Abstract
The effect of the seasonal cycle on the quasi-biennial oscillation (QBO) in the equatorial stratosphere was investigated using a chemistry-climate model (CCM) by fixing the seasonal cycle in CCM simulations. The CCM realistically reproduced the QBO in wind and ozone fields of a 30-month period in a climatological simulation (control run) under annually repeating sea surface temperature (SST) with a seasonal cycle. For the control run, four experimental simulations (perpetual runs) were made by fixing solar declination and SST on the 15th of January, April, July, and October, respectively, for about 20 years. In the three perpetual runs of January, July, and October, the QBO was maintained and persisted throughout the 20-year integration in spite of some small differences in period and amplitude among the three runs. On the other hand, the QBO in the perpetual April run began to weaken after about 15 years and the downward propagation of westerly wind stopped at about 20 hPa, resulting in the QBO’s ceasing. The cause of this QBO disappearance is related to the evolution of the background mean flow in the lower stratosphere, which filtered out the parameterized gravity waves propagating upwards farther.