Enhanced atmospheric shortwave absorption facilitates future AMOC recovery

Abstract

The amount of shortwave radiation absorbed by atmospheric water vapor is highly model dependent. Previous studies showed that this uncertainty affects the spatial pattern of mean climate state as well as the global-mean future projections. However, its influence on the spatial pattern of future projections remains unexplored. This study examines how the CO2-induced climate response pattern depends on the atmospheric water vapor shortwave radiation absorption. The magnitude of atmospheric water vapor shortwave radiation absorption in Community Earth System Model 1.2.2 (CESM1-CAM4-CLM4-POP2) is modulated by altering the water vapor shortwave absorption parameter k. The pre-industrial control simulations with different k values, ranging from 60% to 120% of the default value, were integrated for 150 years. Subsequently, an additional 150 years were integrated under quadrupled CO2 conditions. Regardless of k value, the Atlantic meridional overturning circulation (AMOC) weakens abruptly in response to the quadrupling of CO2. However, the simulation with a higher k value exhibits a faster AMOC recovery after approximately 20 years after quadrupled CO2, with the lowest k simulation exhibiting a persistent AMOC weakening with no sign of recovery for the total 300-year integration period. The faster AMOC restoration with a larger k value is attributed to the climatologically 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 higher surface salinity condition facilitates a more rapid surface density destratification of the subpolar North Atlantic, leading to the AMOC restoration. As a result, the higher k value is associated with a larger northward heat transport anomaly, causing greater northern hemisphere warming compared to the southern hemisphere under quadrupled CO2 conditions. The enhanced hemispheric asymmetry in surface temperature leads to a stronger counter-clockwise cross-equatorial Hadley circulation and a larger northward shift in tropical precipitation. This study elucidates the intricate interplay among various components within the Earth system, encompassing radiation, sea ice, AMOC, and large-scale atmospheric circulation. Thus, it emphasizes the critical need for improving the parameterization of shortwave radiation absorption by water vapor to reduce uncertainties in future climate projections.