Physical Oceanography 
System Nordsee 
23 
spring season, the influence of the Azores High eventually prevails. Weak WNW winds 
at its northern edge from early June characterize typical North Sea summer weather. 
Virtual net displacement in 2005 corresponded to a mean vector wind of 3.5 m/s from 
W (264°), which hardly differed from the climatological vector wind (3.7 m/s, 256°) and 
occupied a medium rank (16) in the 35-year ensemble (rank 1:1.8 m/s (1996); rank 35: 
5.7 m/s (1990)). Scalar wind velocity, at 8.6 m/s, only ranked 9th among values rang 
ing from 7.6 m/s (2003) to 10.0 m/s (1990). The strongest departure from the climato 
logical trajectory occurred in winter. The long period of stormy weather in January, 
which showed in a very high (12.0 m/s) and directionally stable (77 %) vector wind, 
was followed by highly variable wind conditions from February (1.5 m/s, 14 %) through 
April. A strikingly stable phase of SSW winds occurred in October (6.1 m/s, 70 %), 
which extended to the neighbouring months and lasted about 50 days. 
Storm (p. 64 sqs.) 
The development of storm frequency in the period from 1970 to 2005 was investigated 
on the basis of daily atmospheric-pressure fields. The linear dependence of storm fre 
quency in the DJF quarter on the winter NAO index (DJF) was very strong between 
1974 and 1989 (80 % variance), but has been markedly weaker since then (53 %). The 
core of the storm season (October - March, 02M) shifted from December to January 
in the early 1980s, and to February in 1997. The storm frequency in autumn (OND) 
showed a significant linear decrease by 8 storm days since 1970. The storm frequency 
in winter (JFM) increased by 7.1 days per decade until the early 1990s; since then, it 
has returned to the level of the 1970s at a rate of - 9 days per decade. The frequency 
of storms during the full storm season (02M) varied between 54 (1989/90) and 3 days 
(2002/03). Its long-term development grossly resembled that in winter. The relative 
proportion of severe storms in total storm frequency did not show a linear trend in any 
of the seasons (OND, JFM, 02M) but vacillated about an average level of 16 %. 
Air temperature and radiation (p. 71 sqs.) 
The annual cycle of global radiation on the island of Norderney in 2005 featured sur 
plus radiation in April, June, September, and October due to frequent high-pressure 
weather. This surplus compared with a significant deficit in midsummer (July / August) 
that was caused by changeable low-pressure weather. Parallel monthly anomalies 
were observed in the annual cycle of air temperature. However, seasonal tempera 
tures were at normal levels except in autumn (SON), which, at 12.2 °C, was the warm 
est on record (1971 -2005). 
Physical Oceanography 
Currents (p. 81 sqs.) 
Atmospheric circulation in the North Sea region imprints itself on the patterns and in 
tensities of surface currents. Prevailing SW - NW winds generate a large-scale cyclon 
ic (counter-clockwise) rotation pattern that is characteristic of current fields in the cold 
season and in the long-term mean. In 2005, this cyclonic pattern was rather well pro 
nounced in autumn but only faintly identifiable in winter because the forming impact of 
transient depressions in January was largely compensated by highly variable and