Abstract/Summary

The storm tracks in the mid-latitudes are a key component of the general circulation by transporting heat, momentum, and moisture from the equator towards the poles. However, the non-linear dynamics related to the interaction between mean flow and eddies are not yet fully understood. The aim of this thesis is to broaden this understanding by developing models based on minimal necessary ingredients that can capture some of the observed properties of the mid-latitudes. The derived models are based on Phillips’ quasi-geostrophic two-level model, the simplest model that can capture barotropic as well as baroclinic processes together with diabatic heating and surface friction. Both models are comprised of a set of ordinary differential equations, describing the interaction of the zonal flow with the eddies, and are analysed by means of classical dynamical systems theory. The first model, based on a highly simplified flow geometry, exhibits two physically realisable steady states, one purely zonal flow and one where, additionally, finite eddy motions are present. As the diabatic heating increases, the zonal solution loses stability and the eddy solution becomes attracting. After this transition, the zonal components of the solution are independent of the baroclinic forcing, a mechanism called eddy saturation. The second model, based on a zonal flow with variable latitudinal maximum and tilted eddies, enabling a positive momentum flux contrary to the first model, exhibits three states. In addition to a purely zonal state, the system features a barotropic eddy regime with poleward momentum flux, but zero poleward heat flux for lower diabatic heating. Increasing the forcing pushes the system into a baroclinic eddy regime with finite poleward heat flux. In both regimes, the zonal flow maximum is pushed polewards, whereas the growth rate of the flow speed is weakened in the baroclinic eddy regime, exhibiting incomplete eddy saturation.