Preliminary findings 
With the exception of statements concerning the area averaged North Sea sea surface 
temperature (SST), the following assessments are based on CTD raw data collected during 
the cruise. There will be a second check of the CTD data after the cruise and - if necessary - 
temperature and salinity data will be re-calibrated before the final processing and analysis of 
:he data. 
SST 
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 lasts normally until end of August or beginning of 
September. Then the water column will be vertically mixed again by the first fall storms. At 
water depth greater than about 30 m the upper layer is separated from the colder bottom 
layer by a sharp thermocline with vertical gradients in the order of up to 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 greater 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 conditions in both 
layers, determined by the spatial distribution of temperature and salinity, allows the 
calculation of heat and salt budgets. 
During the first eight months of 2017 the monthly anomalies of the area averaged North Sea 
SST varied between 0.7 and 1.0 K compared to the reference period 1971-1993. At 16 °C 
the North Sea SST reached its seasonal Maximum in the beginning of August and virtually 
remained at this level through the first week of September (see red line in Fig. 5). Hence, the 
current seasonal cycle established - like in 2016 - a plateau-shaped maximum rather than a 
distinguished peak. That confirms that the complete survey took place during the seasonal 
heat maximum of the North Sea which can be expected in the bottom layer about 4 weeks 
after reaching its maximum in the surface layer. 
20.0 
1 
18.0 + 
16.0 
1998-2014 
2015 
2016 
2017 
— 14.0 + 
o 
sn 
12.0 
— 
[0] 
% 10.0 
8.0 
PT na U 
"43.16 107 16741 2377 2779 3175 25 37 39 41 43 45 47 ı 
week (Wednesday - Tuesday) 
Fig. 5: Weekly area averaged North Sea SST January from 1998 until end of September 
2017. 
Temperature 
While the temperature sections along 54° and 55° N are vertically mixed due to low water 
depth in their eastern parts, the zonal (east-west) temperature sections between 56° and 60° 
N show a massive homogeneous mixed surface layer with a strong thermocline at about 30 
? psu = practical salinity units 
3