Abstract
A study is made of the importance of horizontally varying static stability and nongeostrophic effects upon the location of Northern Hemisphere storm track instability modes and mature anomaly teleconnection pattern modes during January 1979. The analysis has been conducted with a two-level primitive equation model, and the results compared with corresponding results from one-, two-, and five-level quasigeostrophic models, although the main emphasis is a comparison between the two-level results.January 1979 was a period of frequent and severe storm activity in the Northern Hemisphere and a time of transition from high-latitude blocking over northwestern Canada and the Beaufort Sea area to persistent and large-scale blocking in the North Atlantic region. The three-dimensional instability modes from all models are discussed in the context of these synoptic developments. In particular, it is found that the inclusion of a horizontally varying static stability and nongeostrophic effects influence the structure of the cyclogenesis modes and lead to significant changes in the geographical locations of some of the preferred regions of cyclogenesis in situations of large-scale anomalous flow, such as occurred during January 1979. It is shown that this result can be understood from a consideration of Phillips' criterion for instability generalized to primitive equation and quasigeostrophic models in spherical geometry. In contrast, for more normal climatological flows there is a close correspondence between primitive equation and quasigeostrophic dominant storm track modes, instability criteria, and observations.We have found few differences in the structures of the larger-scale onset-of-blocking, intermediate, and mature anomaly modes that result from the primitive equation and quasigeostrophic instability calculations.
