31 Jan 2022
31 Jan 2022
Status: this preprint is currently under review for the journal CP.

South American Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models

Rebecca Orrison1, Mathias Vuille1, Jason E. Smerdon2, James Apaéstegui3,4, Jose Leandro P. S. Campos5, Francisco W. Cruz5, and Marcela Eduarda Della Libera5,6 Rebecca Orrison et al.
  • 1Department of Atmospheric and Environmental Sciences, University at Albany, Albany, NY, 12226, USA
  • 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, 10964, USA
  • 3Peruvian Institute of Geophysics, Lima, 15012, Peru
  • 4Universidad Nacional Agraria La Molina, Lima, 15024, Perú
  • 5Institute of Geosciences, University of São Paulo, São Paulo, 05508, Brazil
  • 6Institute for Geosciences, Johannes-Gutenberg-Universität, Mainz, Rhineland-Palatinate, 55122, Germany

Abstract. The South American Monsoon System (SAMS) is the main driver of regional hydroclimate variability across tropical and subtropical South America. It is best recorded on paleoclimatic timescales by stable oxygen isotope proxies, which are more spatially representative of regional hydroclimate than proxies for local precipitation alone. Network studies of proxies that can isolate regional influences lend particular insight into various environmental characteristics that modulate hydroclimate, such as atmospheric circulation variability and changes in the regional energy budget as well as understanding the climate system sensitivity to external forcings. To extract the coherent modes of variability of the SAMS over the Last Millennium (LM), we use a Monte Carlo Empirical Orthogonal Function (MCEOF) decomposition of 14 δ18O proxy records and compare them with modes extracted from a similar decomposition using isotope-enabled climate models. The two leading modes reflect the isotopic expression of the upper-tropospheric monsoon circulation (Bolivian High – Nordeste Low waveguide) and the latitudinal displacement of the South Atlantic Convergence Zone (SACZ), respectively. The spatial characteristics of these modes appear to be robust features of the LM hydroclimate over South America and are reproduced both in the proxy data and in isotope-enabled climate models, regardless of the nature of the model-imposed external forcing. Model analyses suggests that the local isotopic composition is primarily a reflection of an upstream rainout processes. The proxy data document that the SAMS was characterized by considerable temporal variability throughout the LM, with significant departures from the mean state during both the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The monsoon was intensified during the LIA over the central and western parts of tropical South America and the South Atlantic Convergence Zone (SACZ) was displaced to the southwest. These centennial-scale changes in monsoon intensity over the LM are underestimated in climate models, complicating the attribution of changes on these timescales to specific forcings and pointing toward areas of important model development.

Rebecca Orrison et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2022-6', Anonymous Referee #1, 08 Feb 2022
    • AC1: 'Reply on RC1', Rebecca Orrison, 06 Apr 2022
  • RC2: 'Comment on cp-2022-6', Anonymous Referee #2, 16 Mar 2022
    • AC2: 'Reply on RC2', Rebecca Orrison, 06 Apr 2022
  • CC1: 'Comment on cp-2022-6', Sebastian Luening, 28 Mar 2022
    • AC3: 'Reply on CC1', Rebecca Orrison, 06 Apr 2022

Rebecca Orrison et al.

Model code and software

SAMS MCEOF Rebecca Orrison

Rebecca Orrison et al.


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Short summary
Leading modes of South American Monsoon System variability over the last millennium are found to be stable features of regional climate. Isotope-enabled climate models successfully reproduce the leading modes of spatial variability and show they are stable regardless of the external forcings imposed. Multi-centennial mean state departures during the Medieval Climate Anomaly and Little Ice Age show regionally coherent patterns of hydroclimatic change in both proxies and models.