Identification of a new dust-stratocumulus indirect effect over the tropical North Atlantic

Abstract

Over the tropical North Atlantic, during boreal summer, both stratocumulus clouds and mineral aerosols are ubiquitous. We find that low cloud fraction increases in response to high mineral aerosols loadings by 3% to 10% over much of the tropical North Atlantic, in International Satellite Cloud Climatology Project (ISSCP) and Pathfinder Atmosphere Extended (PATMOSx) observations. Using the single-column mode of the Community Earth System Model (CESM), we estimate that this indirect effect nets a surface cooling of approximately -3 Wm -2 to -12 Wm -2 per unit of dust optical depth (DAOD) increase in mineral aerosols, similar to observed radiative forcings of -4 Wm -2 to -6 Wm -2 per unit of DAOD in ISSCP and PATMOSx. Increases in stratocumulus clouds are linked to increases in atmospheric stability, reductions in boundary layer height, and moistening of the lower atmosphere in response to increased dust load. Mineral dust is shown to behave similarly to other absorbing aerosols in indirectly forcing a response in stratocumulus clouds. Key Points Dust increases low cloud fraction in observation and GCMCalculated radiative forcing of low cloud responseLow cloud increase due to stability, PBL height, and moisture