A momentum budget study of the semi-annual oscillation

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 jet tropical 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 this study, MERRA2 reanalysis data is used to analyse the contributions of different forcing terms (from the transformed Eulerian mean momentum equation) within the SAO, with a particular focus given to the contributions from wave forcing. This is then compared with the equivalent analysis using data from a climate simulation of the CESM2-WACCM model. Analyses show that lack of strong resolved waves and parameterised gravity waves, along with weakened BDC above 1hPa might be the major model deficiencies. Of particular interest is how the QBO phases affect the biases seen in the upper stratosphere. A first comparison is made between UMGA7 from QBOi phase 1 and reanalysis data. A further analysis will expand to identify likely improvements, including improved convective gravity wave parametrisation and whether the tropical stratosphere circulation biases are sensitive to these.