The SAO and Kelvin waves in the EuroGRIPS GCMS and the UK Met. Office analyses

We compare the tropical oscillations and planetary scale Kelvin waves in fo ur troposphere-stratosphere climate models and the assimilated dataset prod uced by the United Kingdom Meteorological Office (UKMO). The comparison has been made in the GRIPS framework "GCM-Reality Intercomparison Project for SPARC", where SPARC is Stratospheric Processes and their Role in Climate, a project of the World Climate Research Program. The four models evaluated a re European members of GRIPS: the UKMO Unified Model (UM), the model of the Free University in Berlin (FUB-GCM), the ARPEGE-climat model of the French National Centre for Meteorological Research (CNRM), and the Extended UGAMP GCM (EUGCM) of the Centre for Global Atmospheric Modelling (CGAM). The int egrations were performed with different, but annually periodic external con ditions (e.g., sea-surface temperature, sea ice, and incoming solar radiati on). The structure of the tropical winds and the strengths of the Kelvin wa ves are examined. In the analyses where the SAO (Semi-Annual Oscillation) a nd the QBO (Quasi-Biennal Oscillation) are reasonably well captured, the am plitude of these analysed Kelvin waves is close to that observed in indepen dent data from UARS (Upper Atmosphere Research Satellite). In agreement wit h observations, the Kelvin waves generated in the models propagate into the middle atmosphere as wave packets, consistent with a convective forcing or igin. In three of the models, slow Kelvin waves propagate too high and thei r amplitudes are overestimated in the upper stratosphere and in the mesosph ere, the exception is the UM which has weaker waves. None of the modelled w aves are sufficient to force realistic eastward phases of the QBO or SAG. A lthough the SAO is represented by all models, only two of them are able to generate westerlies between 10 hPa and 50 hPa. The importance of the role p layed in the SAO by unresolved gravity waves is emphasized. Although it exh ibits some unrealistic features, the EUGCM, which includes a parametrizatio n of gravity waves with a non-zero phase speed, is able to simulate clear e asterly to westerly transitions as well as westerlies with downward propaga tion. Thermal damping is also important for the westerly forcing in the str atosphere.