Speaker: Aleena Moolakkunnel Jaison (Oxford)
Topic: 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 this study, ERA5, ERA-interim and MERRA2 reanalysis data are used to analyse the contributions of different forcing terms (from the transformed Eulerian mean momentum equation) within the SAO. A first comparison is made between CESM2-WACCM model from CMIP6 and reanalysis data. Analyses show that lack of strong resolved waves and parameterised gravity waves, along with weakened BDC above 1hPa might be the major model deficiencies. A further analysis is in progress to identify likely improvements, including improved convective gravity wave parameterisation, vertical resolution and whether the tropical stratosphere circulation biases are sensitive to these. Preliminary experiments by constraining the winds in lower-middle stratosphere and improving the vertical resolution are showing promising results towards improving SAO representation in models.