Dynamical Aspects of Extratropical Tropospheric Low-Frequency Variability

Abstract

The nature of extratropical tropospheric low-frequency variability remains an important, unresolved problem in the overall dynamics of the climate system. Primarily, this is due to the complexity of dynamics operating on low-frequency timescales of 10–100 days; both synoptic- and planetary-scale dynamical processes are fully active and strongly interactive. This review explores two issues that frequently arise in the study of low-frequency variability, and emphasizes the continuing value of idealized dynamical models in interpreting low-frequency variability. The first issue concerns the extent to which the extratropical atmosphere supports planetary-scale instabilities, and whether a simple picture of such instabilities can be developed. It is argued that under certain circumstances such instabilities do exist, and result from the accumulation of stationary wave energy in local resonant cavities that emerge in zonally varying barotropic flow. The second issue concerns the interaction of synoptic transients with the zonally varying planetary-scale flow. Simple dynamical settings reveal the essence, but also the ambiguity underlying the interaction between these two scales. The implications of these simple model arguments for the current understanding of low-frequency variability in more complicated models as well as nature is discussed, along with the role of such simple models in the overall climate modeling hierarchy.