Nordseezustand 2004 
19 
Summary 
The North Sea is a complex dynamic system whose state is changing subject to both, 
natural processes and environmental pollution due to a multitude of industrial activities 
in the densely populated riparian states. Contaminants and nutrients that enter the 
system through river discharge and atmospheric deposition are well-known to ad 
versely effect its ecological state. Meanwhile, evidence has grown, however, that an 
thropogenic activities are interfering with the intensity and course of natural physical 
processes as well. 
For example, analyses of sea surface temperature observations, which locally date 
back to the 19 th century, revealed that cold and warm periods of 5 - 15 years in length 
have been alternating since at least the past 130 years in the North Sea region. These 
natural regimes and abrupt regime shifts of temperature together with the underlying 
changes in atmospheric circulation have been termed »mishmash climate«. The cur 
rent warm regime that jumped into existence 1988 is the longest and warmest period 
on record since the beginning of measurements in 1873. There is no serious denying 
now of the impact of anthropogenic climate warming. 
Not only temperatures and temperature-dependent variables of the North Sea are af 
fected by climate change but ultimately all system variables coupled immediately or 
obliquely to the state of the atmosphere. Among the variables investigated more close 
ly in this report are water level and sea ice. However, regime shifts and climate change 
are also reflected in phytoplankton biomass, community structure, and fish migration. 
»Maintenance and repair« of technical systems such as motorcycles or biological sys 
tems, among them humans, are best performed by mechanics or physicians distin 
guished by their holistic understanding of the system at hand, in particular its function 
ality and the entanglement of interactive processes. The joint presentation of the 
atmospheric, oceanographic, and chemical state attempts to integrate the results from 
these disciplines and thus aims at a better, holistic understanding of the North Sea sys 
tem. Such understanding is a key prerequisite for sustainable management and imple 
mentation of effective measures to protect the marine environment. 
The main findings as to the state of the North Sea in 2004 are summarized below. 
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. 
The difference in atmospheric pressure between the Azores and Iceland is a simple 
measure of the strength of the zonal circulation over the North Atlantic. While, on av 
erage, pressure decreases from south (Azores High) to north (Icelandic Low) such that 
westerly winds prevail, short-term pressure gradients in the opposite direction are pos 
sible as well (easterly winds). This pressure see-saw is known as North Atlantic Oscil 
lation (NAO), while the index characterizing the state of the oscillation is called NAO 
index. In accord with the state of the NAO, enhanced (+) and weakened (-) zonal cir