Adinrichs et al. 
To summarize: 
Before 1950, the spatial coverage is not suflicient to produce 
reasonable interpolated fields. 
The merging of box averages in a time window of 10 years 
vields a satisfying spatial coverage. For this reason, the 
‘nterpolated fields are created based on decadal mean monthly 
and annual mean values of the box averages. The following 
decades are chosen for this: 
» 1956-1965, 
» 1966-1975, 
» 1976-1985, 
» 1986-1995, 
» 1996-2005, 
2 2006-2015 
The seasonal signal only reaches to a certain depth. For the BNSC, 
the standard depth level of 101m marks the depth to which 
decadal monthly box averages are taken as a basis; for greater 
depths, the decadal annual mean is applied. 
Sensitivity Analysis 
The BNSC box averages, ie., the core of the data product, 
is sensitive to the available observations. A lack of some 
observations might alter the data product as well as additional 
observations becoming available in future. To estimate the 
influence of the observational basis on the data product, a 
sensitivity analysis is performed. 
The sensitivity analysis is applied to all parameters for the time 
periods 1950-2015 and 1960-2015 for the atmospheric and the 
hydrographic part, respectively. 
The analysis works with fields of anomalies as deviations from 
fields of long-term mean values. Based on the box averages of 
the atmospheric and the hydrographic data product, long-term 
climatological monthly mean values are calculated by temporally 
averaging over the available box values in the complete respective 
period. As a reference for the sensitivity analysis, the anomaly 
fields are then calculated as the difference between the long-term 
mean fields and fields of box averages of the BNSC product. 
Additionally, anomaly fields are calculated that are not based 
on the entity but on only 90% of the BNSC observations. The 
reduction of observations is applied randomly and repeated 100 
times. In the following, each 10%-reduction and the subsequent 
box averaging is referred to as a “run.” 
Furthermore, the BNSC region is divided into 
eight sub-regions: 
a the Atlantic part of the BNSC is divided into a 
» northern (north of 60°N), 
» a southern (south of 53°N) and 
e a middle part; 
es the central North Sea, 
e the Skagerrak/Kattegat area (east of 8°E and north of 56°N), 
» the Belt Sea 
» the central Baltic Sea (east of 13°E and south of 59°N, 
including the Gulf of Riga), 
» and finally the two greater gulfs of the Baltic Sea (north 
of 59°N). 
trontiers in Barth Science | www.frontiersin.or 
Baltic and North Seas Climatology 
[t has to be mentioned that the Skagerrak/Kattegat and the 
Belt Sea area are regarded separately only for the hydrographic 
parameters; for the atmospheric variables, those two regions are 
treated as one. 
For each of the above mentioned runs, the anomaly fields are 
averaged horizontally in each of these eight (seven) sub-regions. 
The same is applied to the anomaly fields of the reference. This 
yields time series for each run, which form a kind of “hose” 
around the time series of the reference, with an upper and a 
lower envelope. 
The distance of the envelopes from the reference, however, 
critically depends on the number of runs that are performed. To 
illustrate that the chosen number of 100 runs is suflicient, the 
envelope-reference distances are investigated with respect to their 
dependency on the number of runs. The envelope as a function 
of the number of runs (not shown) reveals that both the distance 
to the lower and the upper envelope increases considerably from 
one to about 10 runs but experiences a rather low augmentation 
after the number of runs exceeds 60. With even more runs, 
the distances converge to a maximum. The respective negative 
maximum for the lower envelope and the positive maximum for 
the upper envelope define the measure for the minimum number 
of required runs: it is the number of runs corresponding to a value 
of 95% of the maximum of the respective envelope. For all the 
three atmospheric parameters in all seven regions it shows, that 
the minimum number of required runs lies between 70 and 80. 
For the hydrographic parameters, envelopes exist around the 
time series of reference anomalies on each depth level and 
also here a clear convergence of the envelope distances can 
be seen for all regions, both parameters and the two different 
temporal resolutions. This confirms that the number of 100 runs 
is sufhcient. 
Comparison With Other Data Products 
The BNSCatm is compared to different data products. These 
include the KNSC, three different reanalyses and station data. In 
addition, the BNSCatm was compared with available temperature 
data from Schmager et al. (2008), and with 30-year means 1981—- 
2010 from ICOADS. 
KNSCatm, Re-analyses and Station Data 
The BNSC is compared to the previous project, the KNSC 
(Sadikni et al., 2018) to identify changes in the new version. The 
KNSC has the same spatial and temporal resolutions as BNSC, 
and covers the period from 1950 to 2010. 
The sea level pressure is compared to three different reanalysis 
products. The first two re-analyses are ERA40 and ERA-Interim, 
both from ECMWF, and are analyzed for the overlapping time 
period 1979-2001. ERA-40 (Uppala et al., 2005) has a 125km 
spatial resolution and the original data was uploaded in 6h 
values. ERA-Interim (Berrisford et al., 2011; Dee et al., 2011) is 
the following project, which analyzes data from 1979 on with 
constant updates to the present. It has a spatial resolution of 
79km and a temporal resolution of 6 h. The climatologies of the 
variables temperature and sea level pressure of BNSCatm are 
also compared to ERA-Interim over the time period from 1981 
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