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 composite patterns of the eddy-induced tendencies were compared with the corresponding monthly mean anomaly patterns. It is seen that the forcing due to eddy vorticity transports exhibits a distinctive barotropic character, and reinforces the monthly averaged geopotential height anomalies throughout the tropospheric column. On the other hand, the eddy heat fluxes lead to dissipation of the monthly mean height anomalies in the upper troposphere, and enhancement of the height anomalies in the lower troposphere. Hence, the eddy heat fluxes exert a strong impact on the baroclinic component of the circulation by destroying the concurrent local monthly mean temperature and geopotential thickness anomalies. The above relationships based on model data are in agreement with the corresponding observational results. A linear stationary wave model was then used to mimic the individual EOF modes appearing in the GCM experiment, and to diagnose the relative importance of different types of forcing in the generation of such modes. As suggested by the tendency calculations, the transient eddy forcing due to heat fluxes was parameterized as a thermal diffusion mechanism in the stationary wave model. When the model was linearized about the climatological zonally averaged basic state, it failed to reproduce the EOF patterns appearing in the GCM experiment. However, when the same model was linearized about the zonally varying GCM climatology, the response to the total forcing (which includes vorticity fluxes by eddies on submonthly time scales, diabatic heating, and nonlinearity in those months when the individual EOF modes are active) bears a considerable resemblance to the corresponding anomaly patterns in the GCM. By evaluating the individual contributions of each of the three forcing mechanisms to the total linear model solution, it is concluded that the transient eddy vorticity fluxes exert the strongest influences. The response to nonlinear effects is negligible, while the forcing due to diabatic heating is weak and acts in opposition to the anomaly patterns in the upper troposphere. The forcing associated with vorticity fluxes by synoptic-scale transient eddies accounts for approximately half of the total vorticity forcing due to all submonthly fluctuations. Both the tendency calculations and the stationary wave model results indicate the crucial role of vorticity transports by transient eddies. The linear model solutions also illustrate the importance of incorporating the climatological stationary waves in the basic state. These findings hence suggest that the monthly mean anomalies in this GCM experiment are intimately linked to barotropic interactions between transient fluctuations of different time scales, and between the monthly mean anomalies and the climatological stationary waves.