This study attempts to depict east Atlantic cyclones specificity using ECMWF 1984–1994 cold season analyses. Due to the strong low frequency variability in this area, a climatology of transient events has to take the “background” weather regime into account. North Atlantic weather regimes have therefore been recovered through cluster analysis techniques. An objective criterion defining the occurrence of subsynoptic cyclones is then designed. It uses the high frequency signature of vorticity. The events thus retained include frontal waves, cold air cyclogenesis and other small scale developments. The preferred location of these events is found to be quite distinct from the classical storm‐tracks. They appear downstream and slightly south of them, reaching the north‐western part of Europe during Zonal weather regimes, and a zone centred some 500 km north of the Azores during Greenland Anticyclone regimes. In order to gain some idea of the various types of subsynoptic cyclones captured by the criterion, a one‐point selection of cases was performed and the resulting vorticity fields were partitioned by a cluster analysis. Two types of waves out of three clusters are documented. A Type 1 is shown to be a modified baroclinic development, with reduced scale and features well accounted for by recent theoretical work on frontal waves. A Type 2 composite, with its typical warm front, is more difficult to set into an existing theoretical framework. It seems connected to the presence of an actively diffluent background flow.

A series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the field phase of the Fronts and Atlantic Storm-Track Experiment (FASTEX). The tracking enables the various cases to be set into a common frame of reference that moves with the system. Time-filtering is used to further separate the events into a basic-state and perturbations. Information is obtained on environmental properties such as strain or baroclinicity.Combining these diagnostics, it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases, but do not seem to characterize specific types nor to determine a systematic behaviour.On the other hand, it is shown that rapid-deepening phases leading Eastern Atlantic systems to their maximum amplitude happen suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper-level feature. For cold-air systems, development results from meeting a diffluent zone. This is unlike the American east-coast systems that are associated with jet exidentrance complex.Another remarkable feature is the generation of a frontal wave as a result of the parent vorticity maximum being apparently stretched and split. This has occurred in 25% of cases.A number of other properties of the FASTEX cases are shown, such as several Occurrences of large phase changes in sharp contrast to the idea of phase locking.

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