Processes Controlling the South American Monsoon Response to Climate Change

Future projections of South American (SA) monsoon precipitation from the Coupled Model Intercomparison Project phase 6 (CMIP6) show a consistent drying during the early part of the monsoon season (September-November), which is also seen in a convection-permitting model simulation. Using a set of idealized atmosphere-only general circulation model (GCM) experiments, this drying signal is shown to be mainly driven by sea surface temperature (SST) changes: uniform SST warming and patterned SST change. Different processes appear to be more important in different months for the ensemble mean drying signal, with this primarily driven by SST pattern change in October and by uniform SST warming in November. There is significant intermodel uncertainty in the SA monsoon precipitation response to each of these drivers, particularly SST pattern change. For uniform SST warming, an existing hypothesis, which suggests that SA monsoon drying is driven by the enhanced land-sea temperature contrast, is tested, but we find that this process is not dominant. For patterned SST warming, moderate intermodel correlations (across the coupled CMIP6 models) are found between SA monsoon precipitation change and changes in meridional and zonal Atlantic SST gradients. In November, a combined zonal and meridional Atlantic SST gradient index can explain more than half of CMIP6 intermodel uncertainty in SA monsoon core region precipitation change.