Summary 
20 
Nordseezustand 2004 
culation states occurred across the North Sea in September (+), October (-), and De 
cember (+). Extreme NAO states in April (+) and June (-), however, took place concur 
rently with opposing circulation anomalies in the North Sea region. The correspon 
dence among the states of zonal circulation in both regions is statistical in nature and 
is limited to the winter half-year. The winter NAO index (DJF quarter), at -0.6, was 
weakly negative, pointing to a relatively strong meandering of the circulation. 
The calendar of weather types for the North Sea region, which was derived from daily 
atmospheric sea level pressure fields, has proved useful a tool in the interpretation of 
anomalies and special events in variables (in)directly related to atmospheric circula 
tion. As to the very weather types, it is worth noting that anticyclonic conditions A, 
which form the strongest fraction among the 27 different types, occurred much less fre 
quently than in the preceding year (19% vs. 27%). With 15 % the proportion of circu 
lation types NW, ANW, and CNW has about doubled since 2002. The frequency of 
storm events went up from 16 days in 2003 to 20 days. Severe and very severe storms 
did not occur, however. 
Robust weather type statistics on monthly and seasonal time scales were performed 
on only the main types A, C (low pressure weather), NE, SE, SW, and NW, and were 
evaluated on the basis of empirical distributions. Over the year, only NW and NE types 
were unusually frequent (80 days) or rare (13 days), respectively, while on shorter time 
scales significant departures from normal also occurred with other circulation types. 
Particularly remarkable are alternations between SE and NW anomalies, some of 
them extreme, in April / May and from August (SE) through November (NW). While in 
the relative cold winter 2003 (JFM) A types had been dominant at 46 days, strong cy 
clonic activity in January and February 2004 led to considerably milder air tempera 
tures. 
The analysis of seasonal atmospheric pressure fields revealed only minor departures 
from climatological distributions (1971 - 2000) in terms of patterns and intensities. At 
mospheric circulation on these time scales may thus be rated »normal«. This normal 
cy, however, resulted from mutual cancellation of complementary, in part, substantial 
anomalies on monthly time scales. The monthly anomaly fields had three basic recur 
rent patterns, viz. N (2 = Feb, 5, 11), W (6, 9, 12), and SE (1,4, 8, 10). The majority of 
these affected the meridional component of the wind fields, indicating an enhanced ex 
change of thermal energy between higher and lower latitudes through the North Sea 
sector. 
Seasonal shifts in the influence of the Icelandic Low and Azores High were apparent 
from the climatological trajectory of the 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). In the course of spring, the 
influence of the Azores High eventually prevails. Weak WNW winds at its northern 
edge from early June characterize typical North Sea summer weather. 
In 2004, the trajectory of the North Sea wind undulated about the climatological path. 
The crossings were usually linked to monthly alternating anomalies of the meridional 
wind component, which tended to cancel out one another. The total distance along the 
current trajectory was covered at an average velocity of 8.4 m/s, which exceeded the 
annual mean velocity of 2003 (7.6 m/s) but clearly fell short of the climatological wind 
speed (9 m/s). The net displacement in 2004 corresponded to a mean vector wind of