The extratropical transition (ET) of tropical cyclones in the North Atlantic basin is explored using the framework of the Cyclone Phase Space (CPS). Analyses from the U. S. Navy Operational Global Atmospheric Prediction System (NOGAPS) and National Centers for Environmental Prediction Global Forecast System (GFS) are used to diagnose CPS locations of 19 tropical cyclones undergoing ET (determined from National Hurricane Center best track data) in the years 1998-2002. A non-hierarchical cluster analysis technique, the k-means method, is employed to determine physically meaningful clusters of cyclone structure in this CPS dataset. Results indicate that cluster analysis successfully partitions observations into meaningful subsets representing distinct cyclone and environment structures. It is also found that while current operational classification schemes are successful at defining unique portions of the CPS, cluster analysis assists in defining a “grey area” of transitioning tropical cyclones, where current operational definitions overlap.

A mean path through the CPS of tropical cyclones undergoing ET is proposed and explored using a synoptic climatology developed from NOGAPS-derived clusters. This climatology highlights distinct stages of ET, beginning with a tropical cyclone isolated from midlatitude influences to its north and ending with an extratropical cyclone, deeply embedded in the midlatitude westerlies. Similar cluster solutions are derived using data from both the NOGAPS and GFS models suggesting that the clusters found in this analysis are not model specific and indicate natural stages of the ET process. However, the differences that do exist between these solutions suggest the use of an observational approach to defining cyclone phase.