Wind Speed as a Dominant Source of Periodicities in Reported Emission Rates of Volcanic SO2

Volcanoes have been found to display periodicities or cyclic trends in a wide range of phenomena. These include the eruptive activity itself, but also in the time series of geophysical and geochemical monitoring data such as volcanic degassing. Here, we test the existence of periodicities of volcanic degassing at 32 volcanoes using the time series of sulfur dioxide (SO2) emission rates from data of the Network of Volcanic and Atmospheric Change (NOVAC). We use the Lomb-Scargle periodogram to analyze the SO2 data which allows efficient computation of a Fourier-like power spectrum from unevenly sampled data. We were able to calculate False-Alarm Probabilities in 28 of the 32 volcanoes, and we identified significant periodicities in the SO2 emission rates in 17 of the 28 volcanoes. However, we find that most of these periodicities are also present in the plume speeds used to determine SO2 emission rates. Periodicities at about 30-70, similar to 120, and similar to 180 days were identified at volcanoes located between 16 degrees N and 16 degrees S and are related to intraseasonality and interseasonality in global trade winds and not volcanic in origin. Periodicities between 30 and 70 days in both plume speed and SO2 emission rates are associated to the Madden-Julian Oscillation that is responsible for intraseasonal variability in the tropical atmosphere. Our study highlights the importance of using local wind data for deriving realistic SO2 emissions and the identification of short-term periodicity in volcanic behavior. Plain Language Summary Volcanoes may show cycling patterns in their eruption patterns but also in the geophysical and geochemical data used to monitor them. When these patterns occur at fixed-time intervals, volcanic behavior may be described as periodic. Periodic patterns in volcanic phenomena may reveal the existence of a fundamental process which regulates the inner workings of the volcano and ultimately aid forecasts of eruptive activity. We analyzed the sulfur dioxide degassing patterns of 32 volcanoes from a global database and we reveal significant periodic patterns in 17 of them. However, we also find similar periodic patterns the wind speed of the volcanic plume. We propose that periodic patterns in the degassing rates do not represent physical-chemical processes inside the volcano but result from atmospheric dynamics.