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 investigation s focus on the organization of localized strom tracks, their impact on low-frequency variability, and on the respo nse to external thermal forcing. A localized storm track in agreement with observations is forced by a heating d pole embedded in a zonally symmetric field if the dipole orientation corresponds to the Northern Hemis phere winter case. The interaction of two storm tracks leads to low-frequency variability. Spatial resona nce between a low-frequency large scale retrograde travelling wave and the storm track eddies is identified caus ing the fluctuations. Teleconnection pattern remarkably similar to the observed North Atlantic Oscillatio n (NAO) and Pacific North American (PNA) pattern emerge when the distance of the two storm tracks is set to the bserved value of about 150 ◦. While the spatial resonance mechanism forces the NAO-like pattern, b aroclinic processes are related to the PNA-like teleconnection. Anomalies induced by large scale thermal f orcing strongly depend on the background flow. A non-linear response is observed in the model depending on t he sign of the forcing and its position relative to the storm track. A baroclinic and an equivalent barotropi c component defines the response. In addition, the change of the space-time variability is affected by eddy fee dbacks.

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