Analysing Major Sudden Stratospheric Warmings in the coupled middle atmosphere ocean model MAECHAM5/MPI-OM

In this study, the stratospheric winter circulation in the coupled middle atmosphere ocean model MAECHAM5/MPI-OM, is analysed. Due to the dynamical and thermodynamical interaction with the ocean, the simulated atmospheric circulation is affected by the internal variability of the ocean. Differences of the stratospheric winter circulation between MAECHAM5/MPI-OM and former MAECHAM5 simulations may be attributed to the interactive ocean. This work is divided into three parts: first the climatology of the model is examined, then major Sudden Stratospheric Warmings (SSWs), and at last the relationship between these warmings and tropospheric blockings. To examine how the model reproduces the stratospheric winter circulation, the climatology of the zonal mean zonal wind and of planetary waves of zonal wavenumber 1 to 3, are carefully analysed and compared with those obtained from ERA-40 observations. While the zonal mean zonal wind is in good agreement with observations, amplitudes and phases of zonal wavenumber 2 are not well represented. Major SSWs are analysed because of the strong impact that such phenomena can have throughout the atmosphere, influencing the weather at the surface for several weeks after the onset of the warming. To identify major SSWs, a new algorithm based on the 10 hPa zonal mean zonal wind at 60°N, is developed. This is done because a comparison between a recent study by Charlton and Polvani (2007) and the Freie Universitat Berlin climatology of mid-winter major SSWs, reveals a significant disagreement. The new algorithm is applied to two databases: one obtained from the model simulation and one from the ERA-40 assimilation. ERA-40 data are used for validation of MAECHAM5/MPI-OM. Comparison of the obtained frequencies of major SSWs shows that in the model a slightly higher number of events occurs. While in ERA-40 the average frequency is of 0.60 events per year, in the model it is of 0.70 events per year. The seasonal distributions show also that the highest number of major SSWs occurs in January and in February respectively for ERA-40 and model data, which is improved for MAECHAM5/MPI-OM compared to former MAECHAM5 simulations. In this work, unlike previous model studies, the state of the polar vortex is also examined during the pre-warming phase of major SSWs, by analysing the planetary wave activity, to determine the behaviour of waves with different zonal wavenumbers. Only planetary waves of zonal wavenumber 1 and 2 appear to have a key role in the development of major SSWs, with wavenumber-1 events being more frequent than wavenumber-2 events and a ratio of 57:13 similar as observed. Because of the influence of tropospheric blockings on major SSWs via alteration of planetary waves, a correlation analysis is performed to determine if the model represents this relationship well. It appears that Pacific blockings are correlated with wavenumber-2 major SSWs although a larger number of wavenumber-2 events would be necessary to make such assertion. No significant correlation is instead obtained for wavenumber-1 major warmings.