Shutdown of northern Atlantic overturning after 2100 following deep mixing collapse in CMIP6 projections

Abstract Several, more recent global warming projections in the Coupled Model Intercomparison Project (CMIP)6 contain extensions beyond year 2100 to 2300/2500. The Atlantic Meridional Overturning Circulation (AMOC) in these projections shows transitions to extremely weak overturning below the surface mixed layer (< 6 Sv; 1 Sv = 106 m3/s) in all models forced by a high-emission (SSP585) scenario and sometimes also forced by an intermediate- (SSP245) and low-emission (SSP126) scenario. These extremely weak overturning states are characterised by a shallow maximum overturning at depths less than 200 m and a shutdown of the circulation associated with North Atlantic Deep Water (NADW) formation. Northward Atlantic heat transport at 26°N decreases to 20-40% of the current observed value. Heat release to the atmosphere north of 45°N weakens to less than 20% of its present-day value and in some models completely vanishes, leading to strong cooling in the subpolar North Atlantic and Northwest Europe. In all cases, these transitions to a weak and shallow AMOC are preceded by a mid-21st century collapse of maximum mixed-layer depth in Labrador, Irminger and Nordic Seas. The convection collapse is mainly caused by surface freshening from a decrease in northward salt advection due to the weakening AMOC but is likely initiated by surface warming. Maximum mixed-layer depths in the observations are still dominated by internal variability but notably feature downward trends over the last 5–10 years in all deep mixing regions for all data products analysed. This could be merely variability but is also consistent with the model-predicted decline of deep mixing.