A Diagnostic and Modeling Study of the Monthly Mean Wintertime Anomalies Appearing in a 100-Year GCM Experiment

Abstract The nature of simulated atmospheric variability on monthly time scales has been investigated by analyzing the output from a 100-year integration of a spectral GCM with rhomboidal wavenumber 15 truncation. In this experiment, the seasonally varying, climatological sea surface temperature was prescribed throughout the world oceans. The principal modes of variability in the model experiment were identified by applying a rotated empirical orthogonal function (EOF) analysis to the Northern Hemisphere monthly averaged 515-mb geopotential height for the winter season (November through March). The individual leading spatial modes are similar to the observed north-south dipoles over the North Atlantic and North Pacific, as well as wavelike patterns in the Pacific/North American and Northern Asian sectors. Quasigeostrophic geopotential tendencies forced by synoptic-scale (2.5–6 day) eddy vorticity and heat fluxes were computed for those months when the individual EOF modes are particularly active. The comp...