Future South American monsoon changes are sensitive to Atlantic SST pattern changes

Future projections of South American Monsoon (SAM) precipitation from CMIP6
(Coupled Model Intercomparison Project phase 6) show a consistent drying during the early part
of the monsoon season (September to November), which is also seen in a convection-permitting
model simulation. Using a set of idealised atmosphere-only 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 inter-model uncertainty in the
SAM precipitation response to each of these drivers, particularly SST pattern change. For uniform
SST warming, an existing hypothesis which suggests that SAM 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 inter-model correlations (across the coupled CMIP6 models) are found
between SAM 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 inter-model uncertainty in SAM core region precipitation change.