ALBERT

Description: The quality of eddy ux-gradient parametrizations in models with coarse resolution depends on whether the generated diffusivity is similar to that of reference solutions produced by eddy resolving models. It is therefore essential to accurately describe the transport properties of eddy resolving ocean models. This thesis is a survey of the lateral transport of passive tracers induced by mesoscale eddies in the velocity field of a 1/12° numerical model of the North Atlantic. Statistical tools are used to relate particle trajectories of Lagrangian floats to the effective eddy diffusivity: Both Taylor's theory of turbulent dispersion, which forms the foundation for the analysis, and further refinements thereof, are discussed and used for computations. The underlying theories rely on several restrictive assumptions about the statistics of the flow field, and one objective of this thesis is to make the reader aware of how difficult an interpretation of the results can be.

Description: Mesoscale variability of velocities is an important part of the global ocean circulation, as it contains more kinetic energy than the mean flow over most of the ocean. Understanding its generation, dissipation and modulation processes therefore is crucial to better understand ocean circulation in general. In this thesis, a global 1/12◦ ocean model (ORCA12) is used to study the distribution of mean surface Eddy Kinetic Energy (EKE), its seasonal cycle and possible driving mechanisms, averaged over 26 years (1981-2007). For the calculation of EKE, the deviations from yearly mean horizontal velocities u, v are found to be best suitable. The model is then evaluated using EKE derived from satellite altimetry (AVISO). The total EKE from the model, including geostrophic parts, realistically reproduces the observed geostrophic mean EKE and its seasonal cycle. Seasonal cycles of surface EKE in the subtropical gyres, including most of the Western Boundary Currents (WBCs), peak in the summer months in both hemispheres. The mean EKE and amplitudes of the annual cycle are generally larger in the Pacific, compared to the Atlantic. The seasonal variations of EKE in the WBCs are driven by dissipation processes at the sea surface, namely the wind stress and thermal interactions with the atmosphere in winter. Only in the core regions of the currents other processes play a role as the surface EKE there peaks in winter/spring, not consistent with the dissipation hypothesis. The balance of dissipation and generation terms in the strong, chaotic WBCs, however, varies from year to year. In the subtropical gyres’ interior, dissipation is not solely responsible for the annual cycle. Instead, the vertical shear of near-surface horizontal velocities is found to peak in summer, in phase with the EKE. This seasonal cycle of the shear can be observed down to ∼ 150m depth, depending on the region. Inspections of profiles of horizontal velocity and EKE reveal the vertical shear to be associated with the velocity differences between the Mixed Layer and the interior ocean, possibly leading to instabilities which locally generate surface intensified EKE, largest in summer. Therefore, the seasonal cycle of near-surface vertical shear of horizontal velocities seems to be responsible for the seasonal variations of surface EKE, although the general source of EKE in the subtropical gyres remains unclear.

Description: 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 Universität 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.