Atlantic Ocean thermal forcing of Central American rainfall over 140,000 years

Domain: earth_science
Published: 2024-12-04
Editorial outcome: minor_revision

Abstract
Tropical hydroclimate in monsoonal regions has been largely understood according to the orbital monsoon hypothesis, in which rainfall exhibits strong covariation with local summer insolation on precessional (~21,000 years) time scales, as exemplified in the Asian and South American monsoon stalagmite records. However, paleo-rainfall variations in some tropical regions are poorly explained by the orbital hypothesis, suggesting alternative forcing mechanisms of regional monsoon changes. Here, we show a 140,000-year record of Central American rainfall from oxygen-isotope (δ18O) time series of precisely dated stalagmites which reveal two dominant thermally-controlled monsoon regimes in which the Atlantic Ocean thermal state linked to the meridional overturning circulation (AMOC) is the primary driver, and local orbital summer insolation control is limited. Our reconstruction, supported by isotope-enabled climate model simulations, pinpoints the potential impacts of future AMOC weakening on the Central American and Caribbean climate.

Introduction
The orbital monsoon hypothesis predicts that local summer insolation on precessional time scales drives monsoon circulation, for example by heating the large Asian and South American landmasses1. Superimposed on precessional-scale rainfall variations in these regions are anti-phased abrupt monsoon events linked to ice-rafted debris deposition in the North Atlantic Ocean called Heinrich events, and associated with monsoon weakening in Asia2 and strengthening in South America3, as revealed by cave calcite δ18O. In the tropics, changing rainfall amounts and oxygen-isotope composition (δ18O p) have been attributed both to changes in local summer insolation that influence atmospheric circulation and to latitudinal shifts in the Intertropical Convergence Zone (ITCZ), which favors the warmer hemisphere4. Such coherent antiphasing of rainfall characterizes a unified circulation system known as the Global Monsoon5,6. However, rainfall variations in some major tropical regions, notably in Central America and the Caribbean7–9, eastern Mexico10, and other regions11, lack a clear precessional scale forcing, possibly because of their smaller continental area relative to surrounding oceans hampered large-scale land-sea thermal gradients (typical of classic monsoon systems), or because of a stronger ocean control on atmospheric dynamics.

Testing for alternative hypotheses on primary drivers of rainfall variations, such as Atlantic Ocean temperature or dynamics12 requires hydroclimate reconstructions near the tropical North Atlantic Ocean, a key center of action for abrupt climate change13. For example, a strong coupling between millennial-scale climate variability recorded in marine sediments of the Cariaco Basin of northern South America and similar events in Greenland ice cores demonstrates a strong coupling between high and low latitude climate in the Atlantic/Caribbean region14,15. However, variations in the strength of tropical convection over and upwind of Central America may not behave similarly to the Cariaco Basin, particularly as Central America is outside the core oceanic region of the ITCZ and responds to more complex ocean-atmosphere dynamics other than solely north-south ITCZ latitudinal changes. Thus, precisely-dated paleoclimate records from continental locations north of 10° N would provide key information on the temporal and dynamical response of Central American regional monsoon to Atlantic Ocean forcing.

The Atlantic meridional Overturning circulation (AMOC) is one of the core tipping elements of the Earth’s climate system16 capable of triggering far-reaching, potentially irreversible global changes. Paleoclimate evidence documents rapid transitions between stable circulation modes of AMOC throughout the last glacial cycle17: a strong (interglacial) mode in which warm surface waters are transported to the northern high latitudes, release heat to the atmosphere, cool and sink at depths to form deep water masses moving southward (North Atlantic Deep Water formation); and a weak (glacial) circulation mode where northward heat transport is reduced, oceanic convection is shallower, and heat accumulates in the tropics. Tropical hydroclimate reconstructions have shown that changes between AMOC states cause shifts in the latitudinal position of the rain belt and the intensity of monsoon precipitation18. Presently, AMOC transports heat from the tropics to the northern hemisphere high latitudes via the Gulf Stream, contributing to a northern mean position of the tropical rain belt19,20. Current state-of-the-art climate models project future AMOC weakening in response to greenhouse gas warming21, albeit with widespread inter-model discrepancy regarding magnitude and rate of slowdown22. For associated impacts, key uncertainties are the role of AMOC on past and future tropical rainfall variability4,23 and the regions most sensitive to future AMOC decline22,24,25.

Central America is an ideal location to test for AMOC-tropical climate controls because it is teleconnected to the AMOC upper limb via the northeasterly trade winds, downwind of the large heat reservoirs of the tropical North Atlantic, Caribbean Sea, and Gulf of Mexico (the broader Atlantic Warm Pool7,26). The rainfall annual cycle responds to interactions between regional SST, ITCZ latitudinal changes, and near-surface wind dynamics, primarily northeasterlies and the Caribbean Low-Level Jet (CLLJ), linked to east-west excursions of the North Atlantic Subtropical High (NASH)27–29. About ~65% of the 2500 mm of our site’s annual rainfall30 occurs during the May through November wet season when SSTs exceed 27.5 °C, the Atlantic Warm Pool is at its maximum areal extension31, and a weaker CLLJ over the Caribbean contributes to deep convective systems. During winter (December-April), an intensified CLLJ linked to the westward expansion of NASH and lower SST increases atmospheric subsidence over the Caribbean suppressing regional convection27. Winter rainfall, which accounts for ca. 35% of the annual total precipitation, consists mostly of localized convection and northerly-sourced (nortes) air masses. We refer to the strong seasonality of Central American rainfall as part of the Global ‘monsoon’, while recognizing that the region lacks some of the distinguishing features of classic monsoon circulations such as large-scale wind reversals.

Here, we show from a speleothem-based paleo-rainfall record over the last glacial cycle (140,000 years to present) from Guatemala that the Atlantic Ocean overturning circulation (AMOC) and sea surface temperature (SST) drive Central American hydroclimate variations and propose a strong link between AMOC and monsoon strength. This thermally-driven Central American monsoon is distinct from precession-driven monsoon systems elsewhere in the tropics, and highlights the direct forcing of Atlantic Ocean thermal states on Central American regional climate.

Results
Glacial-interglacial monsoon convective states
We reconstruct Central American hydroclimate from calcite δ18O time series of precisely dated speleothems from three Maya highland caves at ~15° N latitude on the Caribbean slope of Guatemala (Supplementary Fig. 1), Grutas del Rey Marcos (RM)12, Cueva Jul Iq (JQ), and Cueva Bombil Pek (BO). Our reconstruction is anchored by ninety-three 230Th radiometric ages on nine replicating stalagmites to produce a composite timeseries, which was adjusted for inter-cave differences (related to the elevation difference between caves sites) and for the ice-volume corrected seawater isotopic composition (δ18O ivc, in ‰ Vienna Pee Dee Belemnite, VPDB throughout)32 (Supplementary Fig. 2; Methods). We interpret δ18O ivc as primarily a proxy for δ18O of precipitation (δ18O p) reaching the cave site, which in turn is controlled by rainfall amount along the prevailing source-to-cave moisture trajectory33.

At our cave sites, lower monthly δ18O p values are associated with enhanced rainfall from deep convection during May-November (r = −0.68, p < 0.0001; Supplementary Fig. 3, 4). Although seasonal changes of the ITCZ latitudinal position modulate regional moisture convergence across tropical America, today the ITCZ does not reach the latitude of our cave sites (15° N) during boreal summer7. Rather, a broad zone of convection persists over Central America north of the oceanic ITCZ. We thus discuss δ18O variations in our record in terms of regional convective intensity to distinguish it from proxy records from lower latitudes, such as the Cariaco Basin15 (10 °N), that are more directly affected by north-south ITCZ changes. Cave drip waters integrate amount-weighted precipitation δ18O values, thus lower δ18O indicates more regional convection and a stronger monsoon and higher δ18O reflecting less convection (Supplementary Fig. 5). Our study sites are particularly sensitive to tropical North Atlantic processes (Supplementary Fig. 6) because ~75% of the annual precipitation originates in the Caribbean/western tropical Atlantic region34, with smaller contributions from northern extra-tropical cold-fronts (~15%) and eastern tropical Pacific (~10%).

Central American paleoclimate is defined by a distinct bi-modal glacial-interglacial topology over the last 140,000 years (Fig. 1), suggesting the existence of two primary convective regimes: a weak glacial monsoon and a strong interglacial monsoon, either of which may be interrupted by even more extreme abrupt episodes of monsoon weakening. The prolonged strong monsoon between MIS 5e and the first half of MIS 4 lasted more than 60,000 years and was punctuated by several millennial-scale weak monsoon events.

Following ice age terminations I and II (T1 and T2 in Fig. 2), peak monsoon strength was delayed for several thousand years behind minimum global ice volume, peaking at ca. 125,500 yr BP and 9000 yr BP, respectively, and in both cases relatively stable and wet conditions were maintained for several millennia. A large and abrupt transition to a glacial weak monsoon state occurred between 66,000 and 60,000 yr BP, contemporaneously to intensified iceberg discharges in the North Atlantic35,36 and to an abrupt AMOC slowdown17 (Fig. 2).

Our observations reveal that convective intensity was decoupled from orbital precession during the last glacial/interglacial cycle (Fig. 1a, b): wet intervals lasted longer than a precessional cycle and spanned insolation lows, whereas dry intervals were commonly shorter than a full precessional cycle. Thus, a strong glacial-interglacial cycle distinguishes Central America from monsoons in Asia2, South America37, and Borneo38 that are more clearly paced by precession and local summer insolation (Supplementary Fig. 7).

AMOC and SST forcing
Comparisons with North Atlantic paleoceanographic reconstructions17,36,39 show that Central American monsoon intensity is primarily responding to tropical and subtropical North Atlantic SST and AMOC strength, with superimposed millennial-scale monsoon collapses during cold North Atlantic Heinrich stadials (Fig. 2). For example, SST records from the Iberian Margin36 and the Guiana Basin39 off northern South America show a thermal structure similar to our rainfall record, with a prolonged last interglacial period punctuated by millennial scale cooling and drying excursions. The peak warming off Iberia at 127,100 yr BP (Fig. 2e) closely overlaps the resumption of strong convection over Central America but is delayed behind the ca. 130,000 yr BP global ice volume minimum.

Three major weak monsoon intervals (high δ18O ivc) coincided with Arctic water penetration to the subtropics during MIS 5 as inferred from high C 37:4 alkenone ratios in marine sediments, at 111,500 to 107,500 yr BP (MIS 5d), 105,600 to 103,600 yr BP (MIS 5c), and 89,000 to 85,000 yr BP (MIS 5b) (Fig. 2a). These monsoon collapses coincide with a period of moderate to low global ice volume (Fig. 2g), suggesting that during relatively warm conditions, Central American rainfall lies close to a tipping point in which slight cooling may produce extensive monsoon suppression.

Further, we show that millennial-scale weak monsoon periods over the last glacial/interglacial cycle were synchronous within dating uncertainties to cooling episodes in the northern high- and mid-latitudes36,40 related to ice-rafted debris (IRD) deposition and meltwater routing to the North Atlantic Ocean, called Heinrich (H) events or stadials (HS)35,41. For instance, the weakest monsoon convection in Central America just prior to Terminations II and I are coeval to the largest North Atlantic HS 11 and HS 1, while the abrupt dry phases at the MIS 5b and MIS 5d coincide with HS 8 and HS 10 (Ref. 42; Fig. 2). Less severe monsoon weakenings are also recorded in the Guatemalan stalagmites throughout both warm MIS 5 and cold stages MIS 4-2. Although our monsoon reconstruction shares Heinrich dry events with the Cariaco Basin, our data lack the distinct millennial-scale Dansgaard–Oeschger (DO) variability evident in the Greenland ice core δ18O43 and Cariaco Basin records15. These differences, along with the stronger correspondence between our record and Atlantic SST, suggest that Central American monsoon strength is responding to different forcings than those inferred from the marine sediments in the Cariaco Basin and has a unique paleoclimatic history separate from the southern Caribbean15,44.

Remarkably, the Central American monsoon responded on a one-to-one basis (Fig. 3) with AMOC over the last deglaciation, as detailed in the 231Pa/230Th ratios in Bermuda rise ocean sediments17,45 in the subtropical North Atlantic. Our record indicates moderate monsoon strength at the Last Glacial Maximum (LGM, ca. 20,000 to 18,000 yr BP in our record), a monsoon collapse at 17,500 to 14,800 yr BP (HS 1), a return to wetter conditions during the Bølling-Allerød (B-A, 14,800 to 12,800 yr BP), another monsoon weakening during the Younger Dryas (YD, 12,800 to 11,600 yr BP), and a gradual monsoon strengthening in the Early Holocene to reach near-modern values by 9000 yr BP12. Each of these weak monsoon periods was associated with cooling and AMOC slowdowns, and vice versa for strong monsoon intervals. The moderate monsoon during the LGM is linked to similarly moderate AMOC strength comparable to that during the B-A, and provides evidence for an active – though weaker than modern – monsoon during the Last Glacial Maximum in Guatemala. Reconciling this observation with proxy data indicating a strong LGM monsoon elsewhere in the neotropics is an important target for future work7,46.

Model constraints on AMOC-monsoon link
We use iTRACE47 data – an ensemble of transient isotope-enabled simulations based on the CESMv1.3 and spanning 20,000 to 11,000 yr BP - to demonstrate that during the last deglaciation the strength of the Central American monsoon can be explained by synchronous changes in simulated δ18O p, convective intensity, and AMOC strength (Fig. 4). For instance, the iTRACE full forcing experiment (including ice sheet, greenhouse gas, orbital, and meltwater forcing) shows that an increase in speleothem δ18O ivc coincides with decreases in AMOC strength, North Atlantic surface temperature, and regional convective rainfall and a convective activity index in Central America and Caribbean (see Methods for details). Simulated δ18O p values increase during HS 1 and the YD, almost perfectly matching the speleothem δ18O ivc and weak convective intervals (Figs. 4b and d).

The iTRACE simulation shows that during HS 1 rainfall was suppressed from Central America east to the Caribbean Sea, coupled with a general weakening of convective activity over the North Atlantic (Figs. 5b and d). The rainfall reduction is clearly linked to Caribbean SST cooling of ~5 °C associated with meltwater influx to the North Atlantic and a pronounced slowdown of the AMOC. The strongest iTRACE precipitation response is seen in the May through November wet season, while weaker for winter (DJFMA) rainfall (Fig. 4c). The eastern tropical Pacific Ocean was not an important moisture source, in agreement with modern moisture back-trajectory analyses (Supplementary Fig. 6). SST cooling within the tropical North Atlantic and Caribbean coincides with near-surface wind intensification describing a southward shift of moisture convergence and convection strengthening over South America (Fig. 5a, b). Similar but less pronounced responses are seen for the YD minus Early Holocene (not shown).

Comparing the iTRACE transient simulations with and without meltwater forcing allows isolation of the AMOC forcing (Fig. 6) on Central American rainfall. We find significant decoupling between simulated δ18O p and precipitation in the experiment without meltwater forcing (Fig. 6b, c). Furthermore, the stalagmite δ18O ivc and iTRACE no meltwater results (black in Fig. 6) are strongly different, demonstrating that the variability observed in speleothem δ18O data is best explained by the AMOC slowdown associated with meltwater forcing in the North Atlantic.

Atlantic mean state control on rainfall
Given the temporal dissimilarity between Central America and other tropical monsoon regions more strongly paced by orbital precession (Supplementary Fig. 7), we posit that the Central American monsoon represents a thermally-controlled monsoon, distinct from the Global Monsoon. Central America may be distinguished from other monsoon regions because of its physiography of a small and narrow isthmus bounded by a major ocean heat reservoir whose energy budget drives regional convection31. We propose that AMOC intensity, tropical Atlantic SST, and northern hemisphere ice sheet extent were the dominant rainfall controls over Central America during the last 140,000 years BP, enhanced by Central America’s proximity to the AMOC center of action. For instance, the positive δ18O ivc excursions between late MIS 4 and MIS 2 (~65,000–17,000 yr BP) occurred coevally with Laurentide Ice sheet expansion and AMOC shifting to a weak glacial circulation mode13,17, which suppressed Caribbean and Central American atmospheric convection. Furthermore, the lack of clear DO events in our record suggests that Central American paleoclimate differs from that inferred from the Cariaco Basin and the two regions should be viewed as distinct climate regions. Hydroclimate observations in the Caribbean and Central American regions can be now understood as a predominantly thermodynamic response to changing Atlantic Ocean thermal states, and our study thus reconciles previous regional hydroclimate interpretations from the Bahamas48, Cuba9,49, Puerto Rico50, Costa Rica8, Mexico10,46,51, and Guatemala7 (Supplementary Fig. 8), which can now be considered a unified regional Central American and Caribbean monsoon system driven primarily by Atlantic Ocean processes.

A bi-modal speleothem δ18O ivc distribution (Supplementary Fig. 9) supports the existence of two main monsoon states, and further implicates the existence of an SST threshold below which widespread atmospheric convection is diminished, possibly related to an SST threshold of ~27 °C that sustains vigorous convection today12,52,53. Considered in relation to the background climate boundary conditions, temperature thresholds within the tropical North Atlantic basin might have been lower than modern during intervals of larger/intermediate Northern Hemisphere ice sheets54 and could potentially increase under future greenhouse warming53,55.

The Central American paleoclimatic record and data-model comparison have implications for future projections of AMOC, Tropical North Atlantic/Caribbean SST, and monsoon changes. The analysis of different model simulations during the last deglaciation isolates AMOC as a dominant forcing mechanism of regional monsoon convection. Further, Tropical North Atlantic/Caribbean SST was tied to AMOC strength variations throughout the last glacial cycle, triggering rapid transitions in the Central American monsoon convection.

Observational and proxy data suggest that AMOC has weakened over the last century56–58 in response to increasing greenhouse gas concentrations, while other studies argue that the observed AMOC slowdown results from its internal variability24,59. Under various future warming scenarios, AMOC is very likely to weaken by the end of the century60, although large uncertainties among models hamper our ability to confidently estimate the temporal rates of decline22,25. Such a discrepancy has been attributed to the over-stability of AMOC in model simulations61 and the lack of dynamical cryosphere (i.e., ice melting) and deep water formation processes25. Early-warning signals of AMOC approaching a tipping point are also found in most recent climate models62,63.

The speleothem data indicate that AMOC weakening alone can trigger widespread reduction of monsoon convection in Central America and the Caribbean, independently of the background thermal conditions. Future rainfall in Central America and the Caribbean is projected to decrease significantly by the end of the century64–66, but few studies have focused on the role of AMOC22,67,68. Whether monsoon regimes weaken in the future will also depend on the direct response to SST warming69. However, separating the competing effects of direct radiative, SST warming, and dynamic forcing on future monsoon variations remains challenging70–74.

In this context, our results stress that if the AMOC- Central American monsoon link documented over the last 140,000 yr BP holds under future warming scenarios, a potential AMOC slowdown or collapse as suggested by climate models23,62,64–68,75 will likely weaken monsoon convection in Central America and impact freshwater availability in the region.

Methods
Cave sites
The stalagmites were collected from three caves located in the Alta Verapaz Department on the Caribbean slope of Guatemala. Grutas del Rey Marcos (RM) is a private-owned touristic cave located at about 20 km from the city of Cobán (15.427° N and −90.280° W, 1460 masl). The cave is characterized by a small (gated) entrance into a narrow passage that opens to a sequence of larger rooms highly decorated in speleothems of vadose origin (stalactites, stalagmites, flowstones, draperies, etc.). Bedrock thickness above the cave is about 80–100 m. All samples were retrieved at about 50-70 m from the cave entrance and ‘dead-and-down’, meaning that no active and/or standing stalagmites were collected for cave preservation reasons. Cave monitoring indicates that relative humidity is nearly 100% year-around and temperature is 17±0.5 °C (Ref. 76). Bombil Pek and Jul Iq caves are located near Chisec, 45 km from Cobán (15.837 °N and −90.277° W, elevation 240 masl). The caves open to the surface through a series of sinkholes, and are characterized by a sequence of extensive, highly-decorated conduits and chambers. Bedrock thickness above the caves is between 40–60 m. Stalagmites from BO and JQ caves were collected between 150–200 m from the cave entrance, in sections that had no perceptible airflow.

230Th dating. Seven new stalagmites from Grutas del Rey Marcos (GU-RM-4, -10, -11, and -35), Jul Iq Cave (GU-JQ-2 and -3), and Bombil Pek Cave (GU-BO-1) were analyzed in this study. Two additional ages were determined on stalagmite GU-RM-1 (ref. 12). The stalagmites consisted of columnar and open compact calcite; no aragonite was observed. 230Th radiometric dating was performed at the Isotope Laboratory facilities of the Institute of Global Environmental Change, Xi’an Jiaotong University. Sub-samples of ~60–150 mg were drilled along the growth axis using a SHERLINE 5410 milling device in clear compact calcite layers whenever possible. Uranium and thorium separation followed the standard chemical procedures described in ref. 77. A triple-spike 229Th-233U-236U isotope dilution method was applied to monitor U-Th concentrations and isotope ratios and correct for instrumental fractionation effects. U/Th isotopic ratios were measured on a Thermo-Finnigan Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using updated techniques described in ref. 77. All ages were calculated using half-lives from refs. 78–80, and corrected for detrital contamination using a 230Th/232Th atomic ratio of 4.4 ppm, assuming secular equilibrium with a bulk earth 232Th/238U value of 3.8 (Ref. 81). All age uncertainties are reported as absolute 2σ and include analytical errors and initial 230Th/232Th ratio. Age corrections are relatively small (~20–100 years) for samples with moderate uranium concentration and little 232Th contents, while samples having higher concentrations of detrital thorium (i.e., 232Th > 1000 ppt) and/or low uranium content are more susceptible to 230Th/232Th correction (~300–1500 years) and exhibit larger age uncertainties. All data are shown in Supplementary Table 1.

Stable isotope analysis. Oxygen and carbon isotope measurements were done at the Las Vegas Isotope Science Laboratory (LVIS) of the University of Nevada Las Vegas, Las Vegas (USA). Sub-samples were drilled/milled along the stalagmite growth axes at intervals between 0.2 and 1 mm, producing a subset of 3169 isotope analyses for the stalagmites used here. Sample powders were reacted with 104% phosphoric acid at 70 °C in a Kiel IV automated carbonate preparation device, the CO 2 gas was separated via cryogenic trapping, and isotope measurements were performed on a Thermo Electron Delta V Plus isotope ratio mass spectrometer in dual inlet mode. Isotope values were corrected with an internal standard (USC-1) and reported as δ-‰ deviations from the Vienna Pee Dee Belemnite (VPDB) standard scale. USC-1 values were previously determined by calibration with international standards NBS-18 and NBS-19. Analytical uncertainties are 0.03–0.05‰ for δ13C and 0.08–0.11‰ for δ18O. Cave and rainwater isotope analyses (Supplementary Tables 4–5) were completed at the Center for Stable Isotopes (CSI) of the University of New Mexico (Albuquerque, NM) using a cavity ring-down spectroscopy L1102-I Water Isotopic Analyzer bracketed with calibrated internal standards. δ2H and δ18O precision are better than ±2.0‰ and 0.06‰, respectively. Because the speleothems were collected from three different cave systems with potentially unique hydrology and vegetation cover the δ13C analyses are not discussed further here.

Age modeling. Individual stalagmite age models were constructed using the COPRA algorithm82, which employs Monte-Carlo age model simulations (n=2000) that can constrain uncertainties in both the time and proxy space. 230Th age outliers with high detrital thorium and ages significantly older than the age/depth trends from cleaner samples were omitted from the final age models (Grey shading in Table S1 indicates excluded dates). In cases where poorly-dated sections of stalagmites were compromised by detrital 230Th but had similar δ18O profiles to well-dated stalagmites, we tuned small sections of the stalagmites to the better-dated stalagmites to provide greater time coverage; the δ18O data from the base of GU-RM-1 and GU-RM-35 were tuned to the better chronologies of GU-RM-10 and GU-RM-11 using tie-points on clearly correlative δ18O anomalies across the interval 14-10.5 kyr BP (see Supplementary Fig. 10). The age tie points assume ±200 years uncertainty and were subsequently used in the final COPRA age model. Age model uncertainty and age resolution varies among the different stalagmites, and are about ±200–600 years and ±11-221 years at the 95% confidence interval, respectively.

Guatemala composite δ18O record. Stalagmite δ18O values were composited by binning into 50-yr average bins. Prior to compositing, stalagmites δ18O for BO and JQ caves were adjusted by −0.6‰ relative to RM, to account for different temperature-dependent equilibrium fractionation based on their lower elevation and higher temperatures relative to RM cave, and assuming that modern water-calcite equilibrium conditions were similar during the last glacial cycle. Despite BO and JQ caves being virtually at the same elevation, a further 0.4‰ correction to the JQ samples was applied to align absolute values, which differed slightly between coeval stalagmites while sharing the same overall structure. The origin of the small offsets is not clear, but soil and/or epikarst processes (i.e., PCP83) may have a small influence on the δ18O of drips feeding the different stalagmite sites while preserving the overall replication and time series structure between stalagmites. Because the fractionation of isotopes in precipitation is relative to the ocean δ18O value, and because the ocean δ18O itself has changed, we corrected composite Guatemala (GUA) δ18O record for the δ18O of the ocean from global ice-volume variations to create a δ18O ivc record using the Waelbroeck, et al.32 reconstruction. The ice-volume correction accounts for up to 1–1.2‰ decrease (more negative) in speleothem δ18O values during the glacial periods (i.e., LGM, MIS, 4, and MIS 6) and is relatively small for interglacial stages (i.e., Holocene, MIS 5). This correction does not influence the interpretations or change the δ18O time series in any substantial way.

iTRACE transient climate ensemble simulations. We use the iTRACE47 model output to support the dynamical interpretation of reconstructed Guatemalan rainfall changes. iTRACE is, to our knowledge, the only publicly available transient climate simulation ensemble with a model that incorporates oxygen-isotopes within the ocean, atmosphere, land surface, and sea-ice allowing to directly compare precipitation and temperature simulations with proxy records. The model is built in the Community Earth System Model CESMv.1.3, with a land and atmosphere spatial resolution of 1.9° and 2.5° (latitude and longitude), and a nominal 1° resolution for the ocean and sea ice84. CESM simulations have been successfully validated against modern observations of rainfall variations across the global tropics85,86. Previous studies have shown that the CESM model ensemble reproduces the overall precipitation variability in Central America and the Caribbean in both modern and paleoclimate timescales10,64,87. However, biases in the estimate of magnitude and rate of seasonal rainfall changes were noticed, particularly for the early-wet season (May-June), possibly related to the low spatial resolution of the models that lack of reliably capture the complex regional topography and divergence in modeling regional SST variations88–90. Although the iTRACE model ensemble only simulates the last deglaciation (20-11 kyr BP), it provides a direct means for testing regional climate dynamics during abrupt millennial-scale dry intervals such as Heinrich stadial 1 (HS1) and Younger Dryas (YD).

iTRACE includes a full-forcing experiment with ice sheets, greenhouse gases, orbital, and meltwater forcing related to ice dynamics into the high latitude oceans, as well as sensitivity experiments where some of the forcings are neglected. We extracted seasonal (DJFMA, MJJ, and ASON) precipitation, surface temperature, wind fields at 200 hPa (upper level) and 1000 hPa (surface), and oxygen isotopes for both convective and large-scale rainfall. The AMOC strength (in Sverdrups-Sv) is obtained from the zonally-integrated meridional overturning transport in the Atlantic Ocean, including its Eulerian mean, eddy-induced, and sub-mesoscale components, averaged over the domain encompassing 500–1200 meters depth from the equator to 49.6° N latitude. Spatially-averaged time series for the Caribbean and Central American region were extracted between 10.4° and 21.88° latitude, and 265° to 300° longitude to emphasize regional climatic processes.

Simulated δ18O p is calculated as follows:
δ18O p = (Pr 18O / Pr 16O − 1) × 1000 (1)
where Pr 18O and Pr 16O are the sum of convective and large-scale rainfall amounts for H 2 18O and H 2 16O, respectively. Local atmospheric convective activity is estimated from the horizontal wind fields as the difference between upper level (200 hPa) and lower level (1000 hPa) divergence. Specifically, given the velocity vector field V and its horizontal u and v components along the zonal and meridional axes identified by x and y, respectively, the horizontal divergence (∇ H) of V is defined as:
∇ H · V = ∂u / ∂x + ∂v / ∂y (2)
A convective activity index is calculated from the horizontal divergence at 200 hPa and 1000 hPa as:
(∇ H · V) 200 − (∇ H · V) 1000 (3)
where positive and negative anomalies of the index indicate enhanced and reduced atmospheric convection, respectively. Values of a variable in two periods are considered to be significantly different if the corresponding distributions do not overlap within the 1-99 percentile range.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "- What are the noteworthy results?\nThe authors present a new, high-quality stalagmite d18O record from Central America showing two dominant monsoon\nstates. One that is strong during periods when AMOC is strong and temperatures are warm in the Atlantic, and a weakened\nstate when AMOC is reduced, and it is regionally cooler. The authors argue that this new record is somewhat different from\npreviously published monsoon records in that local precession is not a major driver of monsoon variability in Central\nAmerica.\n- Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work\nis not original, please provide relevant references.\nFirst, I want to state that this is a very high-quality data set and I applaud their efforts. I have no doubt that this record adds to\nour overall understanding of past millennial-scale regional precipitation changes in Central America. The authors also\npresent a clear, straight-forward interpretation of the data.\nNevertheless, I do question how impactful this new record will be to our overall understanding of the Central American\nmonsoon. I say this because it is not clear to me that this new record is much different from the reflectance record from the\nCariaco Basin (Deplazes et al., 2013). They show a comparison between their new cave record and the Cariaco Basin\nrecord on Extended Data Figure 7i. The Cariaco Basin record already showed that tropical north Atlantic hydroclimate is\nintimately connected to changes in AMOC and millennial-scale climate events over the last ice age through north-south\nchanges in the location of the ITCZ. When I look at the two records together, it seems changes in the ITCZ could explain\ntheir record as well, so what is new here? I suppose you could say that MIS 5 looks wetter in Central America than in the\nCariaco Basin, but even so, N-S changes in the ITCZ could still explain the cave record during MIS 5.\n- Does the work support the conclusions and claims, or is additional evidence needed?\nYes, I think the conclusions are supported by the data.\n- Are there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or require revision?\nThe only conclusion that I question is their suggestion that “future Central American and Caribbean rainfall regimes will\nlikely decrease if Atlantic circulation weakens.” Although AMOC may weaken as the planet warms, it seems this connection\nwill become decoupled as the tropical Atlantic continues to warm due to anthropogenic CO2 increases. During the last\nglacial cycle, the tropical Atlantic warmed when AMOC was strong, shifting the ITCZ north and likely strengthening the\nCentral American monsoon. But under the current greenhouse gas warming, the tropical Atlantic will continue to warm, even\nif AMOC weakens. This is already happening today.\n- Is the methodology sound? Does the work meet the expected standards in your field?\nYes, the methodology is sound and meets the standards expected.\n- Is there enough detail provided in the methods for the work to be reproduced?\nYes.", "reviewer_2": "This manuscript presents a ~140,000-year long speleothem d18O record from Guatemala and uses it to suggest that Central\nAmerican hydroclimate is driven by changes in thermal forcing from the Atlantic Ocean, rather than local insolation changes\nas has previously been suggested. The authors bolster their claims with an isotope-enabled transient climate model\nsimulation, called iTRACE, and several other hydroclimate proxies from around Central America and the Caribbean. Lastly,\nthis updated interpretation is used to suggest that a weaker AMOC in the future will lead to regional drying across Central\nAmerica and the Caribbean.\nI find this manuscript to be well-written, mostly convincing, and agree that this will be a valuable contribution to the literature\non Central American hydroclimate. For example, to my knowledge this manuscript presents the first-ever isotopic record from\nCentral America that spans from the present back through an entire glacial cycle. However, I think there are a few\noutstanding questions regarding their argument that need to be addressed before this manuscript is ready for acceptance.\nAlthough I think it is important for these issues to be addressed, I think they still fall within the scope of a minor revision for\nthe authors.\nMy main concerns are as follows:\n1) iTRACE averaging region and its relevancy for your cave site\nAlthough the authors show remarkable similarities between their speleothem record and the iTRACE simulation between 20\nand 11 ka (Figure 4), I am concerned about the averaging region and its comparability to the cave site in Guatemala. For\nexample, Figure 5 clearly shows that the cave site is not within the averaging region, which spans much of the Caribbean\nSea. The similarities in Figure 4 certainly show that hydroclimate throughout the Caribbean is highly correlated to the time-\nev the time-\nevolution of the speleothem’s isotopes, but the mechanisms for rainfall delivery to the Caribbean (as a region average) and\nGuatemala/Yucatán Peninsula are not necessarily the same.\nI think care needs to be taken to show that the broad climate evolution in iTRACE that matches the speleothem isotopic\nwiggles between 20 and 11 ka are also valid for the area surrounding the cave site. This can be done in a number of ways,\nbut the most relevant, in my perspective, would be to re-do Figure 4 with the model averaged over a choice of region near\nthe cave site (either a single grid cell or collection of land grid cells). An alternative could be to show a climatology from\niTRACE of this region and highlight that the changes in hydroclimate on the Central American landmass near the cave site\nare in sync with the Caribbean area used for the Figure 4 average.\nSince the big argument of this manuscript is that Central American rainfall is largely tied to thermal forcing from the Atlantic\nOcean, I think this connection is really important to make. I currently think the study does a great job of convincing me that\nCaribbean hydroclimate is closely tied to the Atlantic, but I am left unconvinced that this means Central American\nhydroclimate is also tied to the same mechanism.\n2) Which mechanism is responsible for a future decrease in Central American rainfall: Atlantic SSTs or AMOC?\nThe manuscript does an excellent job in tying the past changes in Atlantic SST and AMOC (warm SSTs, strong AMOC = wet\nCentral America; vice versa), but I have trouble connecting this to the future because, unlike the past, SSTs and AMOC will\nnot evolve in conjunction with each other. In the future, Atlantic SSTs will warm but AMOC will weaken.\nI think it is important that the authors go a little further in separating which mechanism is primarily responsible for the\nchanges in Central American hydroclimate because this will give higher confidence to us as a scientific community that we\ncan correctly predict future rainfall conditions in this region.\nOne way to separate the forcing mechanisms could be to look at the meltwater forcing experiment of iTRACE and compare it\nto the full forcing. This would pinpoint the role of AMOC. Alternatively, analyzing reanalysis data (such as ERA5 for 1950 to\nthe present) could give you a correlation over the recent past between Central American rainfall and seasonal average SST\nin the Caribbean/Atlantic.\nRegardless of how you attempt to separate the two, I think it is important for your future implications that we have a better\nquantitative understanding of which mechanism is primarily responsible for the Central American rainfall changes.\nAdditional comments:\n-The term “last glacial cycle” is used several times in the manuscript but is either vaguely defined or is not defined at all. I\nsuggest the authors be clear on what they mean when they use this term. Is is a) the entire history since the Last Interglacial\n(~130 ka), b) the entire 140,000 year period of their speleothem record, or c) some other time period?\n-This comment is tied to Comment #1 above: The North Atlantic Subtropical High (NASH) and Caribbean Low-Level Jet\n(CLLJ) are two primary circulation features that deliver moisture to the Central American landmass and, as such, are\nimportant dynamical features for explaining changes in rainfall in this region. Additionally, these two features are tightly\nlinked to conditions in the Atlantic Ocean. For this reason, I think it would benefit the authors to describe the correlation\nbetween Central American rainfall and Atlantic thermal forcing in the context of changes to these two circulations (the last\nparagraph of “AMOC and tropical climate” is a good candidate for additional language). I believe this will help bolster the\nauthors’ claims with a stronger argument rooted in atmospheric dynamics.\n-I suggest adding a map of the cave site(s) to Figure 1 (or Extended Data Figure 1). I know there is a regional map (with a\nstar) in Figure 5, but as a reader it was hard to comprehend the text earlier in the manuscript without a good visual\nunderstanding of where the speleothem was located.\n-In Figure 4, a “convective index” is shown but never defined. Please include an equation or citation for this quantity.\n-In “Model constraints on AMOC-monsoon link”, the manuscript states that the model results in Figure 5\n(primarily Fig. 5b with regional drying in the Caribbean) demonstrate that “the thermodynamic control of Caribbean rainfall dominates over dynamical changes, whose contribution is more local”. I find this statement to be entirely unconvincing and lacking evidence.\nTo play the other side, the Caribbean Low-Level Jet is a strong dynamical component of hydroclimate in this region and\nwould be southward shifted during HS1 compared to the early Holocene. This southward shift, as demonstrated by the wind\nvectors in Fig. 5a and increase in precipitation at ~ 5°N, 50°W, could be responsible for the Caribbean rainfall reduction. My\npoint is that to make this claim, a more rigorous analysis of the thermodynamic and dynamic components is required, which\nis not currently performed. I suggest removing this statement or performing a more rigorous analysis that can permit the\nauthors to make a strong statement like this.\n-As a follow-up to the previous comment, Figure 5 describes Heinrich stadial 1 minus the Early Holocene from the iTRACE\nsimulations. There can be many definitions of time periods associated with these two events, especially for the Early\nHolocene which I think of as being more recent than 11 ka which iTRACE does not extend to, so a clear time period\ndefinition of these two timeframes would be helpful.\n-Are Extended Data Figures 6 and 7 out of order in the text? I think they should be switched.\n-In Extended Data Figure 7, the authors do a really nice job of showing incredible agreement between many records\nthroughout Central America and the Caribbean with their speleothem record. Because there is such great agreement, one\ndisagreement seems to be really pronounced, which is the wet LGM (~21,000) in the Petén Itzá Lake (Mag. Susc.) record.\nThis disagreement is notable considering that this record is the closest geographically to your cave site and the wet\ninterpretation of this record at the LGM has been a source of confusion for the field even prior to your manuscript’s\ncomparison. It can be brief, but I think it would be fruitful for the authors to provide a possible explanation as to why their\nrecord should be interpreted as showing a drier-than-present LGM in Guatemala (notice I say Guatemala here and not\nCentral America, because these sites are very close to each other) while Hodell’s mag. susc. record is interpreted as\nshowing a wetter-than-present LGM in Guatemala. Can the two be reconciled or is one just wrong?", "reviewer_3": "Please see the attached documents. I have included line numbers in the original manuscript."}, "author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\n- What are the noteworthy results?\nThe authors present a new, high-quality stalagmite d18O record from Central America showing two\ndominant monsoon states. One that is strong during periods when AMOC is strong and temperatures are\nwarm in the Atlantic, and a weakened state when AMOC is reduced, and it is regionally cooler. The authors\nargue that this new record is somewhat different from previously published monsoon records in that local\nprecession is not a major driver of monsoon variability in Central America.\n- Will the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nFirst, I want to state that this is a very high-quality data set and I applaud their efforts. I have no doubt that\nthis record adds to our overall understanding of past millennial-scale regional precipitation changes in\nCentral America. The authors also present a clear, straight-forward interpretation of the data. Nevertheless,\nI do question how impactful this new record will be to our overall understanding of the Central American\nmonsoon. I say this because it is not clear to me that this new record is much different from the reflectance\nrecord from the Cariaco Basin (Deplazes et al., 2013). They show a comparison between their new cave\nrecord and the Cariaco Basin record on Extended Data Figure 7i. The Cariaco Basin record already showed\nthat tropical north Atlantic hydroclimate is intimately connected to changes in AMOC and millennial-scale\nclimate events over the last ice age through north-south changes in the location of the ITCZ. When I look\nat the two records together, it seems changes in the ITCZ could explain their record as well, so what is new\nhere? I suppose you could say that MIS 5 looks wetter in Central America than in the Cariaco Basin, but\neven so, N-S changes in the ITCZ could still explain the cave record during MIS 5.\nWe thank the Reviewer #1 for the constructive comment. In particular, we appreciate that there are some\nsimilarities with the Cariaco reflectance data from off northern South America (particularly Heinrich dry\nevents), and we should include it as a benchmark study for the tropical Atlantic region.\nBriefly, apart from Heinrich dryings which are present in both records (shown below and in Supplementary\nFig. 7), we do not see a clear DO like millennial-scale climate linkage between the two sites, suggesting\neither that Cariaco has a distinct climate ‘flavor’, or that the proxies are registering the rainfall variability\ndifferently in the two regions. We have modified the manuscript to provide additional context on the\ninterpretation of north-south ITCZ movements based on the similarity of Cariaco and the DO variability\nseen in Greenland. We also added the text to new lines 49-58, and a discussion of the key differences in\nnew Lines 112-118, 198-209, and 299-301 in the tracked changes manuscript. We conclude that Central\nAmerica has a distinct response to Atlantic thermal warming that differs from Cariaco and other global\nmonsoon regions.\n- Does the work support the conclusions and claims, or is additional evidence needed?\nYes, I think the conclusions are supported by the data.\n- Are there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or\nrequire revision?\nThe only conclusion that I question is their suggestion that “future Central American and Caribbean rainfall\nregimes will likely decrease if Atlantic circulation weakens.” Although AMOC may weaken as the planet\nwarms, it seems this connection will become decoupled as the tropical Atlantic continues to warm due to\nanthropogenic CO2 increases. During the last glacial cycle, the tropical Atlantic warmed when AMOC was\nstrong, shifting the ITCZ north and likely strengthening the Central American monsoon. But under the\ncurrent greenhouse gas warming, the tropical Atlantic will continue to warm, even if AMOC weakens.\nThis is already happening today.\nWe thank the Reviewer for the comment, and we agree that AMOC and tropical Atlantic SST might not\nvary in sync under future greenhouse warming in a similar fashion to what we observed during the last\nglacial cycle. As such, the forcing mechanisms of Central American monsoon changes might differ from\nthose we document in this study for the past 140,000 years. To address this comment in the manuscript, and\nas suggested by Reviewer #2 (see response below), we performed an analysis of the iTRACE simulations\nboth with and without meltwater forcing to isolate the potential AMOC control. Based our new data-model\ncomparison, we posit that AMOC was the primary forcing of Central American monsoon changes during\nthe last deglaciation (Lines 277-285 and Fig. 6). As such, we adjusted the discussion and conclusion of the\nrevised manuscript to state that if the AMOC forcing on monsoon intensity is hold into the future, Central\nAmerican monsoon regimes will likely weaken following AMOC slowdown. We added this information to\nthe manuscript in Lines 330-340. This statement also is supported by several climate model simulations\nthat focus on future AMOC simulations (Bellomo et al., 2021; Bellomo and Mehling, 2024; Chen et al.,\n2018; Orihuela-Pinto et al., 2022).\n- Is the methodology sound? Does the work meet the expected standards in your field?\nYes, the methodology is sound and meets the standards expected.\n- Is there enough detail provided in the methods for the work to be reproduced?\nYes.\n------------------------------\nReviewer #2 (Remarks to the Author)\nThis manuscript presents a ~140,000-year long speleothem d18O record from Guatemala and uses it to\nsuggest that Central American hydroclimate is driven by changes in thermal forcing from the Atlantic\nOcean, rather than local insolation changes as has previously been suggested. The authors bolster their\nclaims with an isotope-enabled transient climate model simulation, called iTRACE, and several other\nhydroclimate proxies from around Central America and the Caribbean. Lastly, this updated interpretation\nis used to suggest that a weaker AMOC in the future will lead to regional drying across Central America\nand the Caribbean.\nI find this manuscript to be well-written, mostly convincing, and agree that this will be a valuable\ncontribution to the literature on Central American hydroclimate. For example, to my knowledge this\nmanuscript presents the first-ever isotopic record from Central America that spans from the present back\nthrough an entire glacial cycle. However, I think there are a few outstanding questions regarding their\nargument that need to be addressed before this manuscript is ready for acceptance. Although I think it is\nimportant for these issues to be addressed, I think they still fall within the scope of a minor revision for\nthe authors.\nMy main concerns are as follows:\n1) iTRACE averaging region and its relevancy for your cave site\nAlthough the authors show remarkable similarities between their speleothem record and the iTRACE\nsimulation between 20 and 11 ka (Figure 4), I am concerned about the averaging region and its\ncomparability to the cave site in Guatemala. For example, Figure 5 clearly shows that the cave site is not\nwithin the averaging region, which spans much of the Caribbean Sea. The similarities in Figure 4 certainly\nshow that hydroclimate throughout the Caribbean is highly correlated to the time-evolution of the\nspeleothem’s isotopes, but the mechanisms for rainfall delivery to the Caribbean (as a region average) and\nGuatemala/Yucatán Peninsula are not necessarily the same.\nI think care needs to be taken to show that the broad climate evolution in iTRACE that matches the\nspeleothem isotopic wiggles between 20 and 11 ka are also valid for the area surrounding the cave site. This\ncan be done in a number of ways, but the most relevant, in my perspective, would be to re-do Figure 4 with\nthe model averaged over a choice of region near the cave site (either a single grid cell or collection of land\ngrid cells). An alternative could be to show a climatology from iTRACE of this region and highlight that the\nchanges in hydroclimate on the Central American landmass near the cave site are in sync with the\nCaribbean area used for the Figure 4 average.\nSince the big argument of this manuscript is that Central American rainfall is largely tied to thermal forcing\nfrom the Atlantic Ocean, I think this connection is really important to make. I currently think the study does\na great job of convincing me that Caribbean hydroclimate is closely tied to the Atlantic, but I am left\nunconvinced that this means Central American hydroclimate is also tied to the same mechanism.\nThis is a good point. We updated the averaging region to include grid cells containing our cave sites in\nFigure 4 and Box in Figure 5 of the revised manuscript. Overall, the time series shows the same temporal\nevolution between 20 and 11 kyr BP, suggesting that Central America is responding in the same way as the\nbroader Caribbean. Furthermore, in the figure below (summarizing our update in the manuscript) we\ncompare MJJASON rainfall and δ18O from iTRACE in both the Caribbean only and our cave sites during\np\nthe 20-11 kyr BP interval. We clearly observe synchronous changes in both data during the major deglacial\nclimate events, supporting our interpretations of a common forcing mechanism of monsoon rainfall regimes\nin both Caribbean (upstream region) and Guatemala (our cave sites).\n2) Which mechanism is responsible for a future decrease in Central American rainfall: Atlantic SSTs or\nAMOC?\nThe manuscript does an excellent job in tying the past changes in Atlantic SST and AMOC (warm SSTs,\nstrong AMOC = wet Central America; vice versa), but I have trouble connecting this to the future because,\nunlike the past, SSTs and AMOC will not evolve in conjunction with each other. In the future, Atlantic SSTs\nwill warm but AMOC will weaken.\nI think it is important that the authors go a little further in separating which mechanism is primarily\nresponsible for the changes in Central American hydroclimate because this will give higher confidence to\nus as a scientific community that we can correctly predict future rainfall conditions in this region.\nOne way to separate the forcing mechanisms could be to look at the meltwater forcing experiment of\niTRACE and compare it to the full forcing. This would pinpoint the role of AMOC. Alternatively, analyzing\nreanalysis data (such as ERA5 for 1950 to the present) could give you a correlation over the recent past\nbetween Central American rainfall and seasonal average SST in the Caribbean/Atlantic. Regardless of how\nyou attempt to separate the two, I think it is important for your future implications that we have a better\nquantitative understanding of which mechanism is primarily responsible for the Central American rainfall\nchanges.\nWe thank the Reviewer for the comment and the constructive suggestions. As also explained in the second\nresponse to Reviewer #1, regarding future Central American rainfall changes, we agree that our concluding\nremarks on future Central American monsoon changes would benefit from added clarity and a more focused\nanalysis. Following the Reviewer’s suggestions, we modified the manuscript to include iTRACE\nsimulations both with and without the meltwater forcing and compared that to the full forcing results to\npinpoint the role of AMOC. We added Figure 6 to the revised manuscript (and below) to show that simulated\nsurface temperature, precipitation, and δ18O over Central America and Caribbean differ substantially\np\namong the two simulations and our δ18O data. As a result, we have updated the discussion in Lines 277-285\nin the revision and concluding remarks (Lines 315-320 and 330-340) accordingly.\nAdditional comments:\n-The term “last glacial cycle” is used several times in the manuscript but is either vaguely defined or is not\ndefined at all. I suggest the authors be clear on what they mean when they use this term. Is a) the entire\nhistory since the Last Interglacial (~130 ka), b) the entire 140,000 year period of their speleothem record,\nor c) some other time period?\nWe use the term last glacial cycle throughout the text to define the interval from 140,000 years ago to\npresent following Bohm et al. (2015). We define this interval in the text at Lines 59-60 of the revised\nmanuscript.\n-This comment is tied to Comment #1 above: The North Atlantic Subtropical High (NASH) and Caribbean\nLow-Level Jet (CLLJ) are two primary circulation features that deliver moisture to the Central American\nlandmass and, as such, are important dynamical features for explaining changes in rainfall in this region.\nAdditionally, these two features are tightly linked to conditions in the Atlantic Ocean. For this reason, I\nthink it would benefit the authors to describe the correlation between Central American rainfall and Atlantic\nthermal forcing in the context of changes to these two circulations (the last paragraph of “AMOC and\ntropical climate” is a good candidate for additional language). I believe this will help bolster the authors’\nclaims with a stronger argument rooted in atmospheric dynamics.\nTo address this omission of climatic relevance to our conclusions, we included new text in the last paragraph\nof the ‘AMOC and tropical climate’ discussing the interaction between regional SST, NASH, CLLJ, and\nhydroclimate changes in Central America and the Caribbean, in Lines 86-94.\n-I suggest adding a map of the cave site(s) to Figure 1 (or Extended Data Figure 1). I know there is a regional\nmap (with a star) in Figure 5, but as a reader it was hard to comprehend the text earlier in the manuscript\nwithout a good visual understanding of where the speleothem was located.\nWe added a map of the cave sites and other regional records discussed in the manuscript as Supplementary\nFigure 1.\n-In Figure 4, a “convective index” is shown but never defined. Please include an equation or citation for\nthis quantity.\nWe added the following text and equation in the Methods section ‘iTRACE transient climate model’ (Lines\n447- 457) : ‘Local atmospheric convective activity is estimated from the horizontal wind fields as the\ndifference between upper level (200 hPa) and lower level (1000 hPa) divergence. Specifically, given the\nvelocity vector fieldVand its horizontaluandv components along the zonal and meridional axes identified\nbyxandy, respectively, the horizontal divergence ( ) of V is defined as:\nH\n∇\n𝜕𝑢 𝜕𝑣\nA convective activity index is calculated f𝛻ro 𝐻 m∙ 𝑽th=e h oriz+on tal divergence at 200 hPa and 1000 hPa as\n𝜕𝑥 𝜕𝑦\n, where positive and negative anomalies of the index indicate enhanced and\nreduced atmospheric convection, respectively’.\n(𝛻𝐻 ∙𝑽)200− (𝛻𝐻 ∙𝑽)1000\n-In “Model constraints on AMOC-monsoon link”, the manuscript states that the model results in Figure 5\n(primarily Fig. 5b with regional drying in the Caribbean) demonstrate that “the thermodynamic control of\nCaribbean rainfall dominates over dynamical changes, whose contribution is more local”. I find this\nstatement to be entirely unconvincing and lacking evidence. To play the other side, the Caribbean Low-\nLevel Jet is a strong dynamical component of hydroclimate in this region and would be southward shifted\nduring HS1 compared to the early Holocene. This southward shift, as demonstrated by the wind vectors in\nFig. 5a and increase in precipitation at ~ 5°N, 50°W, could be responsible for the Caribbean rainfall\nreduction. My point is that to make this claim, a more rigorous analysis of the thermodynamic and dynamic\ncomponents is required, which is not currently performed. I suggest removing this statement or performing\na more rigorous analysis that can permit the authors to make a strong statement like this.\nWe agree with the reviewer that this statement would need a more rigorous analysis. Because it is not central\nto our main conclusions, we have followed the reviewer’s suggestion and removed the statement from the\nrevised manuscript.\n-As a follow-up to the previous comment, Figure 5 describes Heinrich stadial 1 minus the Early Holocene\nfrom the iTRACE simulations. There can be many definitions of time periods associated with these two\nevents, especially for the Early Holocene which I think of as being more recent than 11 ka which iTRACE\ndoes not extend to, so a clear time period definition of these two timeframes would be helpful.\nWe defined the temporal intervals for the Heinrich Stadial 1 (15,200-16,800 yr BP) and Early Holocene\n(11,001-11,500 yr BP) in Figure 5. This was added in the description of Figure 5 of the revised manuscript\nto provide clarity to the reader.\n-Are Extended Data Figures 6 and 7 out of order in the text? I think they should be switched. We proceeded\nto switch the order of the Figures in the revised version.\nFixed.\n-In Extended Data Figure 7, the authors do a really nice job of showing incredible agreement between many\nrecords throughout Central America and the Caribbean with their speleothem record. Because there is such\ngreat agreement, one disagreement seems to be really pronounced, which is the wet LGM (~21,000) in the\nPetén Itzá Lake (Mag. Susc.) record. This disagreement is notable considering that this record is the closest\ngeographically to your cave site and the wet interpretation of this record at the LGM has been a source of\nconfusion for the field even prior to your manuscript’s comparison. It can be brief, but I think it would be\nfruitful for the authors to provide a possible explanation as to why their record should be interpreted as\nshowing a drier-than-present LGM in Guatemala (notice I say Guatemala here and not Central America,\nbecause these sites are very close to each other) while Hodell’s mag. susc. record is interpreted as showing\na wetter-than-present LGM in Guatemala. Can the two be reconciled or is one just wrong?\nWe agree the LGM is an interesting time interval, and we have modified the manuscript on Lines 227-236\nto add a discussion of the LGM. It would be difficult to fully resolve the discrepancy between an apparently\nstrong monsoon at Petén Itza and southwest Mexico with our data for a moderate monsoon in Guatemala\nin the current manuscript, and we certainly don’t want to imply that one record or another is more ‘right’\nthan the others, as there could be differences in how the proxies are responding to the same rainfall\nvariations. However, we do emphasize that the interpretation of a moderate LGM monsoon in Guatemala\nis fully consistent with the moderate strength of AMOC during the LGM, suggesting, again, that AMOC is\nthe dominant control on Central American rainfall over the last glacial cycle\n---------------------------------------------\nReviewer #3 (Remarks to the Author):\nThe authors present a novel composite speleothem record from Guatemala. They find that this record does\nnot fall in line with the Global Paleomonsoon paradigm of hemispherically anti-phased hydroclimate\nvariability, and instead hypothesize that Atlantic Ocean temperatures related to Atlantic Meridional\nOverturning Circulation strength are a dominant control on Central American and Caribbean rainfall over\nthe past 140 ka. These results are supported by an isotope-enabled model, particularly during abrupt\ndeglacial AMOC weakening events.\nI enjoyed reading this paper and believe is an important record to have published, especially because of the\npaucity of records in this region as well as large uncertainties in past and future AMOC variability.\nBelow are some suggestions which I think could help this work with a focus on method clarification and\nproxy interpretation.\nComments:\nLine 16: I believe anti-phased is more correct than “antiphased,” so this should be updated throughout the\ntext.\nWe have corrected the term throughout the manuscript.\nLine 88: Temperature also has a non-negligible effect on oxygen isotopic fractionation in carbonate\nprecipitates. You could use the equation in (Tremaine et al., 2011) with a dependence of -0.177 ‰/°C. Of\ncourse, this is easiest if there are temperature records nearby, which might be challenging over the full span\nof the Guatemala record.\nTo address this comment, we used the pollen-based temperature reconstruction from Central America\nspanning the last 85,000 years BP (Correa-Metrio et al., 2012) to correct for temperature change to produce\nan estimate the δ18O value of drip water (in VSMOW), shown against the uncorrected records (in VPDB)\nin the graph below. Accounting for temperature changes and their possible effect on equilibrium oxygen\nisotope fractionation in calcite using the equations from Daëron et al. (2019) (turquoise) and Tremaine et\nal. (2011) (red) results in a time series that appears nearly identical to the uncorrected variations, although\nwith some offsets and changes in amplitudes. Overall, the temperature correction would not change the\ninterpretation of the record in any significant way. Because this technique uses some assumptions that\ncannot be readily tested and does not change our interpretation, we have elected to not include a plot of\nδ18O of drip water in the current manuscript, but we do feel that it merits additional attention in the future.\nLines 122-124: This is pretty interesting; it almost looks like much of the record is opposite local summer\ninsolation? Another record that showed something similar and is probably worth citing is from Madagascar\n(Dawson et al., 2024). There is discussion in that work of records from tropical southeastern Africa also not\nfollowing the Global Paleomonsoon paradigm.\nWe thank the reviewer for pointing out this new study, which is adding to the increasing number of sites\nthat do not follow the orbital paradigm. We have added this paper to the Introduction (Line 43, reference\n11).\nLines 124-128: Have you considered trying spectral analysis to see if precessional signals are indeed weaker\nat this site?\nWhile the absence of a precessional-signal is visually apparent in our time series, we followed the\nreviewer’s suggestion to do a continuous wavelet transform (CWT) analysis of our δ18O time series. The\nanalysis, shown below, does not contain a significant precessional signal within the 95% confidence interval\n(CI). As a result, we elected to not include this negative finding in our manuscript. However, we notice\nsignificant (within 95% CI) centennial-to-millennial-scale periodicity throughout the entire length of the\nrecord (bold black line in plot below) coinciding with the major Heinrich stadials, thus supporting our\ninterpretations of a strong and persistent North Atlantic forcing of Central American monsoon changes\nthroughout the entire last glacial cycle.\nFigure 5: I wonder if it would be better to subtract adjacent periods (LGM or B-A) from the H1 response\nrather than early Holocene, considering other forcings like GHGs or orbital variability have changed from\nH1 to the end of the run (and these are of lesser interest here than meltwater forcing). In other words, can\nwe be sure that the temperature, wind, and convective patterns shown represent H1 anomalies, or are these\npatterns mostly representing the early deglaciation mean state compared to that of the early Holocene?\nThis is an interesting suggestion. To explain our reasoning to not include the extra plots, we choose the\nEarly Holocene (11,001-11,500 yr BP) because it represents the most recent interval in iTRACE that is\nclosest to modern climate, and used this interval as common denominator in Figure 5 to show the climatic\nanomalies between the driest interval in the simulation (weak AMOC, low SST and weaker monsoon) and\nnear-modern conditions (strong AMOC, High SST, strong monsoon). We also computed the same analysis\nfor YD minus Early Holocene (not shown), and found similar but less pronounced response. Because the\nsequential period-to-period analyses do not help us with the central finding of our study – that AMOC is\nthe strongest control on Central American rainfall – and would significantly increase the number of figures\nneeded for the manuscript, we decided not to include them in the current manuscript. However, the\nreviewer’s point is salient and the maps we produced for these different time slices are interesting, so we\nmay include them in a follow-up manuscript.\nLine 238: Northern Hemisphere ice sheet extent might also explain why the Guatemala record looks\nstrikingly like the Borneo record (Extended Data Figure 5), perhaps due to the sea-level driven exposure of\nthe Australasian shelves during the last glacial period ~70 ka which is hypothesized to affect Indo- Pacific\nhydroclimate (DiNezio et al., 2018).\nWe observe some striking differences between the Borneo and Guatemala records (for example Borneo has\na precessional influence which is lacking in Guatemala), which suggests that the two regions have some\nimportant differences in forcing. In the case of Guatemala, the effect of sea level changes is relatively minor\nbecause of the lack of a broad bathymetric shelf in the Caribbean, so we question that sea level is a common\nforcing to the two regions. We have elected to not modify the current manuscript for a detailed Western vs.\nEastern Pacific paleoclimate signal to maintain a short format, but we do agree with the reviewer that an\nanalysis of a Walker circulation forcing would be an important contribution for future work.\nLine 273: If there is any information about sample mineralogy that would be helpful to mention here.\nOur assessment of the stalagmite mineralogy was based on visual confirmation of dense calcite with\ncolumnar fabric and the lack of any botryoidal or porous white aragonite, based on several decades of\nexperience working with tropical calcite and aragonite stalagmites. Though we didn’t perform XRD\nanalyses, multiple lines of evidence suggest that all samples consist entirely of calcite. For example, the\nsamples exhibit a shiny surface after polishing, and present low porosity and high density. We did not\nobserve any mineral habits that would suggest the presence of aragonite (i.e., acicular). The low uranium\ncontent (below 500 ppb) further supports an overall calcite composition of the samples, which would exhibit\nsystematically higher concentrations in the presence of aragonite. Further, the strong replication of the δ18O\ntime series of individual stalagmites indicate the absence of post depositional alteration (aragonite to calcite\nrecrystallization) that could be responsible of uranium loss and apparent older ages. We specified the\nmineralogy of the samples in Line 363-364.\nLine 307: How does the composite d13C record look? Any particular reason for not showing it in this work?\nWe have added new lines 395-397 discussing why the δ13C values were omitted from the manuscript.\nBecause this study includes stalagmites from three different caves, each with unique δ13C signatures, any\ncompilation would be largely unusable over the 140,000 year record. However, the reviewer’s suggestion\nis a good one for well-behaved samples, and we anticipate a follow-up study focusing just on the Rey\nMarcos stalagmites, which (mostly) replicate, but that is beyond the scope of the current work on the full\nlast glacial cycle. Because speleothem δ13C is largely affected by site-specific hydrologic and environmental\nprocesses, the difference in vegetation cover and/or hydrology between the cave sites throughout the last\n140 ka might have influenced stalagmite δ13C differently, causing δ13C changes that might have not related\nto regional climate processes and resulting in large δ13C differences and lack of replication.\nLine 323: How many Monte-Carlo age model simulations were used?\nWe performed 2000 Monte-Carlo age model simulations. We included this in the revised manuscript version\nat Line 399.\nLines 360-361: How was AMOC strength calculated?\nWe added the following text to the Methods’ section ‘iTRACE transient climate model (Lines 438-441):\n‘The AMOC strength (in Sverdrups -Sv) is obtained from the zonally-integrated meridional overturning\ntransport in the Atlantic Ocean, including its Eulerian mean, eddy-induced and sub-mesoscale components,\naveraged over the domain encompassing 500-1200 meters depth from the equator to 49.6°N latitude.’ We\nnote that the AMOC signal from iTRACE is largely independent of minor variations in depth and latitude\nbands, producing similar time series, such that our conclusions are robust despite the chosen box\nparameters.\nLine 363: Was simulated d18Op also amount-weighted?\nYes, in the methods’ section ‘iTRACE transient climate ensemble simulations’ (Lines 444-446), we show\nthe equation we used to calculate the simulated δ18O , which results from the sum of Pr and Pr ,\np 18O 16O\nrepresenting convective and large-scale rainfall amount for H 18O and H 16O, respectively.\n2 2\nLines 405-407: This isn’t too surprising considering the strong influence of seasonality on d18O, as the\nauthors suggest is related to the global paleomonsoon. What would be useful is to see whether amount\nweighted seasonal mean (e.g., MJJASO) precipitation has a similar negative relationship with d18O over\ninterannual timescales. This gets at variability beyond the seasonal cycle, which is often most useful for\ninterpreting paleoclimate records (i.e., if one rainy season has more rainfall than another, do we expect\nd18O to be more depleted?). I recognize that data is limited considering there are only two years (2021-\n2022), but are there any nearby GNIP observations to supplement your data? Or perhaps a reanalysis\nproduct like IsoGSM could help shed light on these relationships?\nBecause long-term GNIP stations nearby our cave sites are not available, and our efforts at collecting local\nrainfall only span two years, it is hard to answer this question without more observational data. However,\nwe saw the opportunity to address this comment by comparing the isotope-enable iTRACE model’s\nprecipitation/δ18O response. We added Supplementary Figure 9 to show that MJJASON rainfall and δ18O\np p\nfor the interval 11,001-11,500 year BP (Early Holocene) have a strong and statistically significant\ncorrelation of (r=0.66, p<0.0001), and it is weaker (r=0.10, p<0.0001) for DJFMA rainfall. The new figure\nhighlights the long inter-annual correlation between δ18O and rainfall amount (in the model world) that\np\nconfirms the same relationship in the observational data over much shorter time intervals.\nLines 423-425: I agree that the observed dripwater isotopic values look more like summer rainfall isotopic\nvalues, but when were the drips collected? Might this have an impact on the observed values especially if\ninfiltration rates are fast enough (or epikarst residence times are small enough) to transmit seasonal signals?\nDrip water samples were collected during each field campaign, which occurred during both the summer\n(July 2021, September 2022, and June 2023) and winter (March 2019) season. Independently of the\nsampling period, dripwater δ18O uncertainty (2σ) is within 0.6-1.0 ‰, and lack of a clear seasonal signal\nthat is, instead, more pronounced in the rainfall δ18O values. We added this information in the description\nof Extended Data Figure 4. These data suggest that the epikarst above the cave impart a smoothing of any\npotential seasonal isotopic variations.\nReference\nBellomo, K., Angeloni, M., Corfi, S., and Von Hardenberg, J., 2021, Future climate change shaped by inter-\nmodel differences in Atlanfic meridional overturning circulafion response: Nature\nCommunicafions, v. 12, no. 1.\nBellomo, K., and Mehling, O., 2024, Impacts and State-Dependence of AMOC Weakening in a Warming\nClimate: Geophysical Research Lefters, v. 51, no. 10.\nBohm, E., Lippold, J., Gutjahr, M., Frank, M., Blaser, P., Antz, B., Fohlmeister, J., Frank, N., Andersen, M. B.,\nand Deininger, M., 2015, Strong and deep Atlanfic meridional overturning circulafion during the\nlast glacial cycle: Nature, v. 517, no. 7532, p. 73-76.\nChen, Y., Langenbrunner, B., and Randerson, J. T., 2018, Future Drying in Central America and Northern\nSouth America Linked With Atlanfic Meridional Overturning Circulafion: Geophysical Research\nLefters, v. 45, no. 17, p. 9226-9235.\nCorrea-Metrio, A., Bush, M. B., Cabrera, K. R., Sully, S., Brenner, M., Hodell, D. A., Escobar, J., and\nGuilderson, T., 2012, Rapid climate change and no-analog vegetafion in lowland Central America\nduring the last 86,000 years: Quaternary Science Reviews, v. 38, p. 63-75.\nDaëron, M., Drysdale, R. N., Peral, M., Huyghe, D., Blamart, D., Coplen, T. B., Lartaud, F., and Zanchefta, G.,\n2019, Most Earth-surface calcites precipitate out of isotopic equilibrium: Nature Communicafions,\nv. 10, no. 1.\nOrihuela-Pinto, B., England, M. H., and Taschefto, A. S., 2022, Interbasin and interhemispheric impacts of\na collapsed Atlanfic Overturning Circulafion: Nature Climate Change, v. 12, no. 6, p. 558-565.\nTremaine, D. M., Froelich, P. N., and Wang, Y., 2011, Speleothem calcite farmed in situ: Modern calibrafion\nof δ18O and δ13C paleoclimate proxies in a confinuously-monitored natural cave system:\nGeochimica et Cosmochimica Acta, v. 75, no. 17, p. 4929-4950."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am satisfied with the changes made by the authors, and I recommend publication.", "reviewer_2": "The authors have done an excellent job of addressing my comments from the original review period. I am happy to\nrecommend this revised manuscript for publication.", "reviewer_3": "The authors did a nice job of responding to the first round of Reviewers’ comments and updating the manuscript accordingly.\nI found it useful to see confirmation from iTraCE for the authors’ proxy interpretations. I think this important record is nearly\nready for publication, and only have a few comments.\nIn the main text:\nLines 400-401: Considering the range of different cave environments these samples come from, I can understand why the\nauthors decided to omit the carbon isotopic data from this manuscript. However, I strongly believe this data is important to\nmake available to the community, even if it is not used here. This does not require making a separate composite which is\nbeyond the scope of this study (and likely wouldn’t work as the authors suggest). But it is worthwhile to include somewhere\nin the supplementary data set.\nLines 416-418: I assume the temperature difference mentioned is due to elevation differences between the caves? This is\nworth clarifying. Also, why are different temperature corrections applied to BO and JQ if they are both at the same elevation\nand therefore have the same elevation difference relative to RM? Unless there is some non-elevation factor that I am missing\nwhich should also be mentioned in the text.\nLine 430: Some model validation for these variables in this region would be nice to see. Perhaps there are previous studies\nyou could point to that explore CESM1.3’s strengths/weaknesses in the Atlantic/Caribbean regions? Also, the spatial\nresolution of the model should be specified.\nIn the supplement:\nLine 55 (Figure 3): I think it is worth providing a bit more written detail regarding the source of the precipitation data (only a\nreference is provided at the moment). Is this rain gauge data from a station near your study site? Or is this a gridded product?\nIf so, are you averaging over a certain area? Etc.\nAlso, I understand why only precipitation data from 2021-2022 was chosen to compare with your drip water monitoring data\n(because that is all the data you have). I wonder though if it is worth including all available data for temperature and\nprecipitation to construct a more robust plot of the seasonal cycle (the cycle shown in panel b is understandably quite noisy\nhaving only two years’ worth of data). Perhaps this could be a separate panel because panels (a) and (b) are useful to\ncompare visually. But if this has already been done in another paper, then simply adding the relevant citation should be\nenough."}, "author_response": "REVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI am safisfied with the changes made by the authors, and I recommend publicafion.\nWe thank the reviewer for the valuable suggesfions and comments in the review process, which\nsignificantly improved the quality of the manuscript.\nReviewer #2 (Remarks to the Author):\nThe authors have done an excellent job of addressing my comments from the original review period. I\nam happy to recommend this revised manuscript for publicafion.\nWe appreciate the reviewer’s comments and for the previous review suggesfions that helped improve\nthe quality of our arficle.\nReviewer #3 (Remarks to the Author):\nThe authors did a nice job of responding to the first round of Reviewers’ comments and updafing the\nmanuscript accordingly. I found it useful to see confirmafion from iTraCE for the authors’ proxy\ninterpretafions. I think this important record is nearly ready for publicafion, and only have a few\ncomments.\nWe appreciate the reviewer's contribufion to improving the manuscript. Below is our response to the\ncomments.\nIn the main text:\nLines 400-401: Considering the range of different cave environments these samples come from, I can\nunderstand why the authors decided to omit the carbon isotopic data from this manuscript. However, I\nstrongly believe this data is important to make available to the community, even if it is not used here.\nThis does not require making a separate composite which is beyond the scope of this study (and likely\nwouldn’t work as the authors suggest). But it is worthwhile to include somewhere in the supplementary\ndata set.\nWe included the δ13C data for each stalagmite in the study in the supplementary Data file as well as in\nthe NOAA WDS-Paleo repository.\nLines 416-418: I assume the temperature difference menfioned is due to elevafion differences between\nthe caves? This is worth clarifying. Also, why are different temperature correcfions applied to BO and JQ\nif they are both at the same elevafion and therefore have the same elevafion difference relafive to RM?\nUnless there is some non-elevafion factor that I am missing which should also be menfioned in the text.\nWe clarified in the last version of the manuscript that the correcfion was related to the difference in\nelevafion, which also affects temperature, between the caves. Addifionally, we applied a further\ncorrecfion of 0.4 ‰ to the JQ stalagmites to align them with the BO stalagmites for the overlapping\nperiod. Despite the caves are virtually at the same elevafion, the samples show different absolute\nisotopic values while sfill replicafing the overall climate signal quite strongly. Although we do not explore\nthe cause of this offset in detail, it is possible that processes occurring in the soil and/or epikarst\ncontribute to the variafion in drip water isotope composifion across different drip sites. We provided this\nclarificafion in lines 351-361.\nLine 430: Some model validafion for these variables in this region would be nice to see. Perhaps there\nare previous studies you could point to that explore CESM1.3’s strengths/weaknesses in the\nAtlanfic/Caribbean regions? Also, the spafial resolufion of the model should be specified.\nWe included a new paragraph in lines 375-381, where we discuss previous studies focused on CESM\nvalidafion and model biases that sfill remain in reproducing Central American and Caribbean rainfall\nvariability. We also included the iTRACE spafial resolufion at lines 372-375 of the tracked changes\nmanuscript version.\nIn the supplement:\nLine 55 (Figure 3): I think it is worth providing a bit more wriften detail regarding the source of the\nprecipitafion data (only a reference is provided at the moment). Is this rain gauge data from a stafion\nnear your study site? Or is this a gridded product? If so, are you averaging over a certain area? Etc.\nAlso, I understand why only precipitafion data from 2021-2022 was chosen to compare with your drip\nwater monitoring data (because that is all the data you have). I wonder though if it is worth including all\navailable data for temperature and precipitafion to construct a more robust plot of the seasonal cycle\n(the cycle shown in panel b is understandably quite noisy having only two years’ worth of data). Perhaps\nthis could be a separate panel because panels (a) and (b) are useful to compare visually. But if this has\nalready been done in another paper, then simply adding the relevant citafion should be enough.\nWe have provided addifional informafion about the data collected. Addifionally, we have produced a\ngraph that displays monthly rainfall and temperature from the same monitoring stafion for the period of\n1980 to 2018 (see panel c in Supplementary Fig. 3) to support our 2021-2022 dataset."}]

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