Clustering of cyclones in the ARPEGE general circulation model

Baroclinic waves give major contributions to the climatology and variability of key climate variables in mid- and highlatitudes. Animportant aspect of cyclone variability is the seriality (succession of cyclone occurrence). Serial cyclones are clustered in time and are associated with large economic losses in Europe. Cyclone variability and seriality are intimately linked to the low frequency variability. To have a realistic representation of high and mid-latitude regional climate in general circulation models (GCM), it is of vital importance that the described phenomena are realistically represented. In the present paper we evaluate the ability of the atmospheric GCM ARPEGE to represent various storm track parameters, both with respect to climatology and variability. Cyclone tracks are identified for the Northern Hemisphere during 55 extended winters (1950/51–2004/05) using the NCEP reanalyses by means of an objective tracking method. A corresponding cyclone track data set is derived from a simulation with ARPEGE, forced with observed monthly SSTs from 1950 to 2004. The comparison of the two data sets shows that ARPEGE reproduces the main features of the observed cyclone track fields. The mid-latitude corridors of cyclones and cyclone variability are well reproduced. The model, however, underestimates the mean number of cyclone occurrence and its variance. In the eastern North Atlantic and Nordic Seas the model simulates 70 and 90% of the observed mean cyclone occurrence and its variance, respectively. ARPEGE also substantially underestimates the clustering of cyclones in the eastern North Atlantic (jet exit region). In addition, the model also fails to simulate the relationship between clustering and low-frequency flow patterns at the exit of the North Atlantic storm track.