Secondary frontal waves in the North Atlantic region: A dynamical perspective of current ideas

Abstract The current understanding of the dynamics of secondary cyclogenesis on frontal systems in the North Atlantic region is reviewed. These secondary cyclones are often poorly forecast, and in extreme cases can have damaging consequences, but because of their small scales and potential for rapid growth detailed observations and analyses are limited: in this paper recent work is described and compared. It is argued that, on the synoptic scale, the development of ‘primary’ baroclinic waves is well understood, but on the subscales of these systems, where the secondaries develop, there is a wider variety of mechanisms which may cause or modify cyclone growth. This variety may be reflected in a wider range of resulting systems. Various dynamical models have been postulated: secondary frontal waves are seen as the instability of a low‐level potential‐vorticity strip or warm band at a front, with possible finite amplitude triggering by upper‐level features. Latent heating is thought to provide a suitable unstable potential‐vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent. Secondary systems are often shallow, so boundary‐layer processes are important, but the influence of these is not well understood; laboratory experiments have suggested that the boundary layer will act to suppress instability. In general, frontogenetic strain flow, while acting to intensify the basic‐state front, suppresses wave development—in contrast, recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves. The FASTEX experiment, an international field study of North Atlantic secondary cyclogensis, has recently attempted to observe these systems in detail. It is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field.