Preliminary findings 
The following discussion of scientific results from the North Sea Summer Survey (NSSS) is 
based on CTD data from the real-time data processing, carried out during the survey (see 
‚Equipment and methods”, page 6). 
A delayed-mode processing, including an inspection of discrepancies between the CTD- 
measurements and water sample analyses, will follow. The final, possibly corrected CTD- 
data and a documentation of the delayed-mode processing will be archived at the DOD. 
Temperature: 
Appendix 3, figure 3.1 (top) shows the horizontal temperature distributions for the whole 
North Sea at the surface and bottom. In the surface layer temperatures increased from west 
to east in the central North Sea. Only two local features differed from this pattern. In the 
Dottom layer, lowest temperatures were observed in the northern central part, increasing east 
and west to the coast and especially to the south and to the Channel entrance. The surface 
temperatures were similar to the 2000-2010 summer climatology, but at least 1 K lower in the 
south and southeast (figure 3.1, middle, left). In addition, two small-scale light warm 
anomalies occurred in the west around 57°N. In the bottom layer anomalies were negative 
almost everywhere, but most pronounced around 54°N and off the Danish coast where 
departures exceeded 5 K (figure 3.1, middle, right). Temperatures along the southwest coast 
af Norway were slightly too warm, which may be interpreted as a deep southward intrusion of 
warmer than normal waters from the Atlantic. At 55°N, 3°E a confined strong warm anomaly 
af up to 2 K was observed. It is the position of the Dogger Bank, where the bottom rises up to 
25 m from surrounding depths at 50 m to 70 m. The thermal stratification, due to solar 
radiation, was well developed at all sections during the 2021 NSSS (see Appendix 4). Bottom 
water at the Dogger Bank pertains to the warm mixed layer (Figure 4.1), which is why bottom 
temperatures stand out here from surrounding bottom values. 
BSH’s weekly and monthly analyses of sea surface temperature ” (SST) in the North Sea 
anable us to put the observations during the cruise in a broader context (figure 2, right). The 
seasonal maximum SST was reached during the last week of the survey (table 2). Thus, only 
the northern measurements are representative of the time span of surface temperatures and 
vertical stratification reaching their seasonal peak. The overall temperature anomalies are 
thus not well suited to speculate about warming trends of the area during the last decades, 
which also holds for the 20 NSSS (figure 2, left). 
All sections show a well-developed thermocline at about 20 m depth and a well-mixed layer 
on top, which became weaker along the UK coast due to strong tidal mixing (see Appendix 
4). Even the shallow sections along 54°N and 55°N display a thermocline, at least in central 
parts. On the whole, however, the depths of the thermocline were shallower than during the 
2020 survey, when depths were at least at 30 m and at almost 100 m in the Norwegian 
Trench (section 58°N). 
Salinity: 
An inflow of Atlantic Water (S > 35) (appendix 3, Figure 3.2, top) at the surface was observed 
anly in the northwestern part of the North Sea reaching down to 59°N. In the bottom layer, a 
oroad inflow over the entire northern section (60°N) was observed, reaching farther south on 
the eastern side (58-57°N). An intrusion of Atlantic Water from the southwest through the 
English Channel was not observed, neither in summer 2020. In the Skagerrak and extending 
along the southwestern coast of Norway, the outflow from the Baltic is visible at the surface 
with salinities below 30 PSU. Salinity differences between surface and bottom of up to 6 PSU 
(appendix 3, Figure 3.2, bottom) mark the spreading of the Baltic outflow along the 
Norwegian southwest coast. This feature is also visible as a distinct deviation from the 
overall values around zero in the surface anomaly field (Figure 3.2, middle, left). 
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