QBO modulation of the semiannual oscillation in MAECHAM5 and HAMMONIA

The quasi-biennial oscillation (QBO) signature in the equatorial upper stratosphere and mesosphere is analyzed from MAECHAM5 and HAMMONIA general circulation models. Our results show that this region is significantly influenced by the stratospheric QBO. In the upper stratosphere the QBO modulates the altitude of maximum descent of the stratospheric semiannual oscillation (SSAO) westerly phases. Our results also suggest that the QBO modulates the altitude of maximum descent and also the strength of the SSAO easterly phase. We explore the role of large-scale and small-scale waves and also momentum advection in the forcing of the QBO signature in the SSAO domain. The results show how the vertical propagation of the QBO signature to the middle and upper mesosphere depends on the vertical phase structure of the SAO and consequently on the seasonal cycle. During the solstices when MSAO westerlies prevail in the middle and upper mesosphere no QBO signature can be detected above the stratopause region. However, during the equinoxes, when MSAO easterlies dominate in the middle and upper mesosphere, the QBO signature extends throughout the mesosphere and low thermosphere. The QBO directly modulates MSAO easterlies by modifying the altitude at which they are generated in the upper mesosphere. A QBO signature is also detected on the MSAO westerly phase occurring in the mesopause region during the equinoxes. Copyright © 2010 by the American Geophysical Union