Observed variability of North Atlantic oceanic precipitating systems during winter



[1] The variability of large precipitating weather systems as observed from polar satellites over the North Atlantic Ocean is investigated using a statistical analysis. Nine winters (from 1987 to 1995) have been considered. Cloud systems and troughs over the area 40–60°N, 70–10°W are automatically detected with retrievals of the cloud top pressure, a precipitation index, and the temperature of the lower stratosphere. A classification of the largest precipitating systems as characterized by these variables leads to eight classes, whose occurrence significantly differs over the years. It also shows the systematic presence of a trough upstream of the precipitating area covered by high-level clouds, as expected in the case of baroclinic interaction. In order to understand the large interannual variability an attempt to identify systematic differences in cyclone structures during different flow regimes is then performed. It shows that the large-scale (typically 3000 km wide) and zonally elongated cloud systems are observed mainly to the south of 45°N when the phase of the Arctic Oscillation (AO) is negative (which also corresponds to the Greenland Anticyclone regime). Conversely, zonally elongated cloud systems of smaller scale (1000 km wide) associated with frontal waves are favored also to the south but during the positive AO phase (particularly during zonal regimes). The most tilted weather systems, without any preferential AO phase, are found mostly during the blocking regime. These systems are cyclonically tilted in the northeastern branch of the storm track and anticyclonically tilted in the southwestern branch. It must be noted that similar results are obtained when the AO daily index is replaced by the North Atlantic Oscillation index. This analysis gives observational evidence of previous idealized simulations linking the large-scale circulation to preferential life cycles of weather systems. In addition, a weak response to extreme El Niño Southern Oscillation events has been observed in the location of cyclones but not in their average structure.