Global spread of volcanic dust from the Bali eruption of 1963

The global spread of volcanic dust from the Mt. Agung eruption of 17 March 1963 is assessed on the basis of the decrement of direct solar radiation. The initial injection at 8°S lodged an equatorial reservoir in the stratosphere at a height of 22–23 km. Most of the dust remained in the Southern Hemisphere, but a significant fraction entered the lower Northern Hemisphere stratosphere. Seasonal effects appeared in both hemispheres, the apparent dust amounts showing a ‘winter’ maximum of constant amplitude in any one season with a poleward progression of 40 cm sec −1 between 30° and 90° latitude. At 15°N, seasonal effects were also evident but significantly out of phase with the sequence at higher latitudes, suggesting a hand‐over in the horizontal transport process at 20°–30° latitude. Whilst the ‘winter’ maxima in the Northern Hemisphere were almost identical in amplitude for 1963 and 1964, the Southern Hemisphere data showed a significant reduction in dust amount over these years. The latter effect can be shown to imply an updraught in the lowest 2 km of the equatorial stratosphere of about 0·02 cm sec −1 . By continuity, the poleward outflow from this circulation would be consistent with the observed rate of poleward progression. It is speculated that the seasonal effects could arise from an annual oscillation of the axis of this meridional circulation. Attempts to relate the observations by a diffusion model are less convincing, but in the absence of more direct information, no firm conclusion can be drawn. The paucity of suitable radiation data for equatorial regions severely limits the extent to which inferences can be made.