Slowing Down of the Atlantic Meridional Overturning Circulation Due to Excess Freshwater: Insights from Turbulence-Resolving Simulations

The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in the global climate system by transporting heat, salt, and nutrients across ocean basins. Its stability hinges on the complex interplay between temperature and salinity, although the precise contributions of these factors remain unclear. This highlights the need for systematic investigations to better understand and predict AMOC behavior in a changing climate. In this study, we use turbulence-resolving simulations with a laboratory-scale model of the North Atlantic Ocean to examine how thermal, salinity, and wind forcing influence large-scale ocean circulation. By varying the relative impacts of salinity and temperature forcing, we find that increasing salinity forcing slows the AMOC by weakening deep convection and shifting the subtropical gyre southward. This slowdown reduces northward heat and salt transport, leading to warming and salinification in the northern subtropics and cooling in subpolar regions. Salt-finger convection further amplifies subtropical warming and salinification. On the other hand, a sufficiently strong thermal forcing in a weakened AMOC state can trigger a significant rebound in AMOC strength. Wind stress was also found to enhance both the AMOC and gyre strength. Future climate projections indicate that freshwater forcing will become increasingly significant, and our results suggest that greater salinity forcing will further slow the AMOC and reduce meridional tracer transport. These findings are essential for improving large-scale ocean models and advancing our understanding of temperature-salinity feedback mechanisms in global ocean circulation.