Wednesday, 10 June 2009: 4:10 PM
Pinnacle A (Stoweflake Resort and Confernce Center)
Most current atmospheric general circulation models (AGCMs) do not exhibit a quasi-biennial oscillation (QBO) in tropical stratospheric winds; however, strong correlations that are observed between the state of the QBO and the circulation of the extratropical stratosphere indicate that the QBO contributes significantly to interannual variability in the real stratosphere. We study the influence of the QBO on the extratropical circulation using a 150-year AGCM run in which a QBO is forced spontaneously by both resolved and parameterized tropical waves, and in which no interannual variations in sea-surface temperatures or external forcings (e.g., the 11-year solar cycle) are imposed. The spatial structure and seasonal timing of QBO-extratropical correlations in the model are found to compare favourably with those diagnosed from ERA-40 data. Additionally, the timing of QBO phase transitions with respect to the annual cycle, which serves as a proxy for the vertical structure of tropical stratospheric winds during winter, is found to affect the response of the Northern Hemisphere winter polar vortex to QBO phase. This occurs in both the model and in the ERA-40 data, and suggests that the partial seasonal synchronization of QBO phase transitions may induce decadal-timescale fluctuations in the strength of the polar vortex response to QBO phase. In the model this behaviour must be due to internal atmospheric variability, suggesting that such may also be the case in the real atmosphere.
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