Planetary-wave responses to the composite forcings of the solar cycle and the quasi-biennial oscillation

Previous studies have reported the combined solar cycle-quasi-biennial oscillation (QBO) modulations on extratropical planetary waves in the stratosphere. Holton and Tan [J. Atmos. Sci., 37, 2200-2208, 1980] reported that the amplitudes of wavenumber-1 and -2 at 50 hPa were larger in the easterly phase of QBO during early winter (November-December) while their amplitudes were larger in the westerly phase of QBO during the late winter. Naito and Hirota [J. Met. Soc. Jpn., 75, 925-937, 1997] showed Holton and Tan's results held only when the solar cycle was at minimum. During the solar maximum, Naito and Hirota found that the amplitude of wavenumber-1 were larger in the easterly phase of QBO during both early and late winter (January-March). However, some of the above results are known to be sensitive to the length of the reanalysis data used.

Here, we re-examine the combined solar cycle-QBO modulations using 65 years of NCEP/NCAR reanalysis data and a better QBO index. We find that the QBO modulations on the wave amplitudes are not statistically significant when the solar cycle is at maximum. Furthermore, the QBO modulations on the wave amplitudes are significant during the solar minimum only under two conditions:

(1) for wavenumber-1 during early winter, whose amplitude is larger during the easterly QBO phase; and (2) for wavenumber-2 during the late winter, whose amplitude is larger during the westerly QBO phase.

The relationship of these two conditions with the events of the Stratospheric Sudden Warming will be discussed.