Impact of Inter-Hemispherically Asymmetric Volcanic Aerosol Forcing on Sahelian Rainfall

The semi-arid African Sahel region is highly vulnerable to changes in annual precipitation, as much of the region's workforce is employed in the agricultural industry. Thus, studying the response of rainfall and aridity in this region to radiative signals is a matter of pressing humanitarian relevance. In addition, there is evidence to suggest that spatially asymmetric volcanic aerosol clouds produce different hydroclimatic responses based on their hemispheric symmetry, both globally and in the Sahel. We seek to use two different climate models, GFDL's FLOR model and the CESM model, to characterize the response of rainfall in the Sahel to large volcanic eruptions based on the meridional symmetry of the volcanic eruptions. We find that in both the FLOR experiments simulating three large 20th century eruptions and in the CESM Last Millennium Ensemble simulations of 46 historic volcanic eruptions, asymmetric Northern Hemisphere cooling causes a subsequent drying response in the Sahel, and Southern Hemisphere cooling causes a wetting, or "greening" response. Symmetric volcanic eruptions have a relatively small effect on rainfall in the Sahel. We also find that the FLOR results show a consistent response in the annual rainfall cycle in the Sahel for all three of the eruptions analyzed, with a reduction in rainfall in early summer followed by an increased rainfall in late summer. The annual cycle response of rainfall in the Sahel from the CESM experiments is different from that of the FLOR experiments. Our results show SH eruptions cause a rainfall maximum in September, NH eruptions cause a rainfall minimum in September, and symmetric eruptions show a slight increase in August and a decrease in October. These results highlight the need for accurate meridional structures in historic volcanic forcing data used for climate models as well as the need for further study on regional effects of hemispherically asymmetric radiative forcing, especially as they might pertain to aerosol geoengineering.