Atmospheric Vacillations in a General Circulation Model I: The Large-Scale Energy Cycle

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Abstract
A 124-day time series of simulated atmospheric data has been generated for annual mean conditions with a hemispheric general circulation model. Analysis of these data based on discrete 4-day time means revealed a vacillatory behavior, particularly in the eddy kinetic and available potential energies, with a period of about 20 days in substantial agreement with a number of observational studies. This quasi-periodic behavior was also found to occur in the coupling terms between the various energy components and to a lesser extent in the forcing functions. A fairly complete analysis has been produced which accounts for the various relationships occurring in the vacillation cycle, and essentially explains the basic behavior of the cycle. This indicates that a vacillation is a natural feature of the atmosphere, which occurs because of fluctuations in the intensity of baroclinic activity resulting from a tendency of the atmosphere to overrespond to an initial imbalance in its latitudinal temperature gradient. The analysis in wavenumber space of the synoptic behavior of the 500 mb geopotential surface reveals that drastic changes occur between maximum and minimum phases of the vacillation for wavenumbers associated with baroclinic activity. Finally some implications regarding the possible impact of the vacillation cycle on numerical weather forecasting are noted.

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