Response of the South Asian Monsoon to AMOC Collapse

Paleoclimate 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 Community Earth System Model, version 1 (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 paleoclimate records of past monsoon variability. SIGNIFICANCE STATEMENT: This study explores how the South Asian monsoon responds to a collapse of the Atlantic meridional overturning circulation (AMOC), a large-scale ocean circulation system in the climate system. This is important because climate models project that the AMOC will weaken due to increased anthropogenic warming. While much work has been done on how tropical precipitation changes at global scales, fewer studies have focused on local basins or monsoon systems.We find that the South Asian monsoon weakens in response to an AMOC collapse. However, we also show that the current leading framework of energy budget theory cannot fully explain the regional changes around the Indian Ocean. In particular, there is an increase in precipitation over the Bay of Bengal and the Indochina Peninsula which is driven by local dynamics rather than global energy shifts. These findings highlight the need for region-specific analysis and may help guide future research, including the interpretation of past climate records in this region.