Identification of a New Dust Indirect Effect Over the Tropical North Atlantic

Over the tropical North Atlantic stratocumulus clouds are ubiquitous, and through radiative forcing have large impacts on the regional climate, both directly and through sea surface temperature (SST) induced feedbacks. Mineral aerosols, during the boreal summer season, is also commonly found over the tropical North Atlantic, and is known to alter both the vertical profile of temperatures and moisture in the atmosphere as well as SSTs. Stratocumulus clouds fraction is dependent upon vertical profiles of temperature and moisture, as well as SSTs thus mineral aerosols could have an impact on stratocumulus cloud cover. However, no study has identified a direct relationship between dust and stratocumulus clouds. In this work we find that low clouds fraction increase in response to high mineral aerosols loadings by 3% to 5% over much of the Tropical North Atlantic, in both ISCCP and PATMOSx observations during dusty boreal summers. In idealized CAM runs low cloud fraction shows an increase of 2% to 3% by including mineral aerosols. On daily timescales we find a 10% to 20% increase in low clouds over the Tropical North Atlantic for the three days following mineral aerosol outbreaks in MODIS products. We hypothesize that cooling of SSTs and reductions in entrainment in response to high mineral aerosols load are responsible for increases in stratocumulus clouds. The single-column CESM-SCAM model is applied to test this hypothesis, using observed radiative forcing rates. This new dust indirect effect is a potential cooling mechanism for the tropical North Atlantic. As radiative forcing on the ocean surface by stratocumulus clouds is known to be -1 W/m2 per 1% increase in stratocumulus cloudcover, we estimate this indirect effect nets a cooling of -0.33 W/m2 to -2 W/m2 per unit of AOD increase in mineral aerosols.