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1. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response

   https://doi.org/10.1175/JCLI-D-22-0172.1  

   Bao, Yuntao ; Liu, Zhengyu ; He, Chengfei ( July 2023 , Journal of Climate)

   Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitationδ¹⁸O (δ¹⁸O_p) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation andδ¹⁸O_pshow a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon.

   We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ¹⁸O_pand monsoon intensity change. We highlight a heterogeneous dipole response in precipitation andδ¹⁸O_pon millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future.

    

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2. Variability and Controls on δ 18 O, d‐excess, and ∆′ 17 O in Southern Peruvian Precipitation

   https://doi.org/10.1029/2020JD034009  

   Aron, Phoebe G. ; Poulsen, Christopher J. ; Fiorella, Richard P. ; Levin, Naomi E. ; Acosta, Rene P. ; Yanites, Brian J. ; Cassel, Elizabeth J. ( December 2021 , Journal of Geophysical Research: Atmospheres)

   The isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ¹⁸O_pand δ²H_pvalues from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′¹⁷O_p, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ¹⁸O_pand that seasonal δ¹⁸O_pvariations are related to upstream rainout and local convection. Spatial δ¹⁸O_pvariations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excess_pcycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excess_pand ∆′¹⁷O_pclearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′¹⁷O_pvalues in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ¹⁸O_p, d‐excess_p, and ∆′¹⁷O_pdata and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.

    

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3. The Forest Effects on the Isotopic Composition of Rainfall in the Northwestern Amazon Basin

   https://doi.org/10.1029/2019JD031445  

   Ampuero, A. ; Stríkis, N. M. ; Apaéstegui, J. ; Vuille, M. ; Novello, V. F. ; Espinoza, J. C. ; Cruz, F. W. ; Vonhof, H. ; Mayta, V. C. ; Martins, V. T. S. ; et al ( February 2020 , Journal of Geophysical Research: Atmospheres)

   In the Amazon basin, intense precipitation recycling across the forest significantly modifies the isotopic composition of rainfall (δ¹⁸O, δD). In the tropical hydrologic cycle, such an effect can be identified through deuterium excess (dxs), yet it remains unclear what environmental factors control dxs, increasing the uncertainty of dxs‐based paleoclimate reconstructions. Here we present a 4‐year record of the isotopic composition of rainfall, monitored in the northwestern Amazon basin. We analyze the isotopic variations as a function of the air mass history, based on atmospheric back trajectory analyses, satellite observations of precipitation upstream, leaf area index, and simulated moisture recycling along the transport pathway. We show that the precipitation recycling in the forest exerts a significant control on the isotopic composition of precipitation in the northwestern Amazon basin, especially on dxs during the dry season (r= 0.71). Applying these observations to existing speleothem and pollen paleorecords, we conclude that winter precipitation increased after the mid‐Holocene, as the expansion of the forest allowed for more moisture recycling. Therefore, forest effects should be considered when interpreting paleorecords of past precipitation changes.

    

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4. Precession-band variance missing from East Asian monsoon runoff

   https://doi.org/10.1038/s41467-018-05814-0  

   Clemens, S. C. ; Holbourn, A. ; Kubota, Y. ; Lee, K. E. ; Liu, Z. ; Chen, G. ; Nelson, A. ; Fox-Kemper, B. ( August 2018 , Nature Communications)

   Speleothem CaCO₃δ¹⁸O is a commonly employed paleomonsoon proxy. However, inferring local rainfall amount from speleothem δ¹⁸O can be complicated due to changing source water δ¹⁸O, temperature effects, and rainout over the moisture transport path. These complications are addressed using δ¹⁸O of planktonic foraminiferal CaCO₃, offshore from the Yangtze River Valley (YRV). The advantage is that the effects of global seawater δ¹⁸O and local temperature changes can be quantitatively removed, yielding a record of local seawater δ¹⁸O, a proxy that responds primarily to dilution by local precipitation and runoff. Whereas YRV speleothem δ¹⁸O is dominated by precession-band (23 ky) cyclicity, local seawater δ¹⁸O is dominated by eccentricity (100 ky) and obliquity (41 ky) cycles, with almost no precession-scale variance. These results, consistent with records outside the YRV, suggest that East Asian monsoon rainfall is more sensitive to greenhouse gas and high-latitude ice sheet forcing than to direct insolation forcing.

    

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5. Zonal Indian Ocean Variability Drives Millennial‐Scale Precipitation Changes in Northern Madagascar

   https://doi.org/10.1029/2023PA004626  

   Tiger, Benjamin H. ; Burns, Stephen ; Dawson, Robin R. ; Scroxton, Nick ; Godfrey, Laurie R. ; Ranivoharimanana, Lovasoa ; Faina, Peterson ; McGee, David ( November 2023 , Paleoceanography and Paleoclimatology)

   The low latitude Indian Ocean is warming faster than other tropical basins, and its interannual climate variability is projected to become more extreme under future emissions scenarios with substantial impacts on developing Indian Ocean rim countries. Therefore, it has become increasingly important to understand the drivers of regional precipitation in a changing climate. Here we present a new speleothem record from Anjohibe, a cave in northwest (NW) Madagascar well situated to record past changes in the Intertropical Convergence Zone (ITCZ). U‐Th ages date speleothem growth from 27 to 14 ka. δ¹⁸O, δ¹³C, and trace metal proxies reconstruct drier conditions during Heinrich Stadials 1 and 2, and wetter conditions during the Last Glacial Maximum and Bølling–Allerød. This is surprising considering hypotheses arguing for southward (northward) ITCZ shifts during North Atlantic cooling (warming) events, which would be expected to result in wetter (drier) conditions at Anjohibe in the Southern Hemisphere tropics. The reconstructed Indian Ocean zonal (west‐east) sea surface temperature (SST) gradient is in close agreement with hydroclimate proxies in NW Madagascar, with periods of increased precipitation correlating with relatively warmer conditions in the western Indian Ocean and cooler conditions in the eastern Indian Ocean. Such gradients could drive long‐term shifts in the strength of the Walker circulation with widespread effects on hydroclimate across East Africa. These results suggest that during abrupt millennial‐scale climate changes, it is not meridional ITCZ shifts, but the tropical Indian Ocean SST gradient and Walker circulation driving East African hydroclimate variability.

    

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