Non-stationary Synchronization of Equatorial QBO with SAO in Observation and in Model

It has often been suggested that the period of the Quasi-Biennial Oscillation (QBO) has a tendency to synchronize with the Semi-Annual Oscillation (SAO). Using 45 years of ERA-40 data of the equatorial stratosphere up to the stratopause, we demonstrate that this synchronization is not just a tendency but indeed holds quite well in the upper stratosphere. A QBO period starts when a SAO westerly descends from the stratopause to 7 hPa and initiates the westerly phase of the QBO below, and it ends when another SAO westerly, a few SAO periods later, descends again to 7 hPa to initiate the next westerly QBO. That it is the westerly and not the easterly SAO that initiates the QBO is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the easterly SAO does not create a zero-wind line, unlike the westerly SAO. The currently observed average QBO period of 28 months, which is not an integer multiple of SAO periods, is a result of intermittent jumps of the QBO period from 4-SAO periods to 5-SAO periods. The same behavior is also found in a model, the two-and-a-half dimensional THINAIR model. We find that the non-stationary behavior in both observation and model is not caused by the variable solar-cycle forcing, but is instead caused by the incompatibility of the QBO’s natural period determined by its wave forcing, and the quantized period determined by the SAO. The wave forcing parameter for the QBO period in our current climate probably lies between 4-SAO and 5-SAO periods. If the wave forcing for the QBO is tuned in a parametric study so that its natural period is compatible with the SAO period above, e.g. at 24 months, 30 months or 36 months, non-stationary behavior disappears.