Response of the Atlantic Meridional Overturning Circulation to Climate Change from a suite of Climate Models

Oceans have absorbed most of the heat energy added to the climate system by human activities, which has damped global warming. This heat energy is redistributed into the ocean interior by the Meridional Overturning Circulation (MOC). Changes in the MOC pattern and strength are important to understand due to their large impacts on regional and global climates. Here, we aim to investigate changes in magnitude and spatial distribution of the Atlantic MOC (AMOC) under climate change. For that purpose, a hierarchy of three climate models of varying resolution in the ocean (1${}^{\circ }$, 0.25${}^{\circ }$ and 0.10${}^{\circ }$), the CM2-O suite, is used. Each model of the suite has a control and a sensitivity scenario where the CO${}_2$ level of 286 ppm is kept constant for the former and increased by 1$\%$ per year for the latter.

The coarse and high resolution models show a significant slowdown and shoaling of the AMOC under CO${}_2$ forcing, while the AMOC of the intermediate resolution model is barely sensitive to the forcing. The AMOC is reconstructed from vertical density differences between deep water formation region and the Atlantic basin. This reconstruction, which captures well the upper cell of the AMOC (top 2 km) and the magnitude of the weakening, allows to disentangle how much AMOC change is attributable to temperature versus salinity. We find that changes in salinity drive the AMOC reduction in the coarse and high resolution models.