Future AMOC recovery modulated by atmospheric water vapor shortwave absorption

The amount of shortwave radiation absorbed by atmospheric water vapor is highly model dependent. This study examines how differences in the atmospheric water vapor shortwave radiation absorption affect the CO₂-induced climate response pattern. We control the atmospheric water vapor shortwave radiation absorption in Community Earth System Model 1.2.2 (CESM1-CAM4-POP2) by altering the water vapor shortwave absorptivity parameter k by 60% to 120% of the default value. The pre-industrial control simulations with different k values are integrated for 150 years and additional 150 years are integrated after abruptly quadrupling CO₂ concentrations. Regardless of the k value, the Atlantic meridional overturning circulation (AMOC) weakens in response to the quadrupling of CO₂. However, the simulation with a higher k value exhibits a faster AMOC recovery approximately 30 years after the quadrupled CO₂, with the lowest k simulation exhibiting a persistent AMOC weakening with no sign of recovery for the entire 300-year integration period. The faster AMOC restoration with a larger k value is attributed to the climatologically colder and saltier subpolar North Atlantic sea surface condition arising from the larger Arctic sea ice fraction due to colder temperature associated with stronger atmospheric shortwave absorption. The colder and more saline subpolar North Atlantic sea surface facilitates a more rapid destratification of surface density, establishing a favorable condition for the AMOC restoration. The faster restoration of the AMOC with the higher k value leads to a larger inter-hemispheric energy asymmetry followed by a more northward ITCZ shift as well as a stronger equilibrium climate sensitivity. This study demonstrates the complex interaction among different elements within the Earth system, encompassing radiation, sea ice, AMOC, and large-scale atmospheric circulation, suggesting a way to reduce uncertainties in future climate projections by improving the parameterization of shortwave radiation absorption by atmospheric water vapor.