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FIGURE 2 | Absolute monthly frequency of marine profiles, 1873-2015, box size 0.25° x 0.25°, Y* = 1,194,475 profiles. 
vicinity of the profile position, the last depth level is compared 
to the maximum bottom depth among the “sea” points. In case 
the last observed depth level is deeper than the bottom depth 
including a tolerance value), all observed values below the 
“digital bottom” are flagged. 
Local climatology 
This check tests if the observed parameter values are in a 
local climatological range. The climatological median value 
is determined regionally based on the observations that are 
aot flagged by any of the previous quality checks. The 
median value and the acceptable value range are derived 
from the frequency distribution of the parameter values. 
[t has to be stressed that the skewness of the frequency 
distribution is taken into account when defining the parameter 
(ange, so that for example in regions of strong horizontal 
gradients (for example salinity in estuaries) not too many true 
high, respectively, low values are flagged, see Vandervieren 
ınd Hubert (2004). The local parameter range is defined 
as follows: 
‘x 
Pain = Q1 — (Q3 — QU) -e77MC 
Paax = Q3 + (03 — Q1) - e7?4MC 
where Q1 and Q3 define the location of the first and third quartile, 
tespectively, of the frequency distribution. Consequently, the 
difference Q3 — Q1 corresponds to the interquartile range. MC, 
trontiers in Barth Science | www.frontiersin.or 
the “medcouple”-value, is a measure for the skewness of the 
distribution. For more details on this, see Brys et al. (2003). 
The quality control is performed separately, on the one 
hand, for the parameters salinity and temperature and on the 
other hand, since the North and Baltic Sea differ strongly 
in their hydrographic characteristics, for the two regions. 
The longitudinal segregation line is set at 55.8°N in the 
transition between Kattegat and Belt Sea and splits the entity of 
observational profiles into 792,741 for the North and 402,404 for 
the Baltic Sea. 
Figure 3 shows the results of the quality control: the fraction 
of profiles flagged by the quality control is shown here in spatially 
horizontal resolution. The numerically summarized results of the 
quality control for the two regions and parameters are given 
in Table 2. 
Interpolation on Standard Depth Levels and 
Horizontal Grid 
The temperature and salinity observations, as provided by the 
different data sources, are on observed depth levels. To allow a 
further processing, a spatially vertical interpolation on common 
standard depth levels is necessary. The distribution of those depth 
(evels is determined as follows: in the upper region of the North 
and Baltic Sea: to a depth of 50 m, the difference between the 
depth levels is chosen to be 5m. Further down, the difference 
between the two consecutive levels increases steadily by 1m. 
July 2019 | Volume 7 | Article 155