Response of the South Asian Monsoon to AMOC Collapse

Abstract Palaeoclimate records indicate substantial decreases in precipitation associated with the South Asian monsoon in response to Atlantic Meridional Overturning Circulation (AMOC) perturbations, yet the underlying mechanisms remain poorly understood. In this study, we investigate the impacts of a collapsed AMOC on the South Asian monsoon using a suite of North Atlantic freshwater hosing simulations conducted with the NCAR CESM1 global climate model under preindustrial conditions. Our results show that a weakened AMOC leads to reduced monsoon rainfall and a southward shift of the Southern Hemisphere branch of the Intertropical Convergence Zone (ITCZ). We also see a substantial increase in precipitation over the Indochina peninsula, driven by enhanced winds across the Bay of Bengal. We show that these responses cannot be explained by global energy budget arguments and instead propose a local dynamical mechanism in which enhanced winds over the Bay of Bengal increase orographic precipitation across the Indochina Peninsula. These findings have important implications for both projections of future AMOC weakening or collapse under continued anthropogenic warming and for interpreting palaeoclimate records of past monsoon variability.