The Dynamics of Low-Frequency Variability in a Simple Model of the Global Atmosphere

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Abstract
We examine the dynamics of low-frequency variability in a simple global model with zonally homogeneous boundary conditions. Low-frequency structure comprises trains of nearly equivalent barotropic Rossby waves with weak eastward phase velocities and strong eastward group velocities. These low-frequency eddies are strongly coupled to synoptic activity. The flux of vorticity by synoptic waves acts both to reinforce the low-frequency eddies and to retard their eastward propagation. Mechanistic experiments with an imposed forcing show that the synoptic-scale vorticity fluxes are organized by the low-frequency flow. The behavior is generally similar to that found in previous studies of blocks; the present work shows that strong mutual interactions between the low and synoptic frequencies also characterize less dramatic, low-frequency flows. Abstract We examine the dynamics of low-frequency variability in a simple global model with zonally homogeneous boundary conditions. Low-frequency structure comprises trains of nearly equivalent barotropic Rossby waves with weak eastward phase velocities and strong eastward group velocities. These low-frequency eddies are strongly coupled to synoptic activity. The flux of vorticity by synoptic waves acts both to reinforce the low-frequency eddies and to retard their eastward propagation. Mechanistic experiments with an imposed forcing show that the synoptic-scale vorticity fluxes are organized by the low-frequency flow. The behavior is generally similar to that found in previous studies of blocks; the present work shows that strong mutual interactions between the low and synoptic frequencies also characterize less dramatic, low-frequency flows.

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