An intercomparison experiment involving 15 commonly used detection and tracking algorithms for extratropical cyclones reveals those cyclone characteristics that are robust between different schemes and those that differ markedly.

The Mediterranean storm track constitutes a well-defined branch of the North Hemisphere storm track and is characterised by small but intense features and frequent cyclogenesis. The goal of this study is to assess the level of consensus among cyclone detection and tracking methods (CDTMs), to identify robust features and to explore sources of disagreement. A set of 14 CDTMs has been applied for computing the climatology of cyclones crossing the Mediterranean region using the ERA-Interim dataset for the period 1979Á2008 as common testbed. Results show large differences in actual cyclone numbers identified by different methods, but a good level of consensus on the interpretation of results regarding location, annual cycle and trends of cyclone tracks. Cyclogenesis areas such as the northwestern Mediterranean, North Africa, north shore of the Levantine basin, as well as the seasonality of their maxima are robust features on which methods show a substantial agreement. Differences among methods are greatly reduced if cyclone numbers are transformed to a dimensionless index, which, in spite of disagreement on mean values and interannual variances of cyclone numbers, reveals a consensus on variability, sign and significance of trends. Further, excluding 'weak' and 'slow' cyclones from the computation of cyclone statistics improves the agreement among CDTMs. Results show significant negative trends of cyclone frequency in spring and positive trends in summer, whose contrasting effects compensate each other at annual scale, so that there is no significant long-term trend in total cyclone numbers in the Mediterranean basin in the 1979Á2008 period.

The link between winter (December-January-\ud February) precipitation events at 15 Mediterranean coastal\ud locations and synoptic features (cyclones and Northern\ud Hemisphere teleconnection patterns) is analyzed. A list of\ud precipitation events has been produced; q percentile thresholds\ud (Thq ) and corresponding frequency Nq (for q equal to\ud 25, 50, 90 and 98) have been considered. A negative trend\ud has been detected in total precipitation and N50 at many locations,\ud while no significant trend in N25 , N90 and N98 has\ud been found. The negative phase of the North Atlantic Oscillation\ud (NAO) and the East Atlantic/West Russia pattern\ud (EAWR) compete for exerting the largest influence on the\ud frequency of the 25th, 50th and 90th percentiles, with EAWR\ud and NAO exerting their largest influence in the central and\ud western Mediterranean areas, respectively. All percentiles\ud show a similar behavior except for the 98th percentile, which\ud shows no convincing link to any teleconnection pattern. The\ud cyclone tracks that are associated with precipitation events\ud have been selected using the ERA-40 reanalysis data, and\ud a strong link between intense precipitation and cyclones is\ud shown for all stations. In general, the probability of detecting\ud a cyclone within a distance of 20 from each station increases\ud with the intensity of the precipitation event and decreases\ud with the duration of a dry period. The origin and track\ud of cyclones producing intense precipitation differ among different\ud areas. When precipitation occurs in the northwestern\ud Mediterranean, cyclones are generally either of Atlantic origin\ud or secondary cyclones associated with the passage of major\ud cyclones north of the Mediterranean Basin, while they\ud are mostly generated inside the region itself for events at\ud the eastern Mediterranean coast. An important fraction of intense events in the southern areas is produced by cyclones\ud that are generated over northern Africa. The analysis of sea\ud level pressure and geopotential height at 500 hPa highlights\ud the important role of cyclone depth, circulation strength, surrounding\ud synoptic condition, and of slow speed of the cyclone\ud center for producing intense precipitation events

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 transient AOGCM simulation 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.