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Recent paleoclimate reconstructions have suggested millennial‐scale variability in the Indo‐Pacific Warm Pool region coincident with events of the last deglaciation. Here, we present a new stalagmite oxygen isotope record from northern Borneo, which today is located near the center of the region's mean annual intertropical convergence zone. The record spans the full deglaciation, and reveals for the first time distinct oxygen isotope variations at this location connected with the Bølling‐Allerød onset and the Younger Dryas event. The full deglaciation in the Borneo stalagmite proxy reconstruction appears remarkably similar to a 20–11 ka transient simulation of rainfall over the area produced using the isotope‐enabled Community Earth System Model. In this model, periods of weakened Atlantic Ocean meridional overturning circulation are associated with an anomalous Western North Pacific anticyclone, which is produced in boreal autumn and shifts south over Borneo during boreal winter, causing dry conditions. Plain Language Summary: Here, we aim to resolve conflicting evidence of how tropical convection in Borneo in the Indo‐Pacific oceanic region, a critical region associated with El Niño events and the Asian and Australian‐Indonesian monsoon systems, was affected by past changes in the strength of Atlantic Ocean circulation. Analyzing a 20,000+ year old Borneo cave stalagmite along its growth axis, we find large oxygen isotope shifts coincident with two prominent millennial‐scale periods of reduced Atlantic Ocean overturning circulation strength. The isotope record is similar to a neighboring southwestern Philippine stalagmite record, and we interpret the signal as drier mean annual conditions over the region during these events compared to the background state; an interpretation supported by simulations in three independent state‐of‐the‐art climate models. One model suggests that these conditions in Borneo were driven by the southward shift of an anomalous anticyclone in the boreal winter season. By combining geochemical and model evidence, this study has shown that rainfall decreased for millennial periods during the end of the ice age, when the Atlantic Ocean overturning circulation weakened. In doing so, we have elucidated further how this region responds to major changes in global climate and ocean conditions. Key Points: A new stalagmite record reveals for the first time distinct Bølling‐Allerød and Younger Dryas oxygen isotope variations in northern BorneoThe stalagmite oxygen isotope pattern strongly resembles the recent iCESM transient annual rainfall simulation from 20 to 11 kaBorneo drying during Heinrich 1 and the Younger Dryas may be attributed to an anomalous boreal winter Western North Pacific anticyclone [ABSTRACT FROM AUTHOR]

In tropical paleoclimate studies, paleo‐precipitation is often reconstructed from proxies via the "amount effect," that is, the empirical inverse relationship between local precipitation amount (P) and the oxygen isotopic composition of precipitation (δ18OP). However, recent research has illustrated numerous microphysical and dynamical controls on δ18OP that do not necessarily covary with P, complicating the reconstruction of circulation features like the Intertropical Convergence Zone. Here we introduce a new conceptual and statistical model for δ18OP that better captures the physical foundations for δ18OP as a tracer of hydrological balance. We find that bulk precipitation microphysics and cloud type exert comparable influences on δ18OP. Moisture transport plays an important secondary role in regions of deep atmospheric convection such as the Intertropical Convergence Zone and Indo‐Pacific Warm Pool. Our findings help reconcile conflicting interpretations of Intertropical Convergence Zone excursions, and provide a firm physical grounding for more nuanced, accurate interpretations of past hydroclimate using water isotope proxies. Plain Language Summary: The oxygen isotopic composition of tropical precipitation is a powerful tool for "fingerprinting" the history of evaporation, condensation, and transport that water was subjected to in the atmosphere before it reached the ground as precipitation. For this reason, the oxygen isotopic composition of precipitation is commonly employed as a water cycle tracer, both in modern‐day contexts and in geologic archives. Translating oxygen isotope ratios into metrics of circulation and climate is not always straightforward, however, due to the range of processes that can affect precipitation. Here we introduce a novel conceptual and statistical framework for interpreting oxygen isotope ratios in tropical precipitation by deconvolving its multiple competing influences. We find that the relative importance of each factor varies geographically. Moisture source is particularly important around the Indo‐Pacific Warm Pool, while cloud type exerts strong influence in regions where stratiform clouds are abundant. These results help to reconcile conflicting interpretations of how the Intertropical Convergence Zone and other key features of tropical circulation respond to climate forcings, which are critical questions for past climate reconstructions as well as future climate projections. Key Points: We present a new conceptual framework and statistical model for the oxygen isotopic composition of precipitation at tropical stationsCloud type is leading influence where stratiform rain is abundant; moisture transport plays key role along tropical rain belt perimetersδ18OP and Precip are both water cycle integrators, but δ18OP is a more reliable proxy for large‐scale hydrological processes than Precip [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of tropical coral skeletons (δ18O) is a crucial source of information on past El Niño/Southern Oscillation behavior. Both temperature and the δ18O of the surrounding seawater (δ18Osw) affect coral δ18O; a linear proportionality with δ18Osw is often used to infer past salinity variations, but the degree to which dynamical influences on δ18Osw may affect that relationship is still unclear. Here we use the isotope‐enabled Regional Ocean Modeling System to investigate the dynamics of δ18Osw and salinity variations in different twentieth century climate regimes. The dominant modes of δ18Osw variability are the background trend and eastern/central Pacific El Niño, similar to salinity; likewise, budget analysis reveals a strong impact of ocean dynamics (both advection and vertical mixing/diffusion) on δ18Osw variations during El Niño and La Niña events. These dynamics lead to alterations in the δ18Osw:salinity relationship across the Pacific: the linear approximation is most accurate near the eastern edge of the western Pacific warm pool but with nonlinearities due to large δ18Osw excursions during El Niño. At other sites, the δ18Osw:salinity relationship has substantial scatter and explains relatively little δ18Osw variance. Strikingly, interannual variability can lead to site‐specific δ18Osw:salinity regression coefficient changes of up to 0.4‰/psu on multiyear time scales. This calls the reliability of coral‐based estimates of El Niño/Southern Oscillation extremes into question and highlights the need for increased dynamical understanding as well as sustained in situ observations of δ18Osw and salinity variability. Plain Language Summary: Tropical corals record past El Niño and La Niña events through changes in the isotopic ratios of oxygen in their skeletons, which is affected by the isotopic composition of seawater local to the reef. This seawater δ18O is thought to be closely related to salinity and is used to reconstruct past salinity variations associated with El Niño. However, this paper uses an isotope‐enabled ocean model to show that salinity and seawater δ18O have a complicated relationship. The processes affecting the two variables can be quite different in places, and their relationship varies significantly with time; therefore, it may be much more difficult to use coral records to reconstruct past salinity changes (and thus, El Niño impacts) than was previously thought. Key Points: Salinity and seawater δ18O variations are not always well correlated, and their relationship is affected by ENSO dynamicsThe salinity:seawater δ18O relationship varies strongly on interannual time scalesReconstructing salinity based on seawater δ18O may have large uncertainties [ABSTRACT FROM AUTHOR]