A contracting Intertropical Convergence Zone during the Early Heinrich Stadial 1

Domain: earth_science
Published: 2023-08-04
Editorial outcome: major_revision

Abstract
Despite the fact that the response of tropical hydroclimate to North Atlantic cooling events during the Heinrich Stadial 1 (HS1) has been extensively studied in African, South American and Indonesia, the nature of such responses remains debated. Here we investigate the tropical hydroclimate pattern over the Indo-Asian-Australian monsoon region during the HS1 by integrating hydroclimatic records, and examining a δ18O record from Globigerinoides ruber (white) seawater in the tropical Indian Ocean. Our findings indicate that tropical hydrological conditions were synchronously arid in both hemispheres during the early HS1 (~18.3-16.3 ka) in the Indo-Asian-Australian monsoon region, except for a narrow, wet hydrological belt in northern low latitudes, suggesting the existence of a contracted tropical precipitation belt at that time. This study reveals that the meltwater discharge and resulting changes in global temperatures and El Niño exerted a profound influence on the tropical hydroclimate in the Indo-Asian-Australian monsoon region during the early HS1.

Introduction
During the HS1 (~19–15 ka)1, the North Atlantic region experienced a significant discharge of icebergs and a drastic reduction in the Atlantic meridional overturning circulation (AMOC). The impact of this abrupt cooling in the North Atlantic on the tropical rainfall system has been studied through the analyses of paleoclimatic records and model simulations2,3. Previous research has suggested that the mean position of the Intertropical Convergence Zone (ITCZ) rain-belt shifted southward in response to the cooling in the Northern Hemisphere during the HS14,5. However, evidence from paleoclimatic records in southern Africa6, the southern Indian Ocean7–9 and the southern tropical West Pacific10–12 has shown that severe drought conditions also existed in the southern hemisphere during the HS1 (Fig. 1). McGee et al.13 also argued that the mean ITCZ shifts were less than 1 degree of latitude during the HS1 based on the model results. Furthermore, studies in the southern South China Sea (SCS)14, Flores Sea15 and Northeast Brazil16 have revealed a two-phase structure of hydroclimatic change in the tropics during the HS1, with ITCZ rainfall strengthening (weakening) in the Early HS1 (~19.0–16.1 ka) and becoming weak (strong) during the Late HS1 (~16.1–14.7 ka) in the tropical northern (southern) hemisphere.

Consequently, the direction and magnitude of the shift of the ITCZ in response to North Atlantic cooling events during the HS1 remain controversial5. It is increasingly challenging to explain changes in tropical hydrological climate during the HS1 solely through the mechanisms of ITCZ southward migration. Additionally, the lack of paleoclimatic records from the tropical Indian Ocean, which was influenced by ITCZ precipitation, has severely limited our understanding of the responses of tropical hydroclimate to North Atlantic cooling during the HS1.

Our reconstruction of sea surface temperature (SST) and δ18O (δ18O ) changes relies on the Mg/Ca and δ18O records of planktonic foraminifera Globigerinoides ruber sensu stricto (s.s.) obtained from a deep-sea core located in the southern Bay of the Bengal (BoB). Modern moisture flux observations show that precipitation arrives year-round at this site, with the majority occurring in the latter half of the year (May–December) (Supplementary Fig. 1), correlating with the movements of the ITCZ17. Hence, the location of the study site (Fig. 1, Core I106; 6°14′49.76″N, 90°00′1.04″E; 2,910 m water depth) makes it an ideal location to monitor shifts in the tropical rainfall belt. We assume that precipitation in our study area was mainly controlled by the Indian Ocean Summer Monsoon (IOM) and the ITCZ rain belt system during HS13. We integrated the available hydroclimatic records from a latitudinal transect across the Indo-Asian-Australian (IAA) monsoon region with our results in order to evaluate the responses of the tropical hydrological cycle to the abrupt-onset HS1 cold event that occurred in the high latitudes of the Northern Hemisphere.

Results
δ18O reconstruction as a salinity proxy
The plankton tow samples from the study area indicate that G. ruber is mainly distributed in water depths of 0–50 m, and that it can therefore be classed as a mixed-layer species18. G. ruber δ18O values in Core I106 become gradually negative from −1.09‰ at ~24.0 ka to −2.80‰ at ~1.84 ka, but exhibit an abrupt decline at 18.3–16.3 ka, with a mean value of −1.67‰ (Fig. 2). The Mg/Ca-SSTs from Core I106 show a rapid and steep increase around 19.5 ka, consistent with previous records conducted from the tropical Eastern Indian Ocean3,19 (Fig. 2). The Mg/Ca-SST in Core I106 indicates an increase of about 0.5 °C at 16.3–18.3 ka, which corresponds to a decrease of ~0.12‰ in δ18O (assuming a change of ~0.23‰ in δ18O per 1 °C). Hence, the decrease in δ18O ruber value is primarily attributed to changes in seawater salinity in Core I106. We calculated the δ18O values of Core I106 from Mg/Ca-SST and δ18O using the equation of Bemis et al.20 (see “Methods”), which reflects the sea surface salinity (SSS) associated with regional hydrological changes. Similarly, the most striking characteristic of the calculated δ18O values in Core I106 is an exceptionally abrupt decline at 18.3–16.3 ka (Fig. 2).

The observed SSS and δ18O values in the southern BOB21–23, equatorial East Indian Ocean22, and Andaman Sea24 demonstrate that δ18O values have a linear correlation with salinity in our study area (Supplementary Fig. 2 and Supplementary Dataset 1). Our estimates of δ18O values during the Late Holocene (2–0 ka) fall well within this linear δ18O -salinity correlation (Supplementary Fig. 2). The reconstructed δ18O values for Core I106 are therefore also likely to indicate a regional SSS signal, which is related to varying quantities of fresh surface water.

Wet hydrological conditions in the northern low latitudes during the Early HS1
Multiple δ18O records from the northern BOB25–27 and the northern Arabian Sea28,29, speleothem30 and lake sediment31,32 records from Southern China, and paleoclimatic records from the northern SCS33, all consistently suggest that the hydrological conditions were extremely dry and long-lasting throughout the HS1 in the IAA monsoon region (Figs. 1 and 3 and please see Supplementary Table 1). Many studies have attributed the drought conditions during the HS1 to the retraction of the Asian Summer Monsoon and the southward drift of the ITCZ, which were responses to the cooling in the North Atlantic Ocean during the HS134. However, multiple paleoclimatic records from the equatorial and southern Indian Ocean3,9,35,36 and southern Indonesia11,12,37 also showed that dry conditions were prevalent throughout the entire HS1 period (Figs. 1 and 3). Furthermore, paleoclimatic records from Africa documented a catastrophic drought in Equatorial and Southern Africa at ~17–16 ka6. Therefore, the latitudinal movement of the tropical rain-belt cannot fully explain the hydroclimatic changes observed in the IAA monsoon region during the HS1.

Interestingly, our δ18O record from the tropical BoB exhibited a significant negative shift in the Early HS1 (18.3–16.3 ka), indicating a sudden decrease in SSS and an increase in fresh surface water input (Fig. 3c). The SSS in the BoB is primarily influenced by freshwater discharge and direct precipitation over the ocean24. However, δ18O records from the northern BOB25–27 and lake-sediment records from southwestern China31 revealed that there was weak monsoonal precipitation and thus reduced river runoff inflow into the BOB throughout the HS1 period, (Figs. 1 and 3b). Similarly, δ18O records from offshore Sumatra also indicated a drought during the HS1 period3. Therefore, the increased fresh seawater at Core I106 at about 18.3–16.3 ka was unlikely to have originated from the northern BoB or the south of Sumatra via currents. Additionally, modern hydrological data in the study area suggest that SSS is closely related to precipitation (Supplementary Fig. 1). Furthermore, the Δδ18O archive from Core 758 (5°23.5’N, 90°21.67’E), adjoining Core I106, indicated a general weakening of IOM intensity during the entire HS138. This suggests that there were no significant changes in water stratification at 18.3–16.3 ka. Therefore, the changes in δ18O and SSS at Core I106 during the Early HS1 period are most likely associated with variations in tropical precipitation.

Likewise, the δ18O record from Core ADM-159 (9.27°N, 95.61°E) in the southern Andaman Sea exhibited a significant negative anomaly at about 17.0–18.7 ka39 (Supplementary Fig. 4). The reconstructed SSS values from Core SK129/CR04 (6°29.65’N, 75°58.68’E) in the Equatorial Arabian Sea also indicated a low salinity event at 19.5–16.5 ka40 (Fig. 3e). The δ18O records of multiple planktonic foraminiferal species from the Equatorial Arabian Sea also revealed a negative peak at around 19.0–17.0 ka, which has been attributed to a stronger winter monsoon current41,42. However, the δ18O values in Core SK218/1 from the western BOB, which was influenced by EICC, increased significantly throughout the entire HS1, indicating a weak winter monsoon current25 (Fig. 1). Moreover, if the winter monsoon current had strengthened, more saltwater would have been transported from the south along Sumatra to our study area; on the contrary, the δ18O values at Core I106 declined a lot during the Early HS1. We therefore suggest that the negative δ18O records of planktonic foraminiferal in the Equatorial Arabian Sea during the Early HS1 may also be associated with increased tropical precipitation. Additionally, evidence provided by grain-size populations, dry bulk density, mass accumulation rates, and Si/Al ratios from Core CG2 (6.3928°N, 110.1542°E)14 in the southern SCS suggested strong precipitation during 19.0–18.0 ka and 17.5–16.1 ka (Fig. 3d). In the Sulu Sea, the δ18O records from Core MD97-2141 (8.8°N, 121.3°E)43 indicate that surface water in the Early HS1 was fresher than that during the Late HS1 (Fig. 3f). The X-ray fluorescence-derived log (Fe/Ca) records from MD06-3075 (6°29’N, 125°50’E) at Mindanao, which is a reliable proxy for freshwater runoff, also indicated increased precipitation at Mindanao at 15.7–17.8 ka, but with dry conditions in Borneo and China during this interval44 (Supplementary Fig. 4). The aforementioned records from the northern low latitudes support the notion that tropical precipitation intensified significantly during the Early HS1.

Our newly-integrated paleoclimatic records from the IAA monsoon region therefore reveal that there were mostly drought hydrological conditions in both the northern and southern hemispheres in the Early HS1. However, a wet hydrological condition was identified at ~3–9°N. This evidence suggests a possible contraction of the tropical convection precipitation region during this period.

Possible mechanisms controlling tropical hydroclimatic changes in the Early HS1
Previous studies have reported that the collapsed AMOC and cooling in the Northern Hemisphere during the HS1 resulted in an increase in interhemispheric temperature gradient, leading to a southward shift of the ITCZ4. Model results from the tropical East Indian Ocean suggested that there were drier conditions over the equatorial and north Indian Ocean, and more humid conditions in southern Indonesia, due to the southward displacement of the ITCZ during the HS13. However, our new paleoclimatic records from the northern low latitudes support the existence of a two-phase structure of tropical hydroclimate during the HS1, with remarkable humid conditions occurring in the Early HS1. Paleoclimatic records in Core VM33-80 in south Indonesia show an arid hydrological condition in the early phase of the HS1, and a humid hydrological condition at 16–14.5 ka15. δ18O records in cores MD98-216535, MD01-237810,11, GeoB10069-337 from southern Indonesia, and stalagmite δ18O record from Ball Gown45 all indicated dry hydrological conditions in the early phase of the HS1, which is also supported by paleo-records from the southwest Indian Ocean7,9,36 (Fig. 1). Therefore, variations in tropical precipitation patterns are not only affected by the interhemispheric temperature difference in the IAA monsoon realm, but also associated with other driving factors. In recent years, increasing evidence suggests a hemi-spherically symmetric contraction of tropical precipitation in response to glacial cycle drivers46. Model simulations from Africa have shown that precipitation coherency decreased in both southeastern Equatorial and Northern Africa in response to meltwater-induced reductions in the AMOC during the early phases of the last deglaciation47. Yan et al.48 also pointed out that the latitudinal range of ITCZ rainfall in the Western Pacific contracted over decadal to centennial timescales in response to a cold climate during the Little Ice Age (LIA). Stalagmite record from southwest Madagascar have also shown that the tropical rain-belt simultaneously expands or contracts in both hemispheres in the past49.

Numerous studies have reported an abrupt and early ice recession in the European Ice Sheet during the first part of HS1, leading to meltwater discharge into the Eastern North Atlantic Ocean50,51 (Fig. 4a). This event had a significant impact on the climate both on land and in the ocean52. Evidence from the North Atlantic suggests that the early reduction in AMOC at ~19–16.5 ka was initiated and sustained by the enhanced melting water from Eurasian ice sheets1. Additionally, the melting water from the Laurentide Ice Sheet caused a further reduction in AMOC at ~16.5–15 ka1,53,54 (Fig. 4b, c). The tropical hydroclimate within HS1, located in the northern low latitudes of the IAA monsoon region, also exhibited two distinct phases. Wet hydrological conditions were observed at about 18.3–16.3 ka, followed by dry conditions at ~16.3–14.7 ka, which was consistent with the two-step AMOC slowdown related to meltwater from different ice sheets (Fig. 4e).

During the Early HS1, the cooling of the North Hemisphere, resulting from the meltwater discharge from Eurasian ice sheets and slowdown AMOC, led to the southward migration of the westerlies55 and restricted the northward migration of the tropical rain-belt56. The global surface temperature remained relatively low during this period57,58 (Fig. 4d). At the same time, there was a sudden increase in the advection of heat toward the low latitudes of the Indian Ocean due to the anomalous transportation of heat northward into the northern high latitudes and a more vigorous ITF linked with the expansion of the Indo-Pacific Warm Pool (IPWP)59. This is supported by SST records in cores I106, SO189-39KL3, SK157-460, GeoB1002919 from the low latitudes of the Indian Ocean, which suggest a steep and abrupt rise with a magnitude of >1.0 °C at about 19.5–18.0 ka, and warmer SST events around 20 ka and 17 ka from the northern Arabian Sea61. With enhanced tropical SST warming, the latitudinal migration of the ITCZ in the IAA monsoon region potentially reduced, especially as the seasonally-affected ITCZ generally locates over the warm ocean62.

Collins et al.63 have proposed that the tropical rain-belt in Africa contracted relative to the Late Holocene during the HS1, owing to a latitudinal compression of atmospheric circulation related to a lower mean global temperature. Besides, the tropical precipitation pattern in the IAA monsoon region also has a strong correlation with El Niño-Southern Oscillation (ENSO) activities64. Model studies indicate that there is an anticorrelation between ENSO and the Hadley circulation, which means that narrow and weak Hadley circulation occurs under El Niño conditions65. The zonal SST difference between the West Pacific and East Pacific66 and a transient model simulation67 suggest a more El Niño-like state in the Early HS1 (Fig. 4f). Due to anomalous warming generated by El Niño under this state, the tropical troposphere becomes warmer, and the subtropical troposphere is cooler, which enhances the meridional temperature gradient, and then results in shrinking of the Hadley circulation in both hemispheres68. It was reported that ENSO variability is strongly enhanced in response to meltwater discharges and the resulting substantial slowdown of the AMOC during the Early deglaciation67.

In summary, our new records, in conjunction with existing records, indicate dry hydrological conditions in both the northern and southern parts of the IAA monsoon region during the Early HS1. The synchronous occurrence of drought in both hemispheres suggests that tropical precipitation in the IAA monsoon region likely contracted latitudinally during the Early HS1. Our study demonstrates that the variability in the tropical hydroclimate pattern during the Early HS1 in the IAA monsoon region was a response to the meltwater discharge from the Eurasian ice sheet and the resulting changes in AMOC, global temperatures and El Niño. The cooling in the northern high latitudes hindered the northward expanding of the Hadley circulation, as evidenced by dry condition records in northern hemisphere during the Early HS1. Additionally, strong El Niño also led to a reduction in the extent of the Hadley circulation in the southern hemisphere68.

Methods
Mg/Ca and isotope analyses
Approximately 80 Globigerinoides ruber sensu stricto (s.s.) individuals were selected from 250–350 μm size fractions; they were then crushed before being split into samples ready for stable isotope and Mg/Ca analysis. For Mg/Ca analysis, the pretreatment and analysis procedures followed the standard cleaning protocol developed by Barker et al.69, including ultrasonic cleaning in alternation with washes in Milli-Q water and methanol, removal of organic matter by 2% H2O2 solution, and weak acid leaching with 0.001 M HNO3. The clean samples were then dissolved in 0.075 M HNO3. Samples were centrifuged to remove any remaining insoluble particles and then diluted with Milli-Q water and measured on an ICP-AES at the Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences. The instrumental precision of the ICP-AES was monitored using analysis of an external, in-house standard solution with a Mg/Ca ratio of 4.44 mmol/mol, after every three samples. The relative standard deviation of the external standard was ±0.55%. Analytical reproducibility was estimated by replicate measurements that revealed a reproducibility of Mg/Ca ±1.48% (1σ). The Mn/Ca ratio was ~0.16 mmol/mol, indicating no significant contribution of Mg from Mn-Fe-oxide coating.

For stable isotope analysis, the shell fragments were cleaned by ultrasonication in 2% H2O2 and acetone. Stable isotopic measurements were performed on a Thermo Finnigan MAT 253 mass spectrometer with a Kiel III automatic carbonate preparation device at the Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences. The standard error of the δ18O analyses was <0.05‰. Isotopic values were reported as ‰ Vienna Pee Belemnite (VPDB) and calibrated with the National Bureau of Standards (NBS) 19 standards.

Mg/Ca-SST and δ18O reconstruction
Mg/Ca values were converted to temperature using the equations developed by Anand et al.70: Mg/Ca [mmol mol−1] = 0.38 e0.09T [°C]. δ18O sw values were calculated using the equation proposed by Bemis et al.20: T [°C] = 14.9 – 4.8(δ18O c – δ18O sw). An additional 0.27‰ was added to the δ18O c to convert the Vienna Pee Belemnite (VPDB) values to Vienna Standard Mean Ocean Water (VSMOW) values. δ18O sw values were corrected for sea-level changes using the reconstruction protocol developed by Lambeck et al.71.

Error analysis for SST and δ18O sw
The errors in SST and δ18O sw in this study was estimated using equations proposed by Mohtadi et al.3. The errors in SST and δ18O sw are about ±1.03 °C and ±0.23‰, respectively. The error estimation for SST is carried out by propagating the errors introduced by the equation proposed by Anand et al.70 and Mg/Ca measurement. The SST error estimation is given as3:

σ2 T = (∂T/∂a)2 σa2 + (∂T/∂b)2 σb2 + (∂T/∂Mg/Ca)2 σMg/Ca2 (1)

where a=0.090±0.003 °C−1, b=0.38±0.02 mmol/mol−1, ∂T/∂a = −1/a2 ln(Mg/Ca), ∂T/∂b = −1/ab and ∂T/∂Mg/Ca = 1/(aMg/Ca).

And the uncertainties in δ18O sw is estimated by propagating errors from the δ18O-temperature equation of Bemis et al.20 and δ18O c measurements and SST, which is given following3:

σ2 δ18Osw = (∂δ18Osw/∂T)2 σT2 + (∂δ18Osw/∂a)2 σa2 + (∂δ18Osw/∂b)2 σb2 + (∂δ18Osw/∂δ18Oc)2 σδ18Oc2 (2)

where ∂δ18Osw/∂T = 14.9 ± 0.1 °C, ∂δ18Osw/∂b = −4.8 ± 0.08 °C, ∂δ18Osw/∂T = −1, ∂δ18Osw/∂a = 1, ∂δ18Osw/∂δ18Oc = 1.

Chronological framework
The age model for Core I106 was determined through the utilization of mixed planktonic foraminiferal Accelerated Mass Spectrometry (AMS) radiocarbon data from 17 layers (Supplementary Table 2). Conventional 14C ages were adjusted for isotopic fraction utilizing δ13C values. These ages were further calibrated into calendar ages using CALIB 8.10 software and a MARINE20 dataset, without adjusting for a regional 14C reservoir age. Linear interpolation was then employed to establish chronological continuity between calendar ages. The average sedimentation rate was ~6.25 cm/ka.

Dating uncertainties
The age models utilized in this study for marine sediment records were established through the use of AMS radiocarbon dating on planktonic foraminifera. The AMS 14C dates from marine sediment records were then converted to calendar ages using the CALIB 8.10 program and the MARINE20 curve (please see Supplementary Dataset 2). The age models for terrestrial records referenced in this study were revised using the IntCal20 curve instead of the Marine20 curve. These age models were created through linear interpolating between derived intermediate calendar ages. It is important to note that a regional 14C reservoir age was not applied to all cores in this study. The revised dates are listed in the Supplementary Material. The stalagmite δ18O records were dated using 234U/230Th measurements, as described in the original paper.

Collection of existing paleoclimatic records
Numerous paleoclimate records of the HS1 have been documented in the IAA monsoon regions. In this study, we gathered 43 records that possess four AMS14C age control points ranging from 12 to 24 ka (Supplementary Table 1). The temporal resolution of each sample is generally superior to 500 years, except SK129/CR04, which has three AMS14C with 700 years per sample. Additionally, we collected seven δ18O sw records from the northeast Indian Ocean, seven paleoclimatic records from the northern Arabian Sea, two paleoclimate records from the northern SCS, six paleoclimate records from the southern part of China and the Indian subcontinent, one paleoclimate record from Taiwan, one record from the southern SCS, one record from the Sulu Sea, one record from Mindanao, four records from the southern Arabian Sea and Southern Africa, eleven paleoclimate records and two stalagmite δ18O records from the south tropical Indian Ocean and tropical West Pacific. These records were collected to represent the overall spatial distribution pattern of hydrological conditions during the HS1 ranging from 30° north to 20° south in the IAA Monsoon region.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review: A contracting Intertropical Convergence Zone during the Early HS1\nThis manuscript presents new data and synthesizes a large amount of existing data to\nmake the compelling case that the ITCZ contracted to a narrow zonal band across much\nof the Indian and western-most Pacific, consistent with previously published hypotheses\nregarding ITCZ dynamics over Africa and western Pacific at different time intervals. The\nnew record (1106) is well dated and has compelling structure consistent with the\nauthors interpretation thereof. The cores to which it is compared also have well-defined\nstructure that the authors synthesize and interpret in a consistent, well-reasoned, and\nclearly presented manuscript.\nAs described in detailed comments below, the manuscript would benefit from a Sup Mat\nsection that provides age-model details of the existing cores used in the transect,\nspecifically indicating the age control governing the very short interval of interest in this\nwork (early HS1; 18.7 to 16.5 ka). Is the age control in the cores synthesized sufficient\nto document the synchronous nature of the event seen in 1106 and interpreted as a\ncontracted ITCZ at the narrow latitudinal band inferred?\nSpecific comments\n***\nduring the Heinrich\nIndonesian, the nature of\n16 such responses remains many debates.\nIntent is clear – edit for grammar.\n*****\nThis\n24 study reveals that variable, millennial-scale seasonal insolation patterns, and low\nglobal temperatures\n25 associated with meltwater discharge from the Eurasian ice sheet, both exerted a\nprofound influence on the\n26 tropical hydroclimate during the HS1.\nDischarge from the Eurasian ice sheet into what region? Please specify where the\ndischarge is going- eastern N. Atl?\nIt is not clear what ‘variable millennial-scale seasonal insolation patterns’ means. Please\nclarify. Secular or orbital variations?\n***\n-2 BZW WKITSXYWZIYOTS TL EEF GSJ WKXOJZGQ c*1BXKG\\GYKW #c*1BX\\&WKXOJZGQ$\nResidual relative to what? ‘Residual’ implies something has been subtracted from the\nd18Osw record. If not, then consider dropping the modifier ‘residual’. Clarify how\nd18Osw is constructed.\n***\nModern moisture flux observations show that\n52 precipitation arrives year-round at this site, with the majority sourced from the local\ntropical Eastern Indian\nHence, the location of the\n.- XYZJ^ XOYK #:OM' *% 7TWK =*)/3 /`*-d-2'0/eA% 2)`))d*')-e93 +%2*) R \\GYKW JKUYN$\nmakes it an ideal location\n55 to monitor shifts in the tropical rainfall belt.\nPlease justify assumption that modern moisture source areas and transport paths at the\ncore location also apply during HS1, making the site appropriate for use in this study.\n***\n56 latitudinal transect across the Asian-Indian Ocean\nAn odd geographic term. There is an Indian Ocean but not an Asian Ocean. How about\nthe Indo-Asian-Australian (IAA) monsoon region? It’s a difficult area to define\ngeographically for sure.\n***\nthe calculated 18Osw\n73 and 18Osw-residual values is an exceptionally abrupt decline at 18.7-16.5 ka BP (Fig.\n2).\nThe reader has not been introduced to what these two terms mean and does not yet\nknow the difference between them, leading to confusion. Please define them where each\nis first used or point the reader to the methods section or sup mat section where they\nare clearly defined.\n***\nplanktonic foraminiferal AMS\n402 radiocarbon data from 17 layers (Table S2). Conventional 14C ages were corrected\nLTW OXTYTUOI LWGIYOTS ZXOSM c*,7 [GQZKX'\nand a MARINE13 dataset, adjusting for a\n-)- WKMOTSGQ *-7 WKXKW[TOW GMK #MQTHGQ WKXKW[TOW GMK 4 -)) ^W% fD 4 ,+a0) ^W$' FNK\nmean sedimentation was ~6.25 cm/ka.\nThis age model appears robust. Are the age models from the other cores used in\ncomparison similarly robust? See next comment.\n****\nThe results show that tropical\n21 hydrological conditions were synchronously arid in both hemispheres during the\nearly HS1 (18.7-16.5 ka\nnorthern low latitudes,\n23 suggesting the existence of a contracted Intertropical Convergence Zone (ITCZ) rain-\nbelt at that time.\n18.7 to 16.5 ka is a very short interval of time. This paper does not (yet) present\nsufficient information to demonstrate that this short interval is sufficiently resolve in all\nthe cores utilized. In order to draw the spatial-temporal conclusions in this paper, the\nage models at all the sites used need to be evaluated to demonstrate that they\nidentify/resolve the interval of interest sufficiently.\nFor example, 1106 (this study) has six dates in the interval depicted in figure 2. It is\nwell resolved. Are the other six cores sufficiently dated as well; sufficient to draw the\nconclusions presented in the manuscript? This important topic could be addressed in the\nSup Mat.\n***\n1/ :OMZWK +' ;' WZHKW XNKQQ c*1B [GQZKX% @M(7GWGYOTX% WKITSXYWZIYKJ @M(7G&YKRUKWGYZWK\nWGYOTX%c*1BXKG\\GYKW[GQZKX% GSJ WKXOJZGQ\n10 c*1BXKG\\GYKW [GQZKX LWTR 7TWK =*)/\nPropagated error in the x and y dimensions would be useful in figure 2\n***\nThis would indicate that tropical precipitation fell\nwithin a narrower geographical 132 range during the Early HS1 than during the\nHolocene.\n‘…HS1 compared to the Holocene”?\n***\ntropical Asian-Indian Monsoon region.\n*,/ #5$ ;' WZHKW c*1B WKITWJX LWTR YNK 9VZGYTWOGQ =SJOGS BIKGS -)' #6$ EO(5Q WGYOTX\nfrom Core CG2 from the southern SCS 6. (C)\n*,0 c*1BX\\&WKXOJZGQ WKITWJX LWTR 7TWK =*)/ LWTR YNK XTZYNKWS 6T6 #YNOX XYZJ^$' #8$ \nc*1BX\\&WKXOJZGQ WKITWJX LWTR 7TWK E>*+2&7D)-\n*,1 LWTR YNK YWTUOIGQ =SJOGS BIKGS -+' #9$ c*1BX\\&WKXOJZGQ WKITWJX LWTR 7TWK @8)*&\n2390 from the southern SCS 43.\nThe location of AMS dates on the various cores would be useful on Figures 2, 3, and 4.\nThis would help the reader assess the extent to which the structures in the various cores\nalign or not.\n***\nDuring the early phases of the last deglaciation, boreal insolation was lowest during the\nsummer months, and the ITCZ rain-belt was therefore drawn toward 147 the Equator;\nhowever, boreal winter\n148 insolation reached its maximum during this period, and the ITCZ rain-belt migrated\nfurther north (Fig. 4D). 149 Consequently, the seasonal migration range of the tropical\nrain-belt during the Early HS1 was probably\n150 relatively narrow\nDJF and JJA insolation should probably be plotted on the same Y-axis scale. The\ndiffering scales gives the impression that DJF and JJA are similarly strong when, in\ncomparison, JJA has a larger absolute value and larger range. It is not clear that this\nforcing would substantially narrow the ITCZ rain belt.\n***\nboth the northern\n177 and southern parts of the Asian-Indian Monsoon region during the Early HS1. These\ndry conditions cannot\n178 be explained by a simple southward drift of the tropical rain-belt. Analyzing paleo-\nrecords from the northern\n179 low latitudes in the Asian-Indian Monsoon region, we established that a narrow\ntropical rain-belt existed\n180 during the Early HS1. This study presents a clear picture of the variations in\nhydrological conditions during\n181 the Early HS1 in the Asian-Indian Monsoon region.\nThe last sentence might be replaced by a summary of the mechanisms driving the\nnarrowed ITZC from both Northern- and Southern-Hemisphere perspectives. This would\nmake for a considerably stronger conclusion statement and enhance the relevance to the\nwider scientific community targeted by Nature Communications.", "reviewer_2": "Yang and others focus on the response of the tropical Intertropical Convergence Zone\n(ITCZ) feature over the Indian Ocean during Heinrich Stadial 1 (HS1) by presenting a\nSK\\ WKITWJ TL XKG\\GYKW&cbgB #G UWT]^ LTWXGQOSOY^3 cbgBX\\$ LWTRLTWGROSOLKWGQ\ngeochemistry in marine sediments proximal to the Ninetyeast Ridge and by compiling a\nXKWOKX TL UWK[OTZXQ^ UZHQOXNKJ WKITWJX' FNKOW WKITWJ XNT\\X QT\\KW cbgBX\\ #TW QT\\KW\nsurface-salinity) during the initial part of HS1 and Yang and colleagues suggest that this\nis in accordance with a series of other records showing intensified rainfall offshore the\ntip of peninsular India & in the northern western Pacific Warm Pool region. These\nrecords are in contrast to widespread dual-hemispheric and pan-Indian Ocean records of\ndrought conditions during HS1. Thus, the authors argue that the ITCZ was restricted to a\nnarrow band during the early part of HS1 and that rainfall in this contracted ITCZ was\nmore intense. Whereas the premise is interesting and their record will certainly be of\nvalue to the paleoceanographic community, I am unable to recommend this manuscript\nfor publication in Nature Communications as I am yet unconvinced by their mechanistic\narguments. I detail my points below:\nProxy uncertainties and evidence for a “wetter” yet narrower ITCZ band during the early\npart of HS1: I am not convinced of the purported “wetter” records that are presented in\nthe manuscript (Fig. 3 & Table S1). For one, it is extremely challenging to reconstruct\nUWKIOUOYGYOTS ZXOSM XZWLGIK&TIKGS cbgB GSJ GY YNK [KW^ QKGXY% YNK GZYNTWX RZXY\nUWTUGMGYK YNK ZSIKWYGOSYOKX GUUWTUWOGYKQ^ LWTRUGOWKJ @M(7G&cbgB JGYG #XKK K'M'\nRohling, 2000; Thirumalai et al. 2016; Gray & Evans, 2019 etc.). Thus, it is unclear\nwhether the changes shown in Fig. 3 are actually consistent with less saline conditions\nduring HS1, given that anomalies are on the order of ~0.1–0.3 ‰ across all records.\nPlease perform a robust error propagation exercise to see whether such an anomaly\npersists. Moreover, the authors suggest that they only use the Dekens et al. 2002\nequation for inverting SSTs - they must additionally test for the influence of salinity (e.g.\nsee Gray & Evans, 2019) on Mg/Ca and see whether their result holds. They should\nperform the same analysis across all purported records consistent with “fresher”\nconditions. Finally, the Sulu Sea & Mindanao Dome records seem to show a steady\nOSIWKGXK OS cbgBX\\ T[KWYNK KGWQ^ UGWY TL <E* GSJ ITSYWGXY \\OYN YNK WKITWJ LWTR 7TWK\nI106; yet this discrepancy is not explained. Moreover, I do not know if the average\ncbgBX\\ LWTR YNK ?;@YT YNK KGWQ^ UGWY TL <E* OS YNKXK WKITWJX #DTXKSYNGQ KY GQ' +)),3\nBoillet et al. 2011) are even significantly different from one another. This needs to be\ntested within uncertainty.\nWinter Monsoon Rainfall: Thus, it appears that only the I106 record seems to show a\nXYWZIYZWKJ GSTRGQTZX WKXUTSXK TL QT\\KWcbgBX\\ OS YNK XZH&XKQKIYKJ 0WKITWJX UZWUTWYKJ\nto show higher rainfall during the early part of HS1 (although the robustness is to be\ntested as indicated above). Whereas the Sulu Sea & Mindanao records response are\nuncertain, in the Indian Ocean side, the CR04 (Fig. 3D) also seems to be uncertain and\nnot robust in its response. This leaves potentially the SS3827G record - which is only a\ncbgBIGQIOYK WKITWJ \\OYNTZY G YKRUKWGYZWK RKGXZWKRKSY' FNZX OY OX GQXT ZSIKWYGOS OS OYX\nN^JWTIQORGYOI WKXUTSXK'AK[KWYNKQKXX%OL YNOX QT\\KW cbgB&IGQIOYK \\GX OSJKKJ IGZXKJ H^\nQT\\KW cbgBX\\% YNKS YNK GZYNTWX SKKJ YT GJJWKXX YNK OJKG YNGY YNOX OX JZK YT YNK\nadvection of fresh runoff from the southeastern Indian coast due to strengthened winter\nmonsoon rainfall (see e.g. Sarkar et al. 1990 or Kumar & Ramesh, 2017). Is it possible\nthat these low salinity waters also made their way towards the core site? That said, the\nauthors do not provide enough clarity to rule out that these different core sites might be\nrepresenting rainfall/salinity during different parts of the year. This dimension needs to\nbe discussed in a revised manuscript.\nForcing Mechanisms: The proposed mechanisms for the interaction of orbital forcing\nwith abrupt climate change do not take into account the latest results suggesting that\norbital forcing modulates millennial-scale activity. That said, I was confused while\nreading the mechanism because the authors do not explain what happens to the ITCZ in\nthe southern hemisphere of the tropical Indian Ocean, which when DJF insolation is\nrelatively higher, should exhibit a seasonally more southern ITCZ in the Southern\nHemisphere. How does this affect their proxy comparison?\nComparisons with models: The authors do not present comparisons with climate model\noutput - which by itself is not a problem. However, there are several papers (e.g.\nMohtadi et al. 2014) which use model simulations and show that there is evidence for\nITCZ movement but not intensification - let alone intensification associated with a\ncontraction. Can the authors speculate as to why this may be the case? Can they rule out\nthat the seasonal-bias of different proxies affects this finding and that it can be applied\nto mean-annual ITCZ shifts?\nReferences:\nGray, W. R., & Evans, D. (2019). Nonthermal Influences on Mg/Ca in Planktonic\nForaminifera: A Review of Culture Studies and Application to the Last Glacial Maximum.\nPaleoceanography and Paleoclimatology, 34(3), 306–315.\nhttps://doi.org/10.1029/2018pa003517\nKumar, P. K., & Ramesh, R. (2017). Revisiting reconstructed Indian monsoon rainfall\n[GWOGYOTSX JZWOSM YNK QGXY _+.PG LWTR UQGSPYTSOI LTWGROSOLKWGQ cbgB LWTR YNK 9GXYKWS\nArabian Sea. Quaternary International, 443, 29–38.\nhttps://doi.org/10.1016/j.quaint.2016.07.012\nMohtadi, M., Prange, M., Oppo, D. W., De Pol-Holz, R., Merkel, U., Zhang, X., Steinke, S.,\n& Lückge, A. (2014). North Atlantic forcing of tropical Indian Ocean climate. Nature,\n509(7498), 76–80. https://doi.org/10.1038/nature13196\nRohling, E. J. (2000). Paleosalinity: confidence limits and future applications. Marine\nGeology, 163(1–4), 1–11. https://doi.org/10.1016/s0025–3227(99)00097–3\nRosenthal, Y., Oppo, D. W., & Linsley, B. K. (2003). The amplitude and phasing of climate\nchange during the last deglaciation in the Sulu Sea, western equatorial Pacific.\nGeophysical Research Letters, 30(8), 1428–1424.\nhttps://doi.org/10.1029/2002GL016612\nSarkar, A., Ramesh, R., Bhattacharya, S. K., & Rajagopalan, G. (1990). Oxygen isotope\nevidence for a stronger winter monsoon current during the last glaciation. Nature,\n343(6258), 549–551. https://doi.org/10.1038/343549a0\nFNOWZRGQGO% >'% CZOSS% F' @'% \" @GWOST%;' #+)*/$' 7TSXYWGOSOSM UGXY XKG\\GYKW cbgB GSJ\ntemperature records developed from foraminiferal geochemistry. Paleoceanography.\nhttps://doi.org/10.1002/2016PA002970"}, "author_response": "Thank you very much for your constructive comments. We have revised the manuscript accordingly.\nOur point-by-point responses are detailed below.\nThe questions about the age-model details of the existing cores above have been answered in detail\nin Comment #8 below.\nSpecific comments\n***\n1. Despite the responses of the tropical hydroclimate to a North Atlantic cooling event during the\nHeinrich Stadial 1 (HS1) having been extensively studied in African, South American and\nIndonesian, the nature of such responses remains many debates. Intent is clear -edit for grammar.\n[Response]\nThank you very much for pointing out this error.\nWe have modified this sentence’s grammar, please see Line 14-16.\n2.This study reveals that variable, millennial-scale seasonal insolation patterns, and low global\ntemperatures associated with meltwater discharge from the Eurasian ice sheet, both exerted a\nprofound influence on the tropical hydroclimate during the HS1.\nDischarge from the Eurasian ice sheet into what region? Please specify where the discharge is going-\neastern N. Atl?\n[Response]\nThanks for your suggestion. Yes, the region is Eastern North Atlantic. We have specified that an\nabrupt and early Eurasian fluvial discharge into the Eastern North Atlantic in Line 161-163.\n3. It is not clear what ‘variable millennial-scale seasonal insolation patterns’ means. Please clarify.\nSecular or orbital variations?\n[Response]\nThank you very much for your comment.\nThis term might be not accurate and distractive. The low global temperature, warming SST in\ntropical Indian Ocean and ENSO activities play a more important role in the contraction of tropical\nprecipitation in Asian Monsoon region than the seasonal insolation pattern during the Early HS1\n(Line 191-216), thus we removed this term in the revised manuscript.\n-'B^[[OMXW\\][^M]SXWXPFFGKWN[O\\SN^KUh18B\\OK`K]O[#h18Osw-residual). Residual relative to what?\n‘Residual’ implies something has been subtracted from the d18Osw record. If not, then consider\ndropping the modifier ‘residual’. Clarify how d18Osw is constructed.\n[Response]\nThanks for your suggestion.\nThe stable oxygen isotopic composition (h18O) of planktonic foraminiferal shells is mainly\ninfluenced by seawater temperature and Seawater h18O (h18Osw). We calculated the h18O value\nsw\nbased on the h18O of planktonic foraminiferal and Mg/Ca-SST value from the same sample using\nthe equation of Bemis (1998). h18O signal reflects both salinity of the water mass associated with\nsw\nlocal factors (i.e. runoff, precipitation) and global ice volume. In order to get the salinity of the water\nmass associated with local factors, the effect of global ice volume should be subtracted from h18O ,\nsw\nindicating the local seawater salinity signal. In this revised manuscript, in order not to confuse\nthe reader, we have corrected 18O to 18O , and clarified how 18O was constructed\nsw-residual sw sw\nin Methods part (Line 257-261).\n5. Modern moisture flux observations show that precipitation arrives year-round at this site, with\nthe majority sourced from the local tropical Eastern Indian Ocean during the summer months,\ncorrelating with the movements of the ITCZ. Hence, the location of the study site (Fig. 1, Core I106;\n/d*-i-2'0/jA% 2)d))i*')-j93 +%2*) V `K]O[ NOY]R$ VKTO\\ S] KW SNOKU UXMK]SXW ]X VXWS]X[ \\RSP]\\ SW\nthe tropical rainfall belt.\nPlease justify assumption that modern moisture source areas and transport paths at the core location\nalso apply during HS1, making the site appropriate for use in this study.\n[Response]\nThanks for your suggestion.\nModern moisture flux observations show that precipitation arrives year-round at this site, and the\nmajority occurs in the summer months (Supplementary Fig. 1), correlating with the movements of\nthe ITCZ (Belgaman et al., 2017) (Line 51-53). Hence, the location of the study site makes it an\nideal location to monitor shifts in the tropical rainfall belt.\nWe make the assumption that the hydrologic cycle in this region is largely consistent during HS1\nand present day based on the following situation: 1) marine and continental tectonic during the HS1\nis consistent with it today, and the structure of land-sea thermodynamic between the Indian Ocean\nand Asian Continent has not changed significantly; 2) there also existed Indian Ocean Summer\nMonsoon and ITCZ rain belt system during the HS1. Accordingly, we assume that precipitation in\nour study area in the HS1 was still linked with Indian Ocean Summer Monsoon and the ITCZ rain-\nbelt system; 3) in addition, other paleoclimatic studies also suggest that tropical East Indian\nOcean was affected by the Indian Ocean Summer Monsoon and ITCZ rain belt system in the\nHS1 (Mohtadi et al., 2010, 2011, 2014). These justifications have been added to the main text\"Line\n53-57#.\n6. latitudinal transect across the Asian-Indian Ocean, An odd geographic term. There is an Indian\nOcean but not an Asian Ocean. How about the Indo-Asian-Australian (IAA) monsoon region?\nIt’s a difficult area to define geographically for sure.\n[Response]\nThanks for your suggestion.\nIndo-Asian-Australian monsoon region is a good definition. We have changed “Asian-Indian Ocean”\ninto “Indo- Asian-Australian monsoon region” in the revised manuscript (Line 57-60).\n7. the calculated 18Osw and 18Osw-residual values is an exceptionally abrupt decline at 18.7-16.5\nka BP (Fig. 2). The reader has not been introduced to what these two terms mean and does not yet\nknow the difference between them, leading to confusion. Please define them where each is first used\nor point the reader to the methods section or sup mat section where they are clearly defined.\n[Response]\nThank you for your suggestion.\nFollowing your Comment #4, in this revised manuscript, we have corrected h18O to h18O ,\nsw-residual sw\nand clarified how h18O was constructed in Methods part (Line 256-262).\nsw\n8. Chronological framework. The age model for Core I106 was established using mixed planktonic\nforaminiferal AMS radiocarbon data from 17 layers (Table S2). Conventional 14C ages were\nMX[[OM]ON PX[ S\\X]XYSM P[KM]SXW ^\\SWQ h13C values. Conventional 14C ages were calibrated into\ncalendar ages using BetaCal software and a MARINE13 dataset, adjusting for a regional 14C\n[O\\O[_XS[ KQO #QUXLKU [O\\O[_XS[ KQO 4 -)) b[% kE 4 ,+e0)b[$'GRO VOKW \\ONSVOW]K]SXW `K\\c/'+.\ncm/ka. This age model appears robust. Are the age models from the other cores used in comparison\nsimilarly robust? See next comment.\nThe results show that tropical hydrological conditions were synchronously arid in both hemispheres\nduring the early HS1 (18.7-16.5 ka BP) in the Asian-Indian Monsoon region, except for a narrow,\nwet hydrological belt in northern low latitudes, suggesting the existence of a contracted Intertropical\nConvergence Zone (ITCZ) rain-belt at that time.18.7 to 16.5 ka is a very short interval of time. This\npaper does not (yet) present sufficient information to demonstrate that this short interval is\nsufficiently resolve in all the cores utilized.In order to draw the spatial-temporal conclusions in this\npaper, the age models at all the sites used need to be evaluated to demonstrate that they\nidentify/resolve the interval of interest sufficiently.\nFor example, 1106 (this study) has six dates in the interval depicted in figure 2. It is well resolved.\nAre the other six cores sufficiently dated as well; sufficient to draw the conclusions presented in the\nmanuscript? This important topic could be addressed in the Sup Mat.\n[Response]\nWe are extremely grateful to the reviewer for pointing out this issue.\nFirstly, we have re-sifted the all cores utilized in this study. We set a requiremnt for at least four\nage control points between 12 and 24 ka, and the temporal resolution must be greater than 500\nyr/sample, except for Core SK129/CR04 with 3 age control points and ~700 yr/sample (Line 272-\n293).\nSecondly, we have revised the age models at all the sites used in this study as you suggested.\nFor the marine sediment records, we reconstructed the age models by the CALIB 8.10 program and\nthe Marine 20 curve which are all based on Accelerated Mass Spectrometry (AMS) radiocarbon\ndates measured on planktonic foraminifera. For terrestrial records, the age models were revised with\nthe IntCal 20 curve instead of the Marine 20 curve. Note that we did not apply a regional 14C\nreservoir age in all the cores in this study. All dates are listed in supplementary material. And the\n\\]KUKQVS]O h18O records are dated using 234U/230Th measurements as in their original paper (Line\n272-293)(Please see supplementary material for age models).\nAge information for wet condition records in north of Equator\nAge control temporal\nCore Wet condition event proxy\npoints (12-24 ka) resolution\nI106 16.3-18.3 ka 6 270 yrs/sample h18Osw\nCG2 16.6-17.2 ka, 17.4-18.5 ka 11 95 yrs/sample Si/Al/grain size\nADM-159 16.5-18.7 ka 4 240 yrs/sample h18Osw\nMD97-2141 16.4-18.7 ka 9 73 yrs/sample h18Osw\nMD06-3075 15.7-17.9 ka 4 60 yrs/sample Log(Fe/Ca)\nSK129/CR04 16.3-19.5 ka 3 700 yrs/sample h18Osw\nThirdly, in order to make our results more reliable, we also test the validity of the reconstructed\n/;$,5#223 5@8 J18O of cores I106, ADM-159, MD97-2141, SK129/CR04 using the PSU\nsw\nCA>F9BG=D<DGAC=;?55@5>HD=75>E@79BD5=@DHA:/;$,55@8J18O and ±5% age uncertainty. And\n]RO[O\\^U]\\RX`]RK]KUUh18O demonstrate robust negative signals in the early Heinrich stadial 1,\nsw\nas well as original dates (Supplementary Line 56-81).\n9. Figure 2. G. ruber \\ROUU h18O values, Mg/Ca ratios, reconstructed Mg/Ca-temperature ratios,\nh18B\\OK`K]O[ _KU^O\\% KWN [O\\SN^KU h18Oseawater values from Core I106 Propagated error in the x\nand y dimensions would be useful in figure 2.\n[Response]\nThanks for your suggestion.\nWe have add the propagated errors in the X and Y demensions in Figure 2 and Figure 3.\n10.This would indicate that tropical precipitation fell within a narrower geographical range during\nthe Early HS1 than during the Holocene. ‘…HS1 compared to the Holocene”?\n[Response]\nThank you for your comment.\nWe have modified this sentence, please see Line 145-148.\n11.Figure 3. Records showing wet hydrological conditions during the Early HS1 in the tropical\nAsian-Indian Monsoon region. (A) G. ruberh*1B[OMX[N\\P[XV]RO9Z^K]X[SKU=WNSKWBMOKW-)'#6$\nFS(5U[K]SX\\P[XV7X[O7;+P[XV]RO\\X^]RO[WF7F/'#7$h*1B\\`&[O\\SN^KU[OMX[N\\P[XV7X[O=*)/\nP[XV]RO\\X^]RO[W6X6#]RS\\]^Nb$'#8$h*1B\\`&[O\\SN^KU[OMX[N\\P[XV7X[OF>*+2&7E)-\nP[XV]RO\n][XYSMKU =WNSKW BMOKW -+' #9$ h18Osw-residual records from Core MD01-2390 from the southern\nSCS.\nThe location of AMS dates on the various cores would be useful on Figures 2, 3, and 4. This would\nhelp the reader assess the extent to which the structures in the various cores align or not.\n[Response]\nWe are grateful for your suggestion.\nWe have added the AMS dates on the various cores in Figure 2, 3 and 4.\n12. During the early phases of the last deglaciation, boreal insolation was lowest during the summer\nmonths, and the ITCZ rain-belt was therefore drawn toward the Equator; however, boreal winter\ninsolation reached its maximum during this period, and the ITCZ rain-belt migrated further north\n(Fig. 4D). Consequently, the seasonal migration range of the tropical rain-belt during the Early HS1\nwas probably relatively narrow. DJF and JJA insolation should probably be plotted on the same Y-\naxis scale. The differing scales gives the impression that DJF and JJA are similarly strong when, in\ncomparison, JJA has a larger absolute value and larger range. It is not clear that this forcing would\nsubstantially narrow the ITCZ rain belt.\n[Response]\nThanks for your comment.\nBased on climatic model results, Singarayer et al. (2017) have proposed that there was ocean\ndominated expansion and contraction of the tropical rainbelt during the late Quaternary. This\nexpansion/contraction is the result of the different response of the marine ITCZ when at its northern\nand southern extremes. In boreal summer, when ITCZ is farthest north, if the isolation is the lowest,\nthe ITCZ moves towards the equator; in boreal winter, when the ITCZ is farthest south, if the\ninsolation is higher in both the northern and southern hemispheres, the rainbelt is located further\nnorth (Singarayer et al., 2017). But, on the one hand, this view still need more research to support;\non the other hand, we are not sure whether the ITCZ was located at its northern/southern extremes\nduring the early HS1.\nRecently, mounting evidence suggests that the precipitation anomaly in the tropical west Pacific and\nIndian Ocean has a strong correlation with EI Niño-Southern Oscillation (ENSO) activities (e.g.\nThirumalai et al., 2019). We revised our statement and propose that the low global temperature,\nwarming SST in tropical Indian Ocean and ENSO activities are the driving factors for the\ncontraction of tropical precipitation belt in the Asian Monsoon region (Line 191-216). During\nthe early HS1, the global mean temperature was very low, and the tropical SST warming developed\nrapidly in the low latitude of the Indian Ocean, which may lead to reduced range of latitudal\nmovement of the tropical rain belt. At the same time, the low temperature gradient between West\nand East tropical Pacific indicate that there was a EI Niño-like condition at this interval\n(Zhang et al., 2022, NC; Koutavas and Joanides, 2012, Paleoceanography). And recent work indicates that\nthe range of Hadley circulation would contracte equatorward and become weak under EI Niño\ncondition (Wodzicki and Rapp, 2020, JC; Guo and Li, Advances in Climate Change Rearch).\n13. In conclusion, our findings highlight the presence of dry hydrological conditions in both the\nnorthern and southern parts of the Asian-Indian Monsoon region during the Early HS1. These\ndry conditions cannot be explained by a simple southward drift of the tropical rain-belt. Analyzing\npaleo-records from the northern low latitudes in the Asian-Indian Monsoon region, we established\nthat a narrow tropical rain-belt existed during the Early HS1. This study presents a clear picture of\nthe variations in hydrological conditions during the Early HS1 in the Asian-Indian Monsoon region.\nThe last sentence might be replaced by a summary of the mechanisms driving the narrowed ITZC\nfrom both Northern- and Southern-Hemisphere perspectives. This would make for a considerably\nstronger conclusion statement and enhance the relevance to the wider scientific community targeted\nby Nature Communications.\n[Response]\nThanks very much for your constructive suggestion.\nWe have modified the last paragraph following your suggestion, please see Line 217-225."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Yang 2022 ITCZ HS1 Rev #2\nThe authors have adequately addressed the questions/concerns raised in my initial\nreview of the manuscript. The synthesis supports the contention of an unresolved\nquestion regarding climatic state during HS1 and the data presented and interpretation\nthereof are internally consistent, providing an explanation consistent with both the land\nand ocean evidence for the HS1 interval of time. The proposed mechanisms are broadly\ntestable in climate-model space and sufficiently provocative to motivate that\ncommunity.\nThe manuscript will require substantial editing for grammar, some of which is address\nbelow, along with other editorial, scientific comments and clarifications.\n***\nduring the Heinrich\nIndonesian; the nature of\n15 such responses remains many debates\nDespite the fact that the response of the tropical hydroclimate to a North Atlantic\ncooling event during the Heinrich Stadial 1 (HS1) has been extensively studied in\nAfrican, South American and Indonesian; the nature of\nsuch responses remains debated.\n***\nThis\n21 study reveals that low global temperatures and El Niño activates both exerted a\nprofound influence on the\n22 tropical hydroclimate in the Indo-Asian-Australian monsoon region during the early\nHS1.\nThis study reveals that low global temperatures and El Niño both exerted a profound\ninfluence on the tropical hydroclimate in the Indo-Asian-Australian monsoon region\nduring the early HS1.\n***\nWe assume that precipitation in our study area was also linked with\nis consistent with it\n53 today and the structure of land-sea thermodynamic between Indian Ocean and\nEurasian Continent also\n54 exist.\nThe land-sea continent configuration argument for justification of the assumption is\ninsufficient; a great many factors can come to play in this regard, the least of which is\ntectonics, at the time-scale of this work. Presenting this argument will garner far more\ncontroversy than simply stating the assumption and carrying on.\n***\ndistributed in water\n68 depths of 0-50 m, and that it can therefore be classed as a surface species 18\nA mixed-layer species?\n****\n/1 @LI RFVIUYIH VEOMQMW\\ EQH `)0=VZ YEOXIV RJ VXUJEGI ZEWIUMQ WLI <RUWLIEVWIUQ :QHMEQ\nOcean reported\n0( F\\ 8IFUIKMRUKMV IW EO&*) SURYI WLEW `)0=VZ YEOXIV VLRZ E OMQIEU GRUUIOEWMRQ ZMWL\nsalinity in our study area.\nThe reference is for the coastal Andaman Islands, Andaman Sea region, not the\nequatorial region of the IO. Although very likely linear in this region as well, the d18Osw\n- salinity relationship cited does not prove the correlation in the study area.\nCaley et al., 2015 (10.1002/2014PA002720) and references therein may provide a\nbetter reference?\n***\n*-- ;K'7E%??@ EQH `)0=VZ UIGRQVWUXGWMRQ& ;K'7E YEOXIV ZIUI GRQYIUWIH WR\ntemperature using the equations developed\n*-. F\\ 5QEQH IW EO& \"*((+# /)2 ;K'7E CPPRO PRO%)D 4 (&+0I(&(1@C]7D& ?IEZEWIU `)0=\n\"`)0=VZ# YEOXIV ZIUI GEOGXOEWIH XVMQK WLI\n*-/ ITXEWMRQ SURSRVIHF\\ 6IPMV IW EO& \")110# *( 2 @ C]7D 4 ),&1 % ,&0 \"`)0=G%`)0=VZ#&\nAn additional 0.27‰ was added to them\n258 to convert the Vienna Pee Belemnite (VPDB) values to Vienna Standard Mean Ocean\nBEWIU \"A?;=B# YEOXIV& `)\nPlease explicitly provide the s18Osw-salinity relationship used\n***\nMeanwhile, there would be more salt water\n131 transported from the south into our study area if the winter Monsoon Current\nstrengthened, on the contrary,\n`)0 =VZ YEOXIV HIGOMQIH E ORWMQ 7RUI :)(. HXUMQK WLI IEUO\\ 9?)&\nMeanwhile, there would be more salt water transported from the south into our study\nEUIE MJ WLI ZMQWIU ;RQVRRQ 7XUUIQW VWUIQKWLIQIH3 RQ WLI GRQWUEU\\$ `)0 =VZ YEOXIV\ndeclined a lot in Core I106 during the early HS1.\n***\nTropical\n165 hydroclimate within HS1 from the north of the equator in the IAA monsoon realm\nalso exist two distinct\n166 phases, with wet hydrological condition at about 18.3-16.3 ka and dry condition at\n~16.3-14.7 ka, which\n167 was in line with this two-step AMOC slowdown (Fig. 4E ).\nTropical hydroclimate within HS1 from the north of the equator in the IAA monsoon\nrealm also exhibit two distinct\nphases, with wet hydrological condition at about 18.3-16.3 ka and dry condition at\n~16.3-14.7 ka, which was in line with this two-step AMOC slowdown (Fig. 4E ).\n***\nAccordingly,\n180 variations in tropical precipitation patterns are not only affected by the\ninterhemispheric temperature\n181 difference, but also be associated with other driving factors\nAccordingly, variations in tropical precipitation patterns are not only affected by the\ninterhemispheric temperature difference, but also associated with other driving factors", "reviewer_3": "I have read with great interests the manuscript by Yang and co-workers “A contracting\nIntertropical Convergence Zone during the Early Heinrich Stadial 1” that present new\nforaminferal data from a sediment core located in the southern Bay of Bengal to\ninvestigate the response of the Indian-Asian-Australian monsoon region to global and\nregional changes during Heinrich stadial 1. The authors propose more complex\ndevelopments of the Intertropical Convergence Zone (ITCZ) than previously\ndocumented/thought during Heinrich Stadial 1, for which the occurrence of an early\nphase of contracted ITCZ rain belt north of the equator is claimed. The topic is very\ninteresting and worth of publication in Nature Communications, the data are of an\noverall good quality although their interpretation could be deeper and more robust, and\nthe analysis of the complementing evidence could (and should) be improved. The\npalaeocenographic time series presented lack the Holocene section, leaving the reader\nwith the question of how relevant the amplitude of the discussed palaeoclimate is\nsignals with respect to the glacial-interglacial hydroclimate changes in the region.\nMy review centres on two aspects of the manuscript, namely the (overlooked) role of\nthe monsoon winds on the hydrography of the southern Bay of Bengal and the\nrelationship between the findings of Yang et al. and the global temperature changes.\n>ROI RJ WLI ZMQHV YIUVXV SUIGMSMWEWMRQ& @LI EXWLRUV GOEMP WLEW WLIMU VIEZEWIU `)0=\nreconstructions from the southern Bay of Bengal reflects changes in direct precipitation\nand runoff alone. Besides the doubts I have on the impact of the latter on the\nhydrography of the southern Bay of Bengal, this claim is contrasted with the\nNCEP/NCAR 40-year reanalysis project (Kalnay et al., 1996, Bulletin of the American\nMeteorological Society), which highlights the importance of the winds associated with\nthe Indian monsoon system in that sector of the bay. Accordingly, I wonder what the\nUIVSRQVI RJ WLI VXUJEGI RGIEQ VIEZEWIU `)0= ZRXOH FI WR E GLEQKI \"I&K&$ E ZIENIQMQK#\nof the wind mixing of the upper ocean during HS1? This hypothesis needs to be\nconsidered, because a shallower (smaller volume) mixed layer associated to a stratified\nVXUJEGI RGIEQ ZRXOH ^PE[MPMVI_ WLI QIKEWMYI VIEZEWIU `)0= EQRPEO\\ EOVR MQ EFVIQGI\nof the increase in direct precipitation proposed by Yang et al. to interpret their data.\nLow-resolution and not comparably well-dated evidence from Bolton et al. (2013,\nQuaternary Science Reviews) is discussed quite superficially (lines 120-121) in the\nmanuscript and in the supplementary information (lines 43-45). At glacial terminations,\nweakening of the monsoon circulation causes stratification in the southern Bay of\nBengal. I recommend adding some depth to this aspect of the study, as I think that\ninterpretation is not as straightforward as claimed by the authors. Some sort of\nVIQVMWMYMW\\ WIVWRJ WLI MPSEGW RJ WLI PM[IH OE\\IU HISWL RQ WLI VIEZEWIU `)0= NIISMQK\nprecipitation constant may help addressing this point. Also, I am unaware that there is\nupwelling in the southern Bay of Bengal as suggested by the authors (lines 120-121).\nRelationship between Hadley cell circulation and global temperature. This is a key\naspect of the manuscript and I do not think the authors use the latest, up to date,\nstatistically robust reconstruction of global surface temperature to convincingly tackle\nthis issue. The dataset of Osman et al. (2021, Nature) shows that during the early phase\nof Heinrich stadial 1 discussed by the authors global temperature begins to warm\n(approximately at 17 ka BP), so it was not cold as claimed by Yang et al. (lines 191-192)\nnor is a time of global climate stability. Such a change would occur right in the middle of\nthe early phase of Heinrich stadial 1 targeted by the authors. How would their current\ninterpretation change (or hold) considering this new evidence?\nMinor comments\nThe manuscript would benefit from being read through by an English native speaker."}, "author_response": "Response to Reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThe authors have adequately addressed the questions/concerns raised in my initial review of the\nmanuscript. The synthesis supports the contention of an unresolved question regarding climatic\nstate during HS1 and the data presented and interpretation thereof are internally consistent,\nproviding an explanation consistent with both the land and ocean evidence for the HS1 interval of\ntime. The proposed mechanisms are broadly testable in climate-model space and sufficiently\nprovocative to motivate that community.\n1.The manuscript will require substantial editing for grammar, some of which is address below,\nalong with other editorial, scientific comments and clarifications.\n[Response]\nWe are very sorry for the language problems in this manuscript and inconvenience they caused in\nyour reading. In this revised manuscript, the language presentation was improved with the aid of\nthe Scientific English Rewriting website (https://rewriting.ai/.), which specializes in providing\nscientific English writing services for the fields of oceanography and geology. We have then\nproofread the grammar and scientific expression of the whole manuscript to ensure that it meets\nthe standards of this journal.\n2. Despite the responses of the tropical hydroclimate to a North Atlantic cooling event during the\nHeinrich Stadial 1 (HS1) have been extensively studied in African, South American and\nIndonesian; the nature of such responses remains many debates.\nDespite the fact that the response of the tropical hydroclimate to a North Atlantic cooling event\nduring the Heinrich Stadial 1 (HS1) has been extensively studied in African, South American and\nIndonesian; the nature of such responses remains debated.\n[Response]\nThank you so much for your carefully reading and the improvement of the readability of the\nstatement there. We have revised these sentences following your suggestions in Lines 18-20 (in\nManuscript with revising marks).\n3. This study reveals that low global temperatures and El Niño activates both exerted a profound\ninfluence on the tropical hydroclimate in the Indo-Asian-Australian monsoon region during the\nearly HS1.\nThis study reveals that low global temperatures and El Niño both exerted a profound influence on\nthe tropical hydroclimate in the Indo-Asian-Australian monsoon region during the early HS1.\n[Response]\nThanks for your careful reading.\nThis sentence has been revised following your suggestion in Lines 27-28.\n4.We assume that precipitation in our study area was also linked with IOM and ITCZ rain belt\nsystem during HS1 because that marine-continental tectonic is consistent with it today and the\nstructure of land-sea thermodynamic between Indian Ocean and Eurasian Continent also exist.\nThe land-sea continent configuration argument for justification of the assumption is insufficient;\na great many factors can come to play in this regard, the least of which is tectonics, at the time-scale\nof this work. Presenting this argument will garner far more controversy than simply stating the\nassumption and carrying on.\n[Response]\nThank you for underlining this deficiency.\nWe have modified this sentence and deleted the part of “and the structure of land-sea\nthermodynamic between Indian Ocean and Eurasian Continent also exist” (Lines 74-76).\nOur model simulation has shown that precipitation in study area was also affected by Indian\nOcean Summer Monsoon and ITCZ during the HS1 (Fig. R2 in the first responses to reviewers’\ncomments), as well as Mohtadi et al. (2014) (Fig. R1). Thus, we feel it is plausible to assumed\nthat precipitation in our study area was also linked with IOM and ITCZ rain belt system during\nHS1.\nFig. R1 Results from the CCSM3 simulations of Heinrich stadial 1 (from Mohtadi et al., 2014)\n5. Our plankton tow samples from the study area show that G. ruber is mainly distributed in water\ndepths of 0-50 m, and that it can therefore be classed as a surface species A mixed-layer species?\n[Response]\nThank you very much for this suggestion.\nWe have corrected “a surface species” to “a mixed-layer species” in Line 89.\n,& ?JG PDTGSWGF TCMKOKUZ COF b18O values of surface water in the Northeastern Indian Ocean\nsw\nSGQPSUGF DZ 5GDSGIKPSIKT GU CM& QSPWG UJCU b18O values show a linear correlation with salinity in\nsw\nour study area.\nThe reference is for the coastal Andaman Islands, Andaman Sea region, not the equatorial region\nof the IO. Although very likely linear in this region as well, the d18Osw - salinity relationship\ncited does not prove the correlation in the study area. Caley et al., 2015 (10.1002/2014PA002720)\nand references therein may provide a better reference?\n[Response]\nWe are grateful for this suggestion.\nIn response to the concerns raised by the reviewer, we have incorporated additional modern\nPDTGSWGFb18O and salinity data from Delaygue et al. (2001), Singh et al. (2010) and Achyuthan\nsw\net al. (2013) to cover the southern region of the Bay of the Bengal and the equatorial East Indian\nOcean. These data have been included in the =&9440,1,28(6< /2-361(8/32 32 13+,62 >18O\nsw\nand salinity”&?JGTGb18O and salinity data exhibit a linear correlation, as depicted in Figure S2.\nsw\nWe have made relevant modifications to the main text (in Lines 103-110) and the Supplementary\nInformation (inLines 47-50, 68-72).\n?JG NPFGSO b18O and salinity data from the southern Bay of the Bengal and the Andaman Sea\nsw\nregion demonstrate a strong linear correlation (Figure S2). This implies that the reconstructed\nh18O values for Core I106 are indicative of the regional sea surface salinity (SSS) signal.\nsw\n-&9I'3C%>>?COFb).;TXSGEPOTUSVEUKPO&9I'3CWCMVGTXGSGEPOWGSUGFUPUGNQGSCUVSGVTKOIUJG\nequations developed by Anand et al. (2003) 71: Mg/Ca [mmol mol-1] = 0.38e0.09T[°C]. Seawater\nb18;\"b18O ) values were calculated using the equation proposed by Bemis et al. (1998): T [°C] =\nsw\n)+&/ % +&. \"b18O %b18O ). An additional 0.27‰ was added to them to convert the Vienna Pee\nc sw\nBelemnite (VPDB) values to Vienna Standard Mean Ocean Water (VSMOW) values.\nPlease explicitly provide the s18Osw-salinity relationship used.\n[Response]\nThank you very much for your comment.\nWe are very sorry for the confusion we caused.\n)#;VSTUVFZCKNGFUPCOCMZ[GUJGSGMCUKPOTJKQDGUXGGONPFGSOb18O and salinity in the southern\nsw\n2CZ PH 2GOICM& @G HPVOF C TUSPOI MKOGCS EPSSGMCUKPO DGUXGGO b18O and salinity, indicating that\nsw\nb18O in this region can be used as an indicator of changes in regional sea surface salinity.\nsw\n*# 7O UJKT TUVFZ$ XG SGEPOTUSVEUGF b18O DZ 9I'3C%>>? COF b18O values of planktonic\nsw\nforaminifera G. ruber VTKOIUJGGRVCUKPOPH2GNKTGUCM&\")//.#0?A]3B1)+&/%+&.\"b18O %b18O ).\nc sw\n?JG SGEPOTUSVEUGF b18O KO 3PSG 7)(, EPVMF DG EPOTKFGSGF CO KOFKSGEU TCMKOKUZ QSPYZ$ CT b18O\nsw sw\nand salinity showed a strong correlation in our study area (Figure S2). Previous paleo-records\nHSPNUJG4CTU7OFKCO;EGCOCOFUSPQKECM@GTU<CEKHKEJCWGCMTPVTGFb18O to indicate changes in\nsw\nSSS (Mohtadi et al., 2010; 2014; Rashid et al., 2007, 2011; Gibbons et al., 2014). We did not\ncalculate and provide SSS values for Core I106 in this study, as we wanted to directly\n*314(6, 396 >18O records with previous studies. Therefore, we feel it is not necessary to\nsw\n463:/+, 8., ,59(8/32 3- 8., >18O -salinity relationship in the Methods part because it was\nsw\nnot used.\n3) if necessary, we understand the formula in Figure S2 can be used to convert the reconstructed\nb18O into SSS.\nsw\n8. Meanwhile, there would be more salt water transported from the south into our study area if the\nXKOUGS9POTPPO3VSSGOUTUSGOIUJGOGF$POUJGEPOUSCSZ$b).;TXWCMVGTFGEMKOGFCMPUKO3PSG7)(,\nduring the early HS1.\nMeanwhile, there would be more salt water transported from the south into our study area if the\nwinter Monsoon Current strengthened;POUJGEPOUSCSZ$b).;TXWCMVGTFGEMKOGFCMPUKO3PSG7)(,\nduring the early HS1.\n[Response]\nThanks for your carefully reading.\nWe have corrected this sentence following your suggestion in Lines 178-180.\n9. Tropical hydroclimate within HS1 from the north of the equator in the IAA monsoon realm also\nexist two distinct phases, with wet hydrological condition at about 18.3-16.3 ka and dry condition\nat ~16.3-14.7 ka, which was in line with this two-step AMOC slowdown (Fig. 4E).\nTropical hydroclimate within HS1 from the north of the equator in the IAA monsoon realm also\nexhibit two distinct phases, with wet hydrological condition at about 18.3-16.3 ka and dry\ncondition at ~16.3-14.7 ka, which was in line with this two-step AMOC slowdown (Fig. 4E).\n[Response]\nThank you very much for pointing out our writing shortcomings.\nWe have corrected this sentence following your suggestion in Lines 248-250.\n10. Accordingly, variations in tropical precipitation patterns are not only affected by the\ninterhemispheric temperature difference, but also be associated with other driving factors.\nAccordingly, variations in tropical precipitation patterns are not only affected by the\ninterhemispheric temperature difference, but also associated with other driving factors\n[Response]\nThanks for yourcarefulness and patience.\nWe have corrected this sentence following your suggestion in Lines 233-235.\nReviewer #3 (Remarks to the Author):\nI have read with great interests the manuscript by Yang and co-workers “A contracting\nIntertropical Convergence Zone during the Early Heinrich Stadial 1” that present new foraminferal\ndata from a sediment core located in the southern Bay of Bengal to investigate the response of the\nIndian-Asian-Australian monsoon region to global and regional changes during Heinrich stadial 1.\nThe authors propose more complex developments of the Intertropical Convergence Zone (ITCZ)\nthan previously documented/thought during Heinrich Stadial 1, for which the occurrence of an\nearly phase of contracted ITCZ rain belt north of the equator is claimed. The topic is very\ninteresting and worth of publication in Nature Communications, the data are of an overall good\nquality although their interpretation could be deeper and more robust, and the analysis of the\ncomplementing evidence could (and should) be improved. The palaeocenographic time series\npresented lack the Holocene section, leaving the reader with the question of how relevant the\namplitude of the discussed palaeoclimate is signals with respect to the glacial-interglacial\nhydroclimate changes in the region.\nMy review centres on two aspects of the manuscript, namely the (overlooked) role of the monsoon\nwinds on the hydrography of the southern Bay of Bengal and the relationship between the findings\nof Yang et al. and the global temperature changes.\nThank you for your precious comments and suggestions, which are all valuable and very helpful\nfor revising and improving our paper. We have revised the manuscript accordingly, and our point-\nby-point responses are presented below.\n)& =PMG PH UJG XKOFT WGSTVT QSGEKQKUCUKPO& ?JG CVUJPST EMCKN UJCU UJGKS TGCXCUGS b).;\nreconstructions from the southern Bay of Bengal reflects changes in direct precipitation and runoff\nalone. Besides the doubts I have on the impact of the latter on the hydrography of the southern\nBay of Bengal, this claim is contrasted with the NCEP/NCAR 40-year reanalysis project (Kalnay\net al., 1996, Bulletin of the American Meteorological Society), which highlights the importance of\nthe winds associated with the Indian monsoon system in that sector of the bay. Accordingly, I\nXPOFGS XJCU UJG SGTQPOTG PH UJG TVSHCEG PEGCO TGCXCUGS b18O would be to a change (e.g., a\nweakening) of the wind mixing of the upper ocean during HS1? This hypothesis needs to be\nconsidered, because a shallower (smaller volume) mixed layer associated to a stratified surface\nPEGCO XPVMF ^NCYKNKTG_ UJG OGICUKWG TGCXCUGS b).; COPNCMZ CMTP KO CDTGOEG PH UJG KOESGCTG KO\ndirect precipitation proposed by Yang et al. to interpret their data. Low-resolution and not\ncomparably well-dated evidence from Bolton et al. (2013, Quaternary Science Reviews) is\ndiscussed quite superficially (lines 120-121) in the manuscript and in the supplementary\ninformation (lines 43-45). At glacial terminations, weakening of the monsoon circulation causes\nstratification in the southern Bay of Bengal. I recommend adding some depth to this aspect of the\nstudy, as I think that interpretation is not as straightforward as claimed by the authors. Some sort\nPH TGOTKUKWKUZ UGTU PH UJG KNQCEU PH UJG NKYGF MCZGS FGQUJ PO UJG TGCXCUGS b).; LGGQKOI\nprecipitation constant may help addressing this point. Also, I am unaware that there is\nupwelling in the southern Bay of Bengal as suggested by the authors (lines 120-121).\n[Response]\nWe are extremely grateful to reviewer for pointing out this problem.\nFirstly, in response to your valuable suggestions, we have obtained the modern wind speed and\nprecipitation data from the NCEP reanalysis project (1948-2022,\nhttp://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=16341) and sea surface salinity (SSS) data\nfrom the Hadley analyses project (1948-2022,\nhttp://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=16341). These datasets (1948-2022) reveal\nthat the seasonal cycle in SSS change within a year is closely associated with precipitation in\nthe study area, rather than wind speed (Fig. S1/Fig. R2). The lowest SSS values are observed\nfrom August to November, coinciding with the highest precipitation. We have also calculated the\nprecipitation minus evaporation (P-E) fluxes in study area (Fig. R2), which evidently\nsuggests that the change in SSS in the study area is closely linked to the flux of fresher water.\nFig R2 Mean monthly precipitation (blue triangle), mean monthly SSS (orange circle), mean\nmonthly wind speed (grey bar) and fresher water flux (Precipitation-Evaporation, red diamond) in\nstudy area.\nSecondly$ XG CMTP JCWG EPOFVEUGF 9I'3C COF b18O analyses on planktonic foraminifera\nNeogloboquadrina dutertrei, a thermocline species, in Core I106. The temperature difference\nbetween sea surface (G. ruber) and thermocline layer (N. dutertrei# \"c? ) in Core I106\nruber-dutertrei\nexhibited an increase trend since ~21ka with several millennial-scale fluctuations, indicating a\nrelatively weak water stratification during this period. $38()0<\" ?' in Core I106 did\nruber-dutertrei\nnot exhibit significant changes at 16.3-18.3 ka (Fig. R3). We therefore consider that the abrupt\nKOESGCTG KO b18O in Core I106 at 16.3-18.3 ka was primarily related to precipitation rather than\nsw\nwater stratification.\nFig. R3 The temperature difference between sea surface and thermocline water and G. ruberb18O\nvalues since 24 ka.\nThirdly$ PO UJG POG JCOF$ UJG b18O records in Core RC12-344 (Fig. R3) and ADM-159 in the\nsw\nnortheastern Indian Ocean indicate that SSS was high during the last deglaciation period, and then\nit began to decrease at the onset of the Early Holocene, and SSS increased during the Late\n6PMPEGOG& 6PXGWGS$ UJG ISCFVCM KOESGCTG KO c? in Core I106 since 21 ka, indicates a\nruber-dutertrei\nweakened Indian Ocean Summer Monsoon and water stratification. The changing trend in the\n>18O 6,*36+7-6318.,2368.,(78,62#2+/(2%*,(2/7/2*327/78,28;/8.8.(83-?'\nsw ruber-dutertrei\nin Core I106 (Fig. R3). These findings suggest that precipitation may have played a more\nimportant role in the long-term changes in SSS in this region, rather than wind intensity.\n;O UJG PUJGS JCOF$ b18O records obtained from planktonic foraminifera G. ruber in the northeast\n7OFKCO ;EGCO JCWG TJPXO UJCU UJG b18O value was the highest during the Early Holocene and\ngradually declined since about 5.0 ka due to the weakening of summer monsoon precipitation\nduring the Holocene (Rashid et al., 2007, 2011, Govil et al., 2011, Mahtadi et al., 2014,\n5GDSGIKPSIKTGUCM&$*().#&;VSb18O record in Core I106 also shows a similar changing trend from\nUJG4CSMZ6PMPEGOGUPUJGMCUG6PMPEGOG$TVIIGTUKOIUJGEJCOIGKOb18O record may be also closely\nrelated to precipitation.\n2. Relationship between Hadley cell circulation and global temperature. This is a key aspect of the\nmanuscript and I do not think the authors use the latest, up to date, statistically robust\nreconstruction of global surface temperature to convincingly tackle this issue. The dataset of\nOsman et al. (2021, Nature) shows that during the early phase of Heinrich stadial 1 discussed by\nthe authors global temperature begins to warm (approximately at 17 ka BP), so it was not cold as\nclaimed by Yang et al. (lines 191-192) not is a time of global climate stability. Such a change\nwould occur right in the middle of the early phase of Heinrich stadial 1 targeted by the authors.\nHow would their current interpretation change (or hold) considering this new evidence?\n[Response]\nThank you very much for your comments.\n1) we have added the latest statistically robust reconstruction of global surface temperature\n(Osman et al., 2021) in Figure 4 in this revised version. The modelled global surface\ntemperature shows that the Earth was in a ubiquitously cold glacial state at 24-17 ka, and the\ntemperatures during this period in the high northern (>45°N) and southern (>45°S) latitudes were\nboth cooler than the preindustrial era, below -20°C (Osman et al., 2021). The reconstructed global\nsurface temperature by Shakun et al. (2012) also documents that there was about 0.3°C of global\nwarming before about 17.5 ka. They both suggest that the global surface temperature was still\nquite low in the early phase of the HS1 (about 18.3-16.3), although it began to warm slightly. The\nglobal surface temperature in the latter phase of the HS1 increased remarkably.\n2) meanwhile, during the Early HS1, there was a sudden increase in heat advection into the low\nlatitudes of the Indian Ocean due to the anomalous transportation of heat northward into the\nnorthern high latitudes and a more vigorous Indonesian Throughflow (ITF) associated with the\nexpansion of the Indo-Pacific Warm Pool (IPWP) (Deckker et al.,2012). This is supported by a\nsteep and abrupt rise with a magnitude of > 1.0°C at about 19.5-18.0 ka in the East Indian Ocean\n(Mohtadi et al., 2010, 2014, Saraswat et al., 2005), and warmer SSTs events around 20 ka and 17\nka from northern Arabian Sea(Govil and Naidu, 2010). With enhanced tropical SST warming in\nEast Indian Ocean, the latitudinal migration of the ITCZ potentially reduced, especially as the\nseasonally-affected ITCZ generally locates over the warm ocean (Zhou et al., 2019) (Lines 258-\n275).\n3) During the early deglaciation, ENSO characteristics change drastically in response to meltwater\ndischarge and the resulting changes in the AMOC (Liu et al., 2014). Weakened AMOC lead to\nstronger ENSO activities, and a more El Niño-like state considering generally stronger EI Niño\nUJCO 8C :K\\C& ?JG [POCM c>>? DGUXGGO @GTU <CEKHKE COF 4CTU <CEKHKE COF NPFGM TKNVMCUKPO\nsuggest a more El Niño-like state in the early HS1(Liu et al., 2014, Koutavas and Joanides, 2012).\nDue to anomalous warming generated by EI Niño under this state, the tropical troposphere\nbecomes warmer, and subtropical troposphere is cooler, which enhances the meridional\ntemperature gradient, and then results in shrinking of the Hadley circulation in both Hemispheres\n(Lines 276-284).\nGenerally speaking, we consider that the contracting precipitation belt was not affected by a\nsingle factor, but rather a result of multiple factors working together, including mainly\nmainly related to the meltwater discharge, the resulting changes in the AMOC, the global\ntemperature response and EI Niño.\n3. Minor comments. The manuscript would benefit from being read through by an English native\nspeaker.\n[Response]\nThank you for your meticulous evaluation of our manuscript.\nWe sincerely apologize for any errors that may have impeded your reading experience. We have\ntaken the necessary steps to rectify these issues by thoroughly revising and rewriting the\nmanuscript with the aid of the Scientific English Rewriting website (https://rewriting.ai/.). Our\nteam has meticulously scrutinized each sentence for grammatical accuracy and scientific precision,\nensuring that the manuscript meets the journal’s standard.\nOur deepest gratitude goes to reviewers for reviewers’ careful work and\nthoughtful suggestions that have helped improve this paper substantially."}, {"round": 3, "reviewer_comments": {"reviewer_3": "This is the second time I review the manuscript by Yang et al., on the Intertropical Convergence\nZone latitudinal changes and associate impacts on the tropical hydroclimate during Heinrich Stadial\n1 (HS1), with emphasis on the Indian ocean and the westernmost sector of the Pacific Ocean. The\nmanuscript has greatly improved, notably the improved text (through revision of the English\nsyntax) allows to follow the scientific content way better than in previous version. Accordingly, I\nwould support acceptance in <i>Nature Communications</i>, pending revision related to the point\ndetailed below.\nIn the previous assessment, my concerns centred on 3 main aspects of the study:\n(1) The lack of the Holocene section in the new palaeoceanographic time series presented for core\nI106, leaving the reader wondering about the actual amplitude of the discussed palaeoclimate\nsignals with respect to the glacial-interglacial changes at the same site.\n(2) Role of the winds <i>versus</i> precipitation in the interpretation of the seawater 18O\n\"T-IKG.)+-'IKG.6-IK:.78-'IK:.# ;@9E?=I <AI;KII=< :O J@= 9KJ@FHI, 0A?KH= * F> J@= \nmanuscript. This would allow highlighting the amplitude of the HS1 temperature and\nT-IKG.)+-'IKG.6-IK:.78-'IK:. $ 5?'/9$ 9E< T-IKG.)+-'IKG.6-IK:.78-'IK:.# 0A?KH= * F> J@=\nmanuscript. This would allow highlighting the amplitude of the HS1 temperature and\nT-IKG.)+-'IKG.6-IK:.78-'IK:. ;@9E?=I MAJ@AE J@= ;FEJ=NJ F> J@= >KCC ?C9;A9C%AEJ=H?C9;A9C\ntransition, so to provide a more complete analysis of the hydrographic changes in the study. In\n9<<AJAFE$ AJ MFKC< @=CG LAIK9CAPAE? J@= T-IKG.)+-'IKG.6-IK:.78-'IK:. IA?E9JKH= F> J@= 49J=\nHolocene relative to the early and middle Holocene. Note that Late Holocene\nT-IKG.)+-'IKG.6-IK:.78-'IK:. <9J9 J@9J 9H= KI=< JF ;FHHF:FH9J= J@= H=C9JAFEI@AG :=JM==E\nT-IKG.)+-'IKG.6-IK:.78-'IK:. 9E< I9CAEAJO ?AL=E J@9J RQ=IJAD9J=I F> \nT-IKG.)+-'IKG.6-IK:.78-'IK:. L9CK=I <KHAE? J@= 49J= 2FCF;=E= \"(%* B9# >9CC M=CC MAJ@AE J@AI\nCAE=9H T-IKG.)+-'IKG.6%I9CAEAJO ;FHH=C9JAFES& 3E ;FE;CKIAFE$ GH=I=EJ9JAFE F> J@= >KCC H=;FH< >FH\ncore I106 would make the analysis upon which the manuscript is centred more transparent and\ncompelling.\nPoint 2 seems to be addressed, although some questions remain, I think that existing knowledge\nto agree with the authors’ interpretation.\nPoint 3 has been satisfactorily addressed."}, "author_response": "Response to REVIEWERS' COMMENTS\nReviewer #3 (Remarks to the Author):\nThis is the second time I review the manuscript by Yang et al., on the Intertropical Convergence\nZone latitudinal changes and associate impacts on the tropical hydroclimate during Heinrich\nStadial 1 (HS1), with emphasis on the Indian ocean and the westernmost sector of the Pacific\nOcean. The manuscript has greatly improved, notably the improved text (through revision of\nthe English syntax) allows to follow the scientific content way better than in previous version.\nAccordingly, I would support acceptance in Nature Communications, pending revision related\nto the point detailed below.\nIn the previous assessment, my concerns centred on 3 main aspects of the study:\n(1) The lack of the Holocene section in the new palaeoceanographic time series presented for\ncore I106, leaving the reader wondering about the actual amplitude of the discussed\npalaeoclimate signals with respect to the glacial-interglacial changes at the same site.\n(2) Role of the winds versus?@64:?:B2B:>= := B96 :=B6@?@6B2B:>= >7 B96 A62E2B6@ '(/ \"J18O )\nSW\nchanges discussed by the authors;\n(3) Relationship between Hadley cell circulation and global temperature.\nPoint 1 was somewhat addressed in the response letter but not in the manuscript. Figure R3\ndisplays the Globigerinoides ruberJ18O and the temperature gradient computed using G.\nruber and (the thermocline species) N. dutertrei from core I106. Given that the data exist, my\n@64><<6=52B:>=:AB>6=@:49E:B9B9:A:=7>@<2B:>=\"*%@C36@J18/#-8&)2#2=5J18O ) Figure\nSW\n2 of the manuscript. This would allow highlighting the amplitude of the HS1 temperature and\nJ18O changes within the context of the full glacial-interglacial transition, so to provide a more\nSW\ncomplete analysis of the hydrographic changes in the study. In addition, it would help\nD:AC2;:G:=8B96J18O signature of the Late Holocene relative to the early and middle Holocene.\nSW\n.>B6 B92B ,2B6 +>;>46=6 J18O data that are used to corroborate the relationship between\nSW\nJ18O 2=5A2;:=:BF8:D6=B92BIH6AB:<2B6A>7J18O values during the Late Holocene (0-2 ka)\nSW SW\n72;; E6;; E:B9:= B9:A ;:=62@ J18O-salinity correlation”. In conclusion, presentation of the full\nSW\nrecord for core I106 would make the analysis upon which the manuscript is centred more\ntransparent and compelling.\n[Response]\nWe are grateful for this suggestion.\nAs suggested by the reviewer, we have added the Holocene data of G. ruber J18O, Mg/Ca,\n-8&)2$0012=5J18O in Figure 2 and Source Data file.\nSW\nPoint 2 seems to be addressed, although some questions remain, I think that existing knowledge\nto agree with the authors’ interpretation.\n[Response]\nWe appreciate the reviewer’s positive evaluation of our work and agree with the comments\nregarding the limitations of our study.\nPoint 3 has been satisfactorily addressed.\n[Response]\nOur deepest gratitude goes to you for your careful work and thoughtful suggestions that have\nhelped improve this paper substantially."}]

Decision letter