Mean State AMOC Affects AMOC Weakening through Subsurface Warming in the Labrador Sea

While most models agree that the Atlantic meridional overturning circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the twenty-first century, they disagree on the projected magni-tudes of AMOC weakening. In this work, CMIP6 models with stronger climatological AMOC are shown to project stron-ger AMOC weakening in both 1% ramping CO2 and abrupt CO2 quadrupling simulations. A physical interpretation of this result is developed. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO2 increase, surface warming is mixed to the deeper Labrador Sea in models with stronger upper-ocean mixing. This subsurface warming and corresponding density de-crease drives AMOC weakening through advection from the Labrador Sea to the subtropics via the deep western bound-ary current. Time series analysis shows that most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in the subtropics by several years. Also, idealized experiments conducted in an ocean -only model show that the subsurface warming over 500-1500 m in the Labrador Sea leads to stronger AMOC weakening several years later, while the warming that is too shallow ( ,500 m) or too deep (.1500 m) in the Labrador Sea causes little AMOC weakening. These results suggest that a better representation of mean state AMOC is necessary for narrowing the intermodel uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.

Automated summary

While most models agree that the Atlantic meridional overturning circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the twenty-first century, they disagree on the projected magnitudes of AMOC weakening. CMIP6 models with stronger climatological AMOC are shown to project stronger AMOC weakening in both 1% ramping CO2 and abrupt CO2 quadrupling simulations. This is because models with stronger mean state AMOC exhibit weaker stratification in the upper Labrador Sea, allowing for stronger mixing of the surface buoyancy flux and leading to subsurface warming and AMOC weakening.