Browsing by Author "Cheng, Hai"
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Item Restricted Precipitation changes over the eastern Bolivian Andes inferred from speleothem (δ¹⁸O) records for the last 1400 years(Elsevier, 2018-07-15) Apaéstegui Campos, James Emiliano; William Cruz, Francisco; Vuille, Mathias; Fohlmeister, Jens; Espinoza, Jhan Carlo; Sifeddine, Abdelfettah; Strikis, Nicolas; Guyot, Jean Loup; Ventura, Roberto; Cheng, Hai; Edwards, R. LawrenceHere we present high-resolution δ¹⁸ records obtained from speleothems collected in the eastern Bolivian Andes. The stable isotope records are related to the regional- to large-scale atmospheric circulation over South America and allow interpreting changes in δ¹⁸ during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical Andes. Two distinct phases with more negative δ¹⁸ values, interpreted as periods of increased convective activity over the eastern Andean Cordillera in Bolivia are observed concomitantly with periods of global climate anomalies during the last millennium, such as the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) respectively. Changes in the Bolivian δ¹⁸ record during the LIA are apparently related to a southward displacement of the Intertropical Convergence Zone (ITCZ), which acts as a main moisture driver to intensify convection over the tropical continent. During the MCA, however, the increased convective activity observed in the Bolivian record is likely the result of a different mechanism, which implies moisture sourced mainly from the southern tropical Atlantic. This interpretation is consistent with paleoclimate records further to the north in the tropical Andes that show progressively drier conditions during this time period, indicating a more northerly position of the ITCZ. The transition period between the MCA and the LIA shows a slight tendency toward increased δ¹⁸ values, indicating weakened convective activity. Our results also reveal a non-stationary anti-phased behavior between the δ¹⁸ reconstructions from Bolivia and northeastern Brazil that confirms a continental-scale east–west teleconnection across South America during the LIA.Item Restricted Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium(Elsevier, 2017-10-15) Ait Brahim, Yassine; Cheng, Hai; Sifeddine, Abdelfettah; Wassenburg, Jasper A.; Cruz, Francisco W.; Khodri, Myriam; Sha, Lijuan; Pérez-Zanón, Núria; Hassane Beraaouz, El; Apaéstegui Campos, James Emiliano; Guyot, Jean-Loup; Jochum, Klaus Peter; Bouchaou, LhoussaineThis study presents the first well-dated high resolution stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg and Sr) speleothem records from southwestern Morocco covering the last 1000 yrs. Our records reveal substantial decadal to multidecadal swings between dry and humid periods, consistent with regional paleorecords with prevailing dry conditions during the Medieval Climate Anomaly (MCA), wetter conditions during the second part of the Little Ice Age (LIA), and a trend towards dry conditions during the current warm period. These coherent regional climate signals suggest common climate controls. Statistical analyses indicate that the climate of southwestern Morocco remained under the combined influence of both the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) over the last millennium. Interestingly, the generally warmer MCA and colder LIA at longer multidecadal timescales probably influenced the regional climate in North Africa through the influence on Sahara Low which weakened and strengthened the mean moisture inflow from the Atlantic Ocean during the MCA and LIA respectively.