173 kyr (Supplementary Fig. 8). Thus, we isolated the 1.2 Myr, 400 kyr, and 173 kyr components using band-passing filters, and compared them with the AM cycle of precession and obliquity (Supplementary Fig. 21). It was observed that obliquity and precession nodes—characterized by the absence of extremely low/high obliquity and precession values—correspond to intervals of large mean grain size. Therefore, we fine-tuned the initial age model by aligning peaks of filtered mean grain size to troughs of the stacked 1.2 Myr, 400 kyr, and 173 kyr AM cycle to obtain the new age model (Supplementary Fig. 22). Analysis based on both the new and initial age models revealed a weakening of the 400 kyr and 173 kyr signals of mean grain size after ~3 Ma (Supplementary Fig. 8). Noteworthily, 125 kyr and 93 kyr cycles, corresponding to AM cycles of precession, were identified in the mean grain size data based on new age model (Supplementary Fig. 8). The Sr/Ca ratio based on new age model reveals obliquity (40 kyr), precession (20 kyr), and half precession (10-kyr) cycles besides 1.2 Myr, 400 kyr, and 173 kyr AM cycles (Supplementary Fig. 7). Noteworthily, 40-kyr cycle of the Sr/Ca ratio is stronger than 100 kyr and 20 kyr cycles. Bandpass filtering, evolutionary spectrum analysis, and AM analysis are all performed using Acycle version 2.4.1100.

CESM1.2.2 and climate simulation
The CESM1.2.2 contains multiple modules, simulating the dynamics and thermodynamics of the atmosphere, land, river routing, ocean, ocean waves, sea ice, and glaciers, and the modules exchange data through a coupler101. This model, which participated in the CMIP5, has demonstrated a reliable capability for simulating global climate102. The horizontal resolutions are F19 (1.9°×2.5°; latitude by longitude) for the atmosphere (Community Atmosphere Model version 4; CAM4) and land (Community Land Model version 4; CLM4), and gx1v6 (approximately 1°×1°; latitude by longitude) for the ocean (Parallel Ocean Program version 2; POP2) and sea ice (Community Ice Code version 4; CICE4). The atmosphere and ocean contained 26 and 60 vertical levels, respectively. To explore the influence of the Northern Hemisphere sea ice on precipitation, two experiments were carried using the CAM4, coupled with the CLM4, within the framework of the CESM1.2.2. One experiment is the control run (PIcntl) for Pre-industrial (PI) with the default (present day) Northern Hemisphere ice sheet, sea ice and SST. The other experiment (PI_lgmseaice) extended Arctic sea ice to the state of the Last Glacial Maximum (LGM). To explore the influence of the obliquity AM cycle on precipitation, another two experiments were carried out for the PI: PI_Omax1 and PI_Omax2, using the fully coupled CESM1.2.2 model. For PI_Omax1 and PI_Omax2, obliquity was set to 24.538° (the same as at 2474 ka) and 23.620° (the same as at 3081 ka), respectively. The 24.538° and 23.620° are the obliquities when the change amplitude of obliquity is the largest and smallest, respectively, in a 1.2 Myr AM cycle (Supplementary Fig. 23). Each simulation was integrated for 800 years, with the last 100 years of data used for analysis to ensure robust results. All models were run on ‘Songshan’ supercomputer at the National Supercomputing Center in Zhengzhou, Zhengzhou University.

TraCE-21ka model data
Model data from TraCE-21ka experiments51, downloaded from https://rda.ucar.edu/datasets/d651050/, include one full forcing simulation (TraCE-FULL, with transient forcing changes in orbitally driven insolation, atmospheric greenhouse gases, ice sheets, and meltwater fluxes) from 22 ka BP to 1990 CE and four single-forcing simulations (TraCE-ORB, TraCE-GHG, TraCE-ICE, TraCE-MWF)103. For the TraCE-21ka experiments, all simulations were performed using the Community Climate System Model version 3 (CCSM3) from the National Center for Atmospheric Research (NCAR). The atmospheric model is the Community Atmosphere Model (CAM) 3 with T31 (~3.75°×~3.75°) horizon resolution and 26 hybrid coordinate levels. The land model is the Community Land Model (CLM) 3.0 with the coupled Dynamic Global Vegetation Model (DGVM). The ocean model is the Parallel Ocean Program (POP) 1.4.3 with 25 levels in the vertical direction. The sea ice model is the Community Sea Ice Model (CSIM5) that includes a subgrid-scale ice thickness distribution.

Processing and analysis of the simulation and reanalysis data
The processing and analysis of the simulation and reanalysis data in this study were performed using the NCL 6.4.0 (NCAR Command Language version 6.4.0), the NCO 4.9.3 (NetCDF Operators version 4.9.3), and the CDO 1.9.3 (Climate Data Operators version 1.9.3).

Precipitation variability estimation
Precipitation records were first linearly interpolated according to median of the sampling rates, precipitation variability is then estimated as the moving (7-point) standard deviation of precipitation records. All calculations were conducted on the Microsoft Excel 2016. It is noteworthy that, unlike some earlier studies that defined precipitation variability as the periodicity of climatic parameters104, this study emphasizes the magnitude of dry-wet variations.

Estimation of westerly jet waviness
When the waviness of the westerly jet increases, it is often accompanied by an amplification of Rossby wave activity, leading to a broader meridional expansion of the jet (e.g., due to divergence over ridge regions) or a narrowing contraction (e.g., due to convergence over trough regions) (Figs. 1d, 4a). As a result, the standard deviation of the jet width may exhibit a statistically positive correlation with the degree of waviness. In this study, the standard deviation of the meridional width variation of the 200 hPa jet stream (>20 m/s) along the zonal direction (east–west) is used as a metric to quantify the waviness of the jet stream.

Production of the figures
The software used to create the figures in the paper includes Grapher 24, CorelDRAW 2024, ArcMap 10.2 powered by Esri, and NCL 6.4.0.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript entitled ‘Westerly jet waviness modulates mid-latitude hydroclimate variability’ is a high-quality piece of research based on robust data, a long temporal span, well-resolved mechanisms, and strong relevance to present-day climate change. It fills a gap in the long-term record of mid-latitude hydroclimate variability, makes a clear breakthrough in understanding its driving mechanisms, and provides an important early-warning value for future climate risk. I think that it is highly consistent with Nature Communications’s standards for originality, broad interest. I suggest publishing after minor revision. Some comments and specific suggestions for its improvement as below.\nGeneral points:\nTraditionally, precipitation in the East Asian monsoon region has been viewed as primarily driven by the East Asian summer monsoon, with the monsoon strength determining the amount of rainfall. In reality, the driving mechanisms of precipitation in the East Asian monsoon region are more complex— particularly in the mid-latitude areas of this region. Authors first revealed a marked shift in precipitation patterns around ~3 Ma, characterized by fluctuations on a 1.2-Ma cycle. Through comparative records and paleoclimate modeling, it was determined that both changes are linked to the westerly jet, significantly advancing our understanding of precipitation and its dynamical mechanisms in East Asia and across mid-latitude regions.\nPrecipitation variability is a current research hotspot, with most previous studies focusing on modern observational timescales. This manuscript further integrated and calculated precipitation variability across the Northern Hemisphere's mid- and low-latitude regions, revealing that after ~3 Ma, mid-latitude precipitation variability significantly decreased, while low-latitude variability markedly increased—a pattern distinct from modern observations, where both regions exhibit simultaneous increases with global warming. The decline in mid-latitude precipitation variability after ~3 Ma was primarily associated with less wavy westerly jet. This is the FIRST study to clearly demonstrate, on a longer timescale, the significant influence of the waviness of westerly jet on mid-latitude precipitation variability，which provided the valuable and international interesting records\nSome specific points:\n(1) The precipitation variability is the core concern of this manuscript. Regarding the definition of precipitation variability, it seems to be different from previous understandings. Many studies on paleoclimate variability mainly emphasize the periodicity of climate parameter (Ao et al., 2021, Science Advances), while the variability defined in this manuscript mainly refers to the amplitude of dry and wet changes. I suggested that the author clarify this point to avoid misleading readers.\n(2) 1.2 Ma cycle and its linkage with the westerly jet represent a key highlight of this study. To my knowledge, Zhang et al. (2022, JGR-Solid Earth) previously identified this cycle in records from the Chinese Loess Plateau (as I believe you have also noted in their paper). They observed that the 1.2 Ma cycle was primarily evident during the Pliocene but absent in the Pleistocene, which differs from the persistent cyclicity shown in this manuscript. Could you explain the possible reasons for this discrepancy?\n(3) According to Figure 2J, the expansion of sea ice caused an intensification of the EASM over northern China, increased precipitation could also be related to intensified EASM. More explanation should be demonstrated.\n(4) Although you have clearly ruled out the influence of the monsoon on the precipitation in the study area, the Western Pacific Subtropical High and ENSO are also important factors affecting precipitation. At the very least, they should either be mentioned or excluded.\n(5) The description of the luminescence dating method is overly simplistic and requires further elaboration. The authors have only presented the growth curve, while I think the decay curve is equally crucial to prove the reliability of luminescence ages.\nthat future orbital tuning should primarily consider the amplitude modulation cycle?\nLine 23: records should be change to record.\nLine 107: change ‘despite also being located in the mid-latitudes’ into although it is also located in the mid-latitudes.\nLine 151: AM should be expanded into full name when it occurs for the first time in the main text.\nLine 225-249: the conception model proposed in Fig. 1C and 1D should be cited more in this part.\nFig S6, the 1.2 Ma cycle is very significant in Antarctic Circumpolar Current strength, I think this record can also be integrated in Fig. 3.", "reviewer_2": "This manuscript presents a novel and highly ambitious study, combining a new, high-resolution 5.7 Myr lacustrine precipitation record from the Datong Basin with CESM1.2 climate simulations. The work aims to link mid-latitude hydroclimate variability directly to the waviness of the westerly jet offering potentially into long-term climate dynamics and future hydroclimate extremes. The scope and integration of proxy data with modeling are commendable. While the study is promising, I recommend major revisions on several key points to strengthen its methodological rigor, substantiate its central claims and confirm its suitability for publication in Nature Communications.\nMajor Points for Revision\n1. Substantiating the Novelty of the Paleo-Record\nTo firmly establish the novelty of the Datong record, the manuscript would benefit from a more explicit and quantitative comparison with existing long-term climate archives from East Asia. The claim of presenting the \"first\" sub-millennial precipitation record spanning 5.7 million years (line 60) needs to be contextualized. Please add a section in the introduction or discussion that directly compares the Datong core’s temporal resolution (e.g., average samples per millennium) and proxy fidelity (Sr/Ca, grain size) against other key records, such as high-resolution loess sequences (e.g., from the work of An Zhisheng or Sun Youbin) or other lacustrine/marine cores (e.g., Qian et al., 2024; Lu et al., 2021). This comparison will clarify precisely how this new record advances the field.\n2. Methodological Transparency and Proxy Validation\nThe methodological rigor underpinning the proxy data requires further clarification to validate the study’s core findings, particularly concerning the critical transition identified at approximately 3 million years ago. Specifically:\n- Age Model Uncertainty: Please explicitly report the quantitative uncertainties associated with the paleomagnetic and luminescence dating methods (line: 260–279) and discuss how these uncertainties might affect the timing of key transitions.\n- Resolution Confirmation: The claim of sub-millennial resolution should be supported with data on sampling frequency (e.g., a histogram of samples per thousand years) throughout the length of the core.\n- Regional Representativeness: A brief discussion is needed on whether local tectonic or hydrological dynamics within the Datong Basin could influence the proxy records independently of the large-scale climate signal.\nThis is crucial for justifying the extrapolation of these findings to represent mid-latitude East Asia more broadly.\n3. Climate Model Experimental Design and Validation\nThe climate simulations provide the mechanistic foundation for the study's claims, but several aspects of the experimental design warrant clarification and strengthening..\n- Pliocene Boundary Conditions: The use of Last Glacial Maximum (LGM) sea ice extent in the PI_lgmseaice simulation (line: 366–367) as an analogue for Pliocene conditions is an unconventional choice and requires a stronger justification. Please clarify all key Pliocene boundary conditions used (e.g., CO₂ levels, ice sheet geography, and gateways).\n- Model Validation: To build confidence in the model's ability to simulate jet stream dynamics, please provide a quantitative validation of the CESM1.2.2 control run against a modern reanalysis product like ERA5. This could include metrics such as RMSE OR pattern correlations for jet stream position and waviness.\n4. Robustness of the Central Attribution to Jet Waviness\nA key area for revision is the attribution of observed rainfall shifts solely to westerly jet dynamics, which overlooks other \"significant\" climate factor influencing East Asia. The manuscript should more thoroughly assess the potential impact of alternative or compounding mechanisms.\n- Maybe the intensification of the East Asian summer monsoon.\n- The phased uplift of the Tibetan Plateau (as noted on line number 112–114).\n- Direct impacts of declining atmospheric CO₂ on moisture availability and transport.\nThe authors should use their existing proxy data or conduct additional sensitivity simulations to test these alternative hypotheses and demonstrate that jet waviness is the dominant driver, rather than just one of several contributing factors.\n5. Grounding Future Predictions in Direct Evidence\nThe manuscript’s forward-looking statements on future hydroclimate extremes (line 246–249) would be substantially more compelling if supported by direct model evidence instead of relying solely on Pliocene analogues.\n- Future Simulations: I strongly recommend running CESM1.2.2 simulations under a high-emissions future scenario (SSP5-8.5) to directly test how projected Arctic amplification influences jet waviness and mid-latitude precipitation in the model.\n- Global Relevance: The discussion should be expanded to consider the applicability of these findings to other mid-latitude regions sensitive to jet stream dynamics, such as western North America and Europe, thereby enhancing the global relevance of the work.\n6. Citation Accuracy (Minor Comment)\nA minor but important point concerns the citation of Ham et al. (2023) regarding precipitation variability (line: 37). My understanding is that this study focuses on a deep-learning model for predicting linear trends in precipitation, not hydroclimate variability in the past. Please review this citation to maintain scientific accuracy.", "reviewer_3": "This work reports sub-millennial resolution precipitation records obtained by lacustrine sediment core spanning the past 5.7 million years and distributed over a large globe surfaces revealing the significant intensification of precipitation. The authors found that such a period is coinciding with the ice sheet expansion of the Northern Hemisphere. This is potentially a significant result. They suggest that the Pliocene-Pleistocene precipitation (record) is potentially controlled by the 1.2Myr obliquity amplitude modulation. This may clearly open new rooms in the understanding of Plio-Pleistocene climate variation.\nI think that such important conclusion could be better supported if the authors add precisions and clarifications according to the following points.\n1. Numerical point versus proxies. The numerical modelling suggests that precipitation may decrease when obliquity is high while the proxies apparently may indicate an inverse result. The authors invoke the possible the possibility of a nonlinear response that sounds more a ‘normalization’. Could the authors propose other possible explications?\n2. The mathematical analysis adopted in the paper should be justified (or improved). In particular, estimation of the rainfall variability through a simple linear interpolation and rolling standard deviations does not take into account the unequal resolution of chronological uncertainties and measurement errors.\n3. The Age estimation is obtained combining limited number of paleo-magnetic samples and two luminescent age samples and the existence of orbital cycles is already incorporate in the chronology.\n4. If I understand correctly, the Pliocene variability is attributed to a strong jet modulation depending from a warmer Arctic. Is this an extrapolation from modern correlation or there is also a physical explanation?"}, "author_response": "Following parts are our explanations what changes to the manuscript have been made and how our replies (answer, A, Black) to the reviewer’s comments (comments and suggestions, C&S, Blue).\nReviewer 1:\n1. C&S: This manuscript entitled ‘Westerly jet waviness modulates mid-latitude hydroclimate variability’ is a high-quality piece of research based on robust data, a long temporal span, well-resolved mechanisms, and strong relevance to present-day climate change. It fills a gap in the long-term record of mid-latitude hydroclimate variability, makes a clear breakthrough in understanding its driving mechanisms, and provides an important early-warning value for future climate risk. I think that it is highly consistent with Nature Communications’ standards for originality, broad interest. I suggest publishing after minor revision. Some comments and specific suggestions for its improvement as below.\nTraditionally, precipitation in the East Asian monsoon region has been viewed as primarily driven by the East Asian summer monsoon, with the monsoon strength determining the amount of rainfall. In reality, the driving mechanisms of precipitation in the East Asian monsoon region are more complex— particularly in the mid-latitude areas of this region. Authors first revealed a marked shift in precipitation patterns around ~3 Ma, characterized by fluctuations on a 1.2-Ma cycle. Through comparative records and paleoclimate modeling, it was determined that both changes are linked to the westerly jet, significantly advancing our understanding of precipitation and its dynamical mechanisms in East Asia and across mid-latitude regions.\nPrecipitation variability is a current research hotspot, with most previous studies focusing on modern observational timescales. This manuscript further integrated and calculated precipitation variability across the Northern Hemisphere's mid- and low-latitude regions, revealing that after ~3 Ma, mid-latitude precipitation variability significantly decreased, while low-latitude variability markedly increased—a pattern distinct from modern observations, where both regions exhibit simultaneous increases with global warming. The decline in mid-latitude precipitation variability after ~3 Ma was primarily associated with less wavy westerly jet. This is the FIRST study to clearly demonstrate, on a longer timescale, the significant influence of the waviness of westerly jet on mid-latitude precipitation variability, which provided the valuable and international interesting records.\nA: Many thanks for your careful reading, positive evaluation, and constructive advices. We try to revise and respond to your comments. Please see the following for more details.\n2. C&S: The precipitation variability is the core concern of this manuscript. Regarding the definition of precipitation variability, it seems to be different from previous understandings. Many studies on paleoclimate variability mainly emphasize the periodicity of climate parameter (Ao et al., 2021, Science Advances), while the variability defined in this manuscript mainly refers to the amplitude of dry and wet changes. I suggested that the author clarify this point to avoid misleading readers.\nA: Thank you. We have clarified it in method section. “Notably, the definition of precipitation variability in this study differs from previous ones, as it primarily emphasizes the magnitude of dry-wet variations, whereas some earlier studies defined it as the variation cycle of climatic parameters (Ao et al., 2021)”.\n3. C&S: 1.2 Ma cycle and its linkage with the westerly jet represent a key highlight of this study. To my knowledge, Zhang et al. (2022, JGR-Solid Earth) previously identified this cycle in records from the Chinese Loess Plateau (as I believe you have also noted in their paper). They observed that the 1.2 Ma cycle was primarily evident during the Pliocene but absent in the Pleistocene, which differs from the persistent cyclicity shown in this manuscript. Could you explain the possible reasons for this discrepancy?\nA: The findings of Zhang et al. (2022) are primarily based on the magnetic susceptibility record of the Chinese Loess Plateau. This record exhibits a significant 1.2 Ma cycle prior to 3 Ma, but this cycle disappears after 3 Ma. Additionally, the magnetic susceptibility does not show a notable increase after 3 Ma, which contrasts with the precipitation record from Datong Basin. This discrepancy is largely attributable to the multi-interpretation of magnetic susceptibility. Since magnetic susceptibility reflects pedogenic strength, which is influenced by both precipitation and temperature (Nie et al., 2014), the combined effects of increased precipitation and decreased temperature after 3 Ma may have counteracted each other, resulting in minimal change in magnetic susceptibility. Conversely, the disappearance of the 1.2 Ma cycle after 3 Ma may be linked to a greater influence of temperature on magnetic susceptibility during this period, as temperature records (Benthic 18O) lack a 1.2 Ma cycle.\n4. C&S: According to Figure 2J, the expansion of sea ice caused an intensification of the EASM over northern China, increased precipitation could also be related to intensified EASM. More explanation should be demonstrated.\nA: Although the intensification of the East Asian summer monsoon can transport more moisture, where this moisture ultimately precipitates is primarily controlled by convective conditions (He et al., 2021; Long et al., 2025). According to Figures 2K and 2L, the enhancement of the upper-level westerly jet north of 40°N is the fundamental cause for the intensified upward motion of moisture and increased precipitation.\n5. C&S: Although you have clearly ruled out the influence of the monsoon on the precipitation in the study area, the Western Pacific Subtropical High and ENSO are also important factors affecting precipitation. At the very least, they should either be mentioned or excluded.\nA: The sensitivity simulation experiments of sea ice were not fully coupled, but only the atmospheric model (CAM4) was run. It is meaningless to observe Western Pacific Subtropical High (WPSH) and Walker circulation. The sensitivity simulation of the obliquity is carried out in a fully coupled environment. Therefore, we have presented the anomaly of the WPSH and Walker circulation. According to Figure R1, the increased obliquity causes the WPSH to shift eastward. The eastward-shifted WPSH is unlikely to lead to decreased precipitation in the mid-latitudes of East Asia. Therefore, changes in the WPSH are not the primary cause of precipitation changes in East Asia. Although the increased obliquity results in a reduced zonal temperature gradient in the tropical Pacific (Figure R2), leading to a weaker Walker Circulation (Figure R3), the weaker Walker Circulation only causes a reduction in near-surface wind speeds east of 160°W in the equatorial Pacific (Figure R2), rather than a weakening of the East Asian summer monsoon. Thus, changes in the Walker Circulation are also not the main cause of precipitation changes. We have demonstrated these in Line 215-224 in main text.\n6. C&S: The description of the luminescence dating method is overly simplistic and requires further elaboration. The authors have only presented the growth curve, while I think the decay curve is equally crucial to prove the reliability of luminescence ages. that future orbital tuning should primarily consider the amplitude modulation cycle?\nA: Thank you. We have presented decay curve (Fig. R4), which exhibits the typical luminescence signals of K-feldspar minerals, confirming the reliability of our measurements. If the phase relationship between obliquity and climate can be confirmed, tuning directly using obliquity is also feasible. However, the most robust approach remains tuning through amplitude modulation cycles.\n7. C&S: Line 23: records should be change to record.\nA: We have changed it.\n8. C&S: Line 107: change ‘despite also being located in the mid-latitudes’ into although it is also located in the mid-latitudes.\nA: We have changed it.\n9. C&S: Line 151: AM should be expanded into full name when it occurs for the first time in the main text.\nA: Thank you. The AM is abbreviation of amplitude modulation. The AM first appears in Line 71, we have explained it.\n10. C&S: Line 225-249: the conception model proposed in Fig. 1C and 1D should be cited more in this part.\nA: Thank you. We have revised it.\n11. C&S: Fig S6, the 1.2 Ma cycle is very significant in Antarctic Circumpolar Current strength, I think this record can also be integrated in Fig. 3.\nA: Although it can reflect changes in the intensity of the Southern Hemisphere's westerlies, our main focus is on the Northern Hemisphere's westerlies. Therefore, we will still keep it in the supplementary materials.\nReviewer 2:\n1. C&S: This manuscript presents a novel and highly ambitious study, combining a new, high-resolution 5.7 Myr lacustrine precipitation record from the Datong Basin with CESM1.2 climate simulations. The work aims to link mid-latitude hydroclimate variability directly to the waviness of the westerly jet offering potentially into long-term climate dynamics and future hydroclimate extremes. The scope and integration of proxy data with modeling are commendable. While the study is promising, I recommend major revisions on several key points to strengthen its methodological rigor, substantiate its central claims and confirm its suitability for publication in Nature Communications.\nA: Many thanks for your careful reading and constructive advices. We try to revise and respond to your comments. Please see the following for more details.\n2. C&S: Substantiating the Novelty of the Paleo-Record. To firmly establish the novelty of the Datong record, the manuscript would benefit from a more explicit and quantitative comparison with existing long-term climate archives from East Asia. The claim of presenting the \"first\" sub-millennial precipitation record spanning 5.7 million years (line 60) needs to be contextualized. Please add a section in the introduction or discussion that directly compares the Datong core’s temporal resolution (e.g., average samples per millennium) and proxy fidelity (Sr/Ca, grain size) against other key records, such as high-resolution loess sequences (e.g., from the work of An Zhisheng or Sun Youbin) or other lacustrine/marine cores (e.g., Qian et al., 2024; Lu et al., 2021). This comparison will clarify precisely how this new record advances the field.\nA: Thanks for your advices. We compared the precipitation records of Datong with those of the surrounding areas (Fig. R5). The sample resolution of Datong's precipitation records is significantly higher than that of the surrounding areas. Although the R records from the North China Plain boreholes and Sr/Ca ratio in i/b loess carbonate nodule from Chinese loess plateau have low resolution, they—along with the Sr/Ca ratio from the Datong borehole—consistently indicate higher precipitation after ~3 Ma compared to before ~3 Ma. In contrast, the magnetic susceptibility record of the Loess Plateau loess does not show an overall increase in precipitation after 3 Ma. Furthermore, the magnetic susceptibility record exhibits a significant 1.2-Myr cycle before 3 Ma (Qin et al., 2022), which becomes insignificant after ~3 Ma (Fig. R5). This differs markedly from the Sr/Ca ratio of the Datong borehole, where the 1.2-Myr cycle persists throughout. We attribute this discrepancy primarily to the multi-proxy climatic implications of magnetic susceptibility. Magnetic susceptibility reflects pedogenic intensity, which is influenced by both precipitation and temperature (Nie et al., 2014). After ~3 Ma, the combined effect of increased precipitation and decreased temperature resulted in no higher magnetic susceptibility after 3 Ma than before. Additionally, the absence of a 1.2-Myr cycle in magnetic susceptibility after 3 Ma may be because magnetic susceptibility during this period was more strongly influenced by temperature, as temperature records do not show a 1.2-Myr cycle (Fig. R5). We have presented these in supplementary information as Text S1.\n3. C&S: Methodological Transparency and Proxy Validation.\nThe methodological rigor underpinning the proxy data requires further clarification to validate the study’s core findings, particularly concerning the critical transition identified at approximately 3 million years ago.\nSpecifically:\n- Age Model Uncertainty: Please explicitly report the quantitative uncertainties associated with the paleomagnetic and luminescence dating methods (line: 260–279) and discuss how these uncertainties might affect the timing of key transitions.\n- Resolution Confirmation: The claim of sub-millennial resolution should be supported with data on sampling frequency (e.g., a histogram of samples per thousand years) throughout the length of the core.\n- Regional Representativeness: A brief discussion is needed on whether local tectonic or hydrological dynamics within the Datong Basin could influence the proxy records independently of the large-scale climate signal.\nThis is crucial for justifying the extrapolation of these findings to represent mid-latitude East Asia more broadly.\nA: Thank you for pointing these out. The age model was established by orbital tuning based on the paleomagnetic and luminescence chronology, utilizing orbital cycles of 1200 kyr, 400 kyr, and 173 kyr. As a result, the maximum chronological uncertainty does not exceed 173 ka. Moreover, the mutual constraints among multiple orbital cycles further reduce the uncertainty to a level significantly smaller than 173 ka. We have presented a histogram of samples per thousand years (Fig. R5B). Average resolution of records from the Datong core is 2.7 sample/ka (Fig. R5), which is significantly higher than other records. The precipitation records from Datong basin reveal increased rainfall after ~3 Ma, a trend also observed in records from the North China Plain and the Chinese Loess Plateau (Fig. R5). Moreover, its dominant cycle (1.2-Myr cycle) is highly consistent with the meridional sea surface temperature (SST) gradient records across the Northern Hemisphere. Consequently, the climatic pattern revealed in the Datong Basin cannot be attributed to local influences (such as local tectonic and hydrological dynamics) but is demonstrably linked to global climate dynamics.\n4. C&S: Climate Model Experimental Design and Validation.\nThe climate simulations provide the mechanistic foundation for the study's claims, but several aspects of the experimental design warrant clarification and strengthening.\n- Pliocene Boundary Conditions: The use of Last Glacial Maximum (LGM) sea ice extent in the PI_lgmseaice simulation (line: 366–367) as an analogue for Pliocene conditions is an unconventional choice and requires a stronger justification. Please clarify all key Pliocene boundary conditions used (e.g., CO₂ levels, ice sheet geography, and gateways).\n- Model Validation: To build confidence in the model's ability to simulate jet stream dynamics, please provide a quantitative validation of the CESM1.2.2 control run against a modern reanalysis product like ERA5. This could include metrics such as RMSE OR pattern correlations for jet stream position and waviness.\nA: Thank you for pointing this out. It is more scientifically robust to investigate the impact of ice sheet expansion on mid-latitude climate after ~3 Ma by incorporating Pliocene climatic boundary conditions. For Pliocene simulations, the Mid-Pliocene Warm Period is commonly adopted as the climatic reference. During this interval, the global mean annual temperature was 1.8–3.6 °C higher than pre-industrial (PI) levels (Haywood et al., 2021), terrestrial monsoon precipitation was greater than present, sea ice extent was more reduced compared to modern conditions, the Arctic was seasonally ice-free, and atmospheric CO2 concentrations were higher than today (Haywood et al., 2021). Our primary objective is to investigate the impacts of sea ice expansion on mid-latitude climate under warm-period background conditions. The PI period was selected as the baseline climate state due to its status as the most accessible representation of warm conditions. Recently, Zhang et al. (2019) conducted a series of ice sheet sensitivity experiments under Pliocene climatic boundary conditions using CESM 1.0.4. The experimental parameters can be found in Table R1. Their results show that precipitation changes induced by reduced ice sheets are consistent with the precipitation response to sea ice reduction in our CESM1.2.2-based PI climate experiments (Fig. R6). This agreement further supports the validity of using pre-industrial (PI) climate as the baseline state for our study.\n5. C&S: Robustness of the Central Attribution to Jet Waviness.\nA key area for revision is the attribution of observed rainfall shifts solely to westerly jet dynamics, which overlooks other \"significant\" climate factor influencing East Asia. The manuscript should more thoroughly assess the potential impact of alternative or compounding mechanisms.\n- Maybe the intensification of the East Asian summer monsoon.\n- The phased uplift of the Tibetan Plateau (as noted on line number 112–114).\n- Direct impacts of declining atmospheric CO₂ on moisture availability and transport.\nThe authors should use their existing proxy data or conduct additional sensitivity simulations to test these alternative hypotheses and demonstrate that jet waviness is the dominant driver, rather than just one of several contributing factors.\nA: Thank you for your advices. Records from the Datong Basin, the North China Plain, and the Chinese Loess Plateau (Fig. R5) all indicate an increase in precipitation in mid-latitude East Asia after ~3 Ma. This climatic transition around ~3 Ma is unlikely to be related to the uplift of the Tibetan Plateau. First, evidence for significant Pliocene uplift of the Tibetan Plateau remains insufficient (Molnar et al., 2010). Moreover, the basic patterns of the monsoon and westerly circulation had already been established by 26 Ma (Wu et al., 2022).\nThe observed wetting trend after ~3 Ma was accompanied by a significant expansion of Northern Hemisphere ice volume (Expedition-323-Scientists, 2010; Haug et al., 2005; Lisiecki and Raymo, 2005) and a considerable decrease in atmospheric CO2 (Seki et al., 2010). To investigate this relationship, we conducted idealized sea-ice sensitivity experiments and analyzed ice sheet and greenhouse gas (GHG)-forced precipitation anomalies using the TraCE-21ka dataset (Simulation of the Transient Climate of the Last 21,000 Years) (Liu et al., 2014b). The resulting precipitation anomalies forced by increased sea ice exhibit a meridional tripolar pattern over East Asia (Fig. 2J), matching both modern observations and geological records of summer precipitation changes across the ~3 Ma transition. These results are broadly consistent with the ice-sheet-forced precipitation anomalies derived from the TraCE-21ka simulation (Fig. R8). In contrast, the precipitation anomalies forced by reduced GHG exhibit patterns completely opposite to those documented in the geological records across East Asia (Fig. R8). This discrepancy effectively rules out a dominant influence of CO2 on precipitation changes during the ~3 Ma transition. We have demonstrated these in Line 112-132 in main text.\nThe CO2 concentration does not exhibit a 1.2-Myr cycle, whereas the Antarctic Circumpolar Current — which is closely linked to the intensity of the Southern Hemisphere westerly jet (Lamy et al., 2024) — displays a significant 1.2-Myr cycle (Fig. R9). This indicates that the 1.2-Myr cycle is primarily associated with variations in westerly jet strength. Although our study focuses on the Northern Hemisphere westerly jet, the intensity of westerly jet in both hemispheres is governed by their respective meridional temperature gradients. This shared mechanistic driver suggests that the two systems may exhibit similar periodic behavior, potentially including a common dominant cycle.\nThe sea ice sensitivity experiments were not fully coupled, but only the atmospheric model was run, thus, the mechanistic discussion can only focus on atmospheric processes. Although the intensification of the East Asian summer monsoon can transport more moisture, where this moisture ultimately precipitates is primarily controlled by convective conditions (He et al., 2021; Long et al., 2025). According to Figures 2K and 2L, the enhancement of the upper-level westerly jet north of 40°N is the fundamental cause for the intensified upward motion of moisture and increased precipitation. The sensitivity simulation of the obliquity is carried out in a fully coupled environment. Therefore, we have presented the anomaly of the WPSH and Walker circulation. According to Fig. R1, the increased obliquity causes the WPSH to shift eastward. The eastward-shifted WPSH is unlikely to lead to decreased precipitation in the mid-latitudes of East Asia. Therefore, changes in the WPSH are not the primary cause of precipitation changes in East Asia. Although the increased obliquity results in a reduced zonal temperature gradient in the tropical Pacific (Fig. R2), leading to a weaker Walker Circulation (Fig. R3), the weaker Walker Circulation only causes a reduction in near-surface wind speeds east of 160°W in the equatorial Pacific (Fig. R2), rather than a weakening of the East Asian summer monsoon. Thus, changes in the Walker Circulation are also not the main cause of precipitation changes. We have demonstrated these in Line 215-224 in main text.\n6. C&S: Grounding Future Predictions in Direct Evidence.\nThe manuscript’s forward-looking statements on future hydroclimate extremes (line 246–249) would be substantially more compelling if supported by direct model evidence instead of relying solely on Pliocene analogues.\n- Future Simulations: I strongly recommend running CESM1.2.2 simulations under a high-emissions future scenario (SSP5-8.5) to directly test how projected Arctic amplification influences jet waviness and mid-latitude precipitation in the model.\n- Global Relevance: The discussion should be expanded to consider the applicability of these findings to other mid-latitude regions sensitive to jet stream dynamics, such as western North America and Europe, thereby enhancing the global relevance of the work.\nA: Thank you for pointing this out. Despite decades of observations confirming that climate warming has intensified the waviness of the westerlies (Martin, 2021; Sun et al., 2022) and increased mid-latitude precipitation variability (Zhang et al., 2024), the waviness of the westerlies has not been well reproduced in models, primarily due to model uncertainties (Francis, 2017). For instance, some studies have attempted to simulate Arctic amplification by reducing sea ice (Blackport and Screen, 2020), but sea ice loss primarily affects the temperature of the lower atmosphere, while the westerly jets are typically located at higher altitudes (Francis, 2017). If models contain errors in the stratification of the atmospheric boundary layer and underestimate energy exchange between the troposphere and stratosphere (Wu and Smith, 2016), it becomes challenging to influence the waviness of the westerlies by just manipulating sea ice extent. This is evident in our sea ice simulation studies, where even when sea ice was expanded to the Last Glacial Maximum levels, changes in the waviness of the westerlies remained minimal (Fig. R7). Additionally, some models overestimate warming in tropical regions (Santer et al., 2017). Since the waviness of the westerlies depends on the temperature gradient between mid and high latitudes, if tropical warming is overestimated, detecting changes in the waviness of the westerlies becomes difficult (Francis, 2017). The understanding of the influence of jet waviness on precipitation variability relies primarily on statistical correlations from observational and reanalysis data with plausible physical mechanisms (Francis, 2017; Trouet et al., 2018). This is the primary reason why the mechanisms behind mid-latitude precipitation variability remain poorly understood. We presented CESM2 simulated precipitation variability and westerly jet waviness, under a high-emissions future scenario (SSP5-8.5) from CMIP6. Precipitation variability shows a significantly increasing trend (Fig. R10; R11B), while the waviness of westerlies, although also exhibiting an increasing tendency, is not statistically significant, providing further support for our aforementioned statement.\nIn Western Europe, precipitation after ~3 Ma was also higher than before ~3 Ma (Scheidt et al., 2020), possibly reflecting a driving mechanism similar to that in the mid-latitude East Asian region. Currently, no records in North America indicate changes in precipitation around 3 Ma. However, based on modern observational data, the westerly jet stream has a significant influence on precipitation in North America (Wahl et al., 2019), while precipitation in Europe is also carried by the westerlies. This further demonstrates the impact of the westerly jet stream on precipitation across the entire mid-latitude region (Coumou et al., 2018; Francis and Vavrus, 2012; Routson et al., 2019).\n7. C&S: Citation Accuracy (Minor Comment).\nA minor but important point concerns the citation of Ham et al. (2023) regarding precipitation variability (line: 37). My understanding is that this study focuses on a deep-learning model for predicting linear trends in precipitation, not hydroclimate variability in the past. Please review this citation to maintain scientific accuracy.\nA: Thank you for pointing this out. We have deleted this citation. Ham et al. (2023) pointed the influence of anthropogenic warming on precipitation variability on the weather timescale, while we mainly focus on the precipitation variability on the climate timescale.\nReviewer 3:\n1. C&S: This work reports sub-millennial resolution precipitation records obtained by lacustrine sediment core spanning the past 5.7 million years and distributed over a large globe surfaces revealing the significant intensification of precipitation. The authors found that such a period is coinciding with the ice sheet expansion of the Northern Hemisphere. This is potentially a significant result. They suggest that the Pliocene-Pleistocene precipitation (record) is potentially controlled by the 1.2 Myr obliquity amplitude modulation. This may clearly open new rooms in the understanding of Plio-Pleistocene climate variation.\nI think that such important conclusion could be better supported if the authors add precisions and clarifications according to the following points.\nA: Many thanks for your careful reading and constructive advices. We try to revise and respond to your comments. Please see the following for more details.\n2. C&S: Numerical point versus proxies. The numerical modelling suggests that precipitation may decrease when obliquity is high while the proxies apparently may indicate an inverse result. The authors invoke the possible the possibility of a nonlinear response that sounds more a ‘normalization’. Could the authors propose other possible explications?\nA: We present the spectral analysis results of obliquity, obliquity amplitude modulation, and clip obliquity (Fig. R12). The obliquity does not exhibit 1.2-Myr or 173-kyr cycles, whereas clip obliquity, similar to obliquity amplitude modulation, shows significant 1.2-Myr and 173-kyr cycles. This indicates either a nonlinear response of climate to obliquity or a nonlinear response of the proxy to climate. Extremely low or high obliquity values are absent during periods of low AM of obliquity, whereas extreme values occur when the AM is high. Owing to the nonlinear response of proxies to precipitation or lake levels (Rial et al., 2004), such changes are recorded only when they cross special thresholds. For example, soil magnetic parameters are insensitive to precipitation below 500 mm/year (Nie et al., 2014). In mid-latitude East Asia, recorded precipitation/lake level could be attributed to extreme low-obliquity conditions during 1.2-Myr obliquity AM maxima. Within the 173-kyr and 1.2-Myr bands, precipitation variations recorded in the Datong core are in phase with those from the Lop Nur core (Huang et al., 2021; Liu et al., 2014a) and Aktau section (Frisch et al., 2019), all of which are influenced by summer precipitation in the modern climate system. In contrast, obliquity sensitivity simulation experiments suggest that their precipitation variations are anti-phase within the obliquity band, which supports our hypothesis. We have demonstrated it in Line 227-242 in main text.\n3. C&S: The mathematical analysis adopted in the paper should be justified (or improved). In particular, estimation of the rainfall variability through a simple linear interpolation and rolling standard deviations does not take into account the unequal resolution of chronological uncertainties and measurement errors.\nA: Regarding your inquiry, we have previously taken this into consideration. The rolling standard deviation is a commonly used method for analyzing precipitation variability (Zhang et al., 2024). Even if we do not quantitatively assess changes in precipitation variability by calculating the rolling standard deviation, the difference in the magnitude of precipitation changes before and after 3 million years ago is visibly apparent. The variation in magnitude also reflects differences in precipitation variability, which is independent on the resolution of the chronology or proxy indicators.\n4. C&S: The Age estimation is obtained combining limited number of paleo-magnetic samples and two luminescent age samples and the existence of orbital cycles is already incorporate in the chronology\nA: First, we obtained preliminary ages through linear interpolation based on paleomagnetic and luminescence ages. Spectral analysis revealed the presence of 1.2-Myr, 400-kyr, and 173-kyr cycles, and their phase relationships with orbital periods were determined. These three cycles were then used to refine age model. We do not think these three cycles to be artificially introduced, nor are the phase relationships assumed based on subjective premises.\n5. C&S: If I understand correctly, the Pliocene variability is attributed to a strong jet modulation depending from a warmer Arctic. Is this an extrapolation from modern correlation or there is also a physical explanation?\nA: Yes, you are absolutely correct. We do not have observational records of Pliocene westerly jet waviness. However, the relatively prominent mid-latitude precipitation variability (Fig. R11) also underscores the importance of the westerlies in driving this variability, and our sea ice simulation experiments confirm that sea ice expansion leads to a reduction in westerly jet waviness (Fig. R7). Given that the ice sheets during the Pliocene were smaller than those during the Pleistocene, the westerly jet waviness in the Pliocene should logically have been higher than during the Pleistocene."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have made careful revisions based on the reviewers's comments, which has greatly improved the quality of the manuscript.", "reviewer_2": "I have carefully reviewed the author's revised manuscript and their detailed responses to my earlier comments. I want to commend the authors for the clear effort and depth of their revisions. It’s evident that they have taken the feedback seriously and worked thoroughly to strengthen the paper. The integration of proxy data, climate modeling, and interpretation is now much more convincing and transparent, and all major issues have been addressed appropriately.\nOverall, this is a much improved and more coherent manuscript. The revisions have enhanced its scientific rigor, clarity, and flow. The authors have effectively demonstrated both the regional and global relevance of their findings, while improving methodological transparency and interpretive depth.", "reviewer_3": "The manuscript integrates now the majority of the comments suggested by the reviewers.\nThe novel and ambitious study on the 5.7Mmyr lacustrine precipitation record from the Daton Basin and the relative climate simulation are now clearly presented.\nThe key points improving the methodological rigor are clearly detailed in the new version.\nI recommend the publication under the present form."}, "author_response": "Responses to the reviewers and detailed modification\nFollowing parts are our explanations what changes to the manuscript have been made and how our replies (answer, A, Black) to the reviewer’s comments (comments and suggestions, C&S, Blue).\nReviewer 1:\n1. C&S: The authors have made careful revisions based on the reviewers's comments, which has greatly improved the quality of the manuscript.\nA: Many thanks for your positive evaluation.\nReviewer 2:\n1. C&S: I have carefully reviewed the author's revised manuscript and their detailed responses to my earlier comments. I want to commend the authors for the clear effort and depth of their revisions. It’s evident that they have taken the feedback seriously and worked thoroughly to strengthen the paper. The integration of proxy data, climate modeling, and interpretation is now much more convincing and transparent, and all major issues have been addressed appropriately. Overall, this is a much improved and more coherent manuscript. The revisions have enhanced its scientific rigor, clarity, and flow. The authors have effectively demonstrated both the regional and global relevance of their findings, while improving methodological transparency and interpretive depth.\nA: Many thanks for your positive evaluation.\nReviewer 3:\n1. C&S: The manuscript integrates now the majority of the comments suggested by the reviewers. The novel and ambitious study on the 5.7Mmyr lacustrine precipitation record from the Daton Basin and the relative climate simulation are now clearly presented. The key points improving the methodological rigor are clearly detailed in the new version. I recommend the publication under the present form.\nA: Many thanks for your positive evaluation."}]

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