The impacts of hemispheric asymmetric radiative forcings (Invited Presentation)

The Sahelian drought of the 1970s-1990s was one of the largest humanitarian disasters of the last fifty years, and has been attributed to natural variability, over-grazing and the impact of industrial emissions of sulphur dioxide. Each mechanism can influence the Atlantic sea surface temperature gradient which is strongly coupled to Sahelian precipitation. We suggest that sporadic volcanic eruptions in the northern hemisphere also strongly influence this gradient and cause Sahelian drought. Using de-trended observations from 1900-2010, we show that three of the four driest Sahelian summers were preceded by substantial northern hemisphere volcanic eruptions (Figure 1). We use a state-of-the-art coupled global atmosphere-ocean model to simulate both episodic volcanic eruptions and geoengineering via continuous deliberate injection into the stratosphere. In either case, large asymmetric stratospheric aerosol loadings concentrated in the northern hemisphere are a harbinger of Sahelian drought while those concentrated in the southern hemisphere induce a greening of the Sahel. Further analysis is presented of the impacts of hemispherically asymmetric forcings on precipitation associated with the Asian monsoon and also on Atlantic hurricane frequency. Haywood, J.M., A. Jones, N. Bellouin, and D.B. Stephenson, Asymmetric forcing from stratospheric aerosols impacts Sahelian drought, Nature Climate Change, 3, 7, 660-665, doi: 10.1038/NCLIMATE1857, 2013.