Equatorward upper-ocean heat transport from the Southern Ocean boosted interglacial warming

Domain: neuroscience
Published: 2026-04-11
Editorial outcome: minor_revision

Abstract
The Mid-Brunhes Transition (MBT) signifies a pivotal shift in glacial–interglacial cycles with intensified interglacial warming, yet its underlying mechanisms remain elusive. Targeting the most sensitive component of oceanic heat uptake, the southern-sourced sub-surface waters, the post-MBT ocean heat dynamics are investigated using reconstructed deep and bottom thermocline temperatures from the Western Equatorial Pacific. Our results show that, since the MBT, the deep thermocline temperature has been gradually declining in association with cooling within the (sub)tropical Pacific upper ocean circulation. In contrast, the bottom thermocline temperature shows a pronounced rising trend independent of the calibration-derived uncertainties in its absolute estimates. Such post-MBT bottom thermocline warming is consistent with an enhanced advection of southern-sourced subsurface waters, as indicated by strengthened Southern Ocean meridional thermal gradients and concurrent warming in the southern subtropics. It is thus suggested that enhanced equatorward transport of southern-sourced subsurface waters, as a cross-hemispheric heat-redistribution pathway, may have elevated the background thermal state of post-MBT interglacials, highlighting the potential role of tropical-Southern Ocean connection in shaping climate trajectories.

Introduction
The ocean stores more than 90% of the excess heat from anthropogenic forcing, making it the dominant reservoir of the present energy imbalance1–3. The way of the heat being partitioned within the thermocline and intermediate layers of the ocean influences energy exchange between hemispheres and hence climate feedbacks in modern days4–6, while paleo records show that meridional heat transport between the poles and the equator has influenced climate sensitivity on orbital to millennial timescales7–9. At the center of ocean heat redistribution is the Southern Ocean10–12, where formation and subduction of the Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) ventilate the ocean interior with varying heat, carbon, and nutrients13–15. These water masses connect high-latitude surface forcing to low-latitude upper-ocean ventilation. Their past variability therefore provides key constraints on how ocean heat redistribution has shaped long-term climate transitions.

Modern observations highlight the importance of mode and intermediate waters in global heat uptake and reveal a strong hemispheric asymmetry3,16, with most ocean heat gain occurring in the Southern Hemisphere5. Yet instrumental records are too short to resolve their role over large-amplitude climate shifts. The Mid-Brunhes Transition (MBT), occurring around ~430 thousand-years ago (ka), marks a fundamental shift toward warmer interglacials with higher atmospheric CO2, such as during the Holocene17,18. Although the MBT is not a direct analogue for modern warming, it provides a process-based reference for examining how internal reorganization of the climate system amplified interglacial warmth within the range of natural boundary conditions in the Quaternary (Figs. 1a, d, e). Given the relatively consistent orbital configurations over the past million years20, warmer interglacials after the MBT (post-MBT; ~430 ka onward) are generally attributed to stronger internal feedbacks than those before the MBT (~800–430 ka)19 (Figs. 1d, e), especially the processes of heat distribution within the ocean-atmosphere system.

Upper-ocean heat storage in the Pacific, shaped by wind-driven subtropical cells and thermocline dynamics, regulates the distribution of heat and moisture across the ocean-atmosphere system, thereby influencing global climate variability4,21,22. The Indo-Pacific Warm Pool (IPWP) is a key region where the upper-ocean heat, coupling with intense atmospheric convection, modulates the ocean-atmosphere moisture budget on orbital time-scales8,23–25. Given such importance, the upper-ocean heat dynamics across the (sub)tropical Pacific is supposed to contribute to the post-MBT climate amplification. Previous studies have paid greater attention to deep-ocean ventilation26 and carbon cycling27, whereas the role of upper- to intermediate-ocean heat transport has been less explored. However, direct evidence linking variability in SAMW and AAIW formation to equatorward heat transport on orbital timescales remains limited.

Here we examine the changes in upper- to intermediate-ocean circulation through MBT, particularly involving SAMW and AAIW, to assess the associated effect of equatorward heat transport for post-MBT interglacial warming. To test this hypothesis, we reconstructed deep- and bottom-thermocline water temperatures (DTT and BTT, respectively) from International Ocean Discovery Program (IODP) Site U148928 (2°07.19’N, 141°01.67’E, water depth 3421 m), located at the center of the IPWP, spanning the past ~670 kyr (Figs. 1 and Supplementary Fig. 1a). The reconstructions are derived from Mg/Ca ratios of two planktonic foraminiferal species, Globorotalia tumida and Globorotalia crassaformis, which calcify at ~200 m (i.e., deep thermocline) and ~330–600 m (i.e., bottom thermocline)29–32, respectively (see Methods and Supplementary Material 1). Since available Mg/Ca-temperature calibrations for G. crassaformis were developed from Atlantic Ocean, all reported temperature variations focus on their relative variations and long-term trends that are little affected by the choice of calibrations32 (detailed in Supplementary Materials 1–3). The BTT variations are emphasized because of its direct linkage to the penetration depth and thermohaline properties of SAMW and AAIW, while the DTT record is used alongside to illustrate the thermocline dynamics across the MBT. By linking these changes in IPWP thermocline structure to southern-sourced intermediate-water circulation, we identified a mechanism of equatorward heat redistribution with implications for interglacial climate evolution.

Results
Divergent deep and bottom thermocline changes

The DTT record of Site U1489 documents significant cycles with dominant 100-kyr and 23-kyr periodicities (above 90% confidence interval, Supplementary Figs. 2a, c), showing warmer values aligned with precession minima, especially during interglacial peaks (e.g., peaks at Marine Isotope Stages (MIS) 5, 9, 13, and 15; Figs. 1d, 2a; Supplementary Material 4). These patterns match with the shallower thermocline water temperature records from the IPWP and indicate a common modulation of precession-paced inter-hemispheric summer insolation gradient on the Pacific Subtropical Cells (STCs)8,21. Our results reaffirm a similar precession-driven STCs mechanism governing the thermal condition throughout the shallow to deep waters within the IPWP thermocline.

The mean values of our DTT show a slight cooling trend by ~0.9℃±0.2 ℃ (1σ, from 18.6 ℃ to 17.7 ℃) starting from ~470 ka, around the boundary between MIS 13 and 12 (Fig. 1b). This DTT cooling exhibits correspondence with the sea surface temperature (SST) trends in the center of the IPWP33,34 (Supplementary Figs. 3a, b) and with thermocline water temperature reconstructions across the western and eastern equatorial Pacific, pointing to a basin-wide shoaling and cooling of the tropical Pacific thermocline across the Pleistocene35,36(Supplementary Figs. 3c, d). Such changes were likely mediated by variations in the Equatorial Undercurrent and upwelling dynamics37,38.

The DTT cooling is further corroborated by a weak cooling trend in the mid-latitude North Pacific39, and a southward migration of both the Northern Hemisphere westerlies and the North Equatorial Counter Current during the same interval40,41(Figs. 2c, d), thus indicating a cooling and equatorward contraction of Pacific STCs (Fig. 2e). The associated cooling trend in the subtropical North Pacific would enhance thermocline ventilation along isopycnals21,42, thereby facilitating subduction of subtropical cold anomalies into the tropical upper-waters. Both modeling and observational studies indicate that temperature anomalies originating from the subtropics are transmitted equatorward to imprint on the equatorial thermocline, through pathways in the interior and western boundary of Pacific43,44. These lines of evidence allow us to attribute the long-term deep thermocline cooling in the Western Equatorial Pacific since ~470 ka to an intensified northern-sourced cold-water supply through a contracted STCs system. Independent reconstructions of G. tumida from the eastern tropical Pacific (ODP Site 1236) are consistent with our results and confirm the observed long-term DTT cooling as basin-scale thermocline changes rather than a site-specific feature or ecological artifacts45 (Supplementary Fig. 3e).

Differing from DTT, the BTT is dominated by obliquity cyclicities (41-kyr; Figs. 1c, e and Supplementary Figs. 2b, d), indicating sensitivity of the IPWP bottom thermocline to higher-latitude processes, which are more dominated by obliquity-paced cross-latitude insolation-gradient forcing46. Notably, variations in all the proxy records of G. crassaformis display relatively small amplitudes before MIS13 but enlarged variations after MIS12 (Supplementary Figs. 4e–g). The obliquity signal in BTT also strengthens since 400 ka and may be linked to the post-MBT intensification of obliquity forcing47, indicating an increased influence of higher-latitude processes on BTT variability.

Since the MBT, BTT rose on average by ~1.8±0.1 ℃ (1σ), from its prior-MBT foothold (from ~670 to 470 ka) with smaller-amplitude variations (~1 °C) (Fig. 1c). Sensitivity tests among Mg/Ca-temperature calibrations confirm the increase in G. crassaformis Mg/Ca ratios as the primary origin of such post-MBT BTT warming, and the choice of calibrations merely determines the baseline of absolute temperature estimates (Supplementary Material 3 and Supplementary Table 1). To explain the BTT warming trend since ~470 ka, one might speculate a long-term upward movement of the calcification depth of G. crassaformis. However, the δ13C of G. crassaformis closely parallels that of G. tumida and maintains a consistent offset from shallower-dwelling species (Supplementary Figs. 5a, b and Supplementary Material 4), indicating stable biogeochemical and thermohaline gradients vertically across their habitat depths. The δ18O of G. crassaformis do show a progressive negative shift, but it agrees largely with the BTT warming (Supplementary Figs. 4e, f). Moreover, given the consistent inter-species δ18O gradients from the mixed layer to the deep thermocline across the MBT, the progressive lightening of G. crassaformis δ18O may not be derived from a shoaling of its habitat depth but warming and/or freshening of the water-mass (Supplementary Figs. 5c–e). Collectively, the coherent trends since MIS 13/12 among the δ13C, δ18O, and the Mg/Ca-derived BTT at Site U1489, argue against any progressive habitat-depth shift of G. crassaformis, instead indicating changes in the thermohaline properties of the bottom-thermocline water mass.

Southern origin of the IPWP bottom thermocline warming

The substantial warming of bottom-thermocline waters in the center of the IPWP since ~470 ka (Fig. 3a) may be explained by two orthogonal scenarios: (1) vertically increased mixing from the shallower warmer waters, and (2) advectively from an enhanced equatorward transport of warmer waters via the SAMW and/or AAIW. The first scenario contradicts other proxy reconstructions. If downward mixing had dominated, the δ18O gradient between surface- and thermocline-dwelling foraminiferal species would have diminished. However, δ18O records from G. ruber, P. obliquiloculata, and G. tumida at Site U148948 varied parallel to one another, with little systematic change in the δ18O gradient between mixed layer and deep thermocline since ~670 ka (Supplementary Figs. 5c, e, and Supplementary Material 4). In addition, downward mixing would imply thickened warm-water pile-up in IPWP and thereby enlarged thermal contrast zonally across the tropical Pacific. However, the zonal SST gradient has slightly weakened since MIS12 (Fig. 2f), which conflicts with this scenario. Hence, a downward vertical mixing scenario is not supported by the available proxy evidence.

The second mechanism, advection of southern-sourced waters, is supported by multiple independent lines of evidence. An increase of 2–4 ℃ in the SST gradients between the southern Subtropical Front (STF) and Subantarctic Front (SAF) since the MBT (Figs. 3a, c, d, Supplementary Fig. 6) is consistent with a strengthening of Southern Hemisphere westerlies (based on a conceptual estimate, see Supplementary Material 6) and supports a Southern Ocean source for the onset of the BTT warming. Stronger westerlies between the SAF and STF would deepen the mixed layer and enhance the Ekman pumping north of the SAF and thus strengthen the SAMW subduction49. Model experiments further suggest that poleward intensification of westerlies can more than double the rates of SAMW/AAIW subduction50,51. Proxy records from higher latitudes also show stronger subtropical flow of the Antarctic Circumpolar Current (ACC) across the Agulhas Plateau52, an intermediate-water cooling in the southwest Pacific associated with a poleward shift of the ACC53 (Fig. 3e), and intensified deep western boundary currents off New Zealand since ~450 ka54. Taken together, these observations indicate stronger subsurface water export from the sub-Antarctic zone and a more vigorous South Pacific overturning, supporting an intensified AAlW formation in accordance with the observed BTT warming.

After correcting for the temperature and ice-volume effects, the δ18O sw-ice of G. crassaformis still shows a trend toward more negative values (Supplementary Fig. 5d and Supplementary Material 5), likely reflecting an increase in buoyancy in its source water. Concurrently, SST in the southern Subtropical Front region has significantly increased since the MBT46 (Fig. 3c; Supplementary Fig. 7). Similar to modern warming scenarios55–57, warmer and shallower SAMW were likely produced with its subduction zone shifted poleward, due to warming-induced buoyancy increases58,59, thereby facilitating advection of the SAMW/AAIW along isopycnals into the IPWP bottom thermocline under strengthened westerlies. Modern observations also suggest that the equatorward heat transport in associated with Southern Ocean warming60, would be enhanced by intensified zonal flows61 and a deepened mixed layer north of the SAF62. Accordingly, together with an associated strengthening of the upper-layer circulation of the subtropical gyre63, the equatorward advection of the SAMW/AAIW has been further enhanced since the MBT.

It is noted that a “young, high-δ13C” signature in the bottom thermocline, as expected from well-ventilated Southern Ocean–sourced waters, is absent in our G. crassaformis record, largely due to the limitation of this δ13C tracer in this region64,65. This is mainly because, as SAMW and AAIW are transported northward, they experience mixing with subtropical and equatorial water masses64–66, and extensive biogeochemical modifications including remineralization of organic matters and transformations of inorganic carbon species67–70. These processes progressively attenuate the original δ13C signal. Consistent with this interpretation, such an expected “young, high-δ13C” signal can only be detected partially in some benthic δ13C records at mid-latitudes71, and may be further diminished equatorward into the tropical Pacific sub-surface.

In summary, the rise in BTT temperature since the MBT at Site U1489 coincided with SST warming in the Southern Hemisphere STF region and a strengthening of the Southern Hemisphere westerlies (Figs. 3a, c, d), suggesting a linkage to the source of bottom thermocline waters. Based on available evidence, we propose that enhanced subduction of a warmer SAMW, together with intensified AAIW advection and South Pacific overturning, facilitated transport of warmer waters from the subtropical Southern Ocean into the IPWP bottom thermocline.

Equatorward upper-ocean heat transport boosted warmer interglacials

The BTT and DTT records of Site U1489 suggest accelerated upper- to intermediate-ocean circulations towards the equator after the MBT. Consonantly, during the post-MBT interglacials, the southern trade winds may have been weakened, and the southern STF migrated southward, as supported by a warming in the Southern mid-latitudes and a reduced SST gradient between the southern subtropics and the IPWP72(Figs. 3b, c, and Supplementary Fig. 7). Also, an equatorward shift of northern westerly winds41 and a southward displacement of the IPWP since MIS12 (Fig. 3b) has been detected, in concert with our DTT cooling. These together indicate an inter-hemispheric asymmetry in Pacific upper-ocean circulation, which is supposed to have facilitated the heat uptake in southern mid-latitude Pacific, and strengthened the linkage between the IPWP and the Southern Ocean. It leads us to propose an enhanced heat storage in the Mode Waters during the post-MBT interglacial periods, in tandem with the elevated atmospheric pCO2 and orbital insolation heating26,73.

Since the MBT, the difference between our DTT and BTT decreased, while the δ18O sw-ice difference between G. tumida and G. crassaformis at Site U1489 increased (Figs. 4a, b, see Methods). These changes may reflect reduced thermal but enhanced salinity gradients between the two, which is consistent with an intensified AAIW subduction in the Southern Ocean since MIS13/12 (~470 ka) (Fig. 5). The relatively stable global deep-water temperatures74 (Fig. 4e), contrasted with warming in the mid- to high-latitude Southern Hemisphere53,75–79 (Figs. 3c and 4c, d), further support this interpretation. The sub-surface ocean circulation and heat export are often regarded as drivers of decadal variability in modern-climate studies4–6,80, whereas the high-latitude dynamics of AAIW and SRAMW in isolation have been largely examined by palaeoceanographic and observational studies14,81,82. By extending these perspectives from short-term variability to orbital timescales, our records suggest that Southern Ocean forcing operates within a dynamically coupled hemispheric heat-redistribution system, in which subsurface waters of the IPWP transmit high-latitude variability to the tropics. This equatorward subsurface transport increased tropical heat content and elevated the background thermal state, providing a key mechanism that could have contributed to the amplification of interglacial warmth after the MBT.

The proposed feedback that stronger Southern westerly winds intensify upper- to intermediate-ocean circulation and sustain northward ocean heat transport resonates with the observed post-industrial trends such as warming and expansion of subtropical western boundary currents55–57, increased Southern Ocean heat uptake10–12 and the strengthened pole-to-equator heat transport62. These insights provide specific targets for numerical model experiments, particularly sensitivity tests on the subduction rate and equatorward heat transport of SAMW/AAIW under varying climate scenarios. In any case, our findings underscore that Southern Ocean subsurface-water dynamics can effectively absorb the excess heat, and yield cascading effects on the tropical Pacific oceanic heat-structure and global climate state.

Methods
Samples and Chronology

The studied cores from International Ocean Discovery Program (IODP) Site U1489 (02°07.19ʹN, 141°01.67ʹE, 3421 m water depth) were drilled during Expedition 363 on the western slope of the southern Eauripik Rise, within the Caroline Basin north off Papua New Guinea28 (Supplementary Fig. 1a). A total of 332 sediment samples were collected from cores of the site at every ~5 cm interval from the upper 15.31 m core composite depth below seafloor (CCSF), covering the past ~670 kyr with a time resolution of ~2 kyr.

Site U1489 age model follows the published framework based on high-resolution Globigerinoides ruber δ18O48. A preliminary age model and baseline sedimentation rate were first defined using magnetic reversals from Hole D, owing to their consistency in depth scale. We aligned glacial–interglacial boundaries to the LR04 global benthic δ18O stack to define stratigraphic tie points and identify individual glacial cycles. These tie points were then used as age controls for constructing the age–depth model, with linear interpolation applied to all samples between 0 and 15.31 m. The resulting age model yields a relatively stable mean sedimentation rate of 2.1 ± 0.7 cm/kyr (1σ), generally higher during interglacials (1.0–4.0 cm/kyr; Supplementary Fig. 4a).

Subsurface Water Masses

In the western equatorial Pacific, shallow thermocline waters are ventilated from the subtropics through the Pacific Subtropical Cells (STCs)21,45 via the North and South Equatorial Currents83. While the Equatorial Undercurrent transports subsurface waters eastward across the basin, thereby linking hydrographic conditions between the western and eastern Pacific84. For the deeper part of the thermocline, the North Pacific Intermediate Water (NPIW) extends southward to ~8°N but is largely blocked from the equatorial band by the stronger influence of the Antarctic Intermediate Water (AAIW)85. The latter is advected through the Solomon Sea along the north coast of Papua New Guinea and extends to ~15°N86–88, where it dominates the bottom thermocline of the western equatorial Pacific. The Subantarctic Mode Water (SAMW) forms north of the Subantarctic Front (SAF) by wintertime convection and is subsequently advected into the thermocline of the southern subtropical gyres as a key component of the upper limb of the Pacific overturning circulations14,89,90. The SAMW originates mainly from the east and central South Pacific, where it mixes with surrounding waters during formation and northward transport, often together with the AAIW, before entering the subtropical thermocline91,92 (Supplementary Fig. 1c).

At Site U1489, the Equatorial Pacific Intermediate Waters are mainly influenced by AAIW sourced from the southeast Pacific, with additional contributions from the Coral and Tasman Seas and direct Southern Ocean inflow64,65. These circulation pathways make Site U1489 particularly suitable for reconstructing thermocline- and intermediate-water variations and their associated interactions between high and low latitudes. Consistent with previous studies93,94, the GLODAP δ13C distribution shows that newly subducted AAIWP and SAMW exhibit relatively high δ13C values, reflecting quick ventilations from the air-sea interface95 (Supplementary Fig. 1d). In contrast, the deep and bottom thermocline in the western Pacific warm pool have lower δ13C, indicating the attenuation of source water signals due to addition of respired carbon and mixing with older waters.

Planktonic foraminiferal proxies

The subsurface-dwelling species Globorotalia tumida, which calcifies at ~200 m in the western equatorial Pacific29 (detailed in Supplementary Material 1), is commonly used as an indicator of deep thermocline conditions36,96,97. Globorotalia crassaformis calcifies at ~330–600 m in the tropical ocean30,31, and generally above the intermediate waters across ocean basins30–32, so it has been extensively applied to reconstruct deepest thermocline/intermediate-water properties in both the Pacific and Atlantic since the Pliocene30–32,98. Accordingly, we use G. tumida as a proxy for the deep thermocline and G. crassaformis as a proxy for the bottom thermocline, defined relative to their calcification depths (Supplementary Material 1). Proxy records from these species, together with sea surface temperature anomaly (SSTA) stacks (detailed in Supplementary Material 6), therefore provide a means to examine connections between the Indo-Pacific Warm Pool (IPWP), Pacific STCs, and Southern Hemisphere mid-latitude processes mediated by the AAIW and SAMW.

Stable isotope and Mg/Ca-temperature analyses

Between 25 and 30 well-preserved specimens of G. tumida and G. crassaformis were hand-picked from the 355–425 μm and 250–425 μm size fractions for stable isotope and Mg/Ca measurement, respectively. All specimens are well preserved based on scanning electron microscopy examinations (Supplementary Fig. 8 and Supplementary Material 2).

Stable isotope analyses were conducted on a Finnigan MAT253 mass spectrometer with a Kiel IV carbonate preparation device at State Key Laboratory of Marine Geology, Tongji University, following ref. 99. Long-term measurement standard deviation is 0.07‰ for δ18O and 0.05‰ (1σ) for δ13C. Results were calibrated to the Vienna Pee Dee Belemnite (VPDB) scale using the NBS19 standard. The average sampling resolution is ~2–3 kyr for both isotope and Mg/Ca records.

For Mg/Ca analyses, we used a composite cleaning protocol based on the established procedures100 with steps of silicate removal101 and reductive cleaning102. The protocol included sequential removal of clays, oxidative and reductive cleaning, weak acid leaching, and multiple rinses with ultrapure water to eliminate detrital clays, Fe–Mn oxides, and organic matter. Mg/Ca ratios were measured on an Agilent 5110 inductively coupled plasma optical emission spectrometer (ICP-OES) at the Marine Elements Analysis Laboratory, Northwest University, China. Instrumental drift and analytical precision were monitored using ECRM 752-159103, analyzed after every 11 samples. Replicate analyses (n = 30) yielded reproducibility of 5.1% for G. tumida and 3.2% for G. crassaformis.

Cleaning effectiveness was assessed by correlations between Mg/Ca and Al/Ca, Fe/Ca, and Mn/Ca ratios (Supplementary Figs. 9a–h). In G. tumida, a weak positive correlation between Mn/Ca and Mg/Ca was detected (R2 = 0.26). In contrast, Mn/Ca and Mg/Ca of G. crassaformis shows a strong correlation (R2=0.74, Supplementary Figs. 9i–n), primarily due to elevated Mn/Ca values after ~270 ka (Supplementary Fig. 9f), with no apparent link to BTT warming. This relationship, consistent with previous reports of Mn–Mg coupling104–106, is attributed to Mn-rich surface coatings. Following ref. 107, a Mn correction (assuming Mg/Mn = 0.2 mmol/mol) was applied.

Because Site U1489 lies at a depth of ~3421 m, both dissolution and Mn-corrections were applied following refs. 107 and 108. The Mg/Ca ratio in G. tumida was then converted to deep thermocline temperature (DTT) using a species-specific calibration for the 355–425 μm size fraction from western equatorial Pacific29 (Mg/Ca = 0.33exp (0.097 × DTT)). This calibration was selected after evaluating results from alternative equations against modern hydrographic data (Supplementary Figs. 1b, 10a, 11a–d; see Supplementary Material 3 for details). For G. crassaformis, specific calibration for the western equatorial Pacific has not been established. The Atlantic-based calibration, Mg/Ca = 0.658exp (0.122 × BTT)32 with corrections for Mn-contamination and dissolution108,109, gives the most reasonable BTT estimates, in terms of the comparable Late-Holocene BTT values with respect to the modern temperatures at 330–600 m depth in the western equatorial Pacific (Supplementary Figs. 1b, S10b, 11e–h). Uncertainties were calculated following ref. 109, yielding mean errors of 1.7 ℃ for DTT and 0.6 ℃ for BTT (1σ), respectively.

Calculation of seawater δ18O

Seawater δ18O (δ18Osw) was calculated using the equation from ref. 110: δ18Osw (VSMOW, ‰) = (T(°C) -16.5+4.8×δ18Ocalcite(VPDB, ‰))/4.8+0.27. The global ice volume effect, as reported by ref. 111 was then removed to obtain δ18Osw-ice, which reflects local/regional freshwater (or salinity) budget. The δ18Osw-ice errors (see ref. 109 for details) are 0.03‰ for G. tumida and 0.10‰ for G. crassaformis (1σ).

Regional SST stacks and gradients

We compiled subsurface/thermocline records from the IODP Site U1489 and SST reconstructions from Western and Eastern Equatorial Pacific, and Southern Ocean frontal zones (STF and SAF). All datasets, proxies (Mg/Ca or alkenone), coordinates, water depths, and references are listed in Supplementary Table 2. For regional stacks, individual series were linearly interpolated to 4-kyr, adjusted by subtracting the maximum value over the entire time window, smoothed with a 20-kyr running mean, and then averaged. When combining Mg/Ca and alkenone, we used anomalies to minimize method-specific offsets (detailed discussion see Supplementary Material 6). Zonal equatorial Pacific SST gradient is calculated by the difference of SST anomaly between western and eastern equatorial Pacific (ΔSSTAW−E = SSSTAMD97-2140 − SSTODP846), and meridional frontal gradient by those between STF and SAF (ΔSSTASTF−SAF = SST stack (STF) − SST stack (SAF); detailed in Supplementary Material 7).

Time series analyses

To evaluate the orbital pacing and long-term trends, all SSTA records shown in the main text were first linearly interpolated at 4-kyr intervals, and then smoothed with a 20-kyr running average after. Records from IODP Site U1489 were processed using a three-point running average and LOESS smoothing with a Gaussian fit (span = 0.5), and then further smoothed with LOESS (smoothing factor = 0.3) to acquire their long-term trends. Spectral analysis was carried out using the REDFIT procedure112 as implemented in PAST v4.03113. Rectangular windows were applied with an oversampling factor of 6, and the data were divided into two equal-length segments to estimate the red-noise spectra. Wavelet analysis of the temperature reconstructions was performed using Acycle v2.8, following standard procedures114. RAMPFIT regressions were applied to identify changing points in long-term trends115.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Based on high-resolution paleoceanographic reconstructions of deep and bottom thermocline temperatures from Indo-Pacific Warm Pool (IPWP) records, Yang et al. investigate the thermal evolution of the upper-to-intermediate ocean equatorial Pacific over the last 700 ka. The central discussion is based on the differences in behavior between two thermocline depths after ~470 ka, where small deep thermocline cooling is concomitant with substantial warming of bottom thermocline temperatures. Authors argue that this warming originates from the Southern Ocean, driven by Subantarctic Mode Water (SAMW) with intensification of Antarctic Intermediate Water formation. Overall, the paper is really good. Although disagreements among compared records could be better accounted for to avoid simplification of the mechanism, the discussion is well-written and presents a plausible mechanism, is conclusive, providing novel insights into thermal evolution and ocean-atmospheric interactions in the southern Pacific across the Mid-Brunhes Transition (MBT). The new dataset is valuable, and, together with the interpretations, it makes a significant contribution to the scientific community. However, there are a few major issues related to the figures and the method description that need attention. Additionally, there are a few major points that need to be improved to adhere to a global framework and to clarify data comparisons with existing published records. As such, I can`t recommend the manuscript for publication in its present form. As detailed below, the authors should provide more clear descriptions of their results when combined with their respective interpretations. Major comments: First, the manuscript would benefit from better-constructed figures and reorganization of its panels. Although all the used records are there and they look very nice, most of the figure citations in the main text refer to figures in the Supplementary Material, which makes it difficult to follow most of the time. For example, Fig. 1 is almost not used and could be replaced. Figure captions require improvement, as they are often too simplistic and fail to describe their purpose clearly. Second, the introduction needs improvement to contextualize the theme in a broader sense. I am also missing previous paleoceanographic reconstructions and background information related to the dynamics of upper-ocean heat content in the region. Initially, I was confused about whether the paper discussion focused on the causes or effects of the MBT, and I think it would be better clarified from the outset, with a closer tie to the latest topic. A lot of the produced records are in the Supplementary Material and haven't been used in their totality and could be further explored. Third, the methods section is a critical point and needs several improvements. Although the age model for IODP Site 1489 had already been published, I missed at least a general overview of how it was built, including the radiocarbon data used, the calibration curve employed, the alignment method, the interpolation method, and the mean sedimentation rate produced. Although the table listing the used sediment cores is presented in the Supplementary Material, it is not described in the methods, which records were used to calculate the temperature gradients. RAMPFIT and spectral analysis were not even mentioned in the methods and need to be described. Took me a lot of time to understand how trends were estimated. The uncertainties are not mentioned in the manuscript text and or presented in figures, which are also provided in the RAMPFIT results and need to be shown. Finally, the quality of the foraminifera Mg/Ca measurements should be accounted for in the Supplementary Material and other Element/Ca ratios presented, such as Al/Ca, Mn/Ca, and Fe/Ca, and by preferentially evaluating the absolute values and correlation against Mg/Ca to evaluate the cleanning efficiency. A brief explanation of the cleaning procedure performed would be helpful, as a reductive step was included, which may impact the results. Error envelopes could be provided on all temperature records, once they were calculated following Mohtadi et al. (2014). Other comments: Lines 88-89: I see the reference, but you didn`t describe how you calculated the SST gradients from Fig. 2, for example. Lines 88-91: I agree that there is an expressive shift from MIS 12/11 towards the present on Fig. 2b. However, it is not necessarily following the long-term trend seen on DTT. Does it mean that other factors might be controlling BTT? Lines 117-127: Two possibilities were raised to explain BTT warming, one of which is related to downward water mass mixing from surface layers toward the bottom thermocline, which was ruled out by the argument that would result in an intensified east-west Pacific temperature gradient. Supplementary Fig. 2e shows that the increased gradient remains high even when the MBT has already occurred. The timing disagreement weakens your argument. If downward mixing is discarded, why did it not follow the same long-term trend and timing as seen on BTT? Would it be a period when both mechanisms (downward mixing and equatorward transport) are operating? These need to be accounted for. I recommend calculating the d18O gradient between P. obliquiloculata and G. crassaformis as a proxy for stratification, since oxygen and carbon stable isotopes measurements were already performed on both species. Lines 132-137: Although the authors acknowledge that an increased subduction of SAMW should leave a positive d13C signature, which is not observed, this contradiction is then set aside. Can the authors offer a more detailed explanation? Could changes in preformed nutrient concentrations or air-sea gas exchange mask the ventilation signal? Is it possible to evaluate the sea-surface evaporation, sea-water density, and the mixed layer depth possibilities by considering the surface d18Osw, the d18O ice-free corrected, and the d18O gradient between surface and subsurface layers. Lines 148-150: I agree that there is a slightly cooling response in the DSDP Site 593 temperature record (Fig. 2e); however, I`m not sure if it is responding to the MBT, since temperatures from MIS 10 are quite similar to those from MIS 14, 13, and 12. Considering this, the cooling trend would have started around 350 ka and continued to the present. I recommend running RAMPFIT analysis on this record too. In addition, the long-term trend following the tour BBT warming is not clearly seen. Is it related to my comment from lines 117-127? These points need to be accounted for, as AAIW cooling was raised as the main mechanism to explain BTT warming. Lines 162-166: The overall conclusion on the source and mechanisms behind BTT warming needs to be altered accordingly after revision. Minor comments: Lines 20-22: Improve MBT description. Lines 25-29: Separate this into at least two sentences. Lines 30-33: Considering the main discussion point of the manuscript regarding the long-term increasing trends observed in the thermocline temperature records, I recommend improving or clarifying the direct link between your data observations and their role in particurlaly increasing interglacial temperatures. It is challenging to establish a clear connection since no distinct G-IG pattern can be observed at certain stages (e.g., MIS 10-9). Line 44: Add “past” before “…long-term global heat storage changes”. Lines 48-49: MBE was not defined. The MBE can be defined as the event marking the transition between two distinct climatic or oceanographic states. The issue is that the event is not present in all records worldwide, or is marked by concomitant timing among those where it is expressed. It depends on the perspective from which proxy has been used to track it. Please clarify the adoption of MBT rather than MBE. Line 51: Be precise when mentioning “before and after” the MBT by defining a specific time range once in the introduction. Line 52: Remove “are” and change “resulted” to result. Lines 56-57: “Influence global climate” is vague. Line 59: “…regulating the global climate” is vague. Line 65: “To probe” is too strong; consider changing to “To investigate” or similar. Line 77: Include “statistically” to “significant glacial–interglacial cycles with dominant 100-kyr and 23-kyr periodicities”. Or include above 90% CI. Line 86 and lines 96-97: Missing error estimations when reporting temperature values. Lines 88-92: Missing SST data citation connecting your arguments to your figures. Is it related to Fig. 2b? If so, cite it. Lines 96-98: Move temperature trends and amplitude discussion to the next paragraph when you discuss long-term BTT warming. Line 98 and 101: Please be consistent and change to “41-kyr” cycle instead “40-kyr”. Line 101: I recommend filtering these other records from each G. crassaformis to sustain your argument saying that they are mostly paced by a 40-kyr cycle, or to present wavelet or evolutionary spectral analysis to sustain it. If it was raised from the REDFIT analysis presented in the supplementary Fig. 3b, I would recommend it since it is a repetition from line 98. Line 108: Check if Supplementary Fig. 6b is correctly cited. Line 121: Remove “were”. Lines 128-129: Please reformulate this sentence to avoid redundancy. Lines 138-139: The way it is written, it looks like you will add another evidence for why strengthening of SAMW alone cannot give rise to BTTI, not as a conclusion from the arguments you already presented in this paragraph. Please clarify it by including a conclusion statement or presenting additional information arguing that SAMW transport alone cannot be accounted for; if it is not the case, rewrite this sentence accordingly. Line 141: I would recommend changing the “turning point” term to “change point” across your manuscript. Line 141-143: RAMPFIT trend estimation analysis provides you with error bars for your change points and calculated mean values. Please include the precise information on when change points occur with their uncertainty in the text when discussing it, and include the error envelopes in the figure, also in the time domain.", "reviewer_2": "Please see my attached report.", "reviewer_3": "General comments The study by Yang and co-authors presents new reconstructions of deep- and bottom-thermocline water temperatures (DTT and BTT, respectively) from IODP Site U1489, located in the centre of the Indo-Pacific Warm Pool (IPWP), spanning the past ~670 kyr. These new datasets, combined with previously published records, offer the opportunity to investigate key aspects of ocean-atmosphere interactions across the Mid-Brunhes Transition (MBT). Considering that the MBT marks a fundamental reorganisation of glacial–interglacial cycles, characterised by intensified interglacial warming, and that its underlying drivers remain debated, the manuscript addresses a topic of considerable relevance to the scientific community. The authors propose that accelerated upper- to intermediate-ocean circulation in the mid- to low-latitude Pacific may have amplified post-MBT interglacial warming, with the Southern Ocean playing a significant role in shaping these changes. While the manuscript presents valuable new valuable data and addresses an important scientific question, several aspects of the study require clarification and refinement to enhance its scientific rigour and overall readability. In particular, the structure of the introduction would benefit from a more logical progression, beginning with the broader context, followed by knowledge gaps, the working hypothesis, and site justification. Furthermore, some conceptual oversimplifications should be avoided, for example, the direct comparison between the MBT and present-day anthropogenic warming, which is more complex than implied. Methodological details also require improvement, including further details about the species selection, the presentation of the age model, describing the time series analyses, and the description of oceanographic settings. These elements are essential to support interpretations related to regional and large-scale circulation changes. Greater attention to supplementary material, including clearer figure captions, consistent terminology, and acknowledgement of potential methodological limitations (e.g., combining different SST proxies), will strengthen the overall contribution of the study. Finally, while the research addresses a scientifically significant topic and provides valuable new data and insights, the manuscript would benefit from a more in-depth discussion with additional detail regarding the proposed mechanism. Addressing the points outlined below will improve the clarity, methodological robustness, and interpretive strength of the study. Moreover, a more comprehensive comparison with modern oceanographic conditions, as well as the inclusion of paleoclimate modelling to further support the proposed mechanism, would greatly improve the manuscript. Specific comments - Introduction: the introductory text would benefit from a more logical flow and structure. A clearer step-by-step development from global context → knowledge gaps → hypothesis → site justification would enhance readability. Also, the claim that the MBT provides a paleo-analogue for current anthropogenic warming is oversimplified. - Line 71: the calcification depths of the chosen foraminiferal species should be consistent with what is presented in the methods section (lines 218 and 221). - Lines 72–74: at the beginning of this paragraph, you mention that deep- and bottom-thermocline water temperatures were reconstructed from IODP Site U1489. However, in lines 72–74, you state that the focus is on BTT, which sounds somewhat inconsistent. - Lines 80-83: consider showing shallow thermocline records together with your DTT. - Lines 88-90: same as previous comment. Additionally, the authors argue that IPWP DTT and SST show a cooling trend starting around ~470 kyr. The cited publication (i.e. de Garidel-Thoron et al., 2005) actually suggests a long-term stability of western Pacific warm pool SST during the Pleistocene. Check this inconsistency. - Lines 91-92: Regarding the text “The DTT cooling is also corroborated by a similar weak cooling in the mid-latitude North Pacific25 and a strengthening trend of Northern westerly winds26 (Supplementary Fig. 2b, c),”, the mentioned panels of Supplementary Figure 2 indicates a southward shift of the westerlies and not a strengthening. Please, clarify the text to keep consistency. Lines 93-95: provide a reference for this statement. Additionally, this statement could be further detailed, linking the observed cooling to the strength and southward migration of the northern westerlies. Lines 96-103: as far as I can see in Figure 1e, between around 550 and 400 kyr, there is a decoupling between BTT-Band pass filtering and obliquity signal. It should be acknowledged and discussed. - Lines 105–110: The authors state that the G. crassaformis δ¹³C record remains parallel to those of shallower-dwelling species (i.e., G. tumida, G. ruber, and P. obliquiloculata), indicating a persistent ecological gradient throughout the studied interval. While it is evident that δ¹³C records of G. crassaformis and G. tumida follow a parallel trend (with G. crassaformis consistently showing lower δ¹³C values), the δ¹³C records of G. ruber and P. obliquiloculata appear to exhibit similar absolute values to G. crassaformis (around 1‰). How do the authors explain this apparent similarity in δ¹³C values despite the differences in habitat depth among these species? - Lines 120–127: scenario 1 could be more clearly explained in terms of a deepening of the thermocline in the western equatorial Pacific, i.e. intensified trade winds → enhanced east–west transport of warm surface waters → pilling up of warm waters in the western equatorial Pacific -> deepening of the thermocline → DTT warming. - Lines 135-140: to clarify why a strengthening of the SAMW transport cannot account to the observed SAMW warming as well as that a strengthening of the SAMW transport would imprint a positive signature on δ¹³C of bottom thermocline waters in the IPWP, the methods subsection related to modern conditions should include information on the present-day oceanographic settings at both core location and SAMW formation region. - Lines 135-138: An increased export of SAMW wouldn’t be related to a decrease in δ¹³C? - Lines 170-171: Regarding the text “Progressive mid-latitude warming in the Southern Hemisphere”, this warming is proposed considering SST records, right? Please clarify it in the text. - Lines 191-192: Regarding the text “While previous studies focused on high-latitude AAIW dynamics and their role in global heat transport, our study….”, provide reference(s) to this statement. - Methods: much of the interpretation relies on time series analyses (e.g., interpolation, resampling, filtering, running averages, detrending, LOESS regression, spectral analysis, ramp function regression, and so on). It would be interesting to add a “time series” subsection describing each of these techniques within the methodology. If there is no space for it in the main text, it could be inserted in the supplementary information. - Line 207 (Methods subsection “Study site location and sampling”): provide information on the IODP expedition which collected the studied marine core. Additionally, a broad description of the oceanographic setting (specifically the water masses and circulation patterns influencing the study site) is currently missing. This information is crucial to support the discussion, which addresses the influence, for example, of the North Pacific and the Southern Ocean on the study area. Furthermore, the age model for core U1489 requires a more thorough explanation. In particular, the choice of G. ruber over benthic foraminifera should be explained. Providing a supplementary figure illustrating the age model for the target interval is strongly recommended. - Line 216 (subsection “Planktonic foraminiferal ecology and its role in indicating water mass properties”): a more comprehensive description of the planktonic foraminiferal species used in the study is necessary. For example, based on sediment trap studies, is the shell flux to the seafloor seasonal or year-round? Consequently, are the geochemical records derived from these species seasonally biased, or do they reflect annual mean conditions? - Lines 223-227: the sentence “In general, the subsurface-dwelling Pulleniatina obliquiloculata, which calcifies at depths of less than 150 m, is considered an indicator of upper thermocline water properties21,53” seems out of context in this part of the text. - Line 233 (Methods subsection titled “Isotope analysis and Mg/Ca-temperature estimate”): Please (i) provide references related to the species used for the analyses; (ii) include the sampling resolution for both the δ¹⁸O/δ¹³C and Mg/Ca measurements; (iii) specify the carbonate preparation device coupled to the MAT253 mass spectrometer; (iv) be more specific to which Mg/Ca cleaning procedure was implemented and describe any modification if there was one (three slightly different procedures were cited); (v) include information on how potential contamination (e.g. silicates Mn-bearing phases and clay contamination) was assessed in the trace element analyses. - Line 258 (Methods subsection “Calculation of seawater δ¹⁸O”): given the length of the record spanning multiple G-I cycles, the reason for not applying ice volume correction to the δ¹⁸Osw data should be explained. - Line 455 (Figure 2): the caption text related to panel d should specify which cores were used to calculate the anomaly between STF and SAF. Double-check it for all presented figures. - Line 477 (Figure 4): in the section “Divergent deep and bottom thermocline changes”, the authors present and discuss a cooling trend of SST and DTT in the IPWP, starting from at ca. 470 kyr. This cooling trend does not seem to be represented in this figure. - Line 502 (Supplementary Text 1): the residual records should be explained in greater detail, either in the Methods section and/or in Supplementary Information 1. - Lines 512-513: by looking at supplementary Figure 3, it seems that δ¹⁸O-residual from G. crassaformis also exhibits a statistically significant precession signal. Additionally, please double-check Figure 3 regarding the description of the lines indicating the 99% and 95% confidence levels. In the figure itself, these lines are labelled as 90% and 95%, while the caption refers to them as 99% and 95%. - Line 543 (Supplementary text titled “Verification of sea surface temperature stacks”): this section focuses on describing the SSTA stacks. From Supplementary Table 1, it is possible to identify that the records used to construct the SSTA stacks were obtained using two different methods, i.e. Mg/Ca and UK’37. This difference should be acknowledged in the supplementary text, along with a discussion of the potential implications of combining records derived from distinct methods, especially knowing that alkenones and Mg/Ca might show different patterns (e.g. Leduc et al., 2010, 10.1016/j.quascirev.2010.01.004) . - Line 555 (Supplementary figure 1): please clarify in the caption the elements shown in panel b (e.g., white arrow, triangle, foraminifera illustration, selected latitudinal section). - Lines 577: It would be interesting to add in the spectral analysis supplementary figure 3 an evolutionary method (e.g., wavelet analysis) to see whether the identified orbital parameters are constant throughout the record or if there is any change in the dominant cycle that might be related to the findings. Technical corrections - Line 22: “Targeting the most…”. The word “at” is unnecessary here. - Line 48: the acronym MBE was used instead of the already defined MBT. - Line 50: “...for instance, the Holocene”. - Line 50: “…are marked by…”. Use plural once it refers to the interglacials. - Line 51: double-check the cited panels from Figure 1. Maybe, only Fig. 1a would be appropriate to cite here. - Line 52: “...essentially the result of…” instead “...essentially resulted from…” - Line 54: it would be convenient to cite here Fig. 1d, e. - Lines 58 and 559: Homogenise the way you refer to the IPWA, i.e. “Indo-Pacific Warm Pool” or “Indo-Pacific warm pool”. - Line 64: “carbon cycling” instead of “carbon-cycling”. - Line 69: please include a reference to the methods subsection where the study site is described. - Lines 79-80: it would be appropriate to cite also panel d of Figure 1. - Line 103: also cite Figure 1c in this sentence. - Line 107: The word “with” is unnecessary. - Line 113: “...shifting in δ18O of…”. - Line 121: “...were the result of…” instead “...were resulted from…”. - Lines 156-157: “...and warm SAMW would be transported…”. To keep consistent verb tense. - Line 163: “evidences” instead of “evidence”. - Line 179: “contributed” instead of “contribute”. - Line 212: “interval” instead of “intervals”. - Line 230: the acronym SSTA should be presented in full the first time it appears in the text. - Line 256: Delete the word “calculation”. - Line 453: “3-point running average” instead of “3 points running average”. - Line 473: Same as line 455. - Line 511: correct to “~41- and 23-kyr bands”. - Line 536: “...based on of G. tumida…”. Delete the word “of”. - Line 562: “The filed circles…”. Correct to “filled”. - Line 567 (Supplementary Figure 2): I would suggest not invert the y-axis of panel e to facilitate the rationale. - Line 569: Same as line 455. - Line 604: In the sentence “The thin, thin medium, and thick curves…”. You can use just “thin, medium, and thick” instead. - Line 605: Same as line 455. - Line 642: In the supplementary table 1, adjust the first column name to “Region”.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "1 Point-by-point responses to the reviewers’ comments on the manuscript entitled 2 “Equatorward upper-ocean heat transport from the Southern Ocean boosted 3 interglacial warming” (NCOMMS-25-35512) 4 Reviewer #1 (Remarks to the Author): 5 Based on high-resolution paleoceanographic reconstructions of deep and bottom 6 thermocline temperatures from Indo-Pacific Warm Pool (IPWP) records, Yang et al. 7 investigate the thermal evolution of the upper-to-intermediate ocean equatorial Pacific 8 over the last 700 ka. The central discussion is based on the differences in behavior 9 between two thermocline depths after ~470 ka, where small deep thermocline cooling 10 is concomitant with substantial warming of bottom thermocline temperatures. Authors 11 argue that this warming originates from the Southern Ocean, driven by Subantarctic 12 Mode Water (SAMW) with intensification of Antarctic Intermediate Water formation. 13 Overall, the paper is really good. Although disagreements among compared records 14 could be better accounted for to avoid simplification of the mechanism, the discussion 15 is well-written and presents a plausible mechanism, is conclusive, providing novel 16 insights into thermal evolution and ocean-atmospheric interactions in the southern 17 Pacific across the Mid-Brunhes Transition (MBT). The new dataset is valuable, and, 18 together with the interpretations, it makes a significant contribution to the scientific 19 community. However, there are a few major issues related to the figures and the 20 method description that need attention. Additionally, there are a few major points 21 that need to be improved to adhere to a global framework and to clarify data 22 comparisons with existing published records. 23 As such, I can’t recommend the manuscript for publication in its present form. As 24 detailed below, the authors should provide more clear descriptions of their results when 25 combined with their respective interpretations. 26 27 Response: We thank you very much for your insightful comments, which help us 28 substantially improve the quality of our manuscript. We improved figure clarity, 29 expanded methodological details, and placed our results in a broader context. Please 30 see below our revisions according to your specific comments. 31 32 Major comments: 33 First, the manuscript would benefit from better-constructed figures and reorganization 34 of its panels. Although all the used records are there and they look very nice, most of 35 the figure citations in the main text refer to figures in the Supplementary Material, 36 which makes it difficult to follow most of the time. For example, Fig. 1 is almost not 37 used and could be replaced. Figure captions require improvement, as they are often 38 too simplistic and fail to describe their purpose clearly 39 40 Response: We appreciate your suggestions regarding the figure construction and 41 organization. In the revised manuscript, we restructured the figures of the main body to 42 improve the clarity and alignment with the narrative. Specifically: 43 1. The manuscript focuses on the mechanisms underlying DTT and BTT changes, the 44 figures of the main body are thus organized to show their results and comparisons, while 45 supporting datasets are placed in Supplementary Information due to figure number 46 limitation of Nature Communications; 47 2. We moved figures from the previous Supplementary Material that are very much 48 relevant to interpretation to the main body. For example, the original Supplementary 49 Fig. 2 was incorporated into the revised Fig. 2 to improve readability; 50 3. We agree that Fig. 1 was underused in the main text of the previous version. However, 51 this figure presents the primary proxy records from Site U1489 and the relevant climatic 52 background, which are fundamental for contents in both of the introduction and the 53 results. We therefore keep Fig. 1 but refined its layout to clearly guide readers through 54 the relevant climatic context and as well more effectively cited it in the revised 55 manuscript; 56 4. All figure captions were improved to provide clear descriptions that emphasize the 57 scientific context. 58 We hope the above changes effectively improve figure readability and help readers 59 follow the main arguments without frequently referencing the Supplementary 60 Information. 61 62 Second, the introduction needs improvement to contextualize the theme in a broader 63 sense. I am also missing previous paleoceanographic reconstructions and background 64 information related to the dynamics of upper-ocean heat content in the region. 65 Initially, I was confused about whether the paper discussion focused on the causes or 66 effects of the MBT, and I think it would be better clarified from the outset, with a closer 67 tie to the latest topic. A lot of the produced records are in the Supplementary Material 68 and haven't been used in their totality and could be further explored. 69 70 Response: Thanks a lot for your helpful suggestions. We improved the Introduction by 71 the following changes: 1) expanding the background contents to include previous 72 reconstructions of upper-ocean heat content and thermocline evolution in both of the 73 tropical Pacific and Southern Ocean; 2) clarifying from the outset that our study 74 addresses the effects of the MBT, particularly the mechanisms driving amplified 75 interglacial warming after ~430 ka, rather than its causes; and 3) reframing the 76 Introduction to state our main hypothesis, that is, enhanced equatorward transport of 77 thermocline waters from the Southern Ocean played a key role in the post-MBT 78 interglacial warming. These changes place our study in a broader context and sharpen 79 its objectives. Please see details on Lines 35-85 of the revised manuscript. 80 81 Third, the methods section is a critical point and needs several improvements. Although 82 the age model for IODP Site 1489 had already been published, I missed at least a 83 general overview of how it was built, including the radiocarbon data used, the 84 calibration curve employed, the alignment method, the interpolation method, and the 85 mean sedimentation rate produced. Although the table listing the used sediment cores 86 is presented in the Supplementary Material, it is not described in the methods, which 87 records were used to calculate the temperature gradients. RAMPFIT and spectral 88 analysis were not even mentioned in the methods and need to be described. Took me a 89 lot of time to understand how trends were estimated. The uncertainties are not 90 mentioned in the manuscript text and or presented in figures, which are also provided 91 in the RAMPFIT results and need to be shown. 92 93 Response: We appreciate you for pointing out these issues and apologize for having 94 not presented the details in the previous Methods section. In the revised manuscript, we 95 broadened the Methods section to address your concerns specifically as the followings: 96 1. We provide in the revised manuscript an overview of the age model of Site U1489, 97 including tie-points, interpolation method and sedimentation rates. We added a new 98 figure, Supplementary Fig. 8, to illustrate the chronology and stratigraphic alignment 99 (Line 1053). 100 2. We added a subsection, Time series analysis, in the Methods section to describe the 101 procedures for interpolation, LOESS smoothing, RAMPFIT regression and REDFIT 102 spectral analysis, with parameters reported explicitly (Lines 399–413). 103 3. We now report RAMPFIT-derived uncertainties in figure captions (Lines 782 and 104 1042). 105 106 Finally, the quality of the foraminifera Mg/Ca measurements should be accounted for 107 in the Supplementary Material and other Element/Ca ratios presented, such as Al/Ca, 108 Mn/Ca, and Fe/Ca, and by preferentially evaluating the absolute values and correlation 109 against Mg/Ca to evaluate the cleaning efficiency. A brief explanation of the cleaning 110 procedure performed would be helpful, as a reductive step was included, which may 111 impact the results. Error envelopes could be provided on all temperature records, once 112 they were calculated following Mohtadi et al. (2014). 113 114 Response: Thank you for the suggestion. We made the following adjustments in the 115 revised manuscript: 1) added plots of element/Ca ratios (Mg/Ca, Al/Ca, Mn/Ca, Fe/Ca) 116 for G. crassaformis (Supplementary Fig. 10); 2) plotted correlation of Mg/Ca against 117 Al/Ca, Mn/Ca, and Fe/Ca to evaluate the cleaning efficacy (Supplementary Fig. 11); 3) 118 added in the Methods section the description of the cleaning protocol, noting the 119 reductive step and its potential influence on trace metals; 4) discussed in the Methods 120 section impact of Mn/Ca contamination and carbonate dissolution on Mg/Ca-derived 121 temperature estimates; and 5) applied error envelopes to all Mg/Ca temperature 122 reconstructions. Kindly refer to Lines 97–98, 130, 352–375 in the revised manuscript. 123 124 Other comments: 125 Lines 88-89: I see the reference, but you didn’t describe how you calculated the SST 126 gradients from Fig. 2, for example. 127 128 Response: Thank you for pointing out this issue. In the revised manuscript, we 129 explicitly described the procedure to calculate SST gradients shown in the previous Fig. 130 2 (now Fig. 3). The gradients are derived from differences of the latitudinal SST 131 anomaly stacks (based on Mg/Ca and alkenone records) between Subtropical Front 132 (STF) and Subantarctic Front (SAF) sites. The SST stacking procedure is detailed in 133 the revised Methods section (Lines 384–397) and Supplementary Text 6 (Lines 946– 134 967) and site locations are listed in Supplementary Table 1 (Lines 1121–1123). 135 136 Lines 88-91: I agree that there is an expressive shift from MIS 12/11 towards the 137 present on Fig. 2b. However, it is not necessarily following the long-term trend seen on 138 DTT. Does it mean that other factors might be controlling DTT? 139 140 Response: Thanks for the suggestion. We agree with your point. Although a 141 pronounced shift from MIS 12/11 toward the present is evident in Supplementary Fig. 142 2b, this does not mean a direct control of the westerlies on DTT. Rather, we consider 143 the westerlies as a facilitating boundary condition. A detailed description of the 144 underlying mechanism is provided on Lines 107–119 and shown in Fig. 2 of the revised 145 manuscript. 146 147 Lines 117-127: Two possibilities were raised to explain BTT warming, one of which is 148 related to downward water mass mixing from surface layers toward the bottom 149 thermocline, which was ruled out by the argument that would result in an intensified 150 east-west Pacific temperature gradient. Supplementary Fig. 2e shows that the 151 increased gradient remains high even when the MBT has already occurred. The timing 152 disagreement weakens your argument. If downward mixing is discarded, why did it not 153 follow the same long-term trend and timing as seen on BTT? Would it be a period when 154 both mechanisms (downward mixing and equatorward transport) are operating? These 155 need to be accounted for. I recommend calculating the δ18O gradient between P. 156 obliquiloculata and G. crassaformis as a proxy for stratification, since oxygen and 157 carbon stable isotopes measurements were already performed on both species. 158 159 Response: We appreciate you for the insightful comment. We would like to improve 160 the clarity of our argument here. The previous Supplementary Fig. 2e (now Fig. 2f in 161 the revised manuscript) shows pronounced SST gradients across the equatorial Pacific 162 after the MBT, these records would however largely reflect short-term or regional 163 variability rather than the long-term trend. On average, SST anomalies since the MBT 164 are ~1 °C lower than before, consistent with a reduced east–west gradient and a weaker 165 Walker circulation (de Garidel-Thoron et al., 2005; Liu et al., 2019, Sci. Adv.), which 166 does not support enhanced downward mixing. 167 Following your suggestion, we plotted δ13C and δ18O of four planktonic foraminiferal 168 species to indicate the upper ocean stratification (Fig. R1and Supplementary Fig. 5 of 169 the revised manuscript), although we did not directly calculate the δ18O gradient 170 between 171 172 Fig. R1 Planktonic isotope records from IODP Site U1489. a δ13C of Globigerinoides ruber (gray) 173 and Pulleniatina obliquiloculata (gold) (Dang et al., 2024); b δ13C of Globorotalia tumida (blue) 174 and Globorotalia crassaformis (red); c δ18O of G. ruber (gray), P. obliquiloculata (gold), G. tumida 175 (blue) and G. crassaformis (red); d δ18O of G. tumida (blue) and G. crassaformis (red); e δ18O 176 gradient between the mixed-layer and thermocline (δ18O (green) and δ18O 177 (blue). The thin, medium, and thick lines represent the original data, 3-point running 178 average, LOESS Gaussian fit (span = 0.5), respectively. 179 P. obliquiloculata and G. crassaformis. The δ18O offsets among G. ruber, P. 180 obliquiloculata, and G. tumida remain virtually parallel over the study interval (Fig. 181 R1c), indicating a stable stratification from the surface to the deep thermocline. In 182 contrast, the pattern of G. crassaformis δ18O is largely different from that of the above 183 three species (Fig. R1c), which should not be the result of upper water stratification. 184 This explanation could be supported by the δ13C offsets among the four depth dwelling 185 species (Fig. R1a and b). The offsets among thermocline dwellers reflect the biological 186 pump and remain consistent through time, showing no long-term convergence that 187 would be expected under sustained vertical mixing. 188 The negative excursion of G. crassaformis δ18O (Fig. R1c) is however consistent with 189 warming (Fig. 1c) and freshening (Fig. R1d) of the bottom thermocline, more likely 190 pointing to water-mass advection rather than local vertical stratification. If strong 191 equatorial wind-driven mixing or convection had occurred, it should have acted firstly 192 and more strongly in the surface and upper thermocline layers than in the bottom 193 thermocline that G. crassaformis inhabits. Moreover, reconstructed δ18O indicates 194 that waters at this depth not only warmed but also freshened, yielding values more 195 negative than G. tumida. Such anomalies cannot be explained by local vertical density 196 structure and instead reflect the advection of distinct water masses. Together with the 197 stability of δ18O gradients between the mixed-layer and thermocline (Fig. R1e) 198 indicates a limited effect of vertical mixing on the long-term BTT warming. 199 Taken together, SST, δ18O, and δ13C records collectively support equatorward heat 200 transport as the primary driver of BTT warming on orbital timescales. A more detailed 201 description of this mechanism has been included in the revised section Southern origin 202 of the IPWP bottom thermocline warming. Please refer to Lines 145–215 in the revised 203 manuscript. 204 205 Lines 132-137: Although the authors acknowledge that an increased subduction of 206 SAMW should leave a positive 13C signature, which is not observed, this 207 contradiction is then set aside. Can the authors offer a more detailed explanation? 208 Could changes in preformed nutrient concentrations or air-sea gas exchange mask the 209 ventilation signal? Is it possible to evaluate the sea-surface evaporation, sea-water 210 density, and the mixed layer depth possibilities by considering the surface δ18Osw, the 211 δ18O ice-free corrected, and the δ18O gradient between surface and subsurface layers. 212 213 Response: Thank you so much for your insightful view and constructive suggestions 214 as well. We would like to address the issues you concerned point by point as below. 215 1. δ13C signal attenuation 216 We agree that intensified SAMW/AAIW formation and associated Southern Ocean 217 productivity after the MBT would increase preformed δ13C, the Site U1489 records 218 however show that this signal, if any, might be largely overprinted by transport and 219 biogeochemical processes, as indicated by BTT and δ18O of G. crassaformis. 220 An increase in the preformed δ¹³C of newly ventilated SAMW and AAIW is expected 221 to occur through surface removal of 12C, in association with the enhanced SAMW and 222 AAIW formation and Southern Ocean productivity after the MBT (Wu et al., 2017; 223 Tang et al., 2022; Weber et al., 2022; all of the references cited in the revised 224 manuscript). However, Site U1489 δ13C records of thermocline-dwelling species 225 remain virtually parallel with each other and do not show the expected positive anomaly 226 (Fig. R1). This attenuation reflects the combined effects of remineralization, mixing 227 with surrounding waters, and variability in preformed nutrients and air–sea CO 228 exchange, which can obscure the “youth-high-δ13C” signal during northward advection 229 (Carter et al., 2014; Castro et al., 2022; Vollmer et al., 2022; Sigman, 2003; Martínez- 230 Méndez et al., 2013; all cited in the revised manuscript). 231 In the meantime, Mg/Ca-derived BTT warming and negative δ18O at the dwelling 232 depth of G. crassaformis indicate building-up of warm and relatively fresh bottom 233 thermocline waters, underscoring that bottom thermocline temperature and δ18O 234 more robustly capture the intensified SAMW/AAIW influence in the western Pacific. 235 Taken together, the coherent patterns of warming and freshening, coupled with the 236 muted δ13C response, support intensified formation and equatorward advection of the 237 SAMW/AAIW as the dominant mechanism. 238 2. Direct evaluation of E–P, seawater density and the MLD 239 Quantitatively estimates of sea-surface evaporation, seawater density, and the mixed- 240 layer depth are challenging due to lack of paired δ18O records of surface- and 241 thermocline-dwelling species and triple-oxygen isotope data across sites in the STF– 242 SAF region. In the qualitative way, we adopted Mg/Ca- and alkenone-based SST stacks 243 together with the SAF position, which provide robust constraints on surface variability 244 and allow first-order inferences about upper-ocean structure. We revised the text 245 conservatively to avoid unsupported descriptions. 246 3. Changes in the revised manuscript 247 We added a paragraph in the section Southern origin of the IPWP bottom thermocline 248 warming (please see revised manuscript, Lines 188–207), clarifying that the lack of 249 δ13C anomaly most likely reflects biogeochemical overprinting on a physically 250 strengthened SAMW/AAIW connection, rather than indicating its absence. 251 252 Lines 148-150: I agree that there is a slight cooling response in the DSDP Site 593 253 temperature record (Fig. 2e); however, I’m not sure if it is responding to the MBT, 254 since temperatures from MIS 10 are quite similar to those from MIS 14, 13, and 12. 255 Considering this, the cooling trend would have started around 350 ka and continued to 256 the present. I recommend running RAMPFIT analysis on this record too. In addition, 257 the long-term trend following your BBT warming is not clearly seen. Is it related to my 258 comment from lines 117-127? These points need to be accounted for, as AAIW cooling 259 was raised as the main mechanism to explain BTT warming. 260 261 Response: We appreciate you for the helpful comments. We agree that the concurrent 262 DTT cooling and BTT warming during the same interval requires careful justification. 263 Please see below our discussions. 264 1. Timing of the cooling trend in DSDP Site 593 265 Following your suggestion, we performed RAMPFIT analysis on the Site 593 IWTA 266 record (Fig. R2). This identifies a cooling transition in AAIW at ~400 ka, slightly earlier 267 than the ~350 ka as you suggested. This minor difference likely reflects method 268 sensitivity and detection criteria. Importantly, this cooling postdates the initial BTT 269 warming and the establishment of SST gradients between the mid-latitude Southern 270 Hemisphere and the STF–SAF region. 271 272 Fig. R2. Change-point of DSDP Site 593 intermediate water temperature anomaly. 273 2. Role of AAIW cooling in BTT warming 274 We would like to clarify that AAIW cooling is not considered as the primary driver of 275 BTT warming. Instead, we propose a dual mechanism: 1) Surface warming in the STF 276 region increases SAMW formation and its temperature, providing the precondition for 277 heat export to the tropics; 2) Strengthened westerlies (driven by STF-SAF temperature 278 gradients) enhance subduction and equatorward heat transport. 279 AAIW cooling may occur alongside these processes but is not a necessary condition. 280 For instance, intensified westerlies can increase subduction flux without any 281 accompanying temperature change in the AAIW. Episodes of AAIW cooling more 282 often reflect higher-latitude dynamics—such as poleward shifts of the ACC —and thus 283 serve as supporting evidence rather than a primary causal mechanism. 284 3. Apparent lack of a clear long-term trend after initial BTT warming 285 We agree that proxy records do not show a clear monotonic trend after BTT warming. 286 The DSDP 593 record is only supporting evidence and, given the scarcity of 287 intermediate-water records, cannot fully constrain AAIW evolution, but it does not 288 undermine the dual-factor framework discussed in the above section 2. 289 4. Exclusion of vertical mixing as the main driver 290 Multiple lines of evidence suggest that vertical mixing related to equatorial air–sea 291 coupling is unlikely to have played a significant role: 1) Parallel δ18O trends among G. 292 ruber (the mixed layer), P. obliquiloculata (upper thermocline), and G. tumida (deep 293 thermocline) since ~670 ka indicate broadly stable stratification from surface to deep 294 thermocline; 2) Parallel δ13C trends among the species further support the persistence 295 of water column structure (Fig. R1). 296 These lines of evidence collectively argue against enhanced vertical mixing from El 297 Niño-Southern Oscillation-like thermocline dynamics as the main cause of BTT 298 warming. Instead, equatorward heat transport from the Southern Ocean, mediated by 299 SAMW/AAIW formation and westerlies intensification, is supposed to be the dominant 300 mechanism. In the revised manuscript, we clarified this mechanism and provided more 301 details on Lines 145–215. 302 303 Lines 162-166: The overall conclusion on the source and mechanisms behind BTT 304 warming needs to be altered accordingly after revision. 305 306 Response: Many thanks for this recommendation. We revised the conclusion to better 307 clarify the mechanisms driving BTT warming after the MBT. In the revised manuscript, 308 this mechanism is further elaborated in the section Southern origin of the IPWP bottom 309 thermocline warming (please see Lines 208–215). 310 311 Minor comments: 312 Lines 20-22: Improve MBT description. 313 Lines 25-29: Separate this into at least two sentences. 314 Line 44: Add “past” before “…long-term global heat storage changes”. 315 Line 51: Be precise when mentioning “before and after” the MBT by defining a specific 316 time range once in the introduction. 317 Line 52: Remove “are” and change “resulted” to result. 318 Lines 56-57: “Influence global climate” is vague. 319 Line 59: “…regulating the global climate” is vague. 320 Line 65: “To probe” is too strong; consider changing to “To investigate” or similar. 321 Line 77: Include “statistically” to “significant glacial–interglacial cycles with 322 dominant 100-kyr and 23-kyr periodicities”. Or include above 90% CI. 323 Line 86 and lines 96-97: Missing error estimations when reporting temperature values. 324 Lines 88-92: Missing SST data citation connecting your arguments to your figures. Is 325 it related to Fig. 2b? If so, cite it. 326 Lines 96-98: Move temperature trends and amplitude discussion to the next paragraph 327 when you discuss long-term BTT warming. 328 Line 98 and 101: Please be consistent and change to “41-kyr” cycle instead “40-kyr”. 329 Line 108: Check if Supplementary Fig. 6b is correctly cited. 330 Line 121: Remove “were”. 331 Lines 128-129: Please reformulate this sentence to avoid redundancy. 332 Line 141: I would recommend changing the “turning point” term to “change point” 333 across your manuscript. 334 335 Response: We are grateful to you for the very detailed suggestions. All typographical 336 errors, wording changes, figure references, and sentence rearrangements you suggested 337 above were accordingly corrected in the revised manuscript. 338 339 Lines 30-33: Considering the main discussion point of the manuscript regarding the 340 long-term increasing trends observed in the thermocline temperature records, I 341 recommend improving or clarifying the direct link between your data observations and 342 their role in particular increasing interglacial temperatures. It is challenging to 343 establish a clear connection since no distinct G-IG pattern can be observed at certain 344 stages (e.g., MIS 10-9). 345 346 Response: Thank you for the suggestion. We clarified that long-term BTT warming 347 elevates the background state, amplifying interglacial peaks rather than simply 348 following G–IG cycles (please see Lines 240–246 in the revised manuscript). 349 350 Lines 48-49: MBE was not defined. The MBE can be defined as the event marking the 351 transition between two distinct climatic or oceanographic states. The issue is that the 352 event is not present in all records worldwide, or is marked by concomitant timing 353 among those where it is expressed. It depends on the perspective from which proxy has 354 been used to track it. Please clarify the adoption of MBT rather than MBE. 355 356 Response: Thank you so much for your careful reading. The “MBE” on Lines 48–49 357 was a typo; it was corrected to “MBT”. Please refer to Line 53 in the revised manuscript. 358 359 Line 101: I recommend filtering these other records from each G. crassaformis to 360 sustain your argument saying that they are mostly paced by a 40-kyr cycle, or to present 361 wavelet or evolutionary spectral analysis to sustain it. If it was raised from the REDFIT 362 analysis presented in the supplementary Fig. 3b, I would recommend it since it is a 363 repetition from line 98. 364 365 Response: We are grateful to you for the constructive suggestion. We applied wavelet 366 analyses to the other G. crassaformis records, which show no significant 41-kyr signal 367 (Fig. R3 and Supplementary Fig. 2d). This sentence was revised accordingly (Lines 368 123–125 of the revised manuscript). 369 370 Lines 138-139: The way it is written, it looks like you will add another evidence for why 371 strengthening of SAMW alone cannot give rise to BTT, not as a conclusion from the 372 arguments you already presented in this paragraph. Please clarify it by including a 373 conclusion statement or presenting additional information arguing that SAMW 374 transport alone cannot be accounted for; if it is not the case, rewrite this sentence 375 accordingly. 376 377 Response: Thank you for pointing this out. We revised the paragraph with modern 378 literature on the AAIW and SAMW, clarifying our interpretation with current 379 hydrographic knowledge. This mechanism is now elaborated in Southern origin of the 380 IPWP bottom thermocline warming (Lines 188–207). 381 382 Line 141-143: RAMPFIT trend estimation analysis provides you with error bars for 383 your change points and calculated mean values. Please include the precise information 384 on when change points occur with their uncertainty in the text when discussing it, and 385 include the error envelopes in the figure, also in the time domain. 386 387 Response: Thanks for raising this issue. We reported the change-point timing and 388 associated uncertainty from RAMPFIT analysis in the revised figure captions (Fig. 3 389 and Supplementary Fig. 6, Lines 782 and 1041). 390 391 392 Fig. R3 Wavelet analysis on G. crassaformis proxy records from Site U1489. a BTT ; b δ18O ; 393 and c δ13C . 394 395 Reviewer #2 (Remarks to the Author): 396 Yang et al. present planktic foraminiferal elemental (Mg/Ca) and isotope (δ18O and 397 δ13C) records since ~670ka from IODP Sites U1489 in the equatorial west Pacific 398 Ocean, which demonstrate deep and bottom thermocline temperatures. The authors 399 further infer advection of enhanced subduction and advection of Subantarctic Mode 400 Water (SAMW), and contributed to amplified interglacial warmth post- Mid-Brunhes 401 Transition (MBT). 402 The positive aspects represent that the records are novel and significant, not limited 403 to only evoking deep water ventilation or CO₂ feedbacks; the authors highlight changes 404 in upper- to intermediate-ocean circulation, linking these with Southern Hemisphere 405 ocean-atmosphere dynamics. A clear divergence is documented between deep 406 thermocline temperature (DTT) and bottom thermocline temperature (BTT) in the 407 Western Equatorial Pacific since the Mid- Brunhes Transition (MBT; ~430 ka). 408 Specifically, DTT shows a gradual cooling trend, while BTT warms by ~4 °C. This 409 warming of the bottom thermocline is interpreted as evidence of intensified 410 equatorward heat transport from the Southern Ocean, particularly via the Subantarctic 411 Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). 412 Well, this study will hold substantial importance for both the paleoclimate and 413 oceanographic sectors. I believe these are the new records from the equatorial East 414 Pacific Ocean, which will have a major impact. However, I believe that an extensive 415 literature study and the comparison of data concerning East Pacific thermocline 416 dynamics could address some gaps and facilitate additional findings (see major 417 comment 3 and 4). 418 Globorotalia crassaformis derived Mg/Ca temperature may be a concern to rethink 419 (see major comment 1). 420 Yes. The methods are clearly described and meet or exceed current standards in the 421 field. 422 From my point of view, this manuscript requires further modification for enhanced 423 improvement. Therefore, I have provided some comments; please review them. 424 425 Response: Thank you so much for your careful reading and insightful suggestions. We 426 accept your advice by adding a more extensive discussion of East Pacific thermocline 427 dynamics and strengthening the validation of Globorotalia crassaformis-derived 428 Mg/Ca temperatures. We carefully addressed the points you raised in the revised 429 manuscript by expanding the comparison with relevant literatures and providing 430 additional clarification on the Mg/Ca temperature interpretation, which we believe 431 strengthens the study and its implications. Specific changes addressing each point are 432 detailed below. 433 434 Major Comments: 435 1. In line 71, you stated that the depth habitat of Globorotalia crassaformis is ~300– 436 400 m. Then in lines 96-97, you stated “small-amplitude (±2°C) variations from ~670 437 ka to 470 ka”. Provide a depth vs. temperature profile of the study Site in the 438 supplementary file. Do you believe it is feasible to reach ~2°C at a depth of 400 m in 439 the equatorial West Pacific Ocean between ~670 and 470 ka? 440 I believe there is a possibility of diagenesis affecting Globorotalia crassaformis; 441 please verify this with the diagenesis test on those specimens. The justifications are as 442 follows. 443 • According to Rosenthal et al. (2018) (Figure 4), at Site U1489, modern 444 temperatures vary from around 7°C to 2°C at depths ranging from ~500 to 1500 m. 445 Though you suggest 400 m, Globorotalia crassaformis can inhabit depths of up to 600 446 m (Steph et al., 2009; Cléroux et al., 2013), although Globorotalia truncatulinoides 447 may reside at even greater depths. This raises issues regarding the depth habits of 448 Globorotalia crassaformis in your temperature determination. 449 • The δ18Oresidual of Globorotalia crassaformis fluctuates between about -1.5 and 450 1.5‰ (Supplementary Fig. 5e), which is huge and suggesting a less equilibrium 451 calcification. 452 • Using Atlantic-based calibration (Line 255) may result in inaccuracies when 453 measuring the temperature of the Pacific bottom thermocline. 454 455 Response: We are grateful to you for the constructive comments. In the revised 456 manuscript, we added details on the calcification depth and Mg/Ca-temperature 457 calibration of G. crassaformis, and modern hydrography at Site U1489 (Methods, 458 Lines 284–328, and Supplementary Text 1, Lines 835–856, 863–892). Please see below 459 our discussion point by point. 460 1. Plausibility of BTT range 461 We agree that the ~2 °C estimate at 400-m depth for 670–430 ka in the previous 462 calculation is a bit cold for the equatorial western Pacific. In the revised Supplementary 463 Fig. 15, we added the modern temperature–depth profile from WOA23, showing ~7– 464 9°C at 400 m. We reprocessed the G. crassaformis Mg/Ca data with corrections for Mn 465 contamination and dissolution (Hasenfratz et al., 2017; Regenberg et al., 2006; cited in 466 the revised manuscript), yielding BTT values of ~5 °C for 670–470 ka, which is 467 comparable to the modern regional hydrography. While absolute values shift with 468 specific Mg/Ca-T calibrations, the long-term pattern is robust--stable BTT before the 469 MBT and a subsequent warming after ~430 ka. We would like to emphasize that the 470 amplitude and temporal pattern of the Mg/Ca-derived temperatures are more reliable 471 than the absolute values. 472 2. Potential diagenetic influence 473 In consideration of your comment on the possibility of diagenetic alteration, we 474 performed scanning electron microscope (SEM) analyses on foraminiferal shells and 475 evaluated this issue in combination with elemental ratio data. SEM images of G. tumida 476 and G. crassaformis (Fig. R4 and Supplementary Fig. 9 of the revised manuscript) show 477 moderate to good preservation, with minor overgrowth and dissolution but no pervasive 478 recrystallization or anomalously high Mg/Ca values (Stainbank et al., 2020; Regenberg 479 et al., 2007; cited in the revised manuscript). These features are typical of pelagic 480 carbonate sediments from ~3400 m water depth, close to the lysocline in the western 481 equatorial Pacific (Parker and Berger, 1971; Valencia et al., 1973; cited in the revised 482 manuscript). Alteration appears broadly uniform through time, indicating that the 483 observed long-term BTT trend is unlikely to result from diagenesis. Representative 484 SEM images are included in the Supplementary Text 2. 485 486 Fig. R4 Scanning Electronic Microscopic images of G. tumida (a and b) and G. crassaformis (c and 487 d). The specimens are from the post-MBT (a = 7.06 m, c = 3.75 m) and the prior-MBT (b and d = 488 11.95 m) of IODP Site U1489. Scale bars and technical parameter information are indicated in white. 489 3. Depth habitat of Globorotalia crassaformis 490 We acknowledge that G. crassaformis can inhabit a wide vertical range, potentially 491 extending below 400 m (Steph et al., 2009; Cléroux et al., 2013; cited in the revised 492 manuscript). Modern profiles indicate that seawater temperatures between 330 to 600 493 m are close to our reconstructed BTT (Fig. R5). Accordingly, we refined the description 494 of its depth habitat in the Methods. 495 496 Fig. R5 Distribution of seawater temperature (color-shaded) of depth sections at a 330 m and b 600 497 m in the modern Pacific Ocean, based on data from the WOA23 (Locarnini et al, 2024). 498 4. Selection of Mg/Ca–T calibrations 499 We acknowledge the limitations of applying Atlantic-derived calibrations to the Pacific. 500 After evaluating several calibrations (Anand et al., 2003; Regenberg et al., 2009; 501 Cléroux et al., 2011, 2013; cited in the revised manuscript) (Fig. R6 and supplementary 502 Fig. 12 of the revised manuscript), we found that the calibration of Cléroux et al. (2013) 503 best reproduces the modern vertical temperature profiles at Site U1489. Although the 504 absolute values may carry a modest cold bias, the relative variations and long-term 505 trends are robust. 506 507 Fig. R6 Comparison of Mg/Ca-Temperature calibrations. Globorotalia crassaformis Mg/Ca 508 based BTT, calculated from the equation of Anand et al. (2003, gray), Regenberg et al. (2009, 509 brown), Cléroux et al. (2011, plum), and Cléroux et al. (2013, red). Red rectangles indicate 510 modern temperatures at water depth of 330–600 m. 511 512 2. To justify “..upward movement of the calcification depth of G. 513 crassaformis ..(Lines 104- 105)” you stated “G. crassaformis δ13C parallels with that 514 of G. tumida… indicating a persistent ecological gradient between G. 515 crassaformis..(Lines 105-110)”. This seems acceptable. 516 But I am wonder when you discussed and linked BBT warming to strengthened SAMW 517 in ‘Southern origin of the IPWP bottom thermocline warming’ section. In lines 135- 518 138, δ13C signal is ignored by simple statement of “However, an increased subduction 519 of SAMW would imprint a positive signature on δ13C of bottom thermocline waters in 520 the IPWP via ventilation, which, however, is not observed”. Whereas, Spero et al. (2003) 521 suggested that a stronger SAMW causes changes in nutrient and carbon isotopic 522 chemistry. How would you justify strengthened SAMW did not affect δ13C signal at 523 Site U1489? 524 525 Response: Thank you for pointing out this issue, which is similar to that raised by 526 Reviewer #1. Although strengthened SAMW could, in principle, lead a positive δ13C, 527 such an increase is not observed at Site U1489. We attribute this to a combination of 528 physical and biogeochemical factors. Along with our discussion in response to the 529 comments of Reviewer #1, we would like to add the point-by-point details according 530 to your comments. 531 1. Complex advection pathways 532 In contrast to the relatively direct SAMW input to the eastern equatorial Pacific 533 (Toggweiler et al., 1991, J. Geophys. Res. Oceans; Spero et al., 2003), the SAMW 534 reaching the western equatorial Pacific traveled longer, more circuitous routes and 535 extensively mixes with the AAIW (Bostock et al., 2010, 2013; Sallée et al., 2010; cited 536 in the revised manuscript). 537 2. Biogeochemical modulation and mixing, and the role of preformed nutrients 538 and air–sea gas exchange 539 We propose additional processes that modulate the δ13C signal during the northward 540 transport of the SAMW and AAIW. Please see above the detailed discussion in 541 response to the comments of Reviewer #1 (Lines 132–137, Page 8, in this Response). 542 In sum, the absence of a strong δ13C excursion at U1489 does not negate a SAMW 543 contribution to bottom thermocline warming; rather, it reflects attenuation and 544 overprinting of isotopic signals along complex western equatorial Pacific pathways. 545 This explanation is clarified in the revised manuscript section Southern origin of the 546 IPWP bottom thermocline warming (Lines 188–207). 547 548 3. Figure 1b indicates that the average temperature of BTT was ~2ºC (with a minimum 549 of approximately 1ºC) from ~670 ka to 470 ka, followed by a subsequent increase in 550 average temperature to 7ºC. In the Figure 4 Schematic illustrating you have shown 551 Globorotalia crassaformis inhabited within the SAMW since ~670 ka. How would you 552 justify that average temperature of the SAMW could be at ~2ºC (minimum of ~1ºC) in 553 the IPWP from ~670 ka to 470 ka? Or it was AAIW? 554 The question remains: how could the SAMW temperature reach ~2ºC at 400 m? As 555 previous studies suggest SAMW fluctuates between 3 to 9ºC throughout glacial- 556 interglacial periods, while AAIW ranges from 2.5 to 5.5ºC in the equatorial Pacific 557 Ocean (McCartney 1977; Herraiz-Borreguero and Rintoul, 2011; Elmore et al. 2015). 558 In addition, model reconstructions consistently show subsurface tropical temperatures 559 did not cool enough to approach 2°C at 400 m depth (Lea et al., 2000; Medina-Elizalde 560 and Lea, 2005; Regenberg et al., 2006). 561 In lines 162-166, “the substantial warming of the IPWP bottom thermocline from 562 MIS13/12 (~470 ka) is associated with intensified AAIW transport and shoaled 563 advection of the SAMW”. 564 Please ensure to substantiate a temperature range of 2 to 7ºC with the advection of 565 warm SAMW. This large temperature increase in bottom thermocline cannot fully 566 justified by warming SST from other locations. Though, you provided AAIW 567 temperature from Site 593, it will be better if you could refer a thermocline record from 568 the Southern sector of the Pacific Ocean where similar warming (~4ºC) trend is 569 observed for SAMW? 570 571 Response: We appreciate you for this critical issue. As we discussed in response to 572 your Major Comment 1, we acknowledge that our previous BTT estimates as low as 573 ~2°C for the interval between ~670 and 470 ka lie in or are slightly below the expected 574 temperature range for the SAMW. Upon re-evaluation, we applied additional 575 corrections to the Mg/Ca record to account for Mn contamination and potential 576 dissolution effects. After these adjustments, the BTT temperatures fluctuate around 577 ~4.5–5°C during this interval, which is more consistent with modern SAMW/AAIW 578 estimates (See the revised section Methods). 579 In the revised interpretation, we clarified that the BTT record represents a persistent 580 mixture of the AAIW and SAMW, consistent with modern observations. Corrected 581 BTT estimates (~4.5°C) fall within the expected SAMW–AAIW temperature range, 582 and the absence of lower δ18O or distinct δ13C excursions in G. crassaformis further 583 supports a mixed-water-mass signal rather than the sole AAIW influence. Across the 584 study interval, the mean BTT warming is ~2.5°C, comparable to thermocline warming 585 in the STF and consistent with enhanced advection and shoaling of the SAMW driven 586 by westerly wind changes. While long-term thermocline records from the mid-latitude 587 South Pacific are absent, we rely on: 1) tight subsurface temperature coupling between 588 the IPWP and South Pacific during the last deglaciation (Zhang et al., 2022); and 2) 589 modern hydrographic observations and model simulations of subtropical–tropical heat 590 transport via mode and intermediate waters (Tatebe et al., 2013; Newsom et al., 2023; 591 Li et al., 2023; cited in the revised manuscript). Together, these lines of evidence 592 support our interpretation, though we recognize this as an important avenue for future 593 work. 594 In brief, we refined the reconstruction, corrected the low-temperature bias, and 595 provided a clearer explanation of how the mixed SAMW/AAIW influence and the 596 ecology of G. crassaformis together account for the observed BTT signal. Please see 597 the revised section Southern origin of the IPWP bottom thermocline warming (Lines 598 145–215 of the revised manuscript). 599 600 4. The two mechanisms appear to involve concurrently within the same timeframe, 601 suggesting that DTT cooling is associated with “Pacific Subtropical Cells, which would 602 promote subduction of cooler waters from higher latitudes into the deep thermocline of 603 Western Equatorial Pacific (Lines 93-95)” while simultaneously, BBT warming is 604 linked to “..intensified AAIW transport and shoaled advection of the SAMW..(162- 605 166)”. There needs to be some justification rather than speculation. 606 These mechanisms are described with sea surface temperature (SST) from different 607 location, though this work focused on thermocline dynamics. Most importantly, 608 thermocline temperature form the East Pacific is ignored, while records are available 609 in literature (Ronge et al., 2020). 610 611 Response: Thanks for pointing this out. We now include the EEP thermocline records 612 (e.g., Ronge et al., 2020) in the revised manuscript. We agree that concurrent DTT 613 cooling and BTT warming in the same interval needs careful explanation. In the 614 revision, we distinguish depth layers and mechanisms more explicitly: 615 Deep thermocline (≈100–200 m, DTT): Cooling reflects intensified Pacific 616 Subtropical Cells (STCs) that enhance the equatorward subduction of cooler subtropical 617 surface waters into the upper thermocline of the western equatorial Pacific. 618 Bottom thermocline to intermediate depths (≈ 330–600 m, BTT): Warming is linked 619 to strengthened ventilation and shoaling of the AAIW and SAMW, which occupy 620 deeper density horizons. 621 These mechanisms, while temporally overlapping, operate at distinct vertical levels 622 (please see revised Methods, Lines 284–328) and thus can coexist without conflicts, 623 representing a vertically stratified reorganization of the tropical Pacific thermocline 624 during the MBT. 625 Regarding SST data, we clarified that SST gradients are used to infer wind stress and 626 mode-water formation, which drive subduction, rather than to represent subsurface 627 temperatures. To reinforce this interpretation, we cited eastern equatorial Pacific 628 thermocline records: G. tumida–based DTT from Ronge et al. (2020) and Ford et al. 629 (2015) showing Pleistocene cooling, consistent with our Site U1489 record. Although 630 G. tumida calcifies deeper in the west than in the east due to eastward shoaling of the 631 Equatorial Undercurrent, both sites capture upper-thermocline conditions, supporting a 632 zonal coherence in DTT cooling. 633 Collectively, these observations indicate that STC-driven DTT cooling and 634 SAMW/AAIW-driven BTT warming reflect complementary processes of a vertically 635 stratified thermocline reorganization, rather than conflicting dynamics, during the 636 MBT. This explanation is clarified in the revised manuscript section Divergent deep 637 and bottom thermocline changes and Southern origin of the IPWP bottom 638 thermocline warming (Lines 88–143, 208–215). 639 640 Minor Comments: 641 Line 77: Remove the word “Significant”. Could not find any significant correlation. 642 643 Response: Agree. It was done. 644 645 Lines 93–95: To interpret the DTT cooling reflects… from higher latitudes into the deep 646 thermocline of Western Equatorial Pacific” is not justified; please clarify with taking 647 account of records from the East Pacific DTT. 648 649 Response: Agree. In the revision, we explicitly cited eastern equatorial Pacific DTT 650 records (Supplementary Figs. 3d, e; Ford et al., 2012, 2015; Ronge et al., 2020; cited in 651 the revised manuscript) alongside our western equatorial Pacific DTT to document 652 basin-scale coherence and surface–subsurface coupling (Please see Lines 98–106). 653 654 Lines 99-100: “extratropical and possibly higher latitudes” not clear. 655 656 Response: In the revised manuscript, we clarified that the dominant 41-kyr obliquity 657 pacing BTT implies sensitivity to high-latitude obliquity forcing (Please see Lines 120– 658 123). 659 660 Lines 105-110: “…indicating a persistent ecological gradient between G. crassaformis 661 and the shallower dwelling species” only δ13C signal cannot justify this statement. 662 Because if you look into δ18O Globorotalia crassaformis (Supplementary Fig. 5e) 663 residual 664 which varies from ~-1.5 to 1.5‰, it suggests that the foraminiferal δ¹⁸O signal is 665 influenced by more than just temperature and δ¹⁸Osw likely reflecting changes in 666 calcification depth, vertical migration, or ecological adaptation. Please take an 667 account of δ18O . 668 residual 669 670 Response: Thank you for this point. While the δ18O of G. crassaformis could in 671 residual 672 principle reflect depth migration, ecological adaptation, or other non-temperature 673 effects, several lines of evidence suggest these factors are not the primary control: 1) 674 the δ13C record of G. crassaformis parallels that of G. tumida and remains consistently 675 offset from shallower species (Supplementary Fig. 5a, b), indicating a stable vertical 676 gradient; 2) the pronounced negative shift in δ18O (Supplementary Fig. 5d) points 677 sw-ice 678 to source-water mass changes rather than systematic upward migration; and 3) positive 679 δ18O excursions are consistent with the Mg/Ca-derived BTT increase since MIS 13/12, 680 with an additional freshening effect also consistent with the proposed SAMW 681 enhancement scenario. Together, these observations indicate that the long-term δ18O 682 and Mg/Ca trends of G. crassaformis most consistently reflect source-water mass 683 influence rather than habitat shift (Please see lines 129–143 of the revised manuscript). 684 685 Lines 156-157: “As a result, the SAMW would be compressed, and warm SAMW are 686 transported to the bottom thermocline of the IPWP” This statement is not fully justified? 687 688 Response: Thanks for your reminding. Based on modern oceanographic studies, we 689 clarified that SAMW was not physically “compressed,” but instead mixed with other 690 water masses (e.g., the AAIW) and thermally modified, enabling the transmission of 691 warmer intermediate waters into the lower thermocline of the IPWP. Please refer to 692 Lines 188–207 of the revised manuscript. 693 694 Lines 181-182: “imply an increase in heat capacity of the upper- to intermediate ocean 695 for the significant post-MBT interglacial warming” When your DDT is showing a 696 steady cooling whereas BTT is progressive warming, it is very difficult to state upper- 697 to intermediate ocean dynamic with ocean heat capacity within IPWP. I wonder it could 698 be link to equatorial dynamics rather than link to high latitudes. 699 700 Response: Thank you for the suggestion. Our study focuses on the BTT warming, 701 which we interpret as enhanced heat storage in the Mode Waters of the Southern Ocean 702 during the post-MBT interglacial warming. We agree with you that equatorial upper- 703 ocean dynamics are also important, as reflected in the DTT cooling, which likely 704 involves tropical air–sea coupled circulation processes. To avoid conflating these 705 distinct mechanisms, we revised the manuscript to emphasize the role of Mode Water– 706 related heat storage in driving the BTT warming, while acknowledging the influence of 707 tropical dynamics on the DTT variability (Lines 231–232). 708 709 Lines 253-256: The calibration used for G. crassaformis (Atlantic-based; Line 254) 710 raises questions about regional accuracy. While the authors acknowledge the lack of a 711 region-specific calibration, this limitation should be explicitly discussed in terms of 712 potential bias or temperature under/overestimation. 713 714 Response: We agree with you. Following your suggestion and in response to your 715 Major Comment 1, we clarified this issue in the revised Supplementary Text 3 (Lines 716 902–912). Please also see details of our response to your Major Comment 1. 717 718 Lines 258–261: The δ18Osw calculation is methodologically appropriate, but the larger 719 uncertainty in G. crassaformis (±0.10‰) might significantly affect salinity inference, 720 particularly when the δ¹8Osw difference between species is small (Lines 183–186). 721 722 Response: Thanks a lot for raising this important point. The observed large uncertainty 723 of the δ18O (±0.10‰) is actually quite minor compared with the full range (~−2.5‰ 724 sw-ice 725 to +0.5‰). 726 727 Reviewer #3 (Remarks to the Author): 728 General comments 729 The authors propose that accelerated upper- to intermediate-ocean circulation in the 730 mid- to low-latitude Pacific may have amplified post-MBT interglacial warming, with 731 the Southern Ocean playing a significant role in shaping these changes. While the 732 manuscript presents valuable new data and addresses an important scientific 733 question, several aspects of the study require clarification and refinement to enhance 734 its scientific rigour and overall readability. 735 In particular, the structure of the introduction would benefit from a more logical 736 progression, beginning with the broader context, followed by knowledge gaps, the 737 working hypothesis, and site justification. Furthermore, some conceptual 738 oversimplifications should be avoided, for example, the direct comparison between the 739 MBT and present-day anthropogenic warming, which is more complex than implied. 740 Methodological details also require improvement, including further details about the 741 species selection, the presentation of the age model, describing the time series analyses, 742 and the description of oceanographic settings. These elements are essential to support 743 interpretations related to regional and large-scale circulation changes. 744 Greater attention to supplementary material, including clearer figure captions, 745 consistent terminology, and acknowledgement of potential methodological limitations 746 (e.g., combining different SST proxies), will strengthen the overall contribution of the 747 study. 748 Finally, while the research addresses a scientifically significant topic and provides 749 valuable new data and insights, the manuscript would benefit from a more in-depth 750 discussion with additional detail regarding the proposed mechanism. Addressing the 751 points outlined below will improve the clarity, methodological robustness, and 752 interpretive strength of the study. Moreover, a more comprehensive comparison with 753 modern oceanographic conditions, as well as the inclusion of paleoclimate modelling 754 to further support the proposed mechanism, would greatly improve the manuscript. 755 756 Response: We are grateful to you for your insightful and constructive comments, which 757 substantially improve our manuscript. Following your comments, we reorganized the 758 Introduction to follow a clearer progression, from the broader climate context to 759 knowledge gaps, our working hypothesis, and the rationale for Site U1489. We 760 removed wording that could imply a direct paleo-analogue between the MBT and 761 modern warming, clarified the oceanographic settings and time-series methods in the 762 Methods section, and expanded the discussion of Southern Ocean influences with 763 additional references. Finally, we explicitly acknowledged the limitations of combining 764 different SST proxies. Specific changes addressing each point are detailed below. 765 766 Specific comments 767 - Introduction: the introductory text would benefit from a more logical flow and 768 structure. A clearer step-by-step development from global context → knowledge gaps 769 → hypothesis → site justification would enhance readability. Also, the claim that the 770 MBT provides a paleo-analogue for current anthropogenic warming is oversimplified. 771 772 Response: We appreciate you for the helpful suggestions. The Introduction has been 773 reorganized (Lines 35–85) for clarity and readability: 1) global climate background; 2) 774 knowledge gap on post-MBT interglacial intensification; 3) our hypothesis on 775 SAMW/AAIW heat transport; and 4) rationale for Site U1489. Wording was revised to 776 avoid a direct MBT–anthropogenic warming analogy, instead presenting the MBT as a 777 reference for natural interglacial variability under broadly comparable background 778 warmth. 779 780 - Line 71: the calcification depths of the chosen foraminiferal species should be 781 consistent with what is presented in the methods section (lines 218 and 221). 782 783 Response: Thank you for pointing this out. We revised the text to ensure that the 784 calcification depths of all foraminiferal species are consistent between the 785 Introduction and Methods sections (Lines 77, 313–317). 786 787 - Lines 72–74: at the beginning of this paragraph, you mention that deep- and bottom- 788 thermocline water temperatures were reconstructed from IODP Site U1489. However, 789 in lines 72–74, you state that the focus is on BTT, which sounds somewhat inconsistent. 790 791 Response: Thanks a lot for this comment, which helps to improve the clarity of our 792 presentation. We clarified that the warming trend of BTT draws more attention, as it 793 directly reflects the influence of intermediate water masses (SAMW and AAIW), 794 though both DTT and BTT are presented. DTT is discussed in parallel to illustrate 795 contrasting thermocline dynamics, with some other published reconstructions of 796 shallower-dwelling foraminifera species across the MBT interval. Please see Lines 79– 797 82 of the revised manuscript. 798 799 - Lines 80-83: consider showing shallow thermocline records together with your DTT. 800 801 Response: Following your suggestion, we added the upper-thermocline Mg/Ca record 802 to Fig. 2a, clarifying precession-paced variations in the IPWP summer insolation 803 gradient. Please refer to Line 769 in the revised manuscript. 804 805 - Lines 88-90: same as previous comment. Additionally, the authors argue that IPWP 806 DTT and SST show a cooling trend starting around ~470 kyr. The cited publication (i.e. 807 de Garidel-Thoron et al., 2005) actually suggests a long-term stability of western 808 Pacific warm pool SST during the Pleistocene. Check this inconsistency. 809 810 Response: We appreciate your comment and clarified our explanation in the revised 811 manuscript. The relatively stable trend in the SST record derived from Globigerinoides 812 ruber at Site MD97-2140 was proposed in contrary to the larger-amplitude cooling in 813 the East Pacific (de Garidel-Thoron et al., 2005). However, if taking a close look, there 814 is a slight cooling trend after ~400 ka, as identified by the LOESS regression (Fig. R7a). 815 This interpretation is further supported by a recently published SST record based on 816 Trilobatus sacculifer from Site 1490, located near our Site U1489 (Fig. R7b; Jia et al., 817 2025; cited in the revised manuscript). To present our interpretation more clearly and 818 consistently, we added Supplementary Fig. 3 and incorporated additional tropical SST 819 records (Fig. R7c–e; Ford et al., 2012, 2015; Ronge et al., 2020; Brierley et al., 2009) 820 in the revised manuscript (Lines 98–106). 821 822 Fig. R7 Seawater temperature records relevant to the mechanisms of U1489 DTT evolution. a 823 MD97-2140 SST-G. ruber (de Garidel-Thoron et al., 2005); b IODP Site U1490 SST-T. sacculifer 824 (Jia et al., 2025); c ODP Site 806 DTT-G. tumida (Ford et al., 2015); d ODP Site 849 DTT-G. 825 tumida (Ford et al., 2015); e ODP Site 1236 DTT-G. tumida (Ronge et al., 2020). 826 - Lines 91-92: Regarding the text “The DTT cooling is also corroborated by a similar 827 weak cooling in the mid-latitude North Pacific25 and a strengthening trend of Northern 828 westerly winds26 (Supplementary Fig. 2b, c),”, the mentioned panels of Supplementary 829 Figure 2 indicates a southward shift of the westerlies and not a strengthening. Please, 830 clarify the text to keep consistency. 831 832 Response: Thanks for your suggestion. We revised the text to note a southward 833 migration of Northern Hemisphere westerlies during the Mid–Late Pleistocene (Chen 834 et al., 2023), which may contribute to deep thermocline cooling at Site U1489, rather 835 than implying a change in the wind strength. Please see Lines 108–109 in the revised 836 manuscript. 837 838 Lines 93-95: provide a reference for this statement. Additionally, this statement could 839 be further detailed, linking the observed cooling to the strength and southward 840 migration of the northern westerlies. 841 842 Response: We are grateful to you for this comment. We added references (e. g. Luyten 843 et al., 1983; Johnson and McPhaden, 1999) and clarified in the revised manuscript how 844 thermocline shoaling, together with the southward shift of mid-latitude North Pacific 845 cooling and the westerlies, may contribute to the DTT declining in the tropical 846 thermocline through the STC system (Please refer to Lines 110–119 of the revised 847 manuscript). 848 849 - Lines 96-103: as far as I can see in Figure 1e, between around 550 and 400 kyr, there 850 is a decoupling between BTT-band pass filtering and obliquity signal. It should be 851 acknowledged and discussed. 852 853 Response: Thank you for your careful observation. Following other reviewers’ 854 comments, we corrected BTT Mg/Ca temperatures for Mn contamination and 855 dissolution. The reanalyzed data (revised Fig. 1e) show signal attenuation between 500– 856 400 ka, noted in the revised manuscript (Please see Lines 125–128). 857 858 - Lines 105–110: The authors state that the G. crassaformis δ13C record remains 859 parallel to those of shallower-dwelling species (i.e., G. tumida, G. ruber, and P. 860 obliquiloculata), indicating a persistent ecological gradient throughout the studied 861 interval. While it is evident that δ13C records of G. crassaformis and G. tumida follow 862 a parallel trend (with G. crassaformis consistently showing lower δ13C values), the δ13C 863 records of G. ruber and P. obliquiloculata appear to exhibit similar absolute values to 864 G. crassaformis (around 1‰). How do the authors explain this apparent similarity in 865 δ13C values despite the differences in habitat depth among these species? 866 867 Response: Thank you for highlighting this critical point. The observed convergence in 868 δ13C likely reflects the subsurface chlorophyll maximum (~80–150 m), where high 869 productivity and remineralization preferentially remove 12C, enriching δ13C-DIC 870 (Furuya, 1990; Shi et al., 2023), while the surface and deeper layers exhibit lower 871 chlorophyll and reduced activity. This mechanism, together with consistent δ13C offsets 872 and δ18O stratification, supports a stable water column, as detailed in the revised 873 Methods (Lines 301–310) and Supplementary Text 5 (Lines 934–944) 874 875 - Lines 120–127: scenario 1 could be more clearly explained in terms of a deepening 876 of the thermocline in the western equatorial Pacific, i.e. intensified trade winds → 877 enhanced east–west transport of warm surface waters → pilling up of warm waters in 878 the western equatorial Pacific -> deepening of the thermocline → DTT warming. 879 880 Response: We thank and agree with you. Following your advice, we clarified the 881 argument and elaborated the BTT warming mechanism in the revised manuscript 882 (please see revised section Southern origin of the IPWP bottom thermocline warming, 883 Lines 145–215). 884 885 - Lines 135-140: to clarify why a strengthening of the SAMW transport cannot account 886 to the observed SAMW warming as well as that a strengthening of the SAMW transport 887 would imprint a positive signature on δ13C of bottom thermocline waters in the IPWP, 888 the methods subsection related to modern conditions should include information on the 889 present-day oceanographic settings at both core location and SAMW formation region. 890 891 Response: Many thanks to you for this constructive suggestion. We added a new 892 Methods subsection Subsurface Water Masses (Lines 284–310), detailing: 1) the core 893 location and SAMW/AAIW formation regions with main transport pathways; 2) 894 modern T–S and δ13C profiles (WOA, GLODAP; Supplementary Fig. 1; Fig. R8) to 895 clarify water-mass properties and depths; and 3) the expected δ13C response in the 896 bottom thermocline under pure SAMW strengthening (positive δ13C), noting that real 897 water-mass mixing attenuates this signal. This is described in Southern origin of the 898 IPWP bottom thermocline warming in the revised manuscript (Lines 188–207). 899 900 Fig. R8 Vertical distribution of δ13C profiles (color-shaded) in the modern Pacific Ocean, 901 based on data from the GLODAP datasets (Lauvset et al., 2022)9. 902 903 - Lines 135-138: An increased export of SAMW wouldn’t be related to a decrease in 904 δ¹³C? 905 906 Response: Thanks for pointing out this issue. We reorganized this section to clarify 907 that enhanced SAMW export does not necessarily produce a δ13C increase. Detailed 908 explanation is provided in our response to comments of Reviewer #1 (Page 7–8, Lines 909 132–137), including discussion of subsurface advection and source-region processes. 910 Details are provided in section Southern origin of the IPWP bottom thermocline 911 warming (Lines 188–207) of the revised manuscript. 912 913 - Lines 170-171: Regarding the text “Progressive mid-latitude warming in the Southern 914 Hemisphere”, this warming is proposed considering SST records, right? Please clarify 915 it in the text. 916 917 Response: We thank you for highlighting this important point. The revised text 918 explicitly states: “SST reconstructions reveal progressive warming in the Southern 919 Hemisphere mid-latitudes…. ” Please see Lines 220–222 of the revised manuscript. 920 921 - Lines 191-192: Regarding the text “While previous studies focused on high-latitude 922 AAIW dynamics and their role in global heat transport, our study….”, provide 923 reference(s) to this statement. 924 925 Response: Thank you. It was done. Please see Lines 247–251 in the revised manuscript. 926 927 - Methods: much of the interpretation relies on time series analyses (e.g., interpolation, 928 resampling, filtering, running averages, detrending, LOESS regression, spectral 929 analysis, ramp function regression, and so on). It would be interesting to add a “time 930 series” subsection describing each of these techniques within the methodology. If there 931 is no space for it in the main text, it could be inserted in the supplementary information. 932 933 Response: Many thanks to you for pointing out this issue. We added a dedicated 934 subsection in the Methods titled Time series analysis methods (please see Lines 399– 935 413 of the revised manuscript), which includes detailed descriptions of resampling, 936 LOESS regression, ramp-fitting, spectral analysis, and residual construction. 937 References for each technique are also provided. 938 939 - Line 207 (Methods subsection “Study site location and sampling”): provide 940 information on the IODP expedition which collected the studied marine core. 941 Additionally, a broad description of the oceanographic setting (specifically the water 942 masses and circulation patterns influencing the study site) is currently missing. This 943 information is crucial to support the discussion, which addresses the influence, for 944 example, of the North Pacific and the Southern Ocean on the study area. Furthermore, 945 the age model for core U1489 requires a more thorough explanation. In particular, the 946 choice of G. ruber over benthic foraminifera should be explained. Providing a 947 supplementary figure illustrating the age model for the target interval is strongly 948 recommended. 949 950 Response: Thank you for the constructive comments. IODP Site U1489 was recovered 951 during Expedition 363. The revised Methods (Lines 265–310) includes a summary of 952 the modern oceanographic settings, highlighting key surface and subsurface currents 953 and intermediate waters, addressing influences from the North Pacific and Southern 954 Ocean. 955 Reviewer #1 also asked about an overview of the age model. We would like to add 956 specific descriptions here in response to the question on the choice of G. ruber. The age 957 model is based on a previously published, orbitally tuned δ18O record of G. ruber 958 covering the past ~4 Ma (Dang et al., 2024). We adopted this framework because the 959 G. ruber record at Site U1489, and over the Western Tropical Pacific, has been widely 960 applied (Bowman et al., 2024, Earth Syst. Sci. Data) and shows close alignment with 961 the LR04 global benthic stack, providing a reliable chronological basis. Although 962 planktonic and benthic δ18O signals may differ slightly, our analysis focuses on orbital- 963 to multi-orbital-scale variability, for which the G. ruber-based chronology is 964 appropriate. To aid clarity, a description of the age model has been added to the 965 Methods (Lines 273–282), with Supplementary Fig. 8 presenting the age model and tie 966 points. 967 968 - Line 216 (subsection “Planktonic foraminiferal ecology and its role in indicating 969 water mass properties”): a more comprehensive description of the planktonic 970 foraminiferal species used in the study is necessary. For example, based on sediment 971 trap studies, is the shell flux to the seafloor seasonal or year-round? Consequently, are 972 the geochemical records derived from these species seasonally biased, or do they reflect 973 annual mean conditions? 974 975 Response: We appreciate your insightful comments regarding the seasonal versus 976 annual representation of G. tumida and G. crassaformis shell fluxes. For G. tumida, 977 modern observational data are absent in our study region, making it difficult to 978 determine whether its signals reflect seasonal or annual mean conditions. For G. 979 crassaformis, evidence from the Atlantic Ocean indicates flux maxima in winter/spring 980 (Salmon et al., 2015; cited in the revised manuscript). To evaluate the potential bias at 981 IODP Site U1489, we examined modern temperature profiles and found that winter– 982 annual mean differences at the species’ habitat depths—even when extending to ~600 983 m, as noted by another reviewer—are minimal (Fig. R9). Therefore, even if these 984 species predominantly record conditions prone to a certain season, little bias from an 985 annual means in the western equatorial Pacific should be introduced into our DTT and 986 BTT reconstructions. This discussion is added to the revised Supplementary Texts 1 987 and 3 (please see Lines 835–861, 894–912). 988 989 990 Fig. R9 Temperature at different water depth (color-shaded) in the modern Pacific Ocean. a 991 Boreal winter (Jan-Mar) and b annual mean temperature at 330 m depth; c Boreal winter (Jan-Mar) 992 and d annual mean temperature at 600 m depth. Data from the World Ocean Atlas (WOA) 2023 993 (Locarnini et al., 2024). 994 995 - Lines 223-227: the sentence “In general, the subsurface-dwelling Pulleniatina 996 obliquiloculata, which calcifies at depths of less than 150 m, is considered an indicator 997 of upper thermocline water properties21,53” seems out of context in this part of the 998 text. 999 1000 Response: Thanks for the suggestion. We agree that its inclusion may appear tangential 1001 in this section. To improve the clarity and focus, we removed this sentence in the 1002 revised manuscript and defined DTT and BTT directly from the calcification depths of 1003 G. tumida and G. crassaformis. Please see the Methods section (Lines 320–322). 1004 1005 - Line 233 (Methods subsection titled “Isotope analysis and Mg/Ca-temperature 1006 estimate”): Please (i) provide references related to the species used for the analyses; 1007 (ii) include the sampling resolution for both the δ18O/δ13C and Mg/Ca measurements; 1008 (iii) specify the carbonate preparation device coupled to the MAT253 mass 1009 spectrometer; (iv) be more specific to which Mg/Ca cleaning procedure was 1010 implemented and describe any modification if there was one (three slightly different 1011 procedures were cited); (v) include information on how potential contamination (e.g. 1012 silicates Mn-bearing phases and clay contamination) was assessed in the trace element 1013 analyses. 1014 1015 Response: Many thanks to you for pointing out the details. Species references, 1016 sampling resolution (~2–3 kyr), and use of a Kiel IV with MAT253 are specified in the 1017 revised manuscript. We also added Mg/Ca cleaning combined steps from Rosenthal et 1018 al. (1997), Barker et al. (2003), and Martin & Lea (2002), together with Mn/Ca and 1019 dissolution corrections (Hasenfratz et al., 2017; Regenberg et al., 2006). The corrected 1020 BTT estimates agree with modern temperatures at ~330–600 m. Please see the revised 1021 Methods (Lines 312–375). 1022 1023 - Line 258 (Methods subsection “Calculation of seawater δ18O): given the length of the 1024 record spanning multiple G-I cycles, the reason for not applying ice volume correction 1025 to the δ¹8Osw data should be explained. 1026 1027 Response: Thank you for this careful comment. Following your suggestion, we applied 1028 the correction for ice-volume changes and presented the corresponding results in the 1029 revised manuscript. Please see the revised Methods (Lines 377–382). 1030 1031 - Line 455 (Figure 2): the caption text related to panel d should specify which cores 1032 were used to calculate the anomaly between STF and SAF. Double-check it for all 1033 presented figures. 1034 1035 Response: Many thanks to you for drawing our attention to this point. In the revised 1036 manuscript, all figure captions were revised to explicitly indicate which cores were used 1037 in calculating the STF-SAF anomaly, ensuring clarity and reproducibility. Please see 1038 Lines 786–788 of the revised manuscript (the revised Fig. 3). 1039 1040 - Line 477 (Figure 4): in the section “Divergent deep and bottom thermocline changes”, 1041 the authors present and discuss a cooling trend of SST and DTT in the IPWP, starting 1042 from at ca. 470 kyr. This cooling trend does not seem to be represented in this figure. 1043 1044 Response: We are grateful to you for noting this detail. We updated Fig. 5 to more 1045 clearly display the DTT and SST trends starting at ~470 kyr, consistent with the textual 1046 discussion. Please also see Line 807 of the revised manuscript. 1047 1048 - Line 502 (Supplementary Text 1): the residual records should be explained in greater 1049 detail, either in the Methods section and/or in Supplementary Information 1. 1050 1051 Response: Thank you for this comment. In the revised manuscript, we included a 1052 description of the calculation procedure for the U1489 residual records in the Methods 1053 (Lines 400–407). These residuals were primarily used as input for spectral and filtering 1054 analyses to support the main idea of this study on variations of DTT and BTT. To 1055 remain the clarity and conciseness of the manuscript’s core narrative, we did not expand 1056 the discussion of the residuals in Supplementary Information. 1057 1058 - Lines 512-513: by looking at supplementary Figure 3, it seems that δ18O from 1059 G. crassaformis also exhibits a statistically significant precession signal. Additionally, 1060 please double-check Figure 3 regarding the description of the lines indicating the 99% 1061 and 95% confidence levels. In the figure itself, these lines are labelled as 90% and 95%, 1062 while the caption refers to them as 99% and 95%. 1063 1064 Response: We appreciate you for carefully checking the confidence levels in 1065 Supplementary Fig. 3. We confirm that the figure labels (90% and 95%) are correct, 1066 and the figure caption was corrected. The δ18O from G. crassaformis does exhibit 1067 residual 1068 a precession peak, marginally above the 90% confidence level. However, this signal is 1069 relatively subdued compared with the pronounced obliquity-band slope trend in the 1070 BTT record, which remains the primary focus of our interpretation. Finally, we 1071 corrected this typo, as shown in the revised Supplementary Fig. 2 (Lines 1001–1010). 1072 1073 - Line 543 (Supplementary text titled “Verification of sea surface temperature stacks”): 1074 this section focuses on describing the SSTA stacks. From Supplementary Table 1, it is 1075 possible to identify that the records used to construct the SSTA stacks were obtained 1076 using two different methods, i.e. Mg/Ca and Uk' . This difference should be 1077 37 1078 acknowledged in the supplementary text, along with a discussion of the potential 1079 implications of combining records derived from distinct methods, especially knowing 1080 that alkenones and Mg/Ca might show different patterns (e.g. Leduc et al., 2010, 1081 10.1016/j.quascirev.2010.01.004). 1082 1083 Response: Thank you so much for the insightful comment. As shown in Supplementary 1084 Table 1, the SSTA stacks combine Mg/Ca- and Uk' -derived SST records, which differ 1085 37 1086 in sensitivity, depth, and seasonal biases (Leduc et al., 2010; cited in the revised 1087 manuscript). While minor amplitudes or phase differences may occur, both are widely 1088 used for glacial–interglacial SST reconstructions (Clark et al., 2024). The revised 1089 Supplementary Text 6 (Lines 957–967) now explicitly notes these differences, 1090 confirming that overall G–IG patterns remain robust. 1091 1092 - Line 555 (Supplementary figure 1): please clarify in the caption the elements shown 1093 in panel b (e.g., white arrow, triangle, foraminifera illustration, selected latitudinal 1094 section). 1095 1096 Response: Thank you for the suggestion. We revised the caption of Supplementary Fig. 1097 1 to clarify all elements. (Please see Lines 984–999 of the revised manuscript). 1098 1099 - Lines 577: It would be interesting to add in the spectral analysis supplementary figure 1100 3 an evolutionary method (e.g., wavelet analysis) to see whether the identified orbital 1101 parameters are constant throughout the record or if there is any change in the dominant 1102 cycle that might be related to the findings. 1103 1104 Response: Thank you for this constructive suggestion. We added wavelet analyses of 1105 DTT and BTT to Supplementary Fig. 3 (the revised Supplementary Fig. 2 and Fig. R10) 1106 to assess orbital stability. The results confirm that DTT is mainly precession-driven, 1107 while BTT is dominated by obliquity (Please see Line 1001 of the revised manuscript). 1108 1109 Fig. R10 Spectral and wavelet analyses of proxy records from IODP Site U1489. Spectral 1110 analysis of records from a G. tumida and b G. crassaformis; and wavelet analysis of c DTT 1111 residual 1112 and d BTT . 1113 residual 1114 1115 Technical corrections: 1116 - Line 22: “Targeting the most…”. The word “at” is unnecessary here. 1117 - Line 48: the acronym MBE was used instead of the already defined MBT. 1118 - Line 50: “...for instance, the Holocene”. 1119 - Line 50: “…are marked by…”. Use plural once it refers to the interglacials. 1120 - Line 51: double-check the cited panels from Figure 1. Maybe, only Fig. 1a would be 1121 appropriate to cite here. 1122 - Line 52: “...essentially the result of…” instead “...essentially resulted from…” 1123 - Line 54: it would be convenient to cite here Fig. 1d, e. 1124 - Lines 58 and 559: Homogenise the way you refer to the IPWA, i.e. “Indo-Pacific 1125 Warm Pool” or “Indo-Pacific warm pool”. 1126 - Line 64: “carbon cycling” instead of “carbon-cycling”. 1127 - Line 69: please include a reference to the methods subsection where the study site is 1128 described. 1129 - Lines 79-80: it would be appropriate to cite also panel d of Figure 1. 1130 - Line 103: also cite Figure 1c in this sentence. 1131 - Line 107: The word “with” is unnecessary. 1132 - Line 113: “...shifting in δ18O of…”. 1133 - Line 121: “...were the result of…” instead “...were resulted from…”. 1134 - Lines 156-157: “...and warm SAMW would be transported…”. To keep consistent 1135 verb tense. 1136 - Line 163: “evidences” instead of “evidence”. 1137 - Line 179: “contributed” instead of “contribute”. 1138 - Line 212: “interval” instead of “intervals”. 1139 - Line 230: the acronym SSTA should be presented in full the first time it appears in the 1140 text. 1141 - Line 256: Delete the word “calculation”. 1142 - Line 453: “3-point running average” instead of “3 points running average”. 1143 - Line 473: Same as line 455. 1144 - Line 511: correct to “~41- and 23-kyr bands”. 1145 - Line 536: “...based on of G. tumida…”. Delete the word “of”. 1146 - Line 562: “The filed circles…”. Correct to “filled”. 1147 - Line 567 (Supplementary Figure 2): I would suggest not invert the y-axis of panel e 1148 to facilitate the rationale. 1149 - Line 569: Same as line 455. 1150 - Line 604: In the sentence “The thin, thin medium, and thick curves…”. You can use 1151 just “thin, medium, and thick” instead. 1152 - Line 605: Same as line 455. 1153 - Line 642: In the supplementary table 1, adjust the first column name to “Region”. 1154 1155 Response: We are grateful to you for these detailed corrections. All of corrections 1156 above were carefully implemented line-by-line throughout the revised manuscript and 1157 Supplementary Information. The corrections include textual phrasing, figure citations, 1158 abbreviation usage, and grammar errors. 1159 1160 Reviewer #4 (Remarks to the Author): 1161 I co-reviewed this manuscript with one of the reviewers who provided the listed reports. 1162 This is part of the Nature Communications initiative to facilitate training in peer review 1163 and to provide appropriate recognition for Early Career Researchers who co-review 1164 manuscripts. 1165 1166 Response: We thank Reviewer #4 for their involvement in the peer-review process. As 1167 the reviewer co-reviewed this manuscript with one of the previous reviewers, we have 1168 provided responses to all relevant comments and suggestions in our point-by-point 1169 replies. 1170 1171 REVIEWER COMMENTS 1172 Reviewer #2 (Remarks to the Author): 1173 Thank you for your reply. However, I remain unsatisfied with your justifications. 1174 My primary concern relates to the temperature calculation. Your main issue regarding 1175 the temperature difference between G. tumida and G. crassaformis stems from the 1176 application of several temperature calculation references derived from Atlantic sources, 1177 one of them fits with your location. This aspect may cause uncertainty for readers, 1178 making it unlikely to satisfy the publication criteria of Nature Communications. 1179 The purity of the δ13C signal remains a worry about the advection of SAMW/AAIW. 1180 The revised manuscript lacks further discussion on East Pacific dynamics. 1181 The manuscript remains more speculative. 1182 Response: We thank the reviewer for the continued careful evaluation of our 1183 manuscript and for identifying several remaining issues that are important for assessing 1184 the robustness and scope of our interpretations. We agree that these points require 1185 explicit clarification. Below, we address each concern in turn and describe how the 1186 revised manuscript more clearly frames methodological uncertainties, refines the 1187 positioning of our conclusions, and delineates the limits of our interpretations. 1188 (1) Temperature estimates and Mg/Ca calibrations 1189 We fully acknowledge the reviewer’s concern that no Pacific-specific Mg/Ca 1190 calibration currently exists for G. crassaformis, and that the use of Atlantic-based 1191 calibrations introduces uncertainty in absolute bottom-thermocline temperature 1192 estimates. To avoid over-interpretation, we have revised the Abstract and Introduction 1193 to emphasize that our interpretations focus on relative temperature changes and long- 1194 term trends, rather than on absolute temperature values (Lines 29–31, Lines 87–90). 1195 To further address this issue, we have added a sensitivity comparison using several 1196 commonly applied Mg/Ca–temperature calibrations (Results and discussion, 1197 Supplementary Material 5, Lines 146–149, Lines 917–931). Although these 1198 calibrations yield different absolute temperature estimates (1.8–4.3 °C; Table R1), all 1199 consistently reproduce a significant post-MBT warming of the bottom thermocline 1200 (Supplementary Table 1, Lines 1131–1133). Furthermore, the original G. crassaformis 1201 Mg/Ca values (Table R1) show a post-MBT increase (0.35±0.004 mol/mol, 1σ) that is 1202 unaffected by the choice of Mg/Ca-temperature calibrations. Similarly, the post-MBT 1203 cooling inferred from G. tumida reflects a systematic decrease in Mg/Ca ratios (−0.16 1204 ± 0.012 mol/mol, 1σ; Table R1) and is therefore largely independent of the choice of 1205 Mg/Ca–temperature calibration. 1206 The above two clarifications support that the central inference of our study, namely 1207 the divergent post-MBT evolution of the deep and bottom thermocline, is robust and 1208 independent of the specific calibration applied. The revised manuscript now explicitly 1209 acknowledges that absolute temperature estimates remain uncertain and therefore 1210 emphasizes relative changes and long-term trends (Lines 29–31, Lines 87–89, Lines 1211 917–931), and that a Pacific-specific calibration indicates a clear direction for future 1212 work. 1213 Table R1. Pre- and post-MBT averages of raw Mg/Ca ratios and Mg/Ca-derived temperatures 1214 from multiple calibration equations for G. crassaformis and G. tumida. 1215 Mg/Ca ratio 1216 Post- MBT Average variations 1217 Species & Pre- MBT average 1218 averages (Post-MBT minus Pre-MBT) 1219 Calibrations 1220 Mg/Ca 1.24 mol/mol 0.88 mol/mol 0.35±0.004 mol/mol, 1σ 1221 ref. 22 6.4 ℃ 4.6 ℃ 1.8±0.1 ℃, 1σ 1222 G. crassaformis ref. 21 11.8 ℃ 7.5 ℃ 4.3±0.2 ℃, 1σ 1223 ref. 20 6.7 ℃ 4 ℃ 2.7±0.1 ℃, 1σ 1224 ref. 19 14.8 ℃ 12.3 ℃ 2.5±0.2 ℃, 1σ 1225 Mg/Ca 1.86 mol/mol 2.02 mol/mol −0.16±0.012 mol/mol, 1σ 1226 ref. 1 17.7℃ 18.6 ℃ −0.8±0.2 ℃, 1σ 1227 G. tumida ref. 19 17.6 ℃ 18.5 ℃ −0.9±0.2 ℃, 1σ 1228 ref. 17 22.1 ℃ 23.3 ℃ −1.2±0.3 ℃, 1σ 1229 ref. 18 13.9 ℃ 14.8 ℃ −0.9±0.2 ℃, 1σ 1230 (2) Depth habitat constraints and inter-species temperature contrast 1231 We want to stress again the clear different δ18O and Mg/Ca ratios between G. 1232 tumida and G. crassaformis, which are essentially originated from their distinct depth 1233 habitats, rather than calibration-induced artifacts. Thus, we constrained their 1234 calcification depths using site-specific hydrographic profiles in combination with 1235 independent δ18O-based estimates (Lines 831–857). While a minor adjustment to the 1236 estimated habitat depth of G. tumida improves ecological consistency at Site U1489, it 1237 does not alter the timing, amplitude, or overall structure of the reconstructed deep- 1238 thermocline temperature variability. 1239 (3) δ13C signals and influences from the South 1240 We agree with the reviewer that, in principle, a clear δ13C ventilation signal would 1241 provide strong supporting evidence for the influence of southern-sourced waters. 1242 However, in the western equatorial Pacific, such signals are commonly attenuated or 1243 overprinted by remineralization and mixing during the distant transport of these waters. 1244 This may have limited δ13C as a direct tracer in this region, and such point is now 1245 explicitly acknowledged and discussed in the manuscript, with appropriate references 1246 (Lines 212–222). 1247 Accordingly, δ13C is treated as a secondary constraint rather than a primary 1248 diagnostic. In contrast, physical properties such as temperature and salinity, which are 1249 more conservative than δ13C and better constrained by geochemical proxies, form the 1250 primary basis for inferring the influence of southern-sourced subsurface waters at Site 1251 U1489. This distinction is now clearly articulated in the Discussion to avoid confusion, 1252 particularly for non-specialist readers. Please see details in Lines 165-229 of the revised 1253 manuscript. 1254 (4) East Pacific dynamics and study scope 1255 We are grateful for the suggestion by the reviewer. In the revised Introduction and 1256 Discussion, we have framed a conceptual Pacific-wide implications and clarified the 1257 choice to maintain the primary focus of this manuscript on the western equatorial 1258 Pacific (Lines 90–96, Lines 131–134). This is because our study focuses on bottom- 1259 thermocline temperature variability in the western equatorial Pacific warm pool, of 1260 which the upper-ocean thermal structure is strongly modulated by those southern- 1261 sourced intermediate/thermocline waters (e.g., Tsuchiya,1991; Zenk et al., 2005; 1262 Bostock et al., 2010, 2013; Zhang et al., 2022; Lines 292–319). While reconstructions 1263 from the eastern equatorial Pacific (EEP) provide valuable complementary perspectives, 1264 but distinct ventilation routes and equatorial zonal air-sea coupled dynamics exert 1265 intensive controls on the EEP, and we believe it would be better to narrow down the 1266 scope of this study as it is now. 1267 (5) Refinement of the interpretations 1268 Taken together, we acknowledge the critical comments by reviewer #2. 1269 Accordingly, we have revised the manuscript to tighten our interpretations as a 1270 mechanistic framework that is consistent with the available data, explicitly outlined its 1271 limitations, and identified testable predictions for future observational and modeling 1272 studies. These revisions are intended to reduce speculative language while preserving 1273 the explanatory value of the proposed Southern Ocean–tropical coupling mechanism. 1274 Reviewer #3 (Remarks to the Author): 1275 We thank the authors for carefully addressing all major concerns and for considering 1276 the additional suggestions. The authors did a very good job, and the manuscript is now 1277 considerably improved. We find no remaining substantive issues in the revised version 1278 and therefore recommend it for publication. Below, we list only a few minor points that 1279 should be addressed before final acceptance. 1280 Response: We appreciate the reviewer for the positive assessment. All minor 1281 comments have now been addressed in the revised manuscript. This includes 1282 specification of supplementary material sections, corrections to terminology and figure 1283 citations, and consistency in time scale notation. 1284 - Throughout the main text, the authors refer to the supplementary material in a very 1285 general way (e.g., lines 92, 158, 166, 207). However, the supplementary material 1286 contains many different sections, making it difficult to identify where exactly to look 1287 when it is cited. I strongly recommend specifying the relevant section of the 1288 supplementary material when citing it in the main text. 1289 - Line 22: Correct is “remain” 1290 - Line 60: Correct is “regulates” and “moisture” 1291 - Line 99: The MIS definition should be presented in line 92 instead of here 1292 - Line 109: Or “... indicating a colder and contracted STCs…”? 1293 - Line 120: Also cite panel c (i.e., BTT) in this sentence 1294 - Line 130: Correct is “rose” 1295 - Line 135: Wouldn’t the correct panels to be cited from Supplementary Fig. 5 be a, b, 1296 and c? Additionally, Supplementary Text 5 (Lines 934–944) should also be cited. 1297 - Line 138: Correct is “shows” 1298 - Line 228: Correct is “facilitates” and “strengthens” 1299 - Line 367: Correct is “specific” 1300 - Lines 773-774: Delete the duplicated part “and summer interhemispheric insolation 1301 gradient” 1302 - Line 783: Wouldn’t the label of panel d (i.e., “SWW increasing”) be more appropriate 1303 as “SSW strengthening”? 1304 - Line 914: Either “Seawater δ18O variations” or “δ18Osw-ice variations” 1305 - Lines 934-935: This sentence seems to be missing something 1306 - Line 1015: Missing punctuation at the end of sentence “c” 1307 - Line 1033: Correct is “P. obliquiloculata” 1308 - Line 1061: Correct is “electron” 1309 Reviewer #4 (Remarks to the Author): 1310 I co-reviewed this manuscript with one of the reviewers who provided the listed reports. 1311 This is part of the Nature Communications initiative to facilitate training in peer review 1312 and to provide appropriate recognition for Early Career Researchers who co-review 1313 manuscripts. 1314 Response: We thank the reviewer for contributing to the co-review process and for the 1315 careful evaluation of our manuscript. 1316 Reviewer #5 (Remarks to the Author): 1317 Review of the manuscript entitled “Equatorward upper-ocean heat transport from the 1318 Southern Ocean boosted interglacial warming” (NCOMMS-25-35512) 1319 The manuscript is significant because it addresses a major knowledge gap in 1320 paleoclimatology: the mechanisms behind the Mid-Brunhes Transition (MBT), a shift 1321 toward intensified interglacial warming that has remained elusive. By providing a 1322 valuable new dataset of deep and bottom thermocline temperatures from the Western 1323 Equatorial Pacific, the authors provide novel evidence that the Southern Ocean plays 1324 a central role in low-latitude climate sensitivity through the equatorward transport of 1325 Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). This 1326 research is particularly impactful as it offers a framework for understanding modern 1327 oceanic heat uptake, where the ocean stores more than 90% of excess heat from 1328 anthropogenic forcing. 1329 In response to a rigorous first round of peer-review, the authors systematically 1330 strengthened the manuscript through significant data reprocessing and structural 1331 reorganization. A primary concern involved original temperature estimates that 1332 appeared unrealistically cold; the authors reprocessed their Mg/Ca data using 1333 corrections for Mn contamination and dissolution, yielding Bottom Thermocline 1334 Temperatures (BTT) of approximately 4.5–5 °C that align with modern regional 1335 hydrography. To further validate these proxies, they conducted Scanning Electron 1336 Microscope (SEM) analyses, confirming that the observed warming trends were not 1337 artifacts of diagenetic alteration. 1338 The core of the study’s significance lies in its identification of a vertically stratified 1339 reorganization of the ocean. Since the MBT, the deep thermocline (DTT) has gradually 1340 cooled in association with tropical circulation shifts, while the BTT has warmed by 1341 ~1.8 °C. The authors effectively disproved the alternative explanation of local vertical 1342 mixing by demonstrating that gradients between surface and thermocline-dwelling 1343 species remained parallel and stable since 670 ka. Instead, the warming is attributed 1344 to the equatorward advection of Subantarctic Mode Water (SAMW) and Antarctic 1345 Intermediate Water (AAIW), driven by a strengthening of the Southern Hemisphere 1346 westerlies and concurrent warming in the southern subtropics. 1347 The limitations identified by the reviewers and acknowledged by the authors do not 1348 appear to invalidate the study’s primary interpretations or conclusions. Instead, the 1349 authors argue that their multi-proxy approach and focus on relative trends provide a 1350 robust framework that survives these methodological constraints. They explicitly 1351 acknowledged that using an Atlantic-based Mg/Ca calibration for Pacific samples may 1352 introduce a \"modest cold bias,\" but argued that the relative temperature trends remain 1353 robust. Furthermore, they addressed the \"missing\" carbon isotope ventilation signal by 1354 clarifying that is often overprinted or attenuated by remineralization and mixing during 1355 the long, circuitous transport to the Western Equatorial Pacific. 1356 By explicitly adding the limitations (such as the conceptual nature of the wind estimates 1357 and the calibration biases) into the revised manuscript, the authors have shifted these 1358 points from \"fatal flaws\" to \"areas for future work\". This transparency allows the 1359 scientific community to accept the current findings as a valid hypothesis supported by 1360 existing data, even if a perfect, multi-dimensional model or regional calibration is not 1361 yet available. In summary, while the work has inherent limitations (standard for 1362 paleoceanographic reconstructions), the authors successfully demonstrate that their 1363 conclusions are the most consistent explanation for the observed divergent temperature 1364 and salinity trends at Site U1489. 1365 Response: We thank Reviewer #5 for the careful and thoughtful assessment of the 1366 revised manuscript and for recognizing that the major methodological concerns raised 1367 in the first round of review have been largely addressed through Mg/Ca data 1368 reprocessing, proxy validation, and manuscript restructuring. We also appreciate the 1369 reviewer’s assessment that the remaining uncertainties do not undermine the central 1370 interpretation. 1371 Recommendations prior to publication: 1372 Based on the revisions made and the scientific justifications provided in the authors' 1373 responses, a minor follow-up review of the manuscript would further strengthen the 1374 work for publication. 1375 • In the Abstract or Introduction: Introduce the \"Trends vs. Absolute Values\" 1376 distinction 1377 A non-expert may focus on the specific temperature values (e.g., the ~1.8 °C 1378 warming) without realizing the methodological limitations behind those numbers. The 1379 authors admit in the supplement that using an Atlantic-based calibration for Pacific 1380 samples may introduce a \"modest cold bias\" in absolute temperatures. Explicitly 1381 stating early on that the study focuses on relative temperature change amplitudes and 1382 long-term trends rather than absolute values would prevent a non-expert from over- 1383 interpreting the specific degrees Celsius reported. The manuscript would benefit from 1384 this clarification near the end of the abstract or at the end of the Introduction where 1385 the proxies are first introduced. 1386 Response: We thank the reviewer for this helpful suggestion. In the revised manuscript, 1387 a concise clarification has been added near the end of the Abstract and reiterated in the 1388 Introduction, emphasizing that our interpretations focus on relative temperature 1389 changes and long-term trends rather than absolute values. This approach prevents 1390 potential over-interpretation by non-expert readers. Please see details on Lines 29–31 1391 and Lines 87–90 of the revised manuscript. 1392 • Sensitivity Analysis of Mg/Ca Calibrations 1393 In the methods, the authors justify using an Atlantic-based calibration for G. 1394 crassaformis because no Pacific-specific calibration exists. While they show that the 1395 Cléroux et al. (2013) calibration best fits modern hydrography, it still introduces a 1396 \"modest cold bias\". The manuscript would benefit from a small table or text block in 1397 the results comparing how much the total warming amplitude (e.g., 1.8 °C vs. 2.5 °C) 1398 fluctuates depending on the calibration used. This would demonstrate that the primary 1399 conclusion—significant post-MBT warming—is independent of the specific equation 1400 selected. 1401 Also, mentioning briefly that a Pacific-specific calibration for G. crassaformis does not 1402 exist in the Results would clarify that while the 1.8 °C rise is robustly observed across 1403 multiple calibrations, the baseline temperature relies on an Atlantic model. 1404 Response: Following the reviewer’s recommendation, a sensitivity comparison has 1405 been added to the Results and Discussion (Lines 146–149), Supplementary Material 1406 3 (Lines 917–931), and Supplementary Table 1 (Lines 1131–1133), showing how post- 1407 MBT bottom-thermocline warming varies across those published calibrations. While 1408 absolute BTT values differ (1.8–4.3 °C; Table R1), all calibrations yield a consistent 1409 warming signal, indicating that the inferred post-MBT warming is independent of the 1410 specific equation selected. The Results and Discussion now explicitly note that no 1411 Pacific-specific Mg/Ca calibration exists for G. crassaformis and that absolute 1412 estimates rely on Atlantic-based formulations. The original G. crassaformis Mg/Ca 1413 values (Table R1) also show a post-MBT increase that is unaffected by the choice of 1414 temperature equation (0.35±0.004 mol/mol). In addition, we conducted an independent 1415 cross-check using published constraints on the calcification depth of G. crassaformis 1416 against modern temperature profiles in both the Pacific and Atlantic (Supplementary 1417 Material 3, Lines 909–916), supporting the cross-basin comparability of the 1418 reconstructed temperatures (Fig. R1e, f; Supplementary Fig. 1e, f, Lines 1002–1022). 1419 Table R1. Pre- and post-MBT averages of raw Mg/Ca ratios and Mg/Ca-derived temperatures 1420 from multiple calibration equations for G. crassaformis and G. tumida. 1421 Mg/Ca ratio 1422 Post- MBT Average variations 1423 Species & Pre- MBT average 1424 averages (Post-MBT minus Pre-MBT) 1425 Calibrations 1426 Mg/Ca 1.24 mol/mol 0.88 mol/mol 0.35±0.004 mol/mol, 1σ 1427 ref. 22 6.4 ℃ 4.6 ℃ 1.8±0.1 ℃, 1σ 1428 G. ref. 21 11.8 ℃ 7.5 ℃ 4.3±0.2 ℃, 1σ 1429 crassaformis ref. 20 6.7 ℃ 4 ℃ 2.7±0.1 ℃, 1σ 1430 ref. 19 14.8 ℃ 12.3 ℃ 2.5±0.2 ℃, 1σ 1431 Mg/Ca 1.86 mol/mol 2.02 mol/mol −0.16±0.012 mol/mol, 1σ 1432 ref. 1 17.7℃ 18.6 ℃ −0.8±0.2 ℃, 1σ 1433 G. tumida ref. 19 17.6 ℃ 18.5 ℃ −0.9±0.2 ℃, 1σ 1434 ref. 17 22.1 ℃ 23.3 ℃ −1.2±0.3 ℃, 1σ 1435 ref. 18 13.9 ℃ 14.8 ℃ −0.9±0.2 ℃, 1σ 1436 Fig. R1 Modern hydrographic context for IODP Site U1489 in a regional and basin-scale 1437 framework. a Annual mean sea surface temperature (SST, color-shaded) in the Pacific from the 1438 World Ocean Atlas 2023 (WOA32). White lines outline the Indo-Pacific Warm Pool (SST = 28 °C). 1439 Grey arrows indicate annual mean wind stress from NCEP/NCAR reanalysis data33. The white 1440 rectangle in panel a marks IODP Site U1489. Filled circles denote reference sites for Northern 1441 westerly winds (blue), equatorial Pacific (red), southern Subtropical Front (orange), and 1442 Subantarctic Front (white). b Vertical temperature distributions in the IODP Site U1489. The yellow 1443 and blue rectangles denote calcification depths of G. tumida1 and G. crassaformis6–8, respectively. 1444 c Vertical distribution of seawater salinity (color-shaded) in the modern Pacific Ocean above 2000 1445 m along a meridional section crossing IODP Site U1489 (inset map), based on WOA 2023 data34. 1446 White triangle and arrow indicate core locations and circulation features, respectively. Schematic 1447 drawings show the planktonic foraminifera species G. tumida and G. crassaformis. d Vertical 1448 distribution of δ13C profiles (color-shaded) in the modern Pacific Ocean above 2000 m, along a 1449 meridional section crossing IODP Site U1489 (inset map), based on data from the GLODAP 1450 datasets35. Meridional temperature transects across the Pacific e and the Atlantic f. Black squares in 1451 panel e and f denote the calcification depth of tropical G. crassaformis6,7. All plots were generated 1452 using Ocean Data View (ODV36). Original data sources are listed in Supplementary Table 2. 1453 • Quantification of Vertical Stratification 1454 Reviewer #1 specifically requested the calculation of the gradient between P. 1455 obliquiloculata (upper thermocline) and G. crassaformis (bottom thermocline) as a 1456 proxy for stratification. While the authors argued that plotting the species together on 1457 one axis is sufficient to show stable vertical ordering, providing a numerical time-series 1458 of this gradient would offer future researchers a reproducible index of water-column 1459 structure. It would also more rigorously support the authors’ claim that the observed 1460 warming is not a result of increased local vertical mixing. 1461 Response: In response to the reviewer’s constructive recommendation, a time series of 1462 δ18O gradients between P. obliquiloculata and G. crassaformis has now been 1463 incorporated (Fig. R2e) into the main text as an explicit proxy for upper-to-bottom 1464 thermocline stratification. Following the reviewer’s suggestion, it is now presented in 1465 the revised main text to improve accessibility (Supplementary Fig. 5e; Lines 156–159 1466 and Lines 1058–1067). Importantly, these gradients demonstrate that the vertical 1467 differences among the mixed layer, upper thermocline, and deep thermocline remain 1468 stable across the MBT, while the progressive reduction in gradients involving G. 1469 crassaformis reflects a systematic lightening of bottom-thermocline δ18O. This pattern 1470 directly supports that the observed bottom-thermocline warming is not attributable to 1471 enhanced local vertical top-down mixing, but instead reflects changes in the properties 1472 of southern-sourced subsurface waters. 1473 Fig. R2 Isotope records from IODP Site U1489. a δ13C of Globigerinoides ruber (gray) and 1474 Pulleniatina obliquiloculata (gold)45. b δ13C of G. tumida and G. crassaformis. c δ18O of G. ruber, 1475 P. obliquiloculata, G. tumida (blue) and G. crassaformis (red). d δ18O of G. tumida (blue) and 1476 sw-ice 1477 G. crassaformis (red). e δ18O gradient between mixed-layer and thermocline (δ18O 1478 (gray), δ18O (blue), and δ18O (red)). The thin, medium, 1479 P. obliquiloculata-G. ruber G. tumida-G. ruber G. crassaformis-P. obliquiloculata 1480 and thick curves in a–e represent the original data, 3-point running average, LOESS Gaussian fit 1481 (span = 0.5), respectively. Vertical gray shadings indicate glacial periods. Numbers on the top of 1482 panel a label Marine Isotope Stages (MIS). References are listed in the Supplementary Materials 1483 section of the main text. 1484 • In the Discussion: The \"Missing\" Carbon Signal 1485 For a reader looking for clear evidence of Southern Ocean water, the absence of the 1486 standard carbon isotope signature could be confusing. Explicitly labelling that this is 1487 a known limitation of the δ13C tracer in this region (incl. references) would help a non- 1488 expert understand why the authors rely more heavily on physical properties like 1489 temperature and salinity. Perhaps at the beginning of the \"Southern origin of the IPWP 1490 bottom thermocline warming\" section would be a good place for this addition. 1491 Response: Thanks very much. To address this potential source of confusion, we have 1492 added a short paragraph early in this section, at the start of the discussion of water-mass 1493 advection, explicitly noting that attenuation of the expected “young, high-δ13C” signal 1494 during long-distance transport is a well-recognized feature of this region, with 1495 appropriate references. This clarification helps explain why physical properties such as 1496 temperature and salinity provide more robust indicators of SAMW/AAIW influence at 1497 Site U1489. Please refer to Lines 212–214 in the revised manuscript. 1498 • Explicit Integration of Wind Proxy Limitations 1499 Currently, the admission that the temperature gradient between the Subtropical and 1500 Subantarctic Fronts is a \"simplified conceptual estimate\" for westerly wind strength is 1501 located in the Supplementary Material. Because the \"strengthened westerlies\" are a 1502 central driver of the proposed heat-transport mechanism, this caveat should be moved 1503 to the main text. This ensures a non-expert reader understands that the wind-driven 1504 aspect of the hypothesis remains an area of ongoing scientific debate rather than a 1505 settled quantification. 1506 Response: As suggested by the reviewer, the statement describing the STF–SAF SST 1507 gradient as a simplified, conceptual indicator of westerly wind strength has been moved 1508 from the Supplementary Material to the main Discussion, ensuring that wind forcing 1509 is presented as inferred rather than directly quantified. Kindly refer to Lines 182–186 1510 in the revised manuscript. 1511 • Expansion of Future Modeling Goals 1512 Reviewer #3 correctly noted that paleoclimate modelling would significantly bolster 1513 the proposed mechanism. Although the authors argue this is out of scope for the current 1514 paper, the manuscript’s conclusion would be more impactful if it included a specific 1515 sentence outlining exactly what future models should test—for example, sensitivity 1516 experiments on Subantarctic Mode Water (SAMW) subduction rates in response to 1517 shifting westerly wind belts. 1518 Response: Thank you for the suggestion. We agree with your point. A sentence has 1519 been added to the final paragraph outlining how future paleoclimate model experiments 1520 could test the sensitivity of SAMW/AAIW subduction and equatorward heat transport 1521 under MBT-like boundary conditions (please see Lines 266–268). 1522 • Final Technical and Editorial Polishing 1523 Despite the authors' claim of a \"line-by-line\" implementation of minor edits, several 1524 small errors remain: 1525 Abstract Grammar: The subject-verb disagreement (\"mechanisms... remains elusive\") 1526 should be corrected to \"remain\". 1527 Uncertainty Reporting: While the authors state that RAMPFIT-derived uncertainties 1528 are reported in figure captions, they should ensure that numerical values (e.g., ± ka) 1529 are explicitly listed in the caption of Figure 3 to match the crosses shown in the plot. 1530 Typo Consistency: Ensure \"41-kyr\" is used consistently throughout, as \"40-kyr\" still 1531 appears in some sections of the revised text. 1532 Response: All remaining technical and editorial points have been addressed, including 1533 correction of subject–verb agreement in the Abstract, explicit reporting of RAMPFIT- 1534 derived age uncertainties in figure captions, and consistent use of “41-kyr” terminology 1535 throughout the manuscript. 1536 Comments on “Equatorward upper-ocean heat transport from the Southern Ocean 1537 boosted interglacial warming” NCOMMS-25-35512 1538 Yang et al. present planktic foraminiferal elemental (Mg/Ca) and isotope (δ18O and 1539 δ13C) records since ~670ka from IODP Sites U1489 in the equatorial west Pacific 1540 Ocean, which demonstrate deep and bottom thermocline temperatures. The authors 1541 further infer advection of enhanced subduction and advection of Subantarctic Mode 1542 Water (SAMW), and contributed to amplified interglacial warmth post- Mid-Brunhes 1543 Transition (MBT). 1544 The positive aspects represent that the records are novel and significant, not limited 1545 to only evoking deep water ventilation or CO₂ feedbacks; the authors highlight changes 1546 in upper- to intermediate-ocean circulation, linking these with Southern Hemisphere 1547 ocean-atmosphere dynamics. A clear divergence is documented between deep 1548 thermocline temperature (DTT) and bottom thermocline temperature (BTT) in the 1549 Western Equatorial Pacific since the Mid- Brunhes Transition (MBT; ~430 ka). 1550 Specifically, DTT shows a gradual cooling trend, while BTT warms by ~4 °C. This 1551 warming of the bottom thermocline is interpreted as evidence of intensified 1552 equatorward heat transport from the Southern Ocean, particularly via the Subantarctic 1553 Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). 1554 Well, this study will hold substantial importance for both the paleoclimate and 1555 oceanographic sectors. I believe these are the new records from the equatorial East 1556 Pacific Ocean, which will have a major impact. However, I believe that an extensive 1557 literature study and the comparison of data concerning East Pacific thermocline 1558 dynamics could address some gaps and facilitate additional findings (see major 1559 comment 3 and 4). 1560 Globorotalia crassaformis derived Mg/Ca temperature may be a concern to rethink 1561 (see major comment 1). 1562 Yes. The methods are clearly described and meet or exceed current standards in the 1563 field. 1564 From my point of view, this manuscript requires further modification for enhanced 1565 improvement. Therefore, I have provided some comments; please review them. 1566 1567 Major Comments: 1568 1. In line 71, you stated that the depth habitat of Globorotalia crassaformis is ~300– 1569 400 m. Then in lines 96-97, you stated “small-amplitude (±2°C) variations from ~670 1570 ka to 470 ka”. Provide a depth vs. temperature profile of the study Site in the 1571 supplementary file. Do you believe it is feasible to reach ~2°C at a depth of 400 m in 1572 the equatorial West Pacific Ocean between ~670 and 470 ka? 1573 I believe there is a possibility of diagenesis affecting Globorotalia crassaformis; 1574 please verify this with the diagenesis test on those specimens. The justifications are as 1575 follows. 1576 • According to Rosenthal et al. (2018) (Figure 4), at Site U1489, modern 1577 temperatures vary from around 7°C to 2°C at depths ranging from ~500 to 1500 m. 1578 Though you suggest 400 m, Globorotalia crassaformis can inhabit depths of up to 600 1579 m (Steph et al., 2009; Cléroux et al., 2013), although Globorotalia truncatulinoides 1580 may reside at even greater depths. This raises issues regarding the depth habits of 1581 Globorotalia crassaformis in your temperature determination. 1582 • The δ18O of Globorotalia crassaformis fluctuates between about -1.5 and 1.5‰ 1583 residual 1584 (Supplementary Fig. 5e), which is huge and suggesting a less equilibrium 1585 calcification. 1586 • Using Atlantic-based calibration (Line 255) may result in inaccuracies when 1587 measuring the temperature of the Pacific bottom thermocline. 1588 2. To justify “..upward movement of the calcification depth of G. 1589 crassaformis ..(Lines 104- 105)” you stated “G. crassaformis δ13C parallels with that 1590 of G. tumida… indicating a persistent ecological gradient between G. 1591 crassaformis..(Lines 105-110)”. This seems acceptable. 1592 But I am wonder when you discussed and linked BBT warming to strengthened SAMW 1593 in ‘Southern origin of the IPWP bottom thermocline warming’ section. In lines 135-138, δ13C 1594 signal is ignored by simple statement of “However, an increased subduction of SAMW would 1595 imprint a positive signature on δ13C of bottom thermocline waters in the IPWP via ventilation, 1596 which, however, is not observed”. Whereas, Spero et al. (2003) suggested that a stronger 1597 SAMW causes changes in nutrient and carbon isotopic chemistry. How would you justify 1598 strengthened SAMW did not affect δ13C signal at Site U1489? 1599 3. Figure 1b indicates that the average temperature of BTT was ~2ºC (with a minimum 1600 of approximately 1ºC) from ~670 ka to 470 ka, followed by a subsequent increase in 1601 average temperature to 7ºC. In the Figure 4 Schematic illustrating you have shown 1602 Globorotalia crassaformis inhabited within the SAMW since ~670 ka. How would you 1603 justify that average temperature of the SAMW could be at ~2ºC (minimum of ~1ºC) in 1604 the IPWP from ~670 ka to 470 ka? Or it was AAIW? 1605 The question remains: how could the SAMW temperature reach ~2ºC at 400 m? As 1606 previous studies suggest SAMW fluctuates between 3 to 9ºC throughout glacial-interglacial periods, 1607 while AAIW ranges from 2.5 to 5.5ºC in the equatorial Pacific Ocean (McCartney 1977; 1608 Herraiz-Borreguero and Rintoul, 2011; Elmore et al. 2015). In addition, model reconstructions 1609 consistently show subsurface tropical temperatures did not cool enough to approach 2°C at 400 1610 m depth (Lea et al., 2000; Medina-Elizalde and Lea, 2005; Regenberg et al., 2006). 1611 In lines 162-166, “the substantial warming of the IPWP bottom thermocline from MIS13/12 1612 (~470 ka) is associated with intensified AAIW transport and shoaled advection of the SAMW”. 1613 Please ensure to substantiate a temperature range of 2 to 7ºC with the advection of 1614 warm SAMW. This large temperature increase in bottom thermocline cannot fully 1615 justified by warming SST from other locations. Though, you provided AAIW 1616 temperature from Site 593, it will be better if you could refer a thermocline record from 1617 the Southern sector of the Pacific Ocean where similar warming (~4ºC) trend is 1618 observed for SAMW? 1619 4. The two mechanisms appear to involve concurrently within the same timeframe, suggesting 1620 that DDT cooling is associated with “Pacific Subtropical Cells, which would 1621 promote subduction of cooler waters from higher latitudes into the deep thermocline of 1622 Western Equatorial Pacific (Lines 93-95)” while simultaneously, BBT warming is 1623 linked to “..intensified AAIW transport and shoaled advection of the SAMW..(162- 1624 166)”. There needs to be some justification rather than speculation. 1625 These mechanisms are described with sea surface temperature (SST) from different 1626 location, though this work focused on thermocline dynamics. Most importantly, 1627 thermocline temperature form the East Pacific is ignored, while records are available 1628 in literature (Ronge et al., 2022). 1629 1630 Minor Comments: 1631 Line 77: Remove the word “Significant”. Could not find any significant correlation. 1632 1633 Lines 93–95: To interpret the DTT cooling reflects a contraction of the Pacific Subtropical 1634 Cells; references 24 and 25 are used. Could you please explain the reasoning behind comparing 1635 DDT with SST when East Pacific DTT records are available? I observed a decrease in DDT in 1636 the equatorial East Pacific Ocean as reported by Ronge et al. (2022). In addition, “which would 1637 promote subduction of cooler waters from higher latitudes into the deep thermocline of 1638 Western Equatorial Pacific” is not justified; please clarify with taking account of records from the 1639 East Pacific DTT. 1640 1641 Lines 99-100: “extratropical and possibly higher latitudes” not clear. 1642 1643 Lines 105-110: “..indicating a persistent ecological gradient between G. crassaformis and the 1644 shallower dwelling species” only δ13C signal cannot justify this statement. Because if you look 1645 into δ18O Globorotalia crassaformis (Supplementary Fig. 5e) which varies from ~-1.5 to 1646 residual 1647 1.5‰, it suggests that the foraminiferal δ¹⁸O signal is influenced by more than just temperature 1648 and δ¹⁸Osw likely reflecting changes in calcification depth, vertical migration, or ecological 1649 adaptation. Please take an account of δ18O 1650 residual. 1651 1652 Lines 156-157: “As a result, the SAMW would be compressed, and warm SAMW are 1653 157 transported to the bottom thermocline of the IPWP” This statement is not fully justified? 1654 1655 Lines 181-182: “imply an increase in heat capacity of the upper- to intermediate ocean for the 1656 significant post-MBT interglacial warming” When your DDT is showing a steady cooling 1657 whereas BBT is progressive warming, it is very difficult to state upper- to intermediate ocean 1658 dynamic with ocean heat capacity within IPWP. I wonder it could be link to equatorial 1659 dynamics rather than link to high latitudes. 1660 1661 Lines 253-256: The calibration used for G. crassaformis (Atlantic-based; Line 254) 1662 raises questions about regional accuracy. While the authors acknowledge the lack of a region-specific 1663 calibration, this limitation should be explicitly discussed in terms of potential bias or 1664 temperature under/overestimation. 1665 1666 Lines 258–261: The δ¹⁸Osw calculation is methodologically appropriate, but the larger 1667 uncertainty in G. crassaformis (±0.10‰) might significantly affect salinity inference, 1668 particularly when the δ¹⁸Osw difference between species is small (Lines 183–186). 1669 1670 REVIEWER COMMENTS 1671 Reviewer #5 (Remarks to the Author): 1672 Besides, some small editorial issues listed below, I’m happy with how the authors have 1673 addressed the reviewers’ comments and concerns. I recommend the manuscript for 1674 publication. 1675 Response: We thank the reviewer for the positive evaluation of our revision and for 1676 these helpful editorial suggestions. We have carefully revised the manuscript 1677 accordingly. Our point-by-point responses are provided below. 1678 1679 (Line 47): The manuscript mentions that water masses \"regulate transports of heat\". I 1680 recommend revising to \"the transport of heat,\" which is used more accurately 1681 elsewhere in the text (e.g., Line 2). 1682 1683 Response: We thank the reviewer for this suggestion and agree with the revision. The 1684 phrase has been corrected from “regulate transports of heat” to “ventilate the ocean 1685 interior with varying heat, carbon, and nutrients” in the revised manuscript. Kindly refer 1686 to Lines 48–49 in the revised manuscript. 1687 1688 (Line 212): The sentence \"It is noted that, a 'young, high-' signature... is absent\" 1689 contains an unnecessary comma after \"that\" which breaks the natural flow of the 1690 sentence. 1691 1692 Response: We agree and have removed the unnecessary comma after “that”. The 1693 sentence has been revised accordingly in the manuscript. Please see details in Lines 1694 212-214 of the revised manuscript. 1695 1696 (Line 215): The text reads, \"This is mainly because that, as SAMW and AAIW are 1697 transported northward...\". The word \"that\" is grammatically redundant and should be 1698 deleted to read: \"This is mainly because, as SAMW and AAIW...\" 1699 1700 Response: We thank the reviewer for catching this error. We have deleted the 1701 redundant “that”, and the sentence now reads: “This is mainly because, as SAMW and 1702 AAIW are transported northward...”. Please refer to Lines 215–218 in the revised 1703 manuscript. 1704 1705 (Line 247): The authors refer to \"enlarged saline gradients\". In oceanographic 1706 literature, \"saline\" is an adjective describing the nature of the water, whereas \"salinity 1707 gradients\" is the standard noun phrase for the measurement being described. 1708 1709 Response:We agree with the reviewer. The expression “enlarged saline gradients” has 1710 been revised to “enhanced salinity gradients” in the revised manuscript (please see 1711 Lines 247–248). 1712 1713 (Lines 262–263): The sentence \"The proposed feedback, stronger Southern westerly 1714 winds intensify upper- to intermediate-ocean circulation and sustain northward ocean 1715 heat transport, resonates with...\" uses a comma to set off a complex independent clause. 1716 This creates a \"comma splice\" effect. It should be set off by dashes or a \"that\" clause 1717 (e.g., \"The proposed feedback—in which stronger... transport—resonates with...\"). 1718 1719 Response: We thank the reviewer for this helpful suggestion. We have revised the 1720 sentence to avoid the comma splice and improve clarity. It now reads: “The proposed 1721 feedback that stronger Southern westerly winds intensify upper- to intermediate-ocean 1722 circulation and sustain northward ocean heat transport resonates with...” (see Lines 1723 262–264)."}]

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