I will discuss the processes that set the time scales of internal variability in the midlatitude atmosphere, in particular those of the annular modes, which characterize meridional shifts of the midlatitude westerly jets. Even a relatively modest shift in the jet stream can make a big difference on a local level; in the northeastern US, for example, a shift of a couple grid boxes in a typical GCM is the difference between record snow in Washington, DC and a typical wintry weekend for Boston.
The persistence of these shifts on intraseasonal time scales makes the difference between a lone storm and a very cold winter. I will explore the annular modes in a hierarchy of models, from the barotropic vorticity equation on the sphere to coupled atmosphere-ocean climate prediction models and chemistry-climate models with high stratospheric resolution.
Different levels of complexity help illustrate the importance of interactions between baroclinic eddies and the zonal mean flow in setting the time scale of the annular modes, and how events in the upper atmosphere introduce slower, stratospheric time scales to the troposphere.