Analysis of systematic biases in equatorial resolved and parameterized wave forcing of the quasi-biennial oscillation in a multi-model ensemble

Abstract. The equatorial resolved and parameterized wave forcing of the quasi-biennial oscillation (QBO) in models participating in phase 2 of the Atmospheric Processes and their Role in Climate (APARC) Quasi-Biennial Oscillation initiative (QBOi) is analyzed. We compare two experiments performed by the multi-model ensemble, NoNudge and ObsQBO, covering the period from 1979 to 2020. In this study, NoNudge designates experiments where the QBO is generated internally without nudging, whereas ObsQBO represents experiments in which the QBO is bias-corrected by nudging stratospheric zonal-mean zonal winds toward ERA5, allowing investigation of modelling uncertainties originating from biases in the background winds. While the NoNudge simulations exhibit QBO biases typically seen in models, including eastward wind bias in the mid-stratosphere and underestimation of the QBO amplitude in the lower stratosphere, these discrepancies are effectively mitigated by nudging in ObsQBO simulations. In the NoNudge experiments, the models reveal weaker Kelvin and gravity wave forcing in the tropical lower stratosphere than ERA5, but these forcings are enhanced in the ObsQBO experiment, suggesting that weak wave forcing of internally generated QBOs results from reduced critical-level filtering of waves due to unrealistically weak vertical wind shear at these levels. However, in ObsQBO experiments, the models still exhibit insufficient Kelvin and gravity wave forcing in the lower stratosphere. This suggests that the weak QBO amplitude in the lower stratosphere may stem from the inherent deficit in wave forcing in the models. One notable feature of the ObsQBO experiments is the excessive eastward wave forcing in the mid-to-upper stratosphere during the easterly QBO phase in the lower tropical stratosphere. The correlation coefficient between the 10 hPa eastward wave forcing and westerly QBO amplitude is 0.8, indicating that excessive westerly QBO is associated with strong eastward wave forcing in the mid-to-upper stratosphere.