Changes in South American Monsoon During the Holocene

The main feature of South American Monsoon is the precipitations associated to the South Atlantic Convergence Zone (SACZ). The SACZ occurs during SH summer and consists in an organisation of convective activity along a northwest-southeast axis from the main convection region in western Amazonia to the Southeast region of Brazil and adjacent Atlantic Ocean. Several paleoclimate data indicate reduced precipitations in the ZCAS influence zone during the Early and Middle Holocene whereas there are some signals of wetter climate in Northeast Brazil and in the southern part of central Brazil. The simulation of 6k climate by the IPSL coupled Ocean Atmosphere GCM suggests some mechanisms to explain these regional differences: (1) the reduced summer insolation provoked a decrease of SACZ activity (2) the SH summer position of ITCZ is displaced southward explaining a wetter climate in northeast Brazil (3) the analysis of the transient meridional heat transport and of the baroclinicity of the model climate suggests more intense winter and early spring cold outbreaks in the central region of South America, also in agreement with the data from south central Brazil. This last mechanism may be enhanced by vegetation feedback. However, in the ZCAS influence region the model indicate an increase in rainfalls during spring that compensates the summer decrease leading to few changes in annual rainfalls. Such a pattern is also observed in other GCM simulations but is not in agreement with the proxydata which indicate lower levels of the lakes and an opening of the Atlantic rainforest toward a mosaic of Savannas and Gallery Forests during Early and Middle Holocene.