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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. Greenhouse Gas and Ice Volume Drive Pleistocene Indian Summer Monsoon Precipitation Isotope Variability

   https://doi.org/10.1029/2020GL092249  

   McGrath, Sarah M. ; Clemens, Steven C. ; Huang, Yongsong ; Yamamoto, Masanobu ( February 2021 , Geophysical Research Letters)

   Orbital‐scale Indian Summer Monsoon variability is often interpreted as a direct response to northern hemisphere summer insolation. Here we present a continuous (0–640 kyr) orbital scale precipitation isotope (δD_precip) record using leaf wax δD from the core monsoon zone of India. The δD_preciprecord is quantitatively coherent with, and δD_precipminima in phase with, greenhouses gas maxima, and ice volume minima across all orbital bands. The δD_preciprecord is also coherent and in phase with the two existing orbital‐scale Indian speleothem δ¹⁸O records, demonstrating a consistent regional response among independent proxies. These findings preclude interpretation of Indian precipitation isotope records as a direct response to northern hemisphere summer insolation. Rather, they dominantly reflect changes in moisture source and transport paths associated with changes in greenhouse gases and ice volume. The orbital‐scale precipitation isotope responses of the Indian and East Asian monsoon systems are uncoupled and are driven by different forcings.

    

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3. Southern hemisphere forced millennial scale Indian summer monsoon variability during the late Pleistocene

   https://doi.org/10.1038/s41598-022-14010-6  

   Band, Shraddha T. ; Yadava, M. G. ; Kaushal, Nikita ; Midhun, M. ; Thirumalai, Kaustubh ; Francis, Timmy ; Laskar, Amzad ; Ramesh, R. ; Henderson, Gideon M. ; Narayana, A. C. ( December 2022 , Scientific Reports)

   Abstract Peninsular India hosts the initial rain-down of the Indian Summer Monsoon (ISM) after which winds travel further east inwards into Asia. Stalagmite oxygen isotope composition from this region, such as those from Belum Cave, preserve the vital signals of the past ISM variability. These archives experience a single wet season with a single dominant moisture source annually. Here we present high-resolution δ 18 O, δ 13 C and trace element (Mg/Ca, Sr/Ca, Ba/Ca, Mn/Ca) time series from a Belum Cave stalagmite spanning glacial MIS-6 (from ~ 183 to ~ 175 kyr) and interglacial substages MIS-5c-5a (~ 104 kyr to ~ 82 kyr). With most paleomonsoon reconstructions reporting coherent evolution of northern hemisphere summer insolation and ISM variability on orbital timescale, we focus on understanding the mechanisms behind millennial scale variability. Finding that the two are decoupled over millennial timescales, we address the role of the Southern Hemisphere processes in modulating monsoon strength as a part of the Hadley circulation. We identify several strong and weak episodes of ISM intensity during 104–82 kyr. Some of the weak episodes correspond to warming in the southern hemisphere associated with weak cross-equatorial winds. We show that during the MIS-5 substages, ISM strength gradually declined with millennial scale variability linked to Southern Hemisphere temperature changes which in turn modulate the strength of the Mascarene High. 

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4. The Asian Summer Monsoon: Teleconnections and Forcing Mechanisms - A Review from Chinese Speleothem d18O Records

   Zhang, H.W. ( July 2019 , Quaternary)

   Asian summer monsoon (ASM) variability significantly affects hydro-climate, and thus socio-economics, in the East Asian region, where nearly one-third of the global population resides. Over the last two decades, speleothem δ18O records from China have been utilized to reconstruct ASM variability and its underlying forcing mechanisms on orbital to seasonal timescales. Here, we use the Speleothem Isotopes Synthesis and Analysis database (SISAL_v1) to present an overview of hydro-climate variability related to the ASM during three periods: the late Pleistocene, the Holocene, and the last two millennia. We highlight the possible global teleconnections and forcing mechanisms of the ASM on different timescales. The longest composite stalagmite δ18O record over the past 640 kyr BP from the region demonstrates that ASM variability on orbital timescales is dominated by the 23 kyr precessional cycles, which are in phase with Northern Hemisphere summer insolation (NHSI). During the last glacial, millennial changes in the intensity of the ASM appear to be controlled by North Atlantic climate and oceanic feedbacks. During the Holocene, changes in ASM intensity were primarily controlled by NHSI. However, the spatio-temporal distribution of monsoon rain belts may vary with changes in ASM intensity on decadal to millennial timescales. 

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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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