Influence of climate variability on the atmospheric transport of Icelandic tephra in the subpolar North Atlantic

Atmospheric transport of Icelandic tephra has been simulated using data on atmospheric circulation and a theoretical model of tephra fallout. Two major features control the trajectories of tephra from Iceland: (1) persistent westerlies in the lower stratosphere between 9 and 15 km; and (2) a seasonal shift in wind direction above about 15 km that becomes more significant at higher elevations, with strong westerlies during the fall and winter, and relatively weak easterlies during the spring and summer. Model-derived predictions of the distance traveled by Icelandic tephra as a function of particle size have been made for each season, and for eruption columns ranging from 10- to 50-km height. The downwind decrease in grain size appears to be less significant as the eruption cloud rises to higher altitudes where stronger winds prevail. Predictions for fall and winter agree well with observations of historical and post-glacial tephra falls in western Europe, and provide a rough estimate of the height of the related eruption columns. Predictions for spring and summer dispersal may explain why the Greenland ice sheet is relatively barren of tephra fall from Iceland, due to weak stratospheric easterlies. Grain size of submarine tephra fall deposits in the Iceland Basin and on the Iceland Plateau indicates, however, that the present atmospheric circulation may not account for tephra transport during the Pliocene and Pleistocene. The coarser grain size of these older deposits may be related to enhanced atmospheric circulation in the subpolar North Atlantic during these epochs or higher eruption columns due to more powerful eruptions in Iceland.