Period Modulation of the Stratospheric Quasi-biennial Oscillation

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Abstract
The period of the stratospheric quasi-biennial oscillation (QBO) in zonal wind is studied with the aid of rocket observations at Kwaialein (9°N, 168°N) for altitudes 25–35 km and rawinsonde observations for Balboa (9°N, 80°W) for pressure levels 50–10 mb, the latter for 1951–79. Since 1951, the period has varied between 21 and 34 months, with an average value near 28 months (2.3 years) at any height, but with slightly greater variability in the lower stratosphere. While the expected downward propagation of easterly and westerly phase was clearly observed, a downward propagation of period is not evident, consistent with the variable structure of the phase descent rates. A systematic modulation of QBO period occurred approximately and inversely in accord with the level of solar activity. Physical mechanisms that might account for the observed variation in QBO period are identified, within the context of assumed dynamical forcing by tropospheric waves. Comprehensive insight into the actual process accounting for the observed period modulation would be gained from improved observational data, including observations of solar ultraviolet behavior.

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