An apparent periodicity in an index of volcanic activity

Abstract

A STOCHASTIC SIMULATION of Lamb's dust veil index^1,2 (DVI) is used here to show that statistically significant peaks in the variance spectrum of geophysical or climatic data are not necessarily evidence of regular, or even quasi-regular, oscillations. The DVI is an annual measure of the amount of volcanic dust in the stratosphere, based on a variety of information, for the period 1500 to the present. Kelly³ has noted the significant correlation of the DVI with various aspects of climatic change in the North Atlantic–European sector on time scales of 50 yr and more, and has shown that the variance spectrum of the DVI contains a statistically significant peak at 7–8 yr. This peak is also present in the spectra of certain climatic and atmospheric circulation parameters. Kelly³ has suggested that this apparent periodicity in stratospheric dust content is more likely to be due to the chance occurrence of volcanic eruptions at 7-8 yr intervals during the limited period of record, rather than to a forced, regular cycle in volcanic activity. We now propose an alternate explanation: that the formulation of the DVI produces the spectral peak.