The South American Monsoon System (SAMS) is the dominant mode of hydroclimatic variability in South America and the associated dynamics have an important influence on atmospheric organization during pluvial and drought periods. Paleoclimate records from the monsoon domain have helped to describe the isotope hydrology of this region over the last millennium, but are insufficient to explain why the system has changed. Utilizing transient simulations of last millennium climate from isotope-enabled global climate models as well as experiments forced by isolated external forcings made available from the NCAR Community Earth System Model Last Millennium Ensemble (CESM-LME) project, we explore the dynamical drivers of variability documented in the isotopic record. We find that while models continue to underestimate the temporal variability of regional climate, they are able to reproduce documented spatial modes of variability. A Monte Carlo empirical orthogonal function (MCEOF) analysis is used to decompose major modes of isotope variability from both isotope proxy records and isotope-enabled global climate models over the last millennium (850 – 1850 CE). The principal component time series from the isotopic records show clear mean state departures during the early and late part of the record, corresponding to the Medieval Climate Anomaly and Little Ice Age periods. We explore the role of internal variability and different external forcings in driving these departures, and use the model space to describe changes to regional circulation patterns during these periods of extreme isotopic change. This enhanced understanding of the drivers of variability over the last millennium could provide a foundation upon which to interpret the sensitivity of this system to future changes due to external forcings, such as anthropogenic activity.

Due to the many factors controlling δ13C values in stalagmites, complicating their paleoclimatic and paleoenvironmental interpretation, most studies do not present d13C values, but instead focus mainly on δ18O values. This is also the case for most cave studies from tropical South America, where many new δ18O stalagmite records covering the last millennia were recently published. Here, we review the d13C values in stalagmites, investigating the influence on this proxy of local hydroclimate, altitude, temperature and vegetation types, by employing a new dataset composed of published and unpublished carbon isotope records from various sites in tropical South America. The main factors influencing δ13C values are associated with the local hydroclimate, followed by minor effects from temperature. Most of the isotopic records show a significant correlation between the δ13C and δ18O values, indicating a close relationship between local hydroclimate and atmospheric convective processes related to the South American Monsoon System. The predominance of C3 plants above most of the karst systems studied here is responsible for the low δ13C values (≤6‰) in most of the speleothems, while local hydroclimate associated with prior calcite precipitation process is the main driver behind its variability during the last two millennia. Using Monte Carlo Principal Component Analysis, we produce an index of the mean hydrologic conditions and its changes over tropical South America for the last two millennia, which is closely related to monsoon variability for the period prior to 1750 CE. The recent break-down in the relationship between monsoon and local hydroclimate may have been caused by the increase in temperature, CO2, deforestation and fire during the current warm period.