The Influence of Orbital Forcing of Tropical Insolation on the Climate and Isotopic Composition of Precipitation in South America

摘要: The [delta][superscript 18] O of calcite ([delta][superscript18]O[subscript c]) in speleothems from South America is well correlated with austral summer (DJF) insolation, indicating the role of orbitally paced changes in insolation in changing the climate of South America. Using an isotope-enabled atmospheric general circulation model (ECHAM4.6) coupled to a slab ocean model, we study how insolation changes climate and the isotopic composition of precipitation ([delta][superscript 18][subscript p]) of South America. Compared with times of high summertime insolation, times of low insolation feature (i) a decrease in precipitation inland of tropical South America due to an anomalous cooling of the South American continent and a weakening of the South American summer monsoon, and (ii) an increase in precipitation in eastern Brazil that is associated with the intensification and southward movement of the Atlantic Intertropical Convergence Zone, which in turn is caused by the strengthening of African winter monsoon that is induced by the anomalous cooling of northern Africa. The cooling of Africa also intensifies and shifts the South Atlantic convergence zone northward by generating a Rossby wave to the west of southern Africa. In times of low insolation, [delta][superscript 18]O[subscript p] increases in the northern Andes and decreases in northeastern Brazil, consistent with the pattern of [delta][superscript 18] O[subscript c] changes seen in speleothems. Further analysis shows that the decrease in [delta][superscript 18]O[subscript p] in northeastern Brazil is due to the "amount effect"; while the increase in the northern Andes reflects a change in the seasonality of precipitation and in the isotopic composition of vapor that forms the condensate.