This project will study the sources of low-frequency variability in the extra-tropical atmosphere using simple numerical models of the general circulation and focusing on the role of synoptic variability in generating low-frequency motions. These investigations, to be carried out using the two-layer primitive equation model on the sphere, will comprise the following: 1) Detailed diagnostic studies of the dynamics of low-frequency variability generated during long integrations of the full model, with and without orography. These simulations are designated the "controls". 2) Studies of "truncated" models that exclude the scales at which high-frequency variability is generated. Only the average effects, computed from the controls, of the high-frequencies will be included. Low-frequency behavior in these models will, therefore, be autonomous to large spatial and slow time scales. 3) Experiments that include the forcing of low frequencies by synoptic variability: the linear and nonlinear time-dependent barotropic responses to the forcing calculated from the controls and to stochastic models of that forcing. 4) comparisons, between the control and truncated models, of the loss of predictability of low-frequency motions. This work will lead to an understanding of how synoptic scales contribute to low frequency variability of the atmosphere and will enhance our capability to predict seasonal climate.
