North Atlantic Ocean Circulation Changes Under Increased CO2 Concentrations Using a High-Resolution Global Coupled Climate Model

This study aims to analyze the effect of increasing atmospheric CO2 concentrations on the Atlantic Meridional Overturning Circulation (AMOC) and its dependence on North Atlantic deep water formation. We used EC-Earth3-HR, the high-resolution version of the global coupled climate model EC-Earth3, with a spatial resolution of about 0.25 degrees in the ocean and 40 km in the atmosphere. Our configuration has undergone a tuning process, and a multi-centennial spin-up has been performed. The experiments analyzed consist of a pre-industrial control simulation (piControl), a one percent per year increase in CO2 experiment (1pctCO2), branching from year 250 of the piControl simulation, and two experiments with fixed CO2 concentrations (400.9 ppm and 551.5 ppm). These two experiments branch off from points corresponding to global temperature anomalies of around 1°C and 2°C in the 1pctCO2 experiment. Both simulations equilibrate at a higher global warming level. As the climate warms, North Atlantic waters become warmer and fresher, weakening deep convection and deep water formation, which reduces the strength of the AMOC by approximately 10% in the low and 20% in the high fixed-CO2 concentration experiments. Deep water formation is assessed using a novel method based on horizontal volume convergence within the main convective areas. The weakening is primarily driven by the Labrador Sea, followed by the Greenland Sea, while the Irminger Sea sustains the remaining deep water formation. These changes align with variations in AMOC strength and meridional volume transport at 26°N and 45°N. Our study emphasizes the connection between North Atlantic deep water formation and AMOC, offering insights into its expected weakening as CO2 concentrations rise.