A modeling study of differing impacts of black carbon and sulfate aerosols on global and tropical precipitation

Anthropogenic aerosols are important forcing agents on climate change, which are suggested to be responsible for the observed southward shift of ITCZ in the second half of the twentieth century. Black carbon (BC) and sulfate (SF) are two major kinds of aerosols with quite different optic properties and therefore interacting with the climate in different ways. This study explores the climatic effects of BC and SF on global and tropical precipitation using a climate model. Results show that the ways how the climate adapts to the radiative forcings caused by BC and SF are quite different. The total global annual-mean precipitation response (ΔP_total) caused by BC is dominated by the negative component of fast precipitation response (ΔP_fast), which scales with instantaneous atmospheric radiative absorption at the rate of about -0.5%/(W/m²). On the contrary, the ΔP_total caused by SF is dominated by the negative slow component (ΔP_slow), which scales with surface air temperature change at the rate of about 3%/K. The fast and slow precipitation responses to BC and SF forcings also manifest themselves as different characteristics in ITCZ (Inter-Tropical Convergence Zone) shift. BC causes a little northward shift of ITCZ mainly through ΔP_fast, whereas SF causes an obvious southward shift of ITCZ through ΔP_slow. The ITCZ shifts caused by BC and SF are found to linearly correlate with the corresponding changes in cross-equatorial heat transport in the atmosphere, with the regression coefficient of about -3°/PW. Comparing the ITCZ shifts caused by BC and SF between CGCM (atmosphere-ocean fully coupled model) and SOM (atmosphere-slab ocean coupled model), it is found that SOM amplifies both the cross-equatorial heat transports in the atmosphere and the ITCZ shifts, by restricting the ocean's role in balancing interhemispheric energy contrast. This underscores the importance of fully coupled ocean in modeling study of tropical climate response to aerosol forcing.