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"IGURE 6 | (A) Station names, (B) Mean Difference of monthly mean sea level pressure [hPa] in measurements to closest grid cell in BNSCatm (red) and ERA-40 
blue} 
of up to 2°C in winter for some areas in the North. 
One explanation is, as we know, that these are the months 
and areas with lowest data density. A direct comparison 
of the BNSCatm 30-year climatology 1981-2010 with the 
[COADS 30-year mean 1981-2010 (available online at https:// 
icoads.noaa.gov/data.icoads.html) confirms this assumption. In 
principle, the data sets show similar patterns and values 
for all atmospheric parameters and months. However, the 
[COADS data set has a much coarser resolution on a 2 x 
2 degree grid. The ICOADS product is a simple monthly 
mean of all available data, that includes everything without 
checking the quality and number of data per grid point. 
[n the averaged 30-year data product, ICOADS exhibits 
obvious artifacts in areas of low data density that are not 
in the BNSCatm. Sadikni et al. (2018) found similar results 
when KNSC was compared to ICOADS. This shows the 
importance of careful quality checks and averaging methods 
that take several aspects like diurnal cycles and data density 
into account. 
BNSChydr 
The data products for the hydrographic part of the BNSC are 
available as time series of monthly and annual mean values 
on standard depth levels for the parameters temperature and 
salinity. There are three different kinds of data products 
trontiers in Earth Science | www.frontiersin.or 
Time series of box averages (temporal mean values over 
all observations in a 0.25° x 0.25° grid boxes in 1 year, 
respectively 1 month) 
2. Decadal box averages (based on 1) 
3. Interpolated fields (based on 2). 
In the following, those three data products are described in 
more detail. 
Time Series of Box Averages 
As described in section Creation of mean values, box averages 
of temperature and salinity are calculated for the BNSC region. 
This yields a time series of annual and monthly mean values 
for the period 1873-2015. Grid boxes that either do not contain 
any observation or solely observations that do not pass the 
quality control, are left empty. As an example, Figure 7 shows 
Ihe annual mean values at the surface for the year 2000. 
Additionally, the corresponding standard deviations and the 
number of observations contributing to the mean value are 
shown for temperature and salinity. 
Concerning the distribution of populated boxes as a function 
of depth and time (not shown) it can be stated that deeper 
water layers (below approximately 1,000 m) show only very few 
populated boxes until the beginning of the 1950s, in salinity as 
Juky 2019 | Volume 7 | Article 15£