For Northern Hemisphere extra-tropical cyclone activity, the dependency of a potential anthropogenic climate
change signal on the identification method applied is analysed. This study investigates the impact of the used
algorithm on the changing signal, not the robustness of the climate change signal itself. Using one single
transientAOGCMsimulation as standard input for eleven state-of-the-art identification methods, the patterns of
model simulated present day climatologies are found to be close to those computed from re-analysis,
independent of the method applied. Although differences in the total number of cyclones identified exist, the
climate change signals (IPCC SRES A1B) in the model run considered are largely similar between methods for
all cyclones. Taking into account all tracks, decreasing numbers are found in the Mediterranean, the Arctic in the
Barents and Greenland Seas, the mid-latitude Pacific and North America. Changing patterns are even more
similar, if only the most severe systems are considered: the methods reveal a coherent statistically significant
increase in frequency over the eastern North Atlantic and North Pacific.We found that the differences between
the methods considered are largely due to the different role of weaker systems in the specific methods.
