Summary 
22 
Nordseezustand 2004 
in the central North Sea, and wave heights exceeding 2 m occurred only half as often. 
Maximum wave heights were always lower in the German Bight, except for winds and 
wind seas from NW. 
Cuxhaven, located at the mouth of the Elbe River which opens toward NW, has record 
ed highest water levels and most severe storm surges in connection with NW winds, 
and lowest levels with winds from the opposite direction (SE). The unusually high fre 
quency of NW weather types in 2004 was reflected in above-average water levels 
through large parts of the year. NW storms also accounted for the three storm surges 
in the year under review, with the highest water level of 3.54 m above MSL recorded 
on November 18 during storm >Pia<. 
The temporal evolution of high and low water levels since 1971 shows that exceedingly 
high water levels occurred almost exclusively in autumn prior to 1988, but preferably 
during the winter months since then. This regime shift came about at the same time as 
the regime shift in North Sea SST, both of which relate to changes in atmospheric cir 
culation (»mishmash climate«). 
The annual mean North Sea SST (derived from BSH's weekly geographical SST anal 
yses) was just 0.2 К below the record temperature of 11 °C set in 2002; 2004 was the 
fourth warmest year on record dating back to 1969. With a record temperature of 
9.1 °C in December 2003, winter turned out considerably too warm from the start. As 
a consequence, sea ice failed to form on the German North Sea coast. The gradual 
return to close to normal temperatures until July (0.3 K) was followed by a steep tem 
perature rise of 2.1 К above normal (15 °C) in August and a significantly too warm fall. 
During the North Sea survey performed by BSH in August 2004, a stable thermal strat 
ification together with a relatively deep thermocline were found. Deep-water tempera 
tures also exceeded long-term means on account of both, the preceding mild winter 
and a strong northerly inflow of warm Atlantic water. In the period 1988 - 2004, North 
Sea surface temperatures in 78 % of all months were higher than normal, while 70 % 
of the winters were free of sea ice. 
The warm regime, which ended a 10-year cold regime in fall 1987, has continued to this 
day. The regime character of North Sea SST is linked to a hybrid wind climate whose 
properties undergo monsoon-type seasonal shifts, which in turn are subject to spon 
taneous reversals at intervals of 5 to 15 years. The recent warm regime (preceding 
cold regime) is characterized by the alternation of enhanced frequencies of maritime 
westerly winds in winter (summer) and continental easterly winds in summer (winter); 
for brevity, these bi-stable, hybrid climatic conditions were termed »mishmash climate«. 
Further evidence for these findings is seen in corresponding differences in seasonal 
distributions of atmospheric sea level pressure over the North Sea. 
The last sudden phase shift in the wind regime in 1988 was accompanied by a spon 
taneous increase in mean SST of 0.9 K, more frequent ice-free winters, a complete ab 
sence of severe ice winters (except in 1996), higher salinities, and a seasonal shift in 
the occurrence of wind-induced high sea levels. Moreover, as was shown elsewhere, 
the North Sea ecosystem underwent dramatic changes at all trophic levels (plankton, 
fish, benthos) at the same time. 
The observed increase in North Sea salinity is causally related to persistently high sa 
linities in the Northeast Atlantic, the enhanced frequency of NW weather types, and 
the pronounced cyclonicity of the current field. The relative area of the North Sea with 
near-bottom salinities exceeding 35 assumed a record extent of 53 % in 2004. Salinity