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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. Hydroclimate Variability in the Equatorial Western Indian Ocean for the Last 250,000 Years

   https://doi.org/10.1029/2022PA004530  

   Windler, Grace ; Tierney, Jessica E. ; deMenocal, Peter B. ( January 2023 , Paleoceanography and Paleoclimatology)

   Indian Ocean sea surface temperatures impact precipitation across the basin through coupled ocean‐atmosphere responses to changes in climate. To understand the hydroclimate response over the western Indian Ocean and equatorial east Africa to different forcing mechanisms, we present four new proxy reconstructions from core VM19‐193 (2.98°N, 51.47°E) that span the last 250 ky. Sub‐surface water temperatures (Sub‐T; TEX₈₆) show strong precessional (23 ky) variability that is primarily influenced by maximum incoming solar radiation (insolation) during the Northern Hemisphere spring season, likely indicating that local insolation dominates the upper water column at this tropical location over time. Leaf waxes, on the other hand, reflect two different precipitation signals:δ¹³C_wax(in phase with boreal fall insolation) is likely reflecting vegetation changes in response to local rainfall over east Africa, whereasδD_precip(primarily driven by boreal summer insolation) represents changes in regional circulation associated with the summer monsoon. Glacial‐interglacial changes in ocean temperatures support glacial shelf exposure over the Maritime Continent in the eastern Indian Ocean and the subsequent weakening of the Indian Walker Circulation as a mechanism driving 100 ky climate variability across the tropical Indo‐Pacific. Additionally, the 100 ky spectral power inδD_precipsupports a basin‐wide weakening of summer monsoon circulation in response to glacial climates. Overall, the proxy records from VM19‐193 indicate that both precession and glacial‐interglacial cycles exert control over hydroclimate at this tropical location.

    

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3. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part II: δ18Op Response

   https://doi.org/10.1175/JCLI-D-22-0289.1  

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

   Understanding the hydroclimate representations of precipitationδ¹⁸O (δ¹⁸O_p) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall andδ¹⁸O_p, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of suchδ¹⁸O_pdipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to theδ¹⁸O_pdepletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to theδ¹⁸O_penrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship betweenδ¹⁸O_pand rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennialδ¹⁸O_pchange in the central-western Amazon, whileδ¹⁸O_pdoes not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when usingδ¹⁸O_pas a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennialδ¹⁸O_pdipole response and pose a challenge in reconciling the discrepancy.

   We want to comprehensively understand the hydroclimate footprints ofδ¹⁸O_pand the mechanisms of the millennial variability ofδ¹⁸O_pover tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennialδ¹⁸O_pchange in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it.

    

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4. Methane, Monsoons, and Modulation of Millennial‐Scale Climate

   https://doi.org/10.1029/2020GL087613  

   Thirumalai, Kaustubh ; Clemens, Steven C. ; Partin, Judson W. ( April 2020 , Geophysical Research Letters)

   Earth's orbital geometry exerts a profound influence on climate by regulating changes in incoming solar radiation. Superimposed on orbitally paced climate change, Pleistocene records reveal substantial millennial‐scale variability characterized by abrupt changes and rapid swings. However, the extent to which orbital forcing modulates the amplitude and timing of these millennial variations is unclear. Here we isolate the magnitude of millennial‐scale variability (MMV) in two well‐dated records, both linked to precession cycles (19,000‐ and 23,000‐year periodicity): composite Chinese speleothemδ¹⁸O, commonly interpreted as a proxy for Asian monsoon intensity, and atmospheric methane. At the millennial timescale (1,000–10,000 years), we find a fundamental decoupling wherein precession directly modulates the MMV of methane but not that of speleothemδ¹⁸O, which is shown to be strikingly similar to the MMV of Antarctic ice coreδ²H. One explanation is that the MMV of methane responds to changes in midlatitude to high‐latitude insolation, whereas speleothemδ¹⁸O is modulated by internal climate feedbacks.

    

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5. Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Inorganic (Carbonate) Carbon Cycle and Feedbacks

   https://doi.org/10.1029/2020PA004090  

   Vervoort, Pam ; Kirtland Turner, Sandra ; Rochholz, Fiona ; Ridgwell, Andy ( September 2021 , Paleoceanography and Paleoclimatology)

   Astronomical cycles are strongly expressed in marine geological records, providing important insights into Earth system dynamics and an invaluable means of constructing age models. However, how various astronomical periods are filtered by the Earth system and the mechanisms by which carbon reservoirs and climate components respond, particularly in absence of dynamic ice sheets, is unclear. Using an Earth system model that includes feedbacks between climate, ocean circulation, and inorganic (carbonate) carbon cycling relevant to geological timescales, we systematically explore the impact of astronomically modulated insolation forcing and its expression in model variables most comparable to key paleoceanographic proxies (temperature, the δ¹³C of inorganic carbon, and sedimentary carbonate content). Temperature predominately responds to short and long eccentricity and is little influenced by the modeled carbon cycle feedbacks. In contrast, the cycling of nutrients and carbon in the ocean generates significant precession power in atmospheric CO₂, benthic ocean δ¹³C, and sedimentary wt% CaCO₃, while inclusion of marine sedimentary and weathering processes shifts power to the long eccentricity period. Our simulations produce reducedpCO₂and dissolved inorganic carbon (DIC) δ¹³C at long eccentricity maxima and, contrary to early Cenozoic marine records, CaCO₃preservation in the model is enhanced during eccentricity‐modulated warmth. Additionally, the magnitude of δ¹³C variability simulated in our model underestimates marine proxy records. These model‐data discrepancies hint at the possibility that the Paleogene silicate weathering feedback was weaker than modeled here and that additional organic carbon cycle feedbacks are necessary to explain the full response of the Earth system to astronomical forcing.

    

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