skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


  • NSF Public Access
  • Search Results
  • South American Summer Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models
Title: South American Summer Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models
Abstract. The South American Summer Monsoon (SASM) is the maindriver of regional hydroclimate variability across tropical and subtropicalSouth America. It is best recorded on paleoclimatic timescales by stableoxygen isotope proxies, which are more spatially representative of regionalhydroclimate than proxies for local precipitation alone. Network studies ofproxies that can isolate regional influences lend particular insight intovarious environmental characteristics that modulate hydroclimate, such asatmospheric circulation variability and changes in the regional energybudget as well as understanding the climate system sensitivity to externalforcings. We extract the coherent modes of variability of the SASM over thelast millennium (LM) using a Monte Carlo empirical orthogonal function(MCEOF) decomposition of 14 δ18O proxy records and compare themwith modes decomposed from isotope-enabled climate model data. The twoleading modes reflect the isotopic variability associated with (1) thermodynamic changes driving the upper-tropospheric monsoon circulation(Bolivian High–Nordeste Low waveguide) and (2) the latitudinaldisplacement of the South Atlantic Convergence Zone (SACZ). The spatialcharacteristics of these modes appear to be robust features of the LMhydroclimate over South America and are reproduced both in the proxy dataand in isotope-enabled climate models, regardless of the nature of themodel-imposed external forcing. The proxy data document that the SASM wascharacterized by considerable temporal variability throughout the LM, withsignificant departures from the mean state during both the Medieval ClimateAnomaly (MCA) and the Little Ice Age (LIA). Model analyses during theseperiods suggest that the local isotopic composition of precipitation isprimarily a reflection of upstream rainout processes associated with monsoonconvection. Model and proxy data both point to an intensification of themonsoon during the LIA over the central and western parts of tropical SouthAmerica and indicate a displacement of the South Atlantic Convergence Zone(SACZ) to the southwest. These centennial-scale changes in monsoon intensityover the LM are underestimated in climate models, complicating theattribution of changes on these timescales to specific forcings and pointingtoward areas of important model development.  more » « less
Award ID(s):
1805490 2103041 1743738
PAR ID:
10413138
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Climate of the Past
Volume:
18
Issue:
9
ISSN:
1814-9332
Page Range / eLocation ID:
2045 to 2062
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; (, Science Advances)
    The South American summer monsoon (SASM) profoundly influences tropical South America’s climate, yet understanding its low-frequency variability has been challenging. Climate models and oxygen isotope data have been used to examine the SASM variability over the last millennium (LM) but have, at times, provided conflicting findings, especially regarding its mean-state change from the Medieval Climate Anomaly to the Little Ice Age. Here, we use a paleoclimate data assimilation (DA) method, combining model results and δ18O observations, to produce a δ18O-enabled, dynamically coherent, and spatiotemporally complete austral summer hydroclimate reconstruction over the LM for tropical South America at 5-year resolution. This reconstruction aligns with independent hydroclimate and δ18O records withheld from the DA, revealing a centennial-scale SASM intensification during the MCA-LIA transition period, associated with the southward shift of the Atlantic Intertropical Convergence Zone and the strengthening Pacific Walker circulation (PWC). This highlights the necessity of accurately representing the PWC in climate models to predict future SASM changes. 
    more » « less
  2. ; ; ; ; ; ; (, Journal of Geophysical Research: Atmospheres)
    Abstract The South American summer monsoon (SASM) generates important hydroclimatic impacts in (sub‐)tropical South America and isotopic tracers recorded in paleoclimatic archives allow for assessing its long‐term response to Pacific variability prior to modern observations. Stable oxygen isotopes in precipitation integrate hydroclimatic changes during the SASM mature phase from December to February (DJF) in response to the Interdecadal Pacific Oscillation (IPO) and El Niño—Southern Oscillation (ENSO), respectively. Here, results from the isotope‐enabled Community Atmosphere Model v.5 are compared with highly resolved and precisely dated isotopic records from speleothems, tree rings, lake and ice cores during the industrial era (1880–2000 CE) and validated against observations from the International Atomic Energy Agency (IAEA) network. Pacific sea surface temperatures (SSTs) are coupled to the isotopic composition of SASM precipitation through perturbations in the Walker circulation associated with low‐ (IPO) and high‐frequency (ENSO) variability, impacting convective activity over tropical South America and the tropical Atlantic. Changes in convection over this monsoon entrance region ultimately control the downstream oxygen isotopic composition of precipitation recorded in paleoclimate archives. Overall, model results, paleoclimate records and IAEA data agree on the isotopic response to Pacific SST forcing. These results highlight the potential for long isotopic paleoclimate records to reconstruct Pacific climate variability on both high‐ and low‐frequency timescales. Furthermore, the isolation of the IPO signal in a diverse set of isotopic archives invites the reinterpretation of other paleoclimate proxies for identifying this historically overlooked forcing. 
    more » « less
  3. ; ; (, Journal of Climate)
    Abstract 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 (δ18Op) 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 andδ18Opshow 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. Significance StatementWe 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/δ18Opand monsoon intensity change. We highlight a heterogeneous dipole response in precipitation andδ18Opon 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. 
    more » « less
  4. ; ; ; (, Climate of the Past)
    Abstract. The Medieval Climate Anomaly (MCA; ca. 950–1250 CE) and the Little Ice Age (LIA; ca. 1450–1850 CE) were periods generally characterized by respectively higher and lower temperatures in many regions. However, they have also been associated with drier and wetter conditions in areas around the Intertropical Convergence Zone (ITCZ) and the Asian Monsoon region and in areas impacted by large-scale climatic modes like the Northern Annular Mode and Southern Annular Mode (NAM and SAM respectively). To analyze coordinated changes in large-scale hydroclimate patterns and whether similar changes also extend to other periods of the Last Millennium (LM) outside the MCA and the LIA, reconstruction-based products have been analyzed. This includes the collection of tree-ring-based drought atlases (DAs), the Paleo Hydrodynamics Data Assimilation product (PHYDA) and the Last Millennium Reanalysis (LMR). These analyses have shown coherent changes in the hydroclimate of tropical and extratropical regions, such as northern and central South America, East Africa, western North America, western Europe, the Middle East, Southeast Asia, and the Indo-Pacific, during the MCA, the LIA and other periods of the LM. Comparisons with model simulations from the Community Earth System Model – Last Millennium Ensemble (CESM-LME) and phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) show that both external forcing and internal variability contributed to these changes, with the contribution of internal variability being particularly important in the Indo-Pacific basin and that of external forcing in the Atlantic basin. These results may help to identify not only those areas showing coordinated changes, but also those regions more impacted by the internal variability, where forced model simulations would not be expected to successfully reproduce the evolution of past actual hydroclimate changes. 
    more » « less
  5. ; ; ; ; ; ; ; ; (, American Geophysical Union)
    For the past few decades, many researchers have sought to understand how tropical hydroclimate responds to climate change via lakes, marine sediments, and speleothems records. Speleothem δ18O records throughout South America have shown that regional rainfall responds to Northern Hemisphere forcing on the millennial scale. Areas under the influence of the South Atlantic Convergence Zone (SACZ) have also shown a close relationship with local insolation on longer timescales. However, apart from the Cruz et al. (2007) record in Southern Brazil, long-term speleothem records throughout the continent have relied primarily on stable oxygen isotopes and are therefore limited to describing large-scale regional variability in rainfall. As such, many areas in South America still lack long-term records of local hydroclimate, which is critical to understanding how different components of the monsoon system respond to orbital and millennial-scale climate change. One proxy that has gained more attention in recent years is trace metal-to-calcium ratios (TM/Ca). Sr, Mg, and Ba to Ca ratios in speleothems are known in certain situations to respond to the degree of Prior Calcite Precipitation (PCP) above a drip site, a phenomenon directly tied to local aridity. In this study, we have obtained high-resolution TM/Ca measurements to pair with stable isotopes from samples spanning 23 to 66 ka from Huagapo Cave in the Peruvian Andes (11.27°S; 75.79°W). TM/Ca ratios in these samples are strongly correlated (R2>0.89), making them suitable for use as PCP proxies. We see that decreases in δ18O during Heinrich events are accompanied by a drop in TM/Ca. The period defined by the MIS 4/3 transition is accompanied by a simultaneous increase in TM/Ca and δ18O. TM/Ca and δ18O negatively correlate with local insolation for the entire record. Interestingly, the Paraíso Cave record from the Amazon Basin shows no correlation between regional or local hydroclimate and insolation during the last glacial period. The discrepancy between the two records and the close relationship between TM/Ca, δ18O, and local insolation in Huagapo samples, may call for a revised interpretation of Andes speleothem δ18O variability, which was originally thought to reflect rainout over the Amazon Basin. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email