Possible solar modulation of the equatorial quasi‐biennial oscillation: Additional statistical evidence

Abstract

Although the quasi‐biennial oscillation (QBO) in the equatorial zonal wind is dominantly driven by wave forcing originating in the troposphere, a recent study suggests that certain properties of the QBO may vary slightly on the 11‐year solar cycle timescale [Salby and Callaghan, 2000]. Here we report further statistical investigation using both equatorial wind data for levels from 50 to 1 hPa and longterm proxy solar ultraviolet flux time series (10.7‐cm solar radio flux and sunspot numbers). Spectral analysis of the solar time series yields evidence for a significant spectral peak at periods between 25 and 30 months, approximately equivalent to the mean QBO period, as had also been noted by earlier authors [Shapiro and Ward, 1962]. Cross‐spectral analysis of the 10.7‐cm solar radio flux and equatorial zonal wind time series shows significant coherency at the QBO period at all available pressure levels. The phase lag of the wind data relative to the solar flux at the QBO period ranges from 0–1 months near the stratopause (1 hPa) to 20–24 months in the lower stratosphere (50 hPa). The nearly inphase relationship near the stratopause suggests a possible modulation of the QBO at this level by the radiative and photochemical effects of solar ultraviolet variations. The amplitudes of the solar variations at the QBO period tend to be larger under solar maximum than under solar minimum conditions. Composite analysis of the westerly and easterly phases of the equatorial zonal wind shows subtle but consistent differences in the durations of the westerlies and easterlies between solar maximum and minimum conditions.