EC-12. Sea surface temperature control on marine cloud darkening over the North Atlantic Ocean

Abstract
The effect of anthropogenic atmospheric aerosols on global cloud radiative forcing through changes in cloud amount and brightness is a major uncertainty in climate projections. Previous studies have identified cloud-top moisture as an important control on the cloud water adjustment to aerosol-induced perturbations. By analyzing 8 years (2003-2011) of A-Train satellite measurements and ERA5 reanalysis over the North Atlantic Ocean for single-layer warm (liquid-phase) clouds, we find a strong negative correlation between sea surface temperature (SST) and cloud-top relative humidity. This relationship has important implications for cloud albedo susceptibility. Our analysis reveals that on daily and interannual timescales, the SST has a strong control on the relative occurrence of aerosol-induced brightening and darkening of clouds by modulating the PBL height and saturation water vapor pressure. Lower SSTs are associated with shallower boundary layers and moister air at cloud top, which hampers cloud-top evaporation and favors cloud thickening (a cooling effect) in response to higher aerosol conditions. At higher SSTs, the PBL deepens and the cloud amount decreases much more strongly through mixing with drier air from above the cloud; the increased air temperature in the free troposphere associated with the warming SST further facilitates cloud-top evaporation by reducing the relative humidity at cloud top, as per the Clausius-Clapeyron relation. As a result, the cloud darkening associated with cloud thinning overcomes the brightening associated with increased cloud droplet number concentration, leading to a warming effect. Our results suggest a more frequent occurrence of less reflective clouds (warming effect) over the North Atlantic with global warming or with the strengthening phase of the Atlantic meridional overturning circulation.