QBO modulation of traveling planetary waves during northern winter

This study analyzes geopotential height data from the ERA-40 and ERA-Interim reanalyses for the period of 1958-2009 to provide some new insights on the stratospheric Quasi-Biennial Oscillation (QBO) modulation of traveling planetary waves during Northern Hemisphere (NH) winter. In the stratosphere, the zonal wave number 1-3 waves with periods of 22.5-30 days and 45-60 days are found to be significantly stronger at midlatitudes when the QBO at 50 hPa is in its westerly phase than when it is in its easterly phase. The modulation is stronger for eastward propagating and standing waves and weaker for westward propagating waves. A QBO modulation of 11-13 day planetary waves is also found but the effect is dominated by post-satellite data. In the troposphere, significant QBO modulation is only detected in westward propagating waves at zonal wave number 2 with periods of 22.5-30 days in early winter (Oct-Dec). Consistent results in the stratosphere are obtained using the temperature data from SABER/TIMED. The SABER data also show that the QBO effect on the eastward propagating 23-day waves extends into the mesosphere (similar to 70 km) for wave number 1 and only up to the stratopause (similar to 45 km) for wave numbers 2 and 3. We suggest that the 22.5-30 day planetary waves are secondary waves generated by a nonlinear coupling between zonal-mean intraseasonal oscillations (ISO) and the well-known 16-day planetary waves. The QBO modulation of those planetary waves is due to a QBO-ISO interaction. Further studies are needed to prove this hypothesis.