This thesis focuses on the nature of oceanic Eddy Kinetic Energy (EKE), its generation and temporal variability. An Ocean General Circulation Model (OGCM) based on the NEMO code builds the foundation for these investigations. For a first case study, several simulations of a 1/4° configuration are used to investigate the temporal variability of EKE in the South Pacific Subtropical Countercurrent (STCC). Decadal changes in wind stress curl associated with the Interdecadal Pacific Oscillation (IPO) lead to up- and downwelling in the STCC, influencing the meridional density gradient and thereby STCC strength, baroclinic instability and the resulting EKE. An additional 30 to 40% of the local density anomalies can be explained by long baroclinic Rossby waves propagating into the region, modulating the decadal signal of the IPO’s influence in the STCC on interannual time scales. In a second case study, the model’s horizontal resolution is regionally increased to 1/20° in the North Atlantic to investigate different types of mesoscale eddies in the Labrador Sea. On decadal time scales, the temporal variability of EKE in the LS is driven by the large-scale atmospheric circulation. In the case of Convective Eddies (CE), local winter heat loss leads to deep convection, a baroclinically unstable rim-current is established along the edge of the convection area and generates EKE at mid-depth. The variations of EKE associated with the surface intensified Irminger Rings (IR) and Boundary Current Eddies are driven by the large-scale changes of the currents of the subpolar gyre. While IR play a vital role in stratifying large parts of the LS and thus suppressing deep convection, CE are the major driver of rapid restratification during and after deep convection.