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Non stationary Synchronization of Equatorial QBO with SAO in Observation and Model Le Kuai1 Run Lie Shia1 Xun Jiang2 Ka Kit Tung3 Yuk L Yung1 1 Division of Geological and Planetary Sciences California Institute of Technology Pasadena CA 91125 2 Department of Earth and Atmospheric Sciences University of Houston TX 77204 3 Department of Applied Mathematics University of Washington Seattle WA 98195 To whom all correspondence should be addressed E mail kl gps caltech edu Accepted by J Atmos Sci 1 Abstract 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 Apparently the synchronization is better the higher up the observation extends Using 45 years of ERA 40 data of the equatorial stratosphere up to the stratopause we confirm that this synchronization is not just a tendency but a robust phenomenon in the upper stratosphere A QBO period starts when a westerly SAO w SAO descends from the stratopause to 7 hPa and initiates the westerly phase of the QBO w QBO below It ends when another w SAO a few SAO periods later descends again to 7 hPa to initiate the next w QBO That it is the westerly but not the easterly SAO e SAO that initiates the QBO is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the e SAO does not create a zero wind line unlike the w 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 nonstationary behavior in both observation and model is not caused by the 11 year 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

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