Teleconnection Patterns and the Roles of Baroclinic, Barotropic and Topographic Instability

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Abstract
The instability characteristics of a three-dimensional Northern Hemisphere flow for January 1978 have been examined in quasi-geostrophic spherical five-level and barotropic models with and without explicit topography present. The structures of the 30 fastest growing modes in the baroclinic model have been analyzed and the modes have been separated into four classes based on their phase frequencies and structures. Class I consists of rapidly propagating monopole cyclogenesis modes, most of which are shallow westward-tilting disturbances with largest amplitudes in the centers of the North Pacific and Atlantic observed storm tracks. In addition, new modes, which correspond to storm tracks across Siberia and across the Middle East are found. Some of the larger-scale cyclogenesis modes also have amplitudes that peak near the tropopause. Class II modes are dipole onset-of-blocking modes with longer periods and larger scales than the cyclogenesis forms. In this class we find disturbances that would initiate the formation of positive and negative anomalies such as blocks and high zonal index flows in the North Soviet Union region as well as in the Atlantic and Pacific. Class III modes have periods and properties intermediate between the onset-of-blocking and the mature anomaly modes of class IV. They are large-scale, essentially equivalent barotropic modes and are similar to the structures in the middle and late stages of the formation of Dole's Pacific and Atlantic mature anomaly patterns. Class IV modes have the longest periods and largest scales; they are equivalent barotropic and have amplitude peaks in the upper troposphere or lower stratosphere. Modes which resemble the Atlantic (or Eastern Atlantic), North Soviet Union (or Eurasian) teleconnection patterns, the North Atlantic oscillation, the Western Pacific and the Western Atlantic teleconnection patterns are found. Some modes also appear to have the right properties such that if they were produced with sufficient amplitude, they could produce a stratospheric sudden warming. In the baroclinic model, the structures of most of the cyclogenesis modes are little changed by the inclusion of explicit topography and its effect, in general terms, is increasingly felt by the larger-scale modes of increasing periods. Modes similar to some of the class IV disturbances of the baroclinic model are also found in the barotropic model; however, there are quantitative differences in their structures and growth rates, and phase frequencies may differ significantly.

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