The roles of wave-zonal flow interaction and orographic forcing on the generation of low-frequency variability in a newly developed GCM

Abstract

A full-Galerkin GCM is developed for mechanistic studies on the generation mechanisms of low-frequency variability. The numerical implementation and baroclinic dynamics of this newly developed GCM are carefully tested, and its sensitivity to vertical resolution and to parameterized physics are also investigated.The role of wave-zonal flow interaction in generating low-frequency variability is studied in a GCM simulation with zonally symmetric forcing. Low-frequency variations are observed in the strength and the location of the zonal mean jet. A zonal index based on EOF analysis is used to describe this low-frequency phenomenon. Composite analysis shows that both the zonal mean circulations and the eddy properties are changed during the zonal flow vacillation. The Eulerian-mean angular momentum budget analysis indicates that wave-zonal flow interaction through eddy momentum flux convergence is the dominant process to maintain the zonal flow vacillation. Eddy forcing associated with eddy heat flux convergence tends to destroy the zonal wind anomalies associated with the zonal flow vacillation. Comparison between eddy forcing from various time-scales of eddies shows that synoptic time-scale of eddies provide most of the forcing to maintain zonal flow vacillation. Wind anomalies in the low-index and the high-index phases of the vacillation are self maintained by the interaction between the zonal flow and this time-scale eddies. This self-maintenance mechanism is demonstrated by two lifecycle experiments. A time-lag phenomenon in the establishment of zonal wind anomalies suggests that the mechanism to trigger the atmosphere to shift between the self-maintained phases may operate in higher latitudes.Analyses of variability in the mountain simulations show that a large-scale mountain not only affects the geographical distribution but also the amount of atmospheric variability. Orographic forcing influences the amplitudes of high-frequency transients and affects the propagation of intermediate-frequency transients. As a results of its different influences on various time-scales of variability, the high-, intermediate-, and low-frequency variability are separated into three different geographic locations. More high-frequency and low-frequency variability is localized in the jet exit region, while the intermediate-frequency variability is concentrated in the upstream region of the mountain. Although the amounts of high-frequency and intermediate-frequency variability are reduced by the orographic forcing, additional of low-frequency variability is generated by the orographic forcing. Barotropic instability of the orographically forced stationary waves and the direct interaction between the zonal flow vacillation and the mountain itself are possible mechanisms responsible for the increase. EOF analyses shows that zonal flow vacillation generated through the wave-zonal flow vacillation is still an important mechanism to generate low-frequency variability in the mountain hemisphere, but this process is localized in the region of the storm track.