Speleothem records of monsoon interannual-interdecadal variability through the Holocene

Modern observations show considerable interannual to interdecadal variability in monsoon precipitation. However, there are few reconstructions of variability at this timescale through the Holocene, and there is therefore less understanding of how changes in external forcing might have affected monsoon variability in the past. Here, we reconstruct the evolution of the amplitude of interannual to interdecadal variability (IADV) in the East Asian, Indian and South American monsoon regions through the Holocene using a global network of high-resolution speleothem oxygen isotope (δ18O) records. We reconstruct changes in IADV for individual speleothem records using the standard deviation of δ18O values in sliding time windows after correcting for the influence of confounding factors such as variable sampling resolution, growth rates and mean climate. We then create composites of IADV changes for each monsoon region. We show that there is an overall increase in δ18O IADV in the Indian monsoon region through the Holocene, with an abrupt change to present-day variability at ~2 ka. In the East Asian monsoon, there is an overall decrease in δ18O IADV through the Holocene, with an abrupt shift also seen at ~2 ka. The South American monsoon is characterised by large multi-centennial shifts in δ18O IADV through the early and mid-Holocene, although there is no overall change in variability across the Holocene. Our regional IADV reconstructions are broadly reproduced by transient climate-model simulations of the last 6 000 years. These analyses indicate that there is no straightforward link between IADV and changes in mean precipitation, or between IADV and orbital forcing, at a regional scale.

Environmental variability at the margin of the South American Monsoon System recorded by a high-resolution sediment record from Lagoa Dourada (South Brazil)

High-resolution geochemical and sedimentological data were analyzed for a lacustrine sediment record from Lagoa Dourada (South Brazil). Four distinctly different depositional processes were determined: (1) Suspension fallout of fine-grained minerogenic particles transferred via fluvial activity dominates the Early Holocene and relates to open grassland in the catchment area; (2) Activation of the karst hydrological system with deposition of massive sand layers indicates increased precipitation at the onset of the Middle Holocene; (3) Minerogenic sediments are replaced by organic deposition due to wetter climatic conditions with the development of forests, which together fostered pedogenesis with the release of dissolved nutrients during the Middle to Late Holocene; (4) Human-induced land-use change caused destabilization of soils in the catchment area with resulting cultural soil erosion between AD 1800 and 1950. These depositional trends are linked to intensity variations of the South American Monsoon System (SAMS). Two century-long climatic events detected by high-resolution XRF scanning data confirm this relationship and probably provide signals of the 8.2 ka event and the Little Ice Age (LIA). Both events document increased rainfall with complex responses of the environmental system. Our SAMS-induced consequences of past hydroclimatic variability on the environment of South Brazil provide background information for better evaluating model projections of future climate change.

Variability of the low-level circulation of the South American Monsoon analysed with complex networks

Understanding the variability of low-level atmospheric circulation regimes is key for understanding the dynamics of monsoon systems. The South American Monsoon is characterized by strong year-long trade winds that are channeled southward into the South American Low-Level Jet after crossing the Amazon basin, which in turn is elementary for the moisture transport to Southern South America. In this study, we utilize streamflow wind networks, a type of climate networks that tracks the local flow of the wind field, together with the analysis of composites of wind, precipitation, and geopotential height fields, to investigate the variability of the South American low-level circulation. The streamflow wind networks are used here as they are able to directly track the wind flow and encode its spatiotemporal characteristics in their topology. We focus on intraseasonal variations in terms of active and break monsoon phases on the one hand, and on the interannual variability in terms of the impacts of the El Niño-Southern Oscillation on the other hand. Our findings highlight the importance of the South American Low-Level Jet, its spatial position and variability. Our study reveals the relation of the active and break regimes to anomalous high- and low-pressure systems over the southern Atlantic that are connected to Rossby wave trains from the southern Pacific, as well as the impact of these regimes on the cross-equatorial low-level flow. In addition, the streamflow networks that we use demonstrate significant shifts of the dominant wind flow pattern during El Niño and La Niña episodes.

Reconstruction of the last three millennia South American Monsoon variability over the Amazon basin using speleothem isotope records

<p>South American Monsoon System (SAMS) and its main feature, the South American Convergence Zone (SACZ) are responsible for the major distribution of moisture in South America. The current work presents a novel high-resolution oxygen isotope record (δ<sup>18</sup>O) based on speleothems from southwest Amazon basin (Brazil), right at SAMS' core region and SACZ onset, where there is still a gap of high resolution paleoclimate records. The novel δ<sup>18</sup>O record presents an average of 3 year-resolution, composed by 1344 stable isotope analysis performed in two speleothems with a well-resolved chronology (37 U/Th ages) with average errors <1%. This work aims to describe the rainfall variability of the core region of the South American monsoon for the last 3k years and to take a broader look at precipitation patterns over Amazon basin. The Rondônia δ18O record shows three main stages throughout this time period. The first is from -1000 to ~400 CE, where it’s in accordance with most of other paleorecords from the Amazon basin. the second segment  is from ~400 to 1200 CE, when there is a continuous increase in the δ18O record until it reaches its highest values around 850 CE during the MCA (800-1200 CE), which is in accordance with western Amazon records, whilst the record in eastern Amazon presents an opposite trend. Thus, a precipitation dipole over Amazon emerges from ~400 CE onwards, majorly triggered by anomalous climate changes such as MCA, where western (eastern) Amazon is drier (wetter). During LIA (1450-1800 CE), on the other hand, Rondônia record presents its lowest values, also agreeing with western records and with records under the influence of SACZ whilst on eastern Amazon a drier period is established. Therefore, with this novel paleoclimate record located at the core region of SAMS, it's possible to evidence the dynamics of the precipitation dipole over the Amazon region, as well as understand the SACZ intensity variations.</p>

Speleothems of South American and Asian Monsoons Influenced by a Green Sahara

<p>Compared to preindustrial, the mid-Holocene (6 ka) had significantly greater Northern Hemisphere summer insolation, slightly warmer global surface temperature, and slightly lower CO<sub>2</sub> concentration. Vegetation was also different during the mid-Holocene. Possibly most prominent was the growth of temperate vegetation in the now barren Sahara. This Saharan vegetation response was related to intensification of the African Monsoon associated with the mid-Holocene orbital configuration. Hydroclimate of the Asian Monsoon and South American Monsoon also responded to mid-Holocene forcings, with general wetting and drying, respectively.</p><p>The mid-Holocene is frequently used for model-proxy comparison studies. However, climate models often struggle to replicate the proxy signals of this period. Here, we attempt to reduce these model-proxy discrepancies by exploring the significance of a vegetated Sahara during the mid-Holocene. Using the water isotopologue tracer enabled version of the Community Earth System Model (iCESM1), we perform mid-Holocene simulations that include and exclude temperate vegetation in the Sahara. We compare our model results with δ<sup>18</sup>O values from mid-Holocene speleothem records in the Asian and South American Monsoon regions.</p><p>We find that inclusion of vegetated Sahara during the mid-Holocene leads to global warming, alters the hemispheric distribution of energy, and generally amplifies the δ<sup>18</sup>O of precipitation responses in the South American and Asian Monsoon regions; these feedbacks improve the δ<sup>18</sup>O agreement between model outputs and speleothem records of the mid-Holocene. Our results highlight the importance of regional vegetation alteration for accurate simulation of past climate, even when the region of study is far from the source of vegetation change.</p>

Early-Warning Signals for a Critical Transition in the Coupled Amazon Rainforest-South American Monsoon System

<p>The Amazon rainforest is widely recognised as a potential tipping element in the Earth's climate system. While several studies suggest a sudden dieback of the rainforest ecosystem after partial deforestation [e.g., 1, 2], there is still a lack of understanding where to search for early-warning signals that might precede such a dieback. In this work we employ a non-linear model of the moisture transport across the Amazon Basin to propose several statistical and physical early warning signals for a critical transition in the coupled dynamics of the Amazon rainforest and the atmospheric circulation of the South American monsoon. </p><p>Widespread deforestation and its effects on evapotranspiration and radiation have been shown to potentially trigger a collapse of the positive feedback related to latent heat release over the rainforest [3], resulting in substantially reduced rainfall amounts. The model includes a nonlinear contribution representing the acceleration of low-level moisture flow caused by condensational latent heating.  </p><p>Guided by our modelling results, we associate characteristic changes in the hydrological cycle as well as statistical indicators in observed data with deforestation-induced circulation changes that are consistent with the identified early-warning signals. Our findings indicate that in response to deforestation, the coupled atmosphere-vegetation system is destabilising and that further deforestation could trigger a transition of the Amazon rainforest to a savanna state. </p><p>[1] Nobre, C. A., & Borma, L. D. S. (2009). “Tipping points” for the Amazon forest. Current Opinion in Environmental Sustainability. https://doi.org/10.1016/j.cosust.2009.07.003</p><p>[2] Hirota, M., Holmgren, M., Van Nes, E. H., & Scheffer, M. (2011). Global resilience of tropical forest and savanna to critical transitions. Science, 334(6053), 232–235. https://doi.org/10.1126/science.1210657</p><p>[3] Boers, N., Marwan, N., Barbosa, H. M. J., & Kurths, J. (2017). A deforestation-induced tipping point for the South American monsoon system. Scientific Reports, 7. https://doi.org/10.1038/srep41489</p>