Zonal Flow Vacillation and Bimodality of Baroclinic Eddy Life Cycles in a Simple Global Circulation Model

Abstract

A global primitive-equation model of the atmosphere is used to study the relationship between the temporal variations of the zonal mean zonal flow and baroclinic eddies. Nonperiodic low-frequency vacillation of the mean zonal flow is found in longtime integrations of the model under a perpetual condition; the zonal-mean jet in the extratropics changes its position nearly barotropically. A potential vorticity?potential temperature (PV??) analysis is performed for two extreme periods of the zonal flow vacillation. Anticyclonic breakings of upper troughs are dominant in the period of a high-latitude jet, while cyclonic breakings are dominant in the period of a low-latitude jet. A statistically significant relationship between the zonal flow vacillation and the morphology of life cycles of baroclinic eddies is obtained for the entire period analyzed. An index of the life cycles, which is introduced in this study, shows clear bimodality in its frequency distribution function. The relationship is also confirmed by two experimental runs with a different intensity of the surface drag. For the low-drag run, the zonal-mean jet is located in high latitudes through the integration period and life cycles of baroclinic eddies are basically characterized by the anticyclonic breaking. For the high-drag run, on the other hand, the zonal-mean jet is located in low latitudes and life cycles of baroclinic eddies are characterized by the cyclonic breaking. Although these two types of breaking pattern are similar to the two paradigms of baroclinic wave life cycles obtained in some idealized one-shot experiments, there are some differences from the one-shot experiments in the deformation field on an isentropic surface and in the relative location between the zonal-mean jet and the latitude of maximum eddy kinetic energy.