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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. 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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3. Determining the source of speleothem δ18O variability from in situ measurements of seasonal and inter-annual isotope trends in precipitation, cave drip water and modern calcite from sites in the Central Peruvian Andes

   Olson, E.J. ( December 2022 , American Geophysical Union Conference. Chicago, IL)

   In the past decade, Huagapo and Pacupahuain Caves in the Central Peruvian Andes have become sources of speleothem oxygen isotope (δ18O) paleoclimate records. These studies identify the South American Summer Monsoon (SASM) as the main climate system controlling δ18O variability. While this interpretation is verified through inter-proxy record comparisons on millennial scales, interpretation of the high-resolution variability within these records is limited by a lack of modern proxy calibration studies at these sites. Here we present results from a modern cave monitoring study undertaken to address the controls on the δ18O values of precipitation at these sites and how surface and in-cave processes affect the δ18O value of speleothem calcite. Speleothem calcite δ18O values reflect an integrated signal of atmospheric processes (e.g., rainout, Raleigh distillation, upstream moisture recycling, changes in moisture source), evaporation and mixing during infiltration in the soil and epikarst, and in-cave processes such as degassing and evaporation. In consideration of these factors, we compare isotope trends in precipitation, cave drip water and modern farmed calcite from the two cave sites. We find that precipitationδ18O values during peak monsoon activity (January -February) shows considerable inter-annual variation with averages of -16.7‰ for 2020, -18.5‰ for 2021 and -13.8‰ in 2022. We investigate the source of this variability in regional atmospheric circulation patterns using weather station data and back trajectories. The mean annual precipitation (MAP) from outside Huagapo Cave is δ18O = -15.5+/- 6‰, while seasonal samples of drip water δ18O = -14.5+/- 1‰, are offset from MAP possibly due to evaporation during infiltration. Cave drip waterδ18O has low variability over inter-annual and seasonal timescales indicating homogenization in the epikarst. Using geochemical and sensor data (e.g. cave relative humidity, temperature, and drip rate) as inputs for a karst based forward model, we simulate modern speleothem δ18O to quantitatively assess the combined effects of hydroclimate processes integration to the isotope record. 

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4. Data‐Model Comparisons of Tropical Hydroclimate Changes Over the Common Era

   https://doi.org/10.1029/2020PA003934  

   Atwood, A. R. ; Battisti, D. S. ; Wu, E. ; Frierson, D. M. W. ; Sachs, J. P. ( July 2021 , Paleoceanography and Paleoclimatology)

   We examine the evidence for large‐scale tropical hydroclimate changes over the Common Era based on a compilation of 67 tropical hydroclimate records from 55 sites and assess the consistency between the reconstructed hydroclimate changes and those simulated by transient model simulations of the last millennium. Our synthesis of the proxy records reveals several regionally coherent patterns on centennial time scales. From 800 to 1000 CE, records from the eastern Pacific and parts of Mesoamerica indicate a pronounced drying event relative to background conditions of the Common Era. In addition, 1400–1700 CE is marked by pronounced hydroclimate changes across the tropics, including dry and/or isotopically enriched conditions in South and East Asia, wet and/or isotopically depleted conditions in the central Andes and southern Amazon in South America, and fresher and/or isotopically depleted conditions in the Maritime Continent. We find notable dissimilarities between the regional hydroclimate changes and global‐scale and hemispheric‐scale temperature reconstructions, indicating that more work needs to be done to understand the mechanisms of the widespread tropical hydroclimate changes during the LIA. Apropos to previous interpretations of large‐scale reorganization of tropical Pacific climate during the LIA, we do not find support for a large‐scale southward shift of the Pacific Intertropical Convergence Zone, while evidence for a strengthened Pacific Walker Circulation and/or an equatorward contraction of the monsoonal Asian‐Australian rain belt exists from limited geographic regions but require additional paleoclimate constraints. Transient climate model simulations exhibit weak forced long‐term tropical rainfall changes over the last millennium but provide several important insights to the proxy reconstructions.

    

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5. Dynamical and hydrological changes in climate simulations of the last millennium

   https://doi.org/10.5194/cp-16-1285-2020  

   Roldán-Gómez, Pedro José ; González-Rouco, Jesús Fidel ; Melo-Aguilar, Camilo ; Smerdon, Jason E. ( January 2020 , Climate of the Past)

   null (Ed.)

   Abstract. Simulations of climate of the last millennium (LM) show that external forcing had a major contribution to the evolution of temperatures; warmer and colder periods like the Medieval Climate Anomaly (MCA; ca. 950–1250 CE) and the Little Ice Age (LIA; ca. 1450–1850 CE) were critically influenced by changes in solar and volcanic activity. Even if this influence is mainly observed in terms of temperatures, evidence from simulations and reconstructions shows that other variables related to atmospheric dynamics and hydroclimate were also influenced by external forcing over some regions. In this work, simulations from the Coupled Model Intercomparison Project Phase 5 and Paleoclimate Modelling Intercomparison Project Phase 3 (CMIP5/PMIP3) are analyzed to explore the influence of external forcings on the dynamical and hydrological changes during the LM at different spatial and temporal scales. Principal component (PC) analysis is used to obtain the modes of variability governing the global evolution of climate and to assess their correlation with the total external forcing at multidecadal to multicentennial timescales. For shorter timescales, a composite analysis is used to address the response to specific events of external forcing like volcanic eruptions. The results show coordinated long-term changes in global circulation patterns, which suggest expansions and contractions of the Hadley cells and latitudinal displacements of westerlies in response to external forcing. For hydroclimate, spatial patterns of drier and wetter conditions in areas influenced by the North Atlantic Oscillation (NAO), Northern Annular Mode (NAM), and Southern Annular Mode (SAM) and alterations in the intensity and distribution of monsoons and convergence zones are consistently found. Similarly, a clear short-term response is found in the years following volcanic eruptions. Although external forcing has a greater influence on temperatures, the results suggest that dynamical and hydrological variations over the LM exhibit a direct response to external forcing at both long and short timescales that is highly dependent on the particular simulation and model. 

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