The spatial structure and temporal characteristics of prominent anomalies occurring in a 15-year simulation with a GFDL spectral general circulation model are examined using empirical orthogonal functions, teleconnection patterns, composite charts, lagged correlation functions and frequency spectra. Despite the absence of any nonseasonal perturbation in the prescribed forcing such as sea surface temperature, insolation and cloud cover, the simulated circulation exhibits an appreciable degree of temporal variability on monthly time scales. The standing oscillation in the Northern Hemisphere winter which accounts for the largest fraction of this variance has a coherent three-dimensional structure. In the middle and upper troposphere, this preferred mode of oscillation is characterized by a wavelike pattern with multiple centers of action. The corresponding anomaly pattern at the sea level is dominated by north–south pressure seesaws over the North Atlantic and North Pacific. The flow patterns assoc... Abstract The spatial structure and temporal characteristics of prominent anomalies occurring in a 15-year simulation with a GFDL spectral general circulation model are examined using empirical orthogonal functions, teleconnection patterns, composite charts, lagged correlation functions and frequency spectra. Despite the absence of any nonseasonal perturbation in the prescribed forcing such as sea surface temperature, insolation and cloud cover, the simulated circulation exhibits an appreciable degree of temporal variability on monthly time scales. The standing oscillation in the Northern Hemisphere winter which accounts for the largest fraction of this variance has a coherent three-dimensional structure. In the middle and upper troposphere, this preferred mode of oscillation is characterized by a wavelike pattern with multiple centers of action. The corresponding anomaly pattern at the sea level is dominated by north–south pressure seesaws over the North Atlantic and North Pacific. The flow patterns assoc...