The portable university model of the atmosphere (PUMA): Storm track dynamics and low-frequency variability

The ability of analysing atmospheric dynamics by idealized experiments using a simplified circulation model is illustrated in three related studies. The investigations focus on the organization of localized strom tracks, their impact on low-frequency variability, and on the response to external thermal forcing. A localized storm track in agreement with observations is forced by a heating dipole embedded in a zonally symmetric field if the dipole orientation corresponds to the Northern Hemisphere winter case. The interaction of two storm tracks leads to low-frequency variability. Spatial resonance between a low-frequency large scale retrograde travelling wave and the storm track eddies is identified causing the fluctuations. Teleconnection pattern remarkably similar to the observed North Atlantic Oscillation (NAO) and Pacific North American (PNA) pattern emerge when the distance of the two storm tracks is set to the observed value of about 150 degrees. While the spatial resonance mechanism forces the NAO-like pattern, baroclinic processes are related to the PNA-like teleconnection. Anomalies induced by large scale thermal forcing strongly depend on the background flow. A non-linear response is observed in the model depending on the sign of the forcing and its position relative to the storm track. A baroclinic and an equivalent barotropic component defines the response. In addition, the change of the space-time variability is affected by eddy feedbacks.