Preprints
https://doi.org/10.5194/egusphere-2024-1818
https://doi.org/10.5194/egusphere-2024-1818
03 Jul 2024
 | 03 Jul 2024
Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Role of the Quasi-Biennial Oscillation on Alleviating Biases in the Semi-Annual Oscillation

Aleena Moolakkunnel Jaison, Lesley J. Gray, Scott M. Osprey, Jeff R. Knight, and Martin B. Andrews

Abstract. Model representations of the stratospheric semi-annual oscillation (SAO) show a common easterly bias, with a weaker westerly phase and stronger easterly phase compared to observations. Previous studies have shown that resolved and parameterized tropical waves in the upper stratosphere are both too weak. These waves propagate vertically through the underlying region dominated by the stratospheric quasi-biennial oscillation (QBO) before reaching the SAO altitudes. The influence of biases in the modelled QBO on the representation of the SAO is therefore explored. Correcting the QBO biases helps to reduce the SAO easterly bias through improved filtering of resolved and parameterized waves that contribute to improving both the westerly and easterly phases of the SAO. The time averaged zonal-mean zonal winds at SAO altitudes change by up to 25 % in response to the QBO bias corrections. The annual cycle in the equatorial upper stratosphere is improved as well. Most of the improvements in the SAO occur during the QBO easterly phase, coinciding with the period when the model's QBO exhibits the largest bias. Nevertheless, despite correcting for the QBO bias there remains a substantial easterly bias in the SAO, suggesting that westerly wave forcing in the upper stratosphere and lower mesosphere is still severely under-represented.

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Aleena Moolakkunnel Jaison, Lesley J. Gray, Scott M. Osprey, Jeff R. Knight, and Martin B. Andrews

Status: open (until 14 Aug 2024)

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  • RC1: 'Comment on egusphere-2024-1818', Anonymous Referee #1, 17 Jul 2024 reply
Aleena Moolakkunnel Jaison, Lesley J. Gray, Scott M. Osprey, Jeff R. Knight, and Martin B. Andrews
Aleena Moolakkunnel Jaison, Lesley J. Gray, Scott M. Osprey, Jeff R. Knight, and Martin B. Andrews

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Short summary
Models have biases in SAO representation, primarily due to lack of strong enough eastward wave forcing. We investigated if this bias arises from increased wave absorption in low-mid stratosphere due to circulation biases. Using model experiments, we found that removing biases in lower altitudes improve the SAO, but a significant bias remains. Thus, modifications to gravity wave parametrisation is required to improve the modelled SAO, potentially leading to improved predictability of SSW.