| Summary: | The period of quasi-biennial oscillation (QBO) of equatorial stratospheric winds is mainly determined by the magnitude of upward momentum transport by equatorial waves and the intensity of the Brewer-Dobson circulation. The observed QBO period is an average of 28 months, but it can range from 24 to 30 months. The purpose of this study is to find out how sensitive the period and other characteristics of the QBO are to the climate change caused by doubling CO₂ concentration. MAECHAM5 T42L90 experiments are conducted with initial values and boundary conditions (sea ice cover, sea surface temperature) acquired from the AMIP2 climatology and two lower-resolution coupled runs of ECHAM5 and MPI-OM1 with CO₂concentrations of 348 and 696 ppmv, respectively. In a test experiment with unchanged gravity wave parameterization, both a speedup of the Brewer-Dobson circulation and a significant increase in convective precipitation variance observed. Increased variance in the diabatic forcing of the tropical atmosphere leads to strengthened excitation of vertically propagation waves, specifically of gravity waves. Thus two sensitivity experiments with different parameter settings of the subgrid scale gravity wave drag parameterization are analyzed. A speedup of the oscillation from the control experiment value of 34 months to between 22 and 17 minths is observed, where easterly (westerly) phase duration decrease mostly at upper (lower) QBO levels. While the Brewer-Dobson circulation also intensifies due to stronger forcing by the breaking of extratropical planetary waves, it does not offset the enhanced generation of both parameterized and resolved waves. Discrimination between zonal wind forcing by resolved and parameterized waves reveals considerable changes in the parameterized forcing, which extends to lower levels and intensifies, especially maximum westward acceleration. REsolved wave forcing seems to increase mostly between 10 and 20 hPa. Considerable uncertainty about the exact amount of QBO period shorting remains and ...
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