The Effects of Changes in Sea Surface Temperature and CO2 Concentration on the Quasi-Biennial Oscillation

The effects of sea surface temperature (SST) and CO2 on future changes in the quasi-biennial oscillation (QBO) are investigated using a climate model that simulates the QBO without parameterized nonstationary gravity wave forcing. Idealized model experiments using the future SST with the present CO2 (FS run) and the present SST with the future CO2 (FC run) are conducted, as are experiments using the present SST with the present CO2 (present run) and the future SST with the future CO2 (future run). When compared with the present run, precipitation increases around the equatorial region in the FS run and decreases in the FC run, resulting in increased and decreased wave momentum fluxes, respectively. In the midlatitude lower stratosphere, westward (eastward) wave-forcing anomalies form in the FS (FC) run. In the middle stratosphere off the equator, westward wave-forcing anomalies form in both the FS and FC runs. Corresponding to these wave-forcing anomalies, the residual vertical velocity significantly increases in the lower stratosphere in the FS run but decreases to below 70 hPa in the FC run, whereas residual upward circulation anomalies form in both the FS and FC runs in the middle equatorial stratosphere. Consequently, the amplitude of the QBO becomes smaller in the lower stratosphere, and the period of the QBO becomes longer by about 1-3 months in the FS run. On the other hand, in the FC run, the QBO extends farther downward into the lowermost stratosphere, and the period becomes longer by 1 month.