Response of QBO Bias to Convection Scheme in an AGCM

A common bias of the simulated quasi-biennial oscillation (QBO) in the lower stratosphere is present in many atmospheric general circulation models (AGCMs) where its amplitude is weaker than the observed amplitude. To reduce this bias, the response of QBO bias to the convection scheme in an AGCM is investigated using different convection schemes (Emanuel and spectral schemes) and a gravity wave (GW) scheme in which the GW properties are estimated from convective heating and mean wind. Unlike the known QBO bias in previous model intercomparison studies, the easterly wind and QBO amplitude in the lower stratosphere are excessively magnified when the Emanuel scheme is used. The spectral scheme reproduces similar known QBO biases, such as the mean westerly wind bias and the weak QBO amplitude in the lower stratosphere. To distinguish the cause of these biases, several sensitivity experiments using the spectral scheme with tunings are conducted. Application of several tunings mitigates the common QBO biases near the 50 hPa level. The zonal wind bias in the troposphere controls the GW and consequently the QBO properties. Further investigation reveals that improved convective momentum transport with phase speed tuning reduces the zonal wind bias and improved the QBO through modified GW momentum fluxes. The findings of this study may be applicable to other AGCMs if the tunings related to the convection scheme are adopted, potentially reducing zonal wind biases.