The Structure, Energetics and Evolution of the Dominant Frequency-Dependent Three-Dimensional Atmospheric Modes

Abstract

The winter-time circulation of the Northern Hemisphere is examined for the following time-scale classes. A) greater than 45 days, B)20?45 days C) 10?20 days D) 6?10 days and E) 2.5&ndash 6 days. The spatial structure of variability within each of these frequency buds is determined by an empirical orthogonal function expansion of the coupled vertical mean and difference streamfunction fields. In general, the dominant low-frequency modes (clan or filter A) exhibit zonally elongted patterns with an equivalent barotropic structure. The intermediate time scale modes (classes B and C) show a tendency for more circular anomaly patterns accompanied by a more baroclinic (westward tilt with height) structure for the class C modes. The dominant frequency modes (classes D and E) are characterized by a strong baroclinic component. The latter exhibit meridionally elongated patterns in the storm track regions together with an upstream tilt to the north and south of the jet exits. In general, lag cross correlations between dominant low-frequency modes tend to be small and/ or symmetric about zero. In contrast, the highest frequency modes exhibit highly asymmetric sinusoidal crow correlation functions reminiscent of travelling waves, while some intermediate and short-time scale modes exhibit correlations suggesting an association with the decaying phases of blocking in the Pacific. The winter average barotropic and baroclinic energy sources and sinks associated with the time-mean winter flow am examined in the context of a two-layer quasi-geostrophic model. The mean flow kinetic energy (KE) conversion terms reflect the geometry of the anomalies such that the dominant low-frequency modes gain energy, the intermediate frequencies are approximately neutral and the higher frequency modes low energy to the mean flow. The magnitude of the KE conversion is largest in the upper troposphere and is dominated by the lowest-frequency modes. The magnitude conversion from the mean flow to the anomalies is positive for the dominant modes in each band. However, the highest frequency modes associated with traveling waves in the North Atlantic and North Pacific are most efficient in the conversion process. For a typical Rossby radius of deformation, it is found that the net barotropic and baroclinic conversions are of comparable magnitude for periods greater than 10 days while the baroclinic conversion dominates for shorter periods. The majority of the conversions are accomplished by just a few (? five) of the dominant EOFs in each frequency band.