dinrichs et al. 
grid boxes are added to the average. If the number of observations 
is still smaller than the threshold, then, in the second step, 
the data of the next 16 environment boxes are added for the 
averaging. Thus, observations of up to 24 surrounding grid boxes 
can contribute to the result of the center grid box. The results of 
these calculations are time series of monthly mean fields of the 
parameters on the one-degree grid. 
From these monthly mean fields, the BNSC climatology is 
calculated for the 30-year periods 1951-1980, 1961-1990, 1971- 
2000, and 1981-2010. For this, the mean values of the fields for 
each month of the respective period are computed. A grid box 
gets no value if it had no value in 1/6 of the time, which is 6 or 
more months. For the longtime climatology 1950-2015, due to 
the long time series of 66 years, the criterion is slightly less strict 
and needs a value in at least 4/5 of the time steps. 
Hydrographic BNSC 
Sources of Data 
Observed temperature and salinity profiles from different data 
sources are the base of the BNSC data product. The sources of 
observational data are: 
e WOD (World Ocean Data Center, Boyer et al., 2013) 
» DOD (Deutsches Ozeanografisches Datenzentrum, Germany, 
www.bsh.de) 
» IJOW (Leibniz Institute for Baltic Sea Research 
Warnemünde, Germany, 
» www.io-warnemuende.de) 
» ICES (International Council for Exploration of the Sea, www. 
ices.dk) 
IMGW-PIB (The Institute of Meteorology and Water 
Vanagement — National Research Institute, Poland, www. 
‚mgw.pl) 
NIOZ (Royal Netherlands Institute for Sea Research, www. 
nio0z.nl) 
» BODC (British Oceanographic Data Center, www.bodc.ac.uk) 
» SCANFISH (SCANfish data, provided by BSH, www.bsh.de) 
» CTD_DK (Danish National Marine Monitoring Data, 
Bioscience, Aarhus University - http://Mads.dmu.dk) 
» ARGO (International ARGO-Project, www.ifremer.fr). 
The authors do not claim the list of sources of data to be 
complete. By the time starting working on BNSChydr, the data 
sources listed above were the ones the authors were aware of. 
Considering the creation of marine climatologies a process, 
future versions may not only include future observational data 
but also data sources not included yet, as, for example, the 
Baltic Environmental Database (BED, http://nest.su.se/bed/), 
the Copernicus Marine Environment Monitoring Service, the 
SeaDataNet program and the database compiled within the 
international project “The Year of the Gulf of Finland 2014” 
{see “The Gulf of Finland assessment., 2016” in the reference list 
for more details). However, the expansion to more data sources 
has always to be considered with respect to the cost-value ratio: 
there is already a strong overlap between some of the above listed 
data sources. Janssen et al. (1999) also found a great intersection 
between the ICES/DOD data bases (which they chose for their 
data product) and other data sources. Thus, new data sources 
rontiers in Earth Science | www.frontiersin.Ofu 
Baltic and North Seas Climatolog\ 
TABLE 1 | Number of remaining BNSChydr profiles after sorting of duplicates. 
Source 
Number of Profiles 
WOD 
DOD 
OW 
ES 
VIGW-PIB 
NIOZ 
30DC 
SCANFISH 
CTD_DK 
ARGO 
Tr 
765,395 
221,388 
9,614 
146,874 
6,384 
2,798 
3,723 
9,798 
25,564 
2,992 
1.194.475 
might contribute some observations not included yet but, at the 
same time, each new data source also adds workload to the data 
processing procedure which is explained in the following. 
Data Processing 
The observational data are processed in several consecutive steps 
described in detail in the following. 
Duplicate Profiles 
ncorporation of several data sources in the research of 
observational data includes intersection of different sources of 
data, e.g., profiles contained in the WOD data base are also 
provided by ICES. Duplicate profiles, however, would bias the 
result of the BNSC. Therefore, it is necessary to exclude the 
duplicate profiles in the further data processing. The numbers of 
remaining profiles after identification of duplicates are listed in 
Table 1, sorted by the different data sources. 
To get an overview of the distribution of the observations, 
a time series (1873-2015) of the frequency of observed profiles 
per year is displayed in Figure 1A. In the period between 1873 
and 1882, there are only 1-6 profiles recorded in the whole 
area. In general, the number of observations is rather low until 
approximately 1950. The periods of the First and Second World 
War can cClearly be recognized by depressions in the observational 
frequency. From 1950 on, the observation rate moderately rises 
to a maximum in the 1990s. For the year 1989, the number of 
profiles exceeds the value of 30,000 profiles and sticks out. Closer 
analysis revealed, that most of the profiles in this year can be 
found in only five 0.25° x 0.25° boxes close to the German North 
and Baltic Sea coast. 
Additionally, Figure 1B shows the distribution of the 
observed profiles in the 12 months of the year. It can clearly be 
seen that the observational frequency is higher in the summer 
than in the winter months. In Figure2, this distribution is 
horizontally resolved; a distinctly higher data density can be seen 
in the months May-September than in the rest of the year. This 
is most obvious in the area of the Baltic and the central North 
Sea. A rather low number of observations characterizes the outer 
regions of the BNSC area. With respect to the data coverage 
as a function of depth (not shown) it can be stated, that the 
Alb 2019 1 Valııme 7 1 Article 15£