Precessional hydroclimatic synchronicity changes in the Indo-Pacific Warm Pool driven by the intertropical convergence zone over the past 450 kyr

Abstract

The Indo-Pacific Warm Pool (IPWP) significantly influences the global hydrological cycle through its impact on atmospheric-oceanic circulation. However, gaining a comprehensive understanding of the hydrologic climate dynamics within the IPWP and its broader effects on the global climate have been hindered by spatial and temporal limitations in paleoclimate records on orbital timescales. In this study, we reconstructed precipitation records (approximated from δ¹⁸O_sw-ivc) over the past 450 kyr, based on planktonic foraminiferal Mg/Ca and δ¹⁸O data obtained from International Ocean Discovery Program Site U1486 in the western tropical Pacific. The δ¹⁸O_sw-ivc record revealed a generally consistent pattern with precession variations over the past 450 kyr, closely corresponding to changes in boreal summer insolation at the equator. The δ¹⁸O_sw-ivc record displayed an anti-phased relationship with Chinese speleothem δ¹⁸O records on the precession band, with lower precipitation in the western tropical Pacific and higher precipitation in the East Asia summer monsoon region during periods of high Northern Hemisphere summer insolation. This anti-phased correlation primarily resulted from the north-south migration of the Intertropical Convergence Zone (ITCZ), influenced by the interhemispheric insolation contrast. By considering additional δ¹⁸O_sw-ivc records from various locations within the IPWP region, we identified synchronous precipitation changes within the IPWP on the precession band. The synchronization of precipitation on both margins of the ITCZ’s seasonal range and differences between central and marginal regions of the ITCZ within the IPWP revealed the expansion and contraction of the ITCZ on precession band.

1. Introduction

The Indo-Pacific Warm Pool (IPWP), acknowledged as possessing the world’s largest tropical warm water mass, is characterized by sea surface temperatures surpassing 28°C (Clement et al. 2005; Kim et al. 2012; De Deckker, 2016) and plays a crucial role in atmospheric deep convection (Russell et al. 2014). Variations in sea surface temperatures, rainfall patterns and oceanic circulation within the IPWP significantly impact the distribution of heat and moisture, thus influencing global atmospheric circulation patterns (Cane, 1998; Hoerling et al. 2001; Yan et al. 1992; Qian et al. 2023). Modern observations highlight the substantial impact of the seasonal latitudinal shifts of the Intertropical Convergence Zone (ITCZ) and its associated monsoon system, as well as the El Niño-Southern Oscillation (ENSO) variability, on shaping hydrologic characteristics in the IPWP region (Leech et al. 2013; Schneider et al. 2014). Therefore, studying hydrological climate variations in the IPWP is crucial for unraveling its intricate role in regional and global climate systems.

On geological timescales, the impact and mechanisms of ITCZ variations on the regional precipitation changes within the IPWP present conflicting findings. As a narrow band of convective winds and intense precipitation located near the equator, the ITCZ regulates the distribution of precipitation in the middle and low latitudes by seasonally migrating in response to interhemispheric temperature gradients (Schneider et al. 2014; Denniston et al. 2016). With the southward migration of the ITCZ, precipitation decreases in the Northern Hemisphere summer monsoon area, while increasing in the Southern Hemisphere summer monsoon area, and vice versa (Schneider et al. 2014). Several studies propose that tropical precipitation changes were primarily linked to the precession cycles in Earth’s orbit (Clement et al. 2004; Wang et al. 2008; Merlis et al. 2013; Jalihal et al. 2019). Accordingly, the interhemispheric anti-phasing precipitation pattern is shaped by contrasting insolation between the Northern and Southern Hemispheres, causing latitudinal ITCZ migrations (Berger, 1978; Kutzbach et al. 2008). Findings from stalagmite δ¹⁸O records in both hemispheres strongly support this hypothesis (e.g., Wang et al. 2008; Cheng et al. 2016). Reconstructions of precipitation, runoff and sea-surface salinity (SSS) from the IPWP further reveal notable responses to precession-induced alterations (e.g., Kissel et al. 2010; Tachikawa et al. 2011; Dang et al. 2015; Jian et al. 2022). Notably, Earth’s obliquity also appears to be important in driving the latitudinal migration of the ITCZ, similarly resulting in anti-phase variations in interhemispheric precipitation in the IPWP (Liu et al. 2015; Zhang et al. 2020, 2022). Liu et al. (2015) suggest that obliquity forcing may have a greater impact on the western Pacific ITCZ migration, based on planktonic foraminifera REE/Ca records. Therefore, the reconstruction of precipitation records on longer timescales is of great significance for exploring the effects of diverse orbital forcing on the hydroclimate variability.

In contrast to the observed anti-phase precipitation patterns between hemispheres on different timescales, an increasing number of studies demonstrated synchronized precipitation variations between hemispheres in tropical regions on (sub-)millennial timescales (e.g., Konecky et al. 2013; Yan et al. 2015; Denniston et al. 2016; Scroxton et al. 2017; Yang et al. 2023). Such synchronized pattern would contradict the traditional migration theory of the ITCZ. The in-phase precipitation changes can be well explained by the expansion and contraction of the latitudinal extent of the ITCZ (Konecky et al. 2013; Yan et al. 2015; Denniston et al. 2016; Scroxton et al. 2017; Yang et al. 2023). During times of ITCZ expansion, precipitation intensifies synchronously at both northern and southern edges, while precipitation declines simultaneously at both edges when the ITCZ contracts (Yuan et al. 2023). The expansion and contraction of the ITCZ is likely triggered by various interacting factors, such as symmetrical changes in solar irradiance (Yan et al. 2015; Scroxton et al. 2017), strengthening Walker circulation (Konecky et al. 2013) and changes in the meridional atmospheric circulation (Denniston et al. 2016). Further studies, hence, are warranted to explore the impact of spatial ITCZ variations on alterations in precipitation patterns between hemispheres.

As the region with the highest water vapor exchange globally (Pierrehumbert, 2000), the IPWP is ideal to provide insight into the impact of ITCZ migration and expansion/contraction on local precipitation. Recently, Xiong et al. (2022) and Yu et al. (2023) examined the spatial pattern of precipitation variations across the IPWP on millennial scales, highlighting the importance of the ITCZ on the precipitation patterns in different regions of the IPWP. The observed changes in precipitation during the Last Glacial Maximum (LGM) point to an overall drying trend across the IPWP, caused by both the zonal shifts of ENSO and the meridional migration of ITCZ (Xiong et al. 2022). Yu et al. (2023) identified noticeably spatial heterogeneity of precipitation within the IPWP over the past 40 kyr, attributed to the predominant effects of the ENSO-like system in the western Pacific and the migration of the ITCZ in the Indian Ocean. Unfortunately, studies on past ITCZ expansion/contraction changes are not only scarce but also primarily confined to the last few millennia for the IPWP region (e.g., Denniston et al. 2016; Konecky et al. 2013; Scroxton et al. 2017; Yan et al. 2015; Yang et al. 2023), significantly limiting our understanding of the impact of ITCZ variability on spatial patterns of precipitation variations in IPWP.

To determine the temporal and spatial variability of the ITCZ on orbital timescales, we attempted to reconstruct the precipitation patterns within the IPWP region. Past precipitation, approximated from the residual oxygen isotope compositions of seawater, was deduced from combined δ¹⁸O and Mg/Ca compositions of sea surface-dwelling planktonic foraminifera from International Ocean Discovery Program (IODP) Site U1486 within the central IPWP (Bismarck Sea). By integrating published ice-volume corrected sea surface salinity records (δ¹⁸O_sw-ivc) from the IPWP region, we assessed the spatial and temporal patterns of precipitation across the IPWP over the past 450 kyr and explored its driving mechanisms on the precession band.

2. Materials and methods

2.a. Location of site U1486 and modern climate

IODP Expedition 363 Site U1486 (02º22.34ʹS, 144º36.08ʹE) was drilled at a water depth of 1332 m in the Bismarck Sea, located ∼150 km away from the mouth of the Sepik River in Papua New Guinea (Fig. 1a; Rosenthal et al. 2018). This study focused on samples from Site U1486, collected at 10-cm intervals, covering a depth range from the core-top down to 31 m Core Composite Depth Below Sea Floor. The sediment lithology in this section remained consistently homogenous, primarily composed of fine-grained, grayish-white sediment rich in planktonic microfossils. No volcanic ash layers or signs of disturbance were observed.

Figure 1. The Indo-Pacific Warm Pool and regional circulation patterns, alongside the modern climatology of the study area. (a) Map of the IPWP displaying modern annual sea surface temperatures (SST at 0 m; World Ocean Atlas 2013 dataset; Locarnini et al. 2013) of the studied Site U1486 (yellow star) and other relevant sites discussed in the study (white circles) is shown. Bright yellow shading lines denote the mean positions of the Intertropical Convergence Zone (ITCZ) in July and January, respectively (Lutgens & Tarbuck, 2001). Black dotted lines represent annual mean 28°C SST isotherms. Key ocean currents include the North Equatorial Current (NEC), North Equatorial Counter Current (NECC), South Equatorial Current (SEC), Kuroshio Current (KC) and Mindanao Current (MC). (b) The Southern Oscillation Index (SOI, 5-point running average) between 1980 and 2020 (https://www.cpc.ncep.noaa.gov/data/indices/soi) together with the monthly rainfall anomaly over the Site U1486 source area (2.5° spatial resolution centered on 2°S, 144°E, 11-point running average, https://climatedataguide.ucar.edu/climate-data/gpcp-monthly-global-precipitation-climatology-project). (c) Depiction of mean rainfall rate and the 1000 hPa wind field in January (left) and July (right). Rainfall data sourced from https://psl.noaa.gov/data/gridded/data.cmap.html, while land wind data obtained from http://iridl.ldeo.columbia.edu/expert/ds:/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1.

The hydrological cycle in the studied area is influenced both by the Asian-Australian monsoon and ENSO (Webster et al. 1998; Wang et al. 2003). During the boreal summer, southeasterly trade winds prevail, with the ITCZ shifting northward, thereby reducing the precipitation rate in the study area (Fig. 1a, c). Conversely, northwesterly winds dominate during the boreal winter, leading to a southward shift of the ITCZ and an increase in precipitation (Fig. 1a, c). On inter-annual timescales, precipitation is closely linked to ENSO events (Fig. 1b). Unusual dry (wet) conditions occur in the Papua New Guinea region during El Niño (La Niña) phases (Dai & Wigley, 2000).

2.b. Stable oxygen isotope and Mg/Ca analyses

Stable oxygen isotope (δ¹⁸O) and Mg/Ca analyses were conducted on the surface-dwelling planktonic foraminiferal species Globigerinoides ruber (white) extracted from 335 samples collected at 10-cm intervals downcore. Approximately 50 shells of G. ruber were selected from the 250–355 μm size fraction. Subsequently, these foraminiferal shells were gently crushed into several fragments.

For stratigraphical purposes, δ¹⁸O was measured on the benthic foraminifer Cibicidoides wuellerstorfi. Approximately 3–5 individuals were picked from both the 300–355 μm and the 355–500 μm size fractions.

For stable isotope analysis, all fragments from the benthic samples and one-third of the fragments from the planktonic foraminifer G. ruber were utilized, while the remaining two-thirds of the G. ruber fragments were allocated for Mg/Ca analysis. The isotope analysis was performed using a GV IsoPrime mass spectrometer at the Key Laboratory of Marine Geology and Environment, Institute of Oceanography, Chinese Academy of Sciences, Qingdao. Results are presented in δ¹⁸O notation (‰ relative to the Vienna Pee Dee Belemnite standard), calibrated to the National Bureau of Standards 18 standard, analyzed approximately every 10 samples, with a long-term standard deviation of 0.06‰.

For Mg/Ca analysis, the G. ruber underwent pretreatment and cleaning procedures following the methods described by Barker et al. (2003), with the exclusion of a reductive step. The shells were sequentially cleaned using Milli-Q water rinses, methanol rinses and an oxidizing solution. Organic matter was eliminated through oxidation using a heated 1% NaOH-buffered H₂O₂ solution. Samples were dissolved in 0.075-M QD HNO₃ and analyzed using an inductively coupled plasma–optical emission spectrometer (ICP-OES) at the same laboratory. The average standard deviation for replicate measurements was approximately 0.52%. The contamination assessment included Fe/Ca, Mn/Ca and Al/Ca ratios, with most samples registering values below detection limits (Fig. S1). Furthermore, according to Barker et al. (2003), the presence of detrital contamination affecting the measured Mg/Ca ratios can be inferred by evaluating the correlation between estimated Mg/Ca and Fe/Ca or Al/Ca data. No significant correlation was found between Fe/Ca and Mg/Ca (R² = 0.006), Mn/Ca and Mg/Ca (R² = 0.02), or Al/Ca and Mg/Ca (R² = 0.004), ensuring the reliability of the Mg/Ca measurements (Fig. S1).

2.c. Estimates of sea-surface temperature and seawater δ¹⁸O

The planktonic foraminifer G. ruber calcifies within the depths of 30–95 m in the mixed layer of the Western Pacific Warm Pool (WPWP) (Rippert et al. 2016; Raddatz et al. 2017). By comparing the reconstructed sea-surface temperature (SST_Mg/Ca) based on the Mg/Ca ratio in G. ruber shells using available calibration equations (Fig. S2; Table S1), we selected the equation (1) established in the western Pacific by Dekens et al. (2002). This decision was based on the fact that its constructed average core-top SST_Mg/Ca value (∼29.3°C) aligns closely with the modern SST (∼29.1°C; Locarnini et al. 2013).

(1) $$\matrix{ {{\rm{SST}}(^\circ {\rm{C}}) = \;{\rm{ln}}\;\left[ {{\rm{Mg}}/{\rm{Ca}}\;\left( {{\rm{mmol}}/{\rm{mol}}} \right)/0.38} \right]/0.09\;} \hfill \cr {\quad \quad \quad \quad \quad \quad + 0.61\;\left( {{\rm{core}}\;{\rm{depth}}\;{\rm{km}}} \right) + 1.6} \hfill \cr } $$

Calcite dissolution might lead to lowered Mg/Ca-temperature estimates due to the selective removal of Mg-ions from the foraminiferal calcite (e.g., Nürnberg et al. 1996; Dekens et al. 2002; Regenberg et al. 2006). Equation (1) incorporates a depth correction to account for dissolution effects on Mg/Ca. As indicated by Regenberg et al. (2014), foraminiferal Mg/Ca in the study area is notably affected at depths below 2500 m. Given the limited impact of dissolution at our specific location (1332 m), we decided not to apply the dissolution correction in our Mg/Ca data. Moreover, recent calibrations derived from sediment traps and modern core tops in WPWP provide confidence that the dissolution effect is insignificant in our study area (Hollstein et al. 2017).

The error in SST_Mg/Ca reconstructions was assessed by propagating the uncertainties arising from both the Mg/Ca measurements and the Mg/Ca temperature calibration, following Mohtadi et al. (2014). On average, the resulting errors are approximately ±0.97°C.

The oxygen isotope composition of seawater (δ¹⁸O_sw) and SSS exhibit a linear relationship in the western Pacific (e.g., Fairbanks et al. 1997; Gibbons et al. 2014). As a result, the residual δ¹⁸O of surface seawater (δ¹⁸O_sw-ivc), which corrects for the influence of continental ice volume, has been widely utilized in the reconstruction of past regional hydrological changes. We derived δ¹⁸O_sw using Equation (2) established by Bemis et al. (1998), adjusting values to the Vienna Standard Mean Ocean Water standard by adding 0.27‰ (Hut, 1987). The estimated average core-top δ¹⁸O_sw value (∼0.25‰) closely matches the modern seawater δ¹⁸O value at the core site (0.22‰; LeGrande & Schmidt, 2006). Additionally, we compared the estimated δ¹⁸O_sw results derived from different SST_Mg/Ca calibration equations (Fig. S2; Table S1), validating the reliability of the calibration equations. Furthermore, we corrected δ¹⁸O_sw for global ice volume changes based on the studies by Bintanja et al. (2005) and Spratt & Lisiecki (2016), respectively. As shown in Fig. S2, the two estimated δ¹⁸O_sw-ivc results exhibit a high level of consistency. Consequently, the estimated δ¹⁸O_sw-ivc derived from simulated global marine isotopes from Bintanja et al. (2005) is reliable and feasible.

(2) $${{\rm{\delta }}^{18}}{{\rm{O}}_{{\rm{sw}}}} = \left[ {{\rm{SS}}{{\rm{T}}_{{\rm{Mg}}/{\rm{Ca}}}}\left( {^\circ {\rm{C}}} \right) - 16.5 + 4.8 \times {{\rm{\delta }}^{18}}{{\rm{O}}_{G.\;ruber}}} \right]/4.8 + 0.27$$

We calculated uncertainties in δ¹⁸O_sw-ivc using the method described by Mohtadi et al. (2014), accounting for uncertainties from Mg/Ca and δ¹⁸O measurements, temperature equations and global ice volume removal (Bintanja et al. 2005). The resulting uncertainties averaged 0.21‰ for δ¹⁸O_sw-ivc.

2.d. Chronology

The age model for Site U1486 was constructed based on the δ¹⁸O record obtained from the benthic foraminifer Cibicidoides wuellerstorfi (δ¹⁸O _{C. wuellerstorfi} ). The δ¹⁸O _{C. wuellerstorfi} values ranged from 1.68‰ to 3.85‰ over the past 450 kyr BP, revealing clear glacial-interglacial cycles (Fig. 2a). Comparison of δ¹⁸O _{C. wuellerstorfi} values with the global benthic foraminiferal δ¹⁸O stack LR04 (Lisiecki & Raymo, 2005) identified 15 tie points for age control (Fig. 2a). The age model for the entire core was calculated using a MATLAB linear interpolation optimization method. Site U1486 provides a continuous record spanning the past 450 kyr, encompassing five glacial-interglacial cycles (Fig. 2a). The average sedimentation rate over this period was approximately 7.17 cm/kyr the past 450 kyr (Fig. 2b). With a sampling interval of 10 cm, the average sample resolution is approximately 1.3 kyr.

Figure 2. Age model of Site U1486. (a) Benthic foraminifer C. wuellerstorfi δ¹⁸O (blue line) and the global benthic foraminiferal δ¹⁸O stack LR04 (Lisiecki & Raymo, 2005) (red line). Black triangles indicate tie points for δ¹⁸O _{C. wuellerstorfi} . (b) Depth-age model (green line) and sedimentation rates (black line) of sediment Site U1486. Gray vertical shadings indicate even-numbered Marine Isotope Stages (MIS).

3. Results

The oxygen isotope values of G. ruber (δ¹⁸O _{G. ruber} ) exhibit a range from −3.18‰ to −0.93‰ over the past 450 kyr, displaying distinct glacial-interglacial cycles (Fig. 3a). Generally, δ¹⁸O _{G. ruber} values are higher/heavier during glacial periods (MIS 2, 4, 6, 8, 10 and 12) and lower/lighter during interglacial stages. The highest δ¹⁸O _{G. ruber} value (−0.93‰) is observed at ∼433.4 kyr (MIS 12), while the lowest δ¹⁸O _{G. ruber} value (∼−3.18‰) is found at the top of the core.

Figure 3. Planktonic foraminiferal proxy records of Site U1486. (a) G. ruber δ¹⁸O, (b) G. ruber Mg/Ca-based SST, (c) the ice volume corrected-seawater δ¹⁸O (δ¹⁸O_sw-ivc) and (d) C. wuellerstorfi δ¹⁸O (black), superimposed is the global ice volume δ¹⁸O_sw (blue; Bintanja et al. 2005). Gray vertical shadings indicate even-numbered marine isotope stages (MIS), while colored shadings for individual records represent their respective error ranges (see Sections 2.2 and 2.3). (e) Spectral (left) and wavelet transform (right) analyses of the δ¹⁸O_sw-ivc record of Site U1486. The green line indicates the precession cycle.

G. ruber Mg/Ca varies from 3.22 to 5.24 mmol/mol, which yields sea-surface temperature (SST_Mg/Ca) at Site U1486 spanning a range of 25.3 to 30.8 °C, with an average of approximately 27.7 °C. As shown in Figure 3b, the SST_Mg/Ca record shows pronounced glacial-interglacial variations over the past 450 kyr, with higher temperatures occurring during interglacial stages and lower temperatures during peak glacial stages.

Modern seawater δ¹⁸O and SSS display a positive correlation, with higher δ¹⁸O_sw-ivc values corresponding to increased salinity in surface seawater, while lower δ¹⁸O_sw-ivc values indicate fresher seawater (Fairbanks et al. 1997; Rosenthal et al. 2003; Stott et al. 2004; Gibbons et al. 2014). Model simulations also support the assumption that the tropical Pacific has maintained a consistent relationship between δ¹⁸O_sw-ivc and SSS (Holloway et al. 2016). In comparison with δ¹⁸O _{G. ruber} and SST_Mg/Ca, δ¹⁸O_sw-ivc does not exhibit obvious glacial-interglacial cycles, ranging from −0.66‰ to 1.20‰ (Fig. 3c). However, during MIS 2, MIS 6, MIS 10 and MIS 12, δ¹⁸O_sw-ivc is relatively lower compared to the interglacial stages, with the highest value observed near the MIS 10/9 transition (Fig. 3c). During deglacial episodes of rapid SST_Mg/Ca warming (e.g., MIS 12/11, MIS 10/9, MIS 6/5 and MIS 2/1), sea-surface δ¹⁸O_sw-ivc values simultaneously increase, implying a change to more saline conditions at sea surface (Fig. 3b, c).

4. Discussion

4.a. Using site U1486 δ¹⁸O_sw-ivc as a proxy to reconstruct local precipitation variability over the past 450 kyr

δ¹⁸O_sw serves as a common proxy for SSS, which is a crucial parameter in the hydrological cycle (Stott et al. 2004; Leech et al. 2013). Although δ¹⁸O_sw is influenced by multiple factors, including precipitation, evaporation, upwelling and meridional and zonal currents (Conroy et al. 2017), the existence of a linear correlation between δ¹⁸O_sw and SSS is due to both being primarily influenced by evaporation and precipitation (Fairbanks et al. 1997; LeGrande & Schmidt, 2006). Thus, the δ¹⁸O_sw variability is often interpreted in the context of hydroclimate variability (Fairbanks et al. 1997; LeGrande & Schmidt, 2006). Additionally, Conroy et al. (2017) found that the δ¹⁸O_sw-salinity linear relationship appears stronger in the western tropical Pacific based on both observational data and results from isotope-enabled climate model simulations. Therefore, δ¹⁸O_sw primarily reflects changes in the precipitation-evaporation balance. Given the minimal fluctuations in modern SST in the western Pacific (Kim et al. 2012), δ¹⁸O_sw likely reflects local precipitation patterns (Gibbons et al. 2014). Recent studies by Yang et al. (2024) analyzing modern reanalysis data in the South China Sea revealed an inverse correlation between regional SSS and precipitation rate. Additionally, Conroy et al. (2017) identified a weak relationship between SST and SSS in the western Pacific based on modern data.

For the reconstruction of past local precipitation changes in the western Pacific, we consider the ice-volume corrected δ¹⁸O_sw-ivc record from Site U1486. Positive deviations in δ¹⁸O_sw-ivc values indicating more saline conditions at sea surface, suggest reduced local precipitation in the western Pacific, while negative values point to intensified local precipitation (Rosenthal et al. 2003; Stott et al. 2004). The Site U1486 δ¹⁸O_sw-ivc record displays relatively muted glacial-interglacial cycles over the past 450 kyr, which fluctuate between −0.66 ‰ and 1.20 ‰ and differ substantially from the SST_Mg/Ca pattern (Fig. 3c). Moreover, the variation in Site U1486 δ¹⁸O_sw-ivc is similar to the change in the ln(Ti/Ca) record obtained from the adjacent core MD05-2920 (Fig. 4i; Tachikawa et al. 2011, 2014). The ln(Ti/Ca) values were utilized as an indicator of rainfall over the drainage basin of the Sepik River in New Guinea (Tachikawa et al. 2011, 2014). Cross-spectral analysis reveals a significant coherence between Site U1486 δ¹⁸O_sw-ivc and ln(Ti/Ca) from core MD05-2920 on the precession band (Fig. S3), supporting the dominance of precession cycles in controlling precipitation variations in the study area.

Figure 4. The orbital-scale precipitation pattern in the Indo-Pacific Warm Pool over the past 450 kyr. (a) The variation in precession (Laskar et al. 2004). (b) GISS_ModelE2-R simulated δ¹⁸O_sw-ivc records of the IPWP (Jian et al. 2022). (c, d, e) The δ¹⁸O_sw-ivc records of cores MD98-2162, SO18480-3 and U1483 in the eastern tropical Indian Ocean (Jian et al. 2022; Zhang et al. 2022). (f, g) The δ¹⁸O_sw-ivc records of cores MD06-3047B and MD06-3067 in the northern sector of western equatorial Pacific (Bolliet et al. Bolliet et al., 2011; Jia et al. Jia et al., 2018). (h) The δ¹⁸O_sw-ivc records of core MD05-2925 (Lo et al. 2022). (i) The ln(Ti/Ca) record of core MD05-2920 (Tachikawa et al. 2014). (j) The δ¹⁸O_sw-ivc record of Site U1486, superimposed by the July–August insolation at the equator (black) (Laskar et al. 2004). The overlain, bold, dark-colored lines represent the precession band-pass filtered results. Band-pass filter with a central frequency of 0.043 kyr⁻¹ and a bandwidth of 0.01 kyr⁻¹. Vertical grey bars indicate lower precession values.

To study the underlying mechanisms driving local precipitation changes in the western Pacific over the past 450 kyr, we employed redfit spectral and wavelet analysis using the Past3 software with a Hanning window (Hammer et al. 2001). The spectral analysis reveals that the Site U1486 δ¹⁸O_sw-ivc record is characterized by prominent 100 kyr eccentricity and 23 kyr precession cycles. Wavelet analyses further shows that the 23 kyr cycle is most prominent during ∼260−180 and ∼370−300 ka BP (Fig. 3e). The phase difference between the maximum in δ¹⁸O_sw-ivc and the maximum in precession is ∼−148° (Fig. 5h). On the precession band, the Site U1486 δ¹⁸O_sw-ivc record displays precession band-pass filtered patterns that are mostly in phase with a few exceptions, such as δ¹⁸O_sw-ivc changes preceding precession shifts during the periods of ∼175−145 and ∼110−75 ka BP (Fig. 4j). The discrepancies between the δ¹⁸O_sw-ivc and precession may be attributed to terrestrial influences (Hollstein et al. 2020), or due to a relatively weakened 23 kyr cycle during these specific time intervals (Fig. 3e). Nonetheless, the δ¹⁸O_sw-ivc record exhibits an overall consistency with precession variations over the past 450 kyr, where almost every low precession value corresponds to high δ¹⁸O_sw-ivc values (Fig. 4j). We conclude that the past local precipitation variations at Site U1486 were associated with precessional changes.

Figure 5. Comparing the precipitation records in the IPWP and East Asia with the ENSO-like proxy and the migration of the ITCZ. (a) The variation in precession (Laskar et al. 2004). (b) The speleothem δ¹⁸O records in China (Cheng et al. 2016), superimposed on the summer insolation at 15°N (Laskar et al. 2004). (c) The summer interhemispheric tropical insolation gradient, determined by the variation in June insolation between 23° N and 23° S (Laskar et al. 2004). (d) CESM-simulated Niño3 SST of DJF (Zhang et al. 2021). (e) Differences between SST and thermocline water temperature (ΔT) for Site U1486. (f) GISS_ModelE2-R simulated δ¹⁸O_sw-ivc records of the IPWP (Jian et al. 2022). (g) The δ¹⁸O_sw-ivc records of Site U1486 superimposed by the July–August insolation at the equator (black) (Laskar et al. 2004). The records are overlaid by the precession band-pass filtered output (bold, dark-colored lines). Band-pass filter with a central frequency of 0.043 kyr⁻¹ and a bandwidth of 0.01 kyr⁻¹. Vertical grey bars indicate lower precession values (high Northern Hemisphere summer insolation). (h) Phase relationships among U1486 δ¹⁸O_sw-ivc, simulated IPWP δ¹⁸O_sw-ivc, speleothem δ¹⁸O and the precession parameter, illustrated through coherences (left) and phase angles (right).

It is noteworthy that, although Site U1486 δ¹⁸O_sw-ivc record and the ln(Ti/Ca) record from adjacent core MD05-2920 exhibit nearly coherent variation on the precession band (Fig. 4i, j), the ln(Ti/Ca) record is characterized by not only precession but also obliquity signals (Tachikawa et al. 2014). The coarse terrigenous fraction represented by Ti varied with the precession cycle, while CaCO₃ exhibited a pronounced obliquity band (Tachikawa et al. 2011). Additionally, precipitation records based on ln(K/Ca) at Site U1483 (offshore north-western Australia) contain an additional obliquity component (Zhang et al. 2020). The orbital forcing of elemental ratios is evidently different from the predominant precession control observed in the δ¹⁸O_sw-ivc records of Site U1486. This discrepancy in dominant cyclicity may suggest that different precipitation proxies respond differently to climatic forcing. The SSS proxy (δ¹⁸O_sw-ivc), primarily controlled by the precipitation-evaporation balance (Conroy et al. 2017), tends to exhibit stronger precession-driven variability due to its direct linkage with monsoon dynamics and seasonal changes in precipitation (Stott et al. 2004).

Orbital precession is triggered by the Earth’s rotational axis oscillation, leading to a continuous change in the relative positioning of the Earth and the Sun (Berger & Loutre, 1994). This alteration modifies the seasonal and meridional distribution of solar radiation across the Earth’s surface with a 23 kyr periodicity (Berger & Loutre, 1994; Merlis et al. 2013). The equatorial insolation variation is primarily influenced by precession changes on orbital timescales, with an increased summer solar insolation in the equatorial region during low precession periods (Yin et al. 2021). Our δ¹⁸O_sw-ivc record reveals a similar pattern with July–August insolation changes at the equator over the past 450 kyr. The elevated insolation levels corresponded to increased δ¹⁸O_sw-ivc values, implying reduced local precipitation in Papua New Guinea, and vice versa (Fig. 4j). The core MD05-2920 ln(Ti/Ca) also exhibits significant coherence with insolation at precessional periodicities, consistent with our δ¹⁸O_sw-ivc record of Site U1486 (Fig. 4i, j). Additionally, modeled precipitation results further support the simultaneous changes in precipitation and local summer insolation in the western Pacific (Kutzbach et al. 2008). These findings suggest that variations in local precipitation in the Papua New Guinea region were likely driven by the equatorial insolation changes in line with precessional variations. The precession signal might have influenced the local precipitation patterns through northern and southern hemispheric thermal gradients (Braconnot et al. 2008) or the temperature contrast between land and sea surfaces (Ruddiman, 2008; Merlis et al. 2013).

4.b. Precession-driven synchronous precipitation changes in the IPWP over the past 450 kyr

The IPWP as Earth’s largest reservoir of atmospheric water vapor experiences the highest rainfall levels (Cane, 1998; Leung et al. 2022). Modern mean rainfall data show a central region for rainfall within the IPWP (Fig. 1c). To investigate precipitation variations on orbital timescales across the entire IPWP region, we compared precipitation records from different areas within its central precipitation zone (Fig. 1a). These areas encompass the northern IPWP (cores MD06-3047B and MD06-3067; Bolliet et al. Bolliet et al., 2011; Jia et al. Jia et al., 2018), the southern sector of the IPWP (cores U1486 and MD05-2925; this study and Lo et al. 2022) and the Indian Ocean (cores MD98-2162, SO18480-3 and U1483; Jian et al. 2022; Zhang et al. 2022). We distinguish the northern and southern divisions of the IPWP based on the equator. Detailed information about the original datasets for these cores is provided in Table S2. Conroy et al. (2017) identified a linear relationship between δ¹⁸O_sw and SSS in various IPWP areas across multiple sites. δ¹⁸O_sw-ivc likely serves as a reliable proxy to reflect past local precipitation variations due to the limited variability of SST in the IPWP (Gibbons et al. 2014). All δ¹⁸O_sw-ivc records are similarly based on G. ruber Mg/Ca and δ¹⁸O and are considered to reliably reflect local precipitation through time.

All δ¹⁸O_sw-ivc records from various regions of the western tropical Pacific across the equator suggest overall synchronous precipitation patterns on orbital timescales. The δ¹⁸O_sw-ivc record of core MD05-2925 (Lo et al. 2022), situated in the southern region of the western equatorial Pacific, shows a fundamental synchrony with variations in boreal summer insolation at the equator driven by precession over the past 450 kyr (Fig. 4h). Similarly, the precipitation record of core MD06-3047B (Jia et al. Jia et al., 2018), located in the northernmost part of the WPWP, is consistently related to precession variations over the past 450 kyr, except during the ∼410-370 ka BP period (Fig. 4f) (likely due to missing original data in the core MD06-3047B δ¹⁸O_sw-ivc record during this specific period). Furthermore, the filtered 23-kyr component of the δ¹⁸O_sw-ivc record from core MD06-3067 in the northern IPWP also demonstrates synchronous changes with precession variations over the past ∼160 kyr, where each peak in SSS corresponds to both a low in precession and a peak in equatorial insolation during boreal summer (Fig. 4g; Bolliet et al. Bolliet et al., 2011).

In contrast, the δ¹⁸O_sw-ivc record from the central IPWP core KX97322-4 exhibits limited coherence with precession variability over the past 360 kyr (Zhang et al. 2021). This lack of coherence might be attributed to an ENSO-like state or potentially to the fact that δ¹⁸O_sw-ivc is influenced by both precession and obliquity (Zhang et al. 2015, 2021). Obliquity may affect the atmospheric circulation and the associated meridional heat and moisture fluxes by regulating the latitudinal distribution of insolation (Mantsis et al. 2011, 2014; Bosmans et al. 2015). This might explain why obliquity bands are present in the δ¹⁸O_sw-ivc record of core KX97322-4, as obliquity could affect local precipitation through atmospheric circulation and heat and moisture fluxes. Additionally, variations in the δ¹⁸O_sw-ivc signals may be due in part to differences in core locations capturing divergent information. Moreover, Liu et al. (2015) proposed that precipitation variations in the western Pacific are more affected by obliquity than by precession-paced changes in the ITCZ, based on foraminiferal rare earth elements (indicator of precipitation-dependent river runoff). This change in dominant cyclicity could be attributed to differences in proxy-specific bias or tendency. In the eastern IPWP, the δ¹⁸O_sw-ivc records from cores SO225-08 (unpublished data from Nürnberg) and ODP806 (Lea et al. 2000), located in the eastern IPWP, did not exhibit synchronous changes with precession parameters but rather showed distinct glacial-interglacial cycles. This discrepancy might be attributed to the relatively low sample resolution in these records. Cores SO225-08 and ODP806, being situated close to the central equatorial Pacific, might experience more influence on their hydrological conditions by atmospheric circulation, such as the meridional Hadley circulation and the Walker circulation (Lea et al. 2000). In conclusion, the variations in δ¹⁸O_sw-ivc across various regions of the WPWP are not entirely identical, possibly due to regional differences, varying impact of driving factors, and/or sample resolution. However, with respect to precessional cyclicity, all precipitation records within the central precipitation zone across the WPWP generally show similar patterns (Fig. 4).

Notably, the pattern of precipitation changes in the western part of the IPWP (eastern tropical Indian Ocean) is similar to in the western tropical Pacific. The filtered 23-kyr component of the δ¹⁸O_sw-ivc records from the southern part of the Makassar Strait (core MD98-2162) and Timor Sea (cores SO18480-3 and U1483) close to the equator demonstrate a nearly synchronous response to precession-driven boreal summer insolation variations (Fig. 4c–e; Jian et al. 2022; Zhang et al. 2022). Higher insolation levels correspond to higher δ¹⁸O_sw-ivc values, hence more saline sea-surface conditions, while lower insolation levels correspond to lowered δ¹⁸O_sw-ivc values and fresher conditions (Fig. 4c–e). This covariance is consistent with the precipitation record of Site U1486. Overall, nearly all records are characterized by higher δ¹⁸O_sw-ivc values (higher SSS; lower precipitation) at precession minima, implying that both the eastern tropical Indian Ocean and the western tropical Pacific within the IPWP experienced drier conditions simultaneously during lower precession periods (Fig. 4).

Consistently, the simulated annual mean δ¹⁸O_sw-ivc derived from a water isotope (δ¹⁸O_sw)-enabled air-sea coupled climate model (GISS_ModelE2-R; Jian et al. 2022) reveals a dominant 23-kyr cycle with higher δ¹⁸O_sw-ivc values and hence, more saline conditions at sea surface during precession minima (Fig. 4b), supporting our notion on synchronous precession-driven precipitation changes in the IPWP. We conclude that the variations in equatorial insolation, driven by precession parameters, likely influence the SSS through precipitation variability within the IPWP region.

4.c. Effects of the ITCZ changes on the precipitation variations in IPWP

4.c.1. Variations in ITCZ latitudinal migration

The IPWP is a significant moisture source for the East Asian summer monsoon (EASM). Numerous studies demonstrated a strong association between EASM variability and the hydrological conditions as well as convective activity in the IPWP (e.g., Lau et al. 2000; Lu, 2001; Kawamura et al. 2001; Yin et al. 2014). The records of Chinese stalagmite δ¹⁸O, commonly used to reconstruct EASM variability, display distinct precession-related changes with intense precipitation during higher Northern Hemisphere summer insolation over the past 450 kyr (Fig. 5b; Cheng et al. 2016). The strong precession signal observed in Chinese speleothem records corresponding to insolation patterns is consistent with climate model simulations (e.g., Battisti et al. 2014; Rachmayani et al. 2016). The Site U1486 δ¹⁸O_sw-ivc variations generally show a pronounced anti-phased relationship with Chinese stalagmite δ¹⁸O on precessional timescales (Fig. 5b, f). This observation is further corroborated by the coherence analysis (Fig. 5h) and by most of the precipitation records within the IPWP region (Fig. 4). When δ¹⁸O_sw-ivc values increase (lower precipitation) in the IPWP, the δ¹⁸O values in Chinese speleothems decrease (higher precipitation). Conversely, periods of high precipitation in the IPWP correspond to a weakening of the EASM. Additionally, the simulated δ¹⁸O_sw-ivc of the IPWP and measured δ¹⁸O values in Chinese speleothems are anti-correlated on precessional timescales (Fig. 5b, e; Jian et al. 2022), which is further confirmed by cross-spectral analysis (Fig. 5h).

The anti-phased relationship between δ¹⁸O_sw-ivc of the IPWP and Chinese speleothem δ¹⁸O is associated with the ocean heat content of the IPWP (Jian et al. 2022). During low precession phases (high Northern Hemisphere summer insolation), La Niña-like conditions prevail in the western tropical Pacific. This is indicated by both the CESM-simulated Niño3 SST of December–January–February (DJF) (Zhang et al. 2021), and the temperature gradient between sea surface and thermocline waters (ΔT). A deep thermocline at Site U1486 deduced from a small ΔT (unpublished data) implies enhanced ocean heat storage in the mixed layer (Fig. 5d, e). This higher ocean heat content likely amplifies the strength of the EASM, enhancing moisture transport from ocean to continent and resulting in a negative deviation in Chinese speleothem δ¹⁸O values (Jian et al. 2022). The precipitation changes in the IPWP and EASM regions exhibit a strong coherence to local insolation changes. Therefore, their precipitation changes may be attributed to the redistribution of heat between the Northern and Southern Hemispheres triggered/amplified by precessional forcing, a conclusion which is consistent with model simulations (Braconnot et al. 2008; Battisti et al. 2014).

The IPWP, located within the belt of the modern ITCZ’s seasonal migration (Fig. 1a), experiences precipitation primarily regulated by the seasonal latitudinal movement of the ITCZ (Trenberth et al. 2000; Schmidt & Spero, 2011; Schneider et al. 2014; Jo et al. 2014). During periods of elevated summer insolation in the Northern Hemisphere, the ITCZ migrates northward (e.g., McGee et al. 2014), lowering precipitation in IPWP (Schmidt & Spero, 2011; Jo et al. 2014). This low in IPWP precipitation is supported by the high values of δ¹⁸O_sw-ivc records from the IPWP region during the lower precession periods, which correspond to the high summer insolation in the Northern Hemisphere (Fig. 4). Seasonal variations in the thermal contrast between the Northern and Southern Hemispheres prompted the migration of the ITCZ, shifting toward the warmer hemisphere (Schneider et al. 2014). During the boreal summer, increasing insolation warms the subtropical water, strengthening the meridional pressure gradient and enhancing the southerlies flowing across the equator into the ITCZ, leading the ITCZ to move northward (Chang & Philander, 1994; Geen et al. 2020). Therefore, the migration of the mean position of the ITCZ could be indicated by the disparity in summer insolation between hemispheres (Chiang & Friedman 2012; Lu et al. 2013; Schneider et al. 2014). We calculated the summer interhemispheric insolation gradient between 23° N and 23° S (Fig. 5c; Laskar et al. 2004). A higher summer interhemispheric gradient during the precessional minima points to the northward migration of the ITCZ and reduced precipitation in the IPWP (Fig. 5f, g). Conversely, a decreased meridional summer interhemispheric insolation gradient results in the southward shift of the ITCZ, causing increased precipitation in the IPWP (Fig. 5c, f, g). During periods of Northern Hemisphere warming, such as the Bølling–Allerød and the early Holocene epochs characterized by a strong EASM, the ITCZ shifted northward (Wang et al. 2001; Yancheva et al. 2007). Recent research further suggests that the location and migrations of the ITCZ, where the trade winds of the Northern and Southern Hemispheres converge, are closely associated with variations in regional monsoon patterns (Gadgil, 2018, and references therein). Hence, the strength of the EASM is closely linked to the migrations of the ITCZ (e.g., Ding et al. 1995; An, 2000; Yancheva et al. 2007). Chinese stalagmite δ¹⁸O records revealed that the intensification of the EASM during the precessional minima, accompanied by the northward migration of the ITCZ, resulting in high precipitation in the EASM region (Fig. 5b, c; Cheng et al. 2016). These findings emphasize the significant impact of ITCZ variations on hydrological changes within the IPWP and EASM regions. Orbital precession played a pivotal role in determining the seasonal distribution of incoming insolation in both hemispheres. This drives the north-south migration of the ITCZ and causes the subtropical high-pressure anomalies in the Indian and Pacific Oceans (Merlis et al. 2013; Schneider et al. 2014), leading to anti-correlated changes in precipitation between the IPWP and EASM region.

The mechanisms described above, supported by other paleoclimatic and modeling studies spanning millennial to orbital timescales (e.g., Wang et al. 2006; Merlis et al. 2013; Dang et al. 2015; Huang et al. 2019), provide a robust explanation for the interhemispheric anti-phased precipitation pattern between the IPWP and EASM region. Nevertheless, precipitation records in the northern equatorial region of the IPWP (e.g., cores MD06-3047B and MD06-3067; Bolliet et al. Bolliet et al., 2011; Jia et al. Jia et al., 2018) exhibit nearly synchronous interhemispheric changes with the southern IPWP region (Fig. 4). The migration of the ITCZ alone might not adequately account for the synchronous interhemispheric precipitation changes observed. Today, the ITCZ shifts between 9°N in boreal summer and 2°N in boreal winter across the modern Pacific Ocean (Schneider et al. 2014). We speculate that during periods of high precession when the ITCZ migrates southward, the ITCZ might still remain in the Northern Hemisphere, resulting in heightened precipitation over both the northern and southern IPWP regions. Besides, we propose that the synchronized interhemispheric precipitation changes in the northern and southern WPWP could be linked to the expansion and contraction of the ITCZ on the precession band (see below).

4.c.2. Variations in ITCZ expansion and contraction

Numerous studies have revealed the in-phase changes in precipitation between hemispheres over (sub-)millennial timescales, which can be effectively explained by the expansion and contraction of the ITCZ, specifically its width (e.g., Collins et al. 2010; Konecky et al. 2013; Yan et al. 2015; Denniston et al. 2016; Scroxton et al. 2017; Yang et al. 2023). Yang et al. (2023) discovered that both the southern and northern margins of the ITCZ experienced simultaneously arid tropical Indian Ocean hydrological conditions during the early stage of the deglacial Heinrich Stadial 1 period, indicating a contracted tropical precipitation belt during that time. The precession bandpass filtered δ¹⁸O_sw-ivc records of core MD06-3047B (located near the northern margin of the ITCZ), and Site U1483 and core MD05-2925 (located in the southern margin of the ITCZ) show reduced precipitation during most lower precession periods (Fig. 6b–d; Jia et al. Jia et al., 2018; Zhang et al. 2022; Lo et al. 2022), possibly indicating the contraction of the ITCZ in the IPWP region and vice versa. While the precipitation records in the northern and southern marginal ITCZ are not consistently in-phase during certain periods (e.g., 390–360 ka BP), this discrepancy may be attributed to regional influencing factors or stratigraphical issues. However, collectively, these records consistently demonstrate high coherence with precession variability, suggesting that the ITCZ contracts during lower precession periods and expands during higher precession periods. We hypothesize that the synchronization of precipitation in both northern and southern regions of the IPWP on the precession band may also be attributed to the expansion and contraction of the ITCZ. Coincident with the ascending branch of the Hadley circulation, the ITCZ is typically delineated as the convergence zone of the trade winds from the Northern and Southern Hemispheres, thus strongly impacted by regional tropical climate dynamics (Byrne & Schneider, 2016; Geen et al. 2020). Studies suggest that on (sub)-millennial timescales, the ITCZ width may be linked to variations in the Walker circulation (Konecky et al. 2013) and meridional atmospheric circulation (Denniston et al. 2016). Consequently, we propose that synchronous precipitation changes between the hemispheres on orbital timescales might be intricately tied to changes in regional atmospheric circulation, which could affect the expansion and contraction of the ITCZ.

Figure 6. Effects of the ITCZ migration and width on the precipitation variations in the IPWP on the precession band. A. The records of ITCZ migration and width variation: (a) The variation in precession (Laskar et al. 2004). (b, c, d) Precession bandpass filtering curves of δ¹⁸O_sw-ivc records in northern marginal ITCZ (core MD06-3047B; Jia et al. Jia et al., 2018) and southern marginal ITCZ (core MD05-2925 and Site U1483; Lo et al. 2022; Zhang et al. 2022). (e) Gradients of δ¹⁸O_sw−ivc between Site U1486 and core MD06-3047B (Jia et al. Jia et al., 2018). (f) Gradients of δ¹⁸O_sw−ivc between Site U1486 and core MD05-2925 (Lo et al. 2022). (g) The summer interhemispheric tropical insolation gradient, determined by the variation in June insolation between 23° N and 23° S (Laskar et al. 2004). Vertical grey bars indicate lower precession values. B. Schematic diagrams showing ITCZ variability during low-precession (left) and high-precession (right) intervals. The pink shaded band indicates the position and size of the ITCZ, referring to the belt of the modern ITCZ’s seasonal migration.

Yuan et al. (2023) suggested that the larger (smaller) discrepancy in rainfall between the central and marginal regions of the ITCZ corresponds to a more contracted (expanded) ITCZ. In our study, we calculated the differences in δ¹⁸O_sw-ivc (Δδ¹⁸O_sw-ivc) among Site U1486 (central ITCZ), core MD06-3047B (near the northern marginal ITCZ) and core MD05-2925 (southern marginal ITCZ) to explore the variations in ITCZ contraction/expansion on orbital timescales (Fig. 6e, f). It should be noted that although δ¹⁸O_sw-ivc may be influenced by various factors, we hypothesize that it primarily reflects changes in the precipitation-evaporation balance at the core sites in the IPWP, as discussed in Section 4.a. However, we acknowledge the presence of a leading or lagging relationship between Δδ¹⁸O_sw-ivc and the expansion and contraction of the ITCZ on precession bands over the past 450 kyr (Fig. 6a, e, f), which does not exclude the possibility of δ¹⁸O_sw-ivc being influenced by other factors or differences in core age models. We use this approach of Δδ¹⁸O_sw-ivc to solely qualitatively describe the changes in ITCZ contraction and expansion during low-precession and high-precession intervals. Overall, the Δδ¹⁸O_sw-ivc records between central and marginal ITCZ regions both increase during lower precession periods, while smaller precipitation differences (reduced Δδ¹⁸O_sw-ivc) indicate an expanded ITCZ during higher precession periods (Fig. 6e, f).

Yan et al. (2015) proposed that the ITCZ’s expansion and contraction correspond with symmetrical changes in insolation between hemispheres on sub-millennial timescales. Furthermore, recent modeling work by Singarayer et al. (2017) indicate that precessional changes in the ITCZ width may result from different responses of its northern and southern extremes to variations in the interhemispheric temperature gradient. Thus, we suggest that variations in interhemispheric insolation due to precession may influence the expansion and contraction of the ITCZ. These hemispheric insolation variations could affect the interactions between radiation and clouds and thus ITCZ width, as suggested by the climate models (Voigt & Shaw, 2015; Lau & Kim, 2015). The contracted and expanded positions of the ITCZ, as depicted in Figure 6B, were determined based on various precipitation records across different IPWP regions, providing a qualitative overview of the ITCZ positions. Some studies suggests that the ITCZ might be influenced by extratropical forcing, such as the North Hemisphere ice sheet and the Atlantic meridional overturning circulation (Kang et al. 2008). If so, variations in the low/high precession parameter under different climate conditions could lead to differing latitudinal shifts and sizes of the ITCZ. However, in our study we hypothesize that position and size of the ITCZ remain constant under all low (high) precession periods. Here, the position and size refer to the belt of the modern ITCZ’s seasonal migration (Fig. 1a). In summary, we suggest that the precessional hydroclimatic synchronicity changes in the IPWP might be driven by both the latitudinal migration of the ITCZ and the variations in its width over the past 450 kyr. The ITCZ migrated northward (southward) and contracted (expanded) during the periods of low (high) precession, as indicated by enriched (depleted) precipitation in the EASM region and in-phase depleted (enriched) precipitation in the entire IPWP region (Fig. 6B).

5. Conclusions

We utilized high-resolution δ¹⁸O_sw-ivc records approximating SSS from Site U1486, retrieved from the Sepik River mouth in Papua New Guinea, to reconstruct precipitation changes within the IPWP over the past 450 kyr. The δ¹⁸O_sw-ivc records display a dominant 23 kyr periodicity without a distinct glacial-interglacial trend. Our analysis focused on examining the spatial pattern of precipitation variations across the IPWP over orbital timescales, revealing a synchronous precipitation pattern on precession band. We assert that changes in ITCZ played a dominant role in determining the reconstructed precipitation patterns in the IPWP region on precession band. The anti-phased precipitation changes between the IPWP and the EASM region unveiled variations in the ITCZ migration, which was determined by the seasonal distribution of incoming insolation in both hemispheres on precession band. The synchronous precipitation changes between the two hemispheres within the IPWP emphasized the significant impact of the ITCZ’s expansion and contraction on precipitation variability within the IPWP over precession timescales. Our findings suggest that during low precession periods, the ITCZ migrated northward and contracted, leading to increased precipitation in the EASM region and synchronous reduction of precipitation within the various locations across the IPWP.