Robustness of the South American monsoon system to an AMOC collapse in a kilometer-scale atmosphere-only model

The Earth’s monsoon systems are closely linked to seasonal migration of the Intertropical Convergence Zone (ITCZ) while the Atlantic Meridional Overturning Circulation (AMOC) acts as a major control on the ITCZ’s latitudinal position through cross-equatorial heat transport. Under sustained global warming, climate models consistently predict a weakening of the AMOC, with some recent studies suggesting a potential tipping, i.e. an irreversible and substantial decline to approximately 3–5 Sv. Such an AMOC collapse is associated with significant cooling and drying in the Northern Hemisphere and a southward shift of the ITCZ.
To assess the impact of a potential AMOC shutdown on the South American Monsoon System (SAMS), we conducted an atmosphere-only simulation using the ICON model at 10 km horizontal resolution. At this resolution, convection is explicitly resolved, and no convective parameterization is required. Sea surface temperatures (SSTs) were taken from an existing AMOC shutdown experiment conducted with a coupled climate model.
Our results broadly reproduce the large-scale precipitation and temperature anomalies observed in lower-resolution coupled model experiments. The southward displacement of the ITCZ produces a zonally elongated dipole precipitation anomaly over the Atlantic Ocean. However, over the South American continent, this signal is attenuated in the high-resolution simulation compared to the lower resolution coupled simulations, where the dipole extends much further inland. This is consistent with previous research indicating that land–atmosphere interactions differ in convection-resolving models compared to CMIP-type models, potentially altering the precipitation response to large-scale perturbations.
In particular, precipitation associated with the SAMS is remarkably robust to the ITCZ shift. Key features such as the Bolivian High, the South Atlantic Convergence Zone, and the South American Low-Level Jet remain qualitatively unchanged despite the AMOC shutdown. This suggests that other drivers—such as the seasonal solar cycle, the orography and geometry of South America, and moisture recycling from the Amazon rainforest—may dominate the spatiotemporal structure of the SAMS, outweighing the influence of large-scale AMOC-driven changes.