|Title||Identification of a new dust-stratocumulus indirect effect over the tropical North Atlantic|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||Doherty O.M, Evan AT|
|Journal||Geophysical Research Letters|
|Type of Article||Article|
|Keywords||african dust; cloud; marine stratocumulus; microphysics; mineral aerosols; observations; ocean; saharan air layer; satellite; smoke; variability|
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.
Here we conﬁrm previously observed coincidence of stratocumulus clouds and mineral dust over the tropical North Atlantic [Carlson and Prospero, 1972; Ott et al., 1991; Clarke et al., 1996; Powell et al., 1997; Chazette et al., 2001; Ben-Ami et al., 2009; Wang et al., 2011] and the previously identiﬁed statistical link between dust load and stratocumulus clouds [Mahowald and Kiehl, 2003; Kaufman et al., 2005]. More generally, we show that dust can behave like other absorbing aerosols in indirectly forcing a response in stratocumulus clouds. In doing so, this work supports the ﬁndings of Brioude et al.  and Wilcox [2010, 2012] who found absorbing aerosols above low clouds acted to increase the abundance of low clouds, as well as Johnson et al.  who found an increase in low clouds when absorbing aerosols top a stratocumulus-capped boundary layer. Overall we demonstrate observational and modeling evidence for a robust stratocumulus indirect eﬀect in response to mineral dust outbreaks and quantify the radiative forcing. This so-called dust ScIE has potentially large implications for regional climate.