Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

Mariano S. Morales, Edward R. Cook, Jonathan Barichivich,  View ORCID ProfileDuncan A. Christie, Ricardo Villalba, Carlos LeQuesne,  View ORCID ProfileAna M. Srur,  View ORCID ProfileM. Eugenia Ferrero, Álvaro González-Reyes,  View ORCID ProfileFleur Couvreux,  View ORCID ProfileVladimir Matskovsky,  View ORCID ProfileJuan C. Aravena, Antonio Lara,  View ORCID ProfileIgnacio A. Mundo,  View ORCID ProfileFacundo Rojas, María R. Prieto,  View ORCID ProfileJason E. Smerdon,  View ORCID ProfileLucas O. Bianchi, Mariano H. Masiokas, Rocio Urrutia-Jalabert,  View ORCID ProfileMilagros Rodriguez-Catón,  View ORCID ProfileAriel A. Muñoz,  View ORCID ProfileMoises Rojas-Badilla, Claudio Alvarez,  View ORCID ProfileLidio Lopez, Brian H. Luckman,  View ORCID ProfileDavid Lister, Ian Harris, Philip D. Jones,  View ORCID ProfileA. Park Williams, Gonzalo Velazquez, Diego Aliste, Isabella Aguilera-Betti,  View ORCID ProfileEugenia Marcotti,  View ORCID ProfileFelipe Flores,  View ORCID ProfileTomás Muñoz, Emilio Cuq, and José A. Boninsegna

PNAS July 21, 2020 117 (29) 16816-16823; first published July 6, 2020 https://doi.org/10.1073/pnas.2002411117

1. Edited by James C. Zachos, University of California, Santa Cruz, CA, and approved June 1, 2020 (received for review February 18, 2020)

Significance

The SADA is an annually-resolved hydroclimate atlas in South America that spans the continent south of 12°S from 1400 to 2000 CE. Based on 286 tree ring records and instrumentally-based estimates of soil moisture, the SADA complements six drought atlases worldwide filling a geographical gap in the Southern Hemisphere. Independently validated with historical records, SADA shows that the frequency of widespread severe droughts and extreme pluvials since the 1960s is unprecedented. Major hydroclimate events expressed in the SADA are associated with strong El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) anomalies. Coupled ENSO-SAM anomalies together with subtropical low-level jet intensification due to increasing greenhouse gas emissions may cause more extreme droughts and pluvials in South America during the 21st century.

Abstract

South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions.