On alleviating semi-annual oscillation wind biases in climate models

The representation of the semi-annual oscillation (SAO) in models shows up a common easterly bias of several tens of m/s compared to observations. These biases are likely a response to deficiencies in eastward wave forcing or the position and timing of the climatological summertime jet and Brewer-Dobson circulation (Smith et al 2019). This motivates further analysis of the energetics of the upper stratosphere within models and a more detailed comparison with reanalyses to see where these differences are coming from. In our study, MERRA2 reanalysis data is used to analyse the contributions of different forcing terms (from the transformed Eulerian mean momentum equation) within the SAO. A comparison is made between model data from CMIP6 and reanalysis. Analyses show that lack of strong resolved waves and parameterised gravity waves, along with weakened BDC above 1hPa might be the major model deficiencies. The influence of biases in lower-middle stratosphere in the representation of SAO energetics is further explored using QBOi phase 2 data. Study shows that correcting the QBO biases helps reduce the SAO easterly bias through improved filtering of resolved and parameterized waves that contribute to forcing the SAO westerly phase.