System Nordsee 
21 
Compilation of Summaries 
This compilation of summaries is to make the main findings of the >North Sea Systerm 
accessible to a broader readership. As to aid following these findings, all figures and 
tables of the main sections come with English annotations and captions. 
Atmospheric Physics 
The atmosphere is the motor, which substantially drives and controls the development 
of the oceanographic state of the North Sea. Characteristics and anomalies of the at 
mospheric circulation often mark the beginning of cause-effect chains extending from 
oceanographic state variables beyond distribution patterns of pollutants and nutrients 
down to biological anomalies. 
North Atlantic Oscillation (p. 37 sqs.) 
The North Atlantic Oscillation (NAO) is part of a global oscillation system which criti 
cally shapes weather and climate conditions. The NAO index measures the phase 
state of the seesaw of atmospheric pressure between Iceland and the Azores, and 
thus is indicative of the strength and direction of zonal atmospheric flow in the North 
Atlantic sector. In the North Sea region, zonal winds usually intensify or weaken in uni 
son with the NAO index in the winter half year, only. Contrasting with the frequent neu 
tral modes that have been observed since 1997, the winter NAO index (DJF quarter 
2004/5) pointed to a pronounced positive mode (2.03) and intense westerly circulation. 
Due to a sharp drop in the NAO index between December (3.3) and March (- 3.7), air 
temperatures in the North Sea region were clearly too mild until the end of January, 
after which they tended to be slightly too low. The high-amplitude oscillation of 2005, 
which was of similar strength as that of 2002, displayed a spacious elliptical trajectory 
in the phase space of dynamic NAO states since 1879. 
Weather types (p. 41 sqs.) 
The calendar of weather types for the North Sea region, which is derived from daily 
sea-level atmospheric pressure fields, is a useful tool in interpreting anomalies and 
special events related directly or indirectly to atmospheric circulation. As to the distri 
bution of weather types, anticyclonic conditions A (17 %), followed by C, NW, W and 
SW types with relative frequencies of about 10 %, formed the strongest fractions 
among the 27 types of circulation. The rare E type, which in 2004 occurred only once, 
showed the strongest relative increase (1300 %). The frequency of storm events rose 
from 20 days in 2004 to 26 days this year but remained clearly below normal because 
of extremely few autumn storms (5). As many as 15 storm days were counted during 
the 3-week rapid development phase at the beginning of the year. With 7 severe 
storms, the proportion of such events was unusually high. 
Robust weather type statistics on monthly and seasonal time scales were performed 
on the reclassified set of main types A, C, NE, SE, SW, and NW by evaluating current 
frequencies in the context of empirical distributions from the period 1971 - 2000. Over 
the year, apart from a moderate deficit/surplus of A types (86 days; Pi 7 ) and SW 
types (93; P 78 ), only the significant increase in NW types (84; P 97 ) is remarkable. The 
record number of NE types in winter (10) and the scarcity of this weather type (3) in