Understanding the Effect of Precession on South American Climate

Abstract:

The oxygen isotope concentration in calcite (δ¹⁸O_c) in speleothems over South America shows a distinct spatial pattern of change for the past 250,000 years orchestrated by precessional forcing. Using an isotope-enabled model (ECHAM4.6) coupled to a slab ocean model, we study how and why precession changes the climate of South America.

Two experiments, called the “low insolation” experiment and “high insolation” experiment, were performed with the same modern boundary conditions, but forced with the extreme minimum and maximum of Southern Hemisphere (SH) summer insolation, respectively. Differences between these two experiments (“low” minus “high”) display as a dipole pattern: less precipitation and heavier precipitation-weighted δ¹⁸O (δ¹⁸O_p) along the Andes, and more precipitation and lighter δ¹⁸O_p in northeastern Brazil. The differences in δ¹⁸O_p are consistent with δ¹⁸O_c of speleothems, in terms of both sign and magnitude. Further analysis of the δ¹⁸O of precipitation, the δ¹⁸O of water vapor and the probability distribution function (pdf) of precipitation intensity reveals that changes in both the seasonality of precipitation and the “amount effect” contribute to the heavier δ¹⁸O_p along the Andes, while the “amount effect” almost exclusively contributes to the lighter δ¹⁸O_p in northeastern Brazil.

To identify the causes of precipitation response, three additional experiments are performed with localized albedo increase over South America and/or Africa. These show that the decrease in precipitation along the Andes is caused by cooling of South American continent, whereas the increase in precipitation over northeastern Brazil is associated with cooling of northern Africa. Reduction of SH summer insolation cools both South America and northern Africa. Cooling of South America weakens the South American summer monsoon (SASM) and changes the pdf of precipitation intensity over tropical South America and along the Andes; contrary to previous suggestions, changes in the strength of SASM does not account for the increase in precipitation over northeastern Brazil. Instead, the cooling of northern Africa intensifies the northeasterly trades south of the equator over the Atlantic, intensifying moisture convergence and hence precipitation over northeastern Brazil.