Preliminary findings 
With the exception of statements concerning the area averaged North Sea sea surface 
temperature (SST), all assessments are basing on CTD raw data collected during the cruise. 
All temperature and salinity data will be processed and — if necessary — re-calibrated after 
the cruise. 
In 2015 the monthly means of the area averaged North Sea SST started with positive 
anomalies of 0.9 K in January and 0.8 K March (reference period 1971-1993). Then the SST 
was dropping with negative anomalies of -0.3 and -0.2 in June and July. During August there 
was a strong heating due to increasing solar radiation, causing again a positive anomaly of 
0.4 K and a monthly SST mean of 15.4 °C. © 
SST is a reliable representative for the temperature of the seasonal mixed layer. Due to 
increasing solar radiation the North Sea established a seasonal stratification during spring 
over wide areas of the North Sea which last normally until end of August or beginning of 
September. Then the water column will be vertically mixed again by the first fall storms. The 
upper layer is separated from the colder bottom layer by a sharp thermocline with vertical 
gradients of the order of 3 K/m. While the oceanographic conditions in the upper layer are 
mainly determined by local radiation, the conditions in the bottom layer are influenced by the 
inflow of Atlantic Water (AW) with salinities >35 psu via the northern open boundary to the 
Atlantic and to a lesser degree via the English Channel. Only the knowledge of the 
hydrographic condition in both layers, determined by the spatial distribution of temperature 
and salinity, allows the calculation of heat and salt budgets. 
The spatial structure of temperature distribution (see Figs. 4, 6, 8 and 9) corresponds largely 
to that of the reference period (RP) 2000-2010 with upper mixed layer depths between 20 
and 30 m and the shallow south-eastern part of the North Sea vertically mixed. However, in 
2015 the mixed layer temperatures of the 54° and 55°N sections correspond to the PR, while 
the northern section are 1-2 K cooler than the RP. 
Atlantic Water >34 psu intrudes from the northern boundary (see Figs. 5, 7, 8 and 9) 
southward into the North Sea. The salinity sections along 58°, 59°, and 60°N show clearly 
the three main paths marked by the 35.25 isohalines: The western path through the Fair Isle 
Channel, the central path over the East Shetland Shelf and the eastern path at the western 
slope of the Norwegian Trench at depths between 250 and 450m. The tongue of Atlantic 
Water is traceable southward down to 56°N, 3°E reaching the north-westerly boundary of the 
German EEZ. 
Concerning SST the summer 2015 was close to the climatological mean, this holds also for 
the volume temperature of the southern North Sea. The calculation of the total heat and salt 
contents will be possible not before the re-processing of the data at BSH. 
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