The Response of the Tropical Atlantic and West African Climate to Saharan Dust in a Fully Coupled GCM

This study examines the climate response in West Africa and the tropical Atlantic to direct aerosol forcing from Saharan-born mineral dust. We impose a change in mineral dust aerosol burden on an ensemble simulation with the fully coupled GFDL Climate Model 2.1 (CM2.1), exploring two optical property regimes for the dust. The two optical regimes are derived from observations and include an older, more absorbing in-situ dataset (ABS) and a newer, more scattering model-extrapolated dataset (SCT). The sensitivity of climate to dust concentration depends strongly on the optical properties. The direct effect of both simulations over the area of dust forcing is strongest at the surface (SFC) with dust imposing significant reductions in shortwave (SW) radiation. However, at the top of the atmosphere (TOA) the two simulations produce a SW forcing of opposite sign due to the increased scattering of the SCT-forced run. The longwave (LW) changes are smaller than the SW forcings, but the ABS-forced simulations have a slightly larger response than the SCT-forced simulations. These differences culminate in similarly opposing local semi-direct effects. The ABS-forced simulations show a decrease in the West African monsoon whereas the SCT-forced simulations create an increase in the monsoon. This is due to moist enthalpy changes throughout the atmospheric column over the Sahara desert creating either horizontal divergence or convergence over West Africa, respectively. In the tropical North Atlantic, dust acts to cool the surface as expected. However, in the subsurface the ABS-forced simulations show a decrease in upper ocean heat content while the SCT-forced simulations show an increase in upper ocean heat content. These differences primarily arise from the wind stress curl response to a shift in the Atlantic ITCZ and not the direct thermodynamic forcing. These results exemplify the significant changes that occur with different optical regimes of dust and have implications for including varying minearology in future climate simulations.