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 - 
‚emperature and salinity data will be re-calibrated prior to their final processing and analysis. 
SST 
SST is a reliable representative for of the seasonal mixed layer temperature. Due to 
increasing solar radiation a seasonal stratification is established 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 depths 
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 
Dottom layer are influenced by the inflow of Atlantic Water (AW) with salinities greater 35 
osu“ 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 
Dudgets. 
The area-averaged North Sea SSTs started in 2018 with moderate positive anomalies (0.4 K, 
See Table 2) and dropped due to unusually intense cooling to -0.9 K in March. This drop was 
followed by an extremely steep rise of 11.4 K from March to July, which has never been 
observed before (Fig. 4). At 16.3 °C (2 K), the monthly mean SST in July ranked 2"* warmest 
in the past 50 years, falling short of the record high of July 2014 by just 0.1 K. SST averaged 
17.0 °C in August and remained at 2.0 K above normal. During the survey the North Sea had 
an average SST of 15.95 °C. 
'"Yeeklv North Sea SST 1998 - 20°. 
18- 
— 1998-2015 
-2016 
—22017 
2018 
— 14.0 
Ö 
— 120 
E12 
N 
% 40.0 
8.0 
a 
MO MOB 
" °719 21 7325 27 
73 35 °7 17 41 43 45 AT 49 *“ 
week (Wednesday - Tuesday) 
Fig. 4: Weekly area averaged North Sea SST 1998 — 11.09.2018 
? pSsu = practical salinity units 
29