The South American monsoon variability over the last millennium in climate models

Abstract

Abstract. In this paper we assess South American monsoon system (SAMS) variability in the last millennium as depicted by global coupled climate model simulations. Highresolution proxy records for the South American monsoon over this period show a coherent regional picture of a weak monsoon during the Medieval Climate Anomaly and a stronger monsoon during the Little Ice Age (LIA). Due to the small external forcing during the past 1000 years, model simulations do not show very strong temperature anomalies over these two specific periods, which in turn do not translate into clear precipitation anomalies, in contrast with the rainfall reconstructions in South America. Therefore, we used an ad hoc definition of these two periods for each model simulation in order to account for model-specific signals. Thereby, several coherent large-scale atmospheric circulation anomalies are identified. The models feature a stronger monsoon during the LIA associated with (i) an enhancement of the rising motion in the SAMS domain in austral summer; (ii) a stronger monsoon-related upper-tropospheric anticyclone; (iii) activation of the South American dipole, which results in a poleward shift of the South Atlantic Convergence Zone; and (iv) a weaker upper-level subtropical jet over South America. The diagnosed changes provide important insights into the mechanisms of these climate anomalies over South America during the past millennium.

Two Millennia of South Atlantic Convergence Zone Variability Reconstructed From Isotopic Proxies

“…Future efforts to improve our understanding of SACZ variability over the past millennia will require detailed studies analyzing the relative importance of internal versus external forcing in determining the positioning and intensity of the SACZ, and how this convergence zone interacts with ocean‐atmosphere feedbacks on long time scales. For this purpose our new data set can serve as an ideal test bed for climate model simulations over the last millennium (e.g., Rojas et al, ), but it also provides a valuable new contribution from the undersampled southern hemisphere for global paleoclimate reconstructions during the late Holocene.…”

Late Quaternary environmental dynamics in the Atacama Desert reconstructed from rodent midden pollen records

“…Marengo et al ., ). A strengthening (weakening) of the Bolivian high is consistent with a stronger (weaker) SASM circulation (Rojas et al ., ). The southward (northward) shift of this upper‐level anticyclone is related to an enhanced (weakened) summer easterly flow over the central Andes and increases (decreases) the moisture transport and rainfall over the Altiplano and the west (Pacific) slope of the Andes (Aceituno and Montecinos, ; Lenters and Cook, ; Vuille and Ammann, ; Garreaud et al ., ).…”

The Threat of Multi‐Year Drought in Western Amazonia

“…This mismatch cannot be explained by a problem with model resolution at the Amazon-Andes transition zone (e.g., Guimberteau et al, 2013;Yin et al, 2013) because we find similarly 'white' simulated precipitation spectra across much of tropical South America in most CMIP5 models (supporting information Figure S5) . These results suggest that CMIP5 models may underestimate the risk of decadal and longer droughts, and generally underestimate past large-scale shifts in precipitation over South America that have been inferred from the paleoclimate record (e.g., Rojas et al, 2016).…”