Quasi-Biennial Oscillation Modulation of the Semi-Annual Oscillation in QBOi Models

Abstract. As part of the Quasi-Biennial Oscillation initiative (QBOi) phase 1, a model-intercomparison study explored the representation of the semi-annual oscillation (SAO) in participating models and found a common easterly bias of several tens of m/s compared to observations, mostly with weaker westerly phases and stronger easterly phases. QBOi phase 1 results also reported biases in model representations of the QBO, which dominates the region below the SAO, with most models displaying a westerly time mean wind bias, and generally weaker QBO easterly phases. Given that the SAO forcing terms can be influenced by the QBO, in this paper we explore the influence of these QBO biases on the representation of the SAO. A multi-model analysis is conducted here using QBOi phase 2 data from current state-of-the-art climate models to examine changes in representation of the SAO in response to corrections in zonal-mean zonal wind QBO biases. Most models show an improvement with a reduced easterly SAO bias in response to the corrected QBO bias. The extent of this response is found to vary significantly across models with SAO time-mean winds changing by 6 % to 403 % in simulations with a bias-corrected QBO relative to the control simulations. Changed gravity wave drag in response to the improved QBO is attributed as the forcing term having the largest overall impact.