Witnessing the impact of the 1783-1784 Laki eruption in the Southern Hemisphere

The Icelandic Laki eruption in 1783/1784 produced a large volume of lava while the associated aerosols were directly responsible for severe environmental and health effects in Iceland and northern Europe. The intense plume of smoke and sulphurous dry fog has been reported to have affected a considerable fraction of Eurasia and Northeastern Canada but no impact descriptions have been reported for the Southern Hemisphere. Here we reproduce the description of an abnormally high incidence of unusual dry fog and haze days during the years 1784-1786 in Rio de Janeiro (20A degrees S, Brazil) obtained by Bento Sanches Dorta, a Portuguese astronomer. Using monthly averages of fog days registered by Dorta between 1781 and 1788 it is shown that the outstanding peak observed between September and November of 1784 might be linked to the Laki eruption. The vast majority of observational and modeling studies appear to contradict such hypothesis; however recent modeling studies of the impact of large high latitude eruptions support the existence of large-scale climatic anomalies in the Southern Hemisphere tropical region, and in particular the appearance of above-normal cloud cover over central Brazil.