20 
Compilation of Summaries 
tion. The typical wave distribution patterns are determined by the prevailing 
wind conditions and by the geographic/bathymetric shape of the North Sea. In 
winter and autumn 2008 and in autumn 2011 the mean wave height in the 
northern part of the North Sea was significantly higher compared to the refer 
ence period 2000-2011. For the rest of the period 2008-2011 the mean wave 
height corresponded well to the long term average except for the whole year 
2010 when it was generally lower. In all seasons, the main wind and wind sea 
directions were between SW and NW. In contrary easterly directions were 
dominating in winter 2009/10 and autumn 2010. Extraordinarily high waves did 
not occur in the southern North Sea during the four-year period. The strongest 
storms and wave heights appeared in the border area to the North Atlantic. 
Water Levels (Page Seite 108 et seq.) 
The wave climate during the period 2008-2011 was characterized by seasonal 
geographic distribution of significant wave height, wind sea- and swell direc 
tion. The typical wave distribution patterns are determined by the prevailing 
wind conditions and by the geographic/bathymetric shape of the North Sea. In 
winter and autumn 2008 and in autumn 2011 the mean wave height in the 
northern part of the North Sea was significantly higher compared to the refer 
ence period 2000-2011. For the rest of the period 2008-2011 the mean wave 
height corresponded well to the long term average except for the whole year 
2010 when it was generally lower. In all seasons, the main wind and wind sea 
directions were between SW and NW. In contrary easterly directions were 
dominating in winter 2009/10 and autumn 2010. Extraordinarily high waves did 
not occur in the southern North Sea during the four-year period. The strongest 
storms and wave heights appeared in the border area to the North Atlantic. 
Temperature (Page Seite 112 et seq.) 
The thermoclines in the zonal sections for the 2008-2011 summer surveys 
demonstrate the considerable interannual variability of thermal stratification. In 
2008, vertical sections in the north-eastern North Sea show a strong vertical 
gradient which declines apparently towards the west, so that no thermoclines 
are formed in these areas. At the surface, all sections exhibit a horizontal 
gradient with temperatures decreasing from east to west. Due to tidal friction 
and strong winds the 54° N section, the Dogger Bank area, and eastern part of 
the 55° N section are largely vertically mixed. The total North Sea heat content 
deviated by 0.6 standard deviations from the 10-year average. 
In 2009, the vertically mixed areas off the southern and Danish coasts were 
considerably warmer than in 2008 at almost 20 °C. The depth of the upper 
mixed layer in the central North Sea and Skagerrak was over 40 m with maxi 
mum vertical gradients of over 3 K/m. Northwards of 57° N the gradient strength 
of the thermocline declined reaching further north. The thermocline depression 
over the Norwegian Deep at approximately 7° E and 58° N, observed almost 
every year, reached a maximum depth of 80 m in 2009. The total heat content 
exceeded the 10-year average by 1.4 standard deviations. 
In 2010, the vertically mixed ribbon of warmer water parallel to the continental 
coast was approximately 1 K cooler than during the previous year. In the strati-