Non-Local Aerosol-Cloud Effective Radiative Forcing: An Important but Overlooked Process

The impact of anthropogenic aerosols on clouds is a leading source of uncertainty in estimating the effect of human activity on the climate system. Traditionally, aerosol-cloud interactions are considered only in terms of local impacts, meaning the effects of local aerosols on local clouds. However, in this talk, I will provide evidence that this local perspective overlooks an important component—a non-local component.

Specifically, in this research we utilize three novel sets of simulations that resolve convection while also representing large-scale processes. Our findings demonstrate that aerosol-cloud interactions intensify tropical overturning circulation. Using a weak temperature gradient approximation, we attribute variations in circulation to clear-sky humidity changes driven by warm rain suppression by aerosols. In two sets of simulations accounting for subtropical-tropical coupling, we show that aerosol-driven subtropical rain suppression leads to increased advection of cold and moist air from the subtropics to the tropics, thus enhancing tropical cloudiness. The increased tropical cloudiness has a strong cooling effect by reflecting more incoming solar radiation.

The classical “aerosol-cloud lifetime effect” is shown here to have a strong remote effect, where subtropical aerosols increase cloudiness in the tropics. This finding broadens the concept of cloud adjustments to aerosol perturbation and has important implications for marine cloud brightening.