| Summary: | The nonlinear interaction between a long orographically excited wave and a baroclinic wave has been investigated with a spectral low-order model in spherical geometry taking into account latitude and longitude dependent heat and vorticity sources and dissipation. Due to excitation with a zonally symmetric heat source of special meridional form, periodic solution behaviour appears and in dependence on the nonlinearities of the model quasi-stationary circulation states develop. The transitions between these states occur chaotically. The quasi-stationary large-scale circulation patterns are of high-index and low-index types and develop for a zonally symmetric thermal forcing with a meridional temperature gradient greater than 35 K. The frequency variability of the large-scale circulation patterns is determined by two different time scales, one short-scale between 2 and 5 days and a longer one between 20 and 30 days. The model describes realistic transitions between periodic and chaotic solutions, which are connected with topographic, barotropic and baroclinic instabilities and nonlinear wave interactions. The parametrization of the influence of the baroclinic waves is a difficult task, since the nonlinear interaction with the long waves for the development of quasi-stationary wave patterns has to be described in detail.
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