North Atlantic ultra high frequency variability: An introductory survey

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.