Compilation of Summaries 
22 
System Nordsee 
spring would have been normal for the respective other season. Together with frequent 
SE types in the FMA quarter, this led to a normalisation of temperatures which had 
been too high in January due to frequent storms (NW: 13 days; P 95 ). The relatively cold 
midsummer period, which was attributable to frequent C and NW weather types, was 
followed by a mild Indian summer with A types (25 days) and SW types (22 days) pre 
vailing in September and October. As a consequence, the North Sea temperature was 
stagnant at 15 °C from July through September and seasonal cooling set in much later 
than usual. 
Atmospheric pressure distribution (p. 51 sqs.) 
In the analysis of seasonal atmospheric pressure fields, strong departures from clima 
tological distributions (1971 -2000) in terms of patterns and intensities were found 
only in the winter season. The normalcy of the other seasonal distributions resulted 
from the mutual cancellation of opposite, in some cases major, anomalies on monthly 
time scales. The dominant main weather type was usually reflected in the pressure 
distribution or pressure anomaly distribution. The diffuse pressure distributions in Feb 
ruary and March, which resulted from a quasi-uniform distribution of weather types, led 
to anomaly fields (NE) that were inverse to climatological data (SW). Conversely, pres 
sure distributions in April (AS) and May (W) displayed themselves almost unchanged 
in the anomalies because climatological pressure distributions are extremely diffuse in 
these months. Atmospheric circulation in the North Sea sector throughout winter and 
autumn 2005 was roughly but consistently captured by the NAO index. 
North Sea Wind (p. 58 sqs.) 
As the North Sea is geographically situated on the unsteady border between Icelandic 
Low and Azores High, hybrid and rotational circulation patterns tend to average out on 
monthly and longer time scales. As a consequence, the classification of corresponding 
atmospheric pressure distributions is controlled by the wind index rather than the vor- 
ticity index. The >North Sea Winds which is identical to the wind index except for a con 
stant factor, thus is a good quantitative approximation of the atmospheric pressure gra 
dient in the North Sea region. 
The prevalence of westerly winds is reflected in their relative frequency of 72 %, with 
winds from the NW and SW sectors accounting for 36 % each over the year. These 
typical conditions, also on seasonal time scales, cause mutual cancellation of the me 
ridional components (N/S), dominance of the zonal wind component (W), and thus 
clearly reduced vector wind speeds (V) as compared to scalar wind velocities (Vel). 
Directional stability (V/Vel) as a yearly average was 41 %, with seasonal variations be 
tween 35 % in spring and 51 % in summer. In »golden« autumn, however, there was 
a heavy SW:NW predominance of 44 versus 28 %. Moreover, winds from the SE sec 
tor reached an extremum of 20 % during this season. At 2.4 m/s, the meridional com 
ponent of the >North Sea Wind< clearly differed from zero, indicating massive advection 
of warm air from the south. 
Seasonal shifts in the influence of the Icelandic Low and Azores High were apparent 
from the virtual climatological trajectory of >North Sea Wind<. Uniformly strong WSW 
winds from October through March are typical of the cold season, which abruptly gives 
way to a period of stagnation in April and May when winds are changeable and may 
blow from any direction (»April does as it will«, German proverb). With progressing