The genesis of mid-latitude cyclones over the Atlantic ocean: a new climatological perspective.

The homogeneous reanalysis by the European Centre for Medium-Range Weather Forecast of all the weather data collected between 1979 and 1993 is employe d to study the initiation of North-Atlantic mid-latitude cyclones. The appe arance of new individual vorticity maxima is detected automatically. The 12 h period surrounding the detection of a new maximum is considered, here, t o highlight the conditions leading to the birth of a new cyclone. Nearly 6 000 genesis events are distributed in 12 classes. The climatological realit y departs significantly from the idealized situations studied so far in the theoretical literature. Among other things, the results show that the gene sis of a new low and its subsequent development depend on different mechani sms and are quite independent from one another. (C) 2000 Academie des scien ces / Editions scientifiques et medicales Elsevier SAS.