F. Große et al.: Looking beyond stratification 
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Biogeosciences, 13, 2511-2535, 2016 
Figure 5. Taylor diagram of simulated (x) bottom O2 concentra 
tions compared to observations (OBS) for time series (see Fig. 3) at 
(a) Cefas North Dogger and (b) MARNET Ems, and (c) spatially 
resolved data (see Fig. 4). Standard deviations and centred root- 
mean-square differences (RMSD) were normalised by the standard 
deviation of the corresponding observations. 
by low O2 conditions and the southeastern parts, which are 
more vulnerable to low O2 concentrations. In addition, the 
model demonstrated it is capable of capturing variations in 
the bottom O2 evolution between different years. In combina 
tion with the results of the time series validation (Sect. 3.1.1), 
this confirms that the described model setup provides reliable 
information on the internal physical and biological processes 
driving the O2 dynamics in the North Sea. 
The small SD of simulated bottom O2 confirms that using 
the averages over a period of up to 4 weeks provides a reason 
able measure for most areas. In addition, these small values 
imply that measurements taken late August/early Septem 
ber (before the breakdown of stratification) can be consid 
ered as a representative synoptic picture of the late summer 
bottom O2 conditions. However, the time series validation 
showed that in some areas lowest concentrations of bottom 
O2 may occur remarkably later in the year (see Fig. 3a). 
Consequently, the picture obtained from observations taken 
in August/September does not necessarily reflect the spatial 
distribution of minimum bottom O2 concentrations, which 
underlines the importance of choosing the appropriate point 
in time for the monitoring of low O2 conditions. 
The small SD of the observations, which is a result of the 
data gridding, shows that in most regions vertical O2 gradi 
ents near the bottom are negligible. The high values of 0.75 
and 0.59 mg O2 L _ 1 southeast of the Dogger Bank and north 
west of Denmark, respectively, result from the fact that val 
ues above and below the thermocline were taken into account 
for the averaging. 
As for the time series, Fig. 5 (marker c) shows the statisti 
cal measures of the validation for the spatially resolved data. 
Here, COR reaches only about 0.64 which is also indicated in 
Fig. 4 by the variations between year 2008 and the previous 
years, when the simulation revealed a relative change inverse 
to that in the observations in the northern North Sea. The nor 
malised RMSD of 0.77 is about twice as high as for the time 
series, which can be attributed to the greater regional differ 
ences in the observed bottom O2 concentrations with higher 
maximum and lower minimum values. The normalised SD 
equals 0.67 which indicates the less strong spatial gradients 
in the simulation. These statistics confirm that the spatial pat 
terns in the observed bottom O2 concentrations are basically 
reproduced by the model, with only slight shortcomings with 
respect to the amplitude of the bottom O2 concentrations and 
year-to-year variations in some regions of the North Sea. 
3.2 Simulated stratification periods and minimum 
bottom O2 
Figure 6a and b show the spatial distribution of the longest 
persistent stratification periods (after Eq. (1) using simulated 
T) for the years 2002 and 2010, respectively. Both years 
show similar stratification patterns with stratification periods 
of >180 days in large parts of the central and northern North 
Sea. 
Comparing the corresponding minimum concentrations of 
bottom O2 (Fig. 6c and d) shows significant differences. 
The minimum bottom O2 concentrations in 2002 in the re 
gion from 55-56.5° N, 4.5-7.5° E constitute the lowest O2 
concentrations during the entire period 2000-2012 reaching 
values of below 5.8mg02L _1 . In contrast, the duration of 
stratification in this area is similar or even longer in 2010 
than in 2002. The O2 concentrations in 2002 are even below 
the O2 threshold applied by OSPAR (ômgChL -1 ; OSPAR- 
Commission, 2005) and persist for about one month (not 
presented). In contrast, 2010 represents a year with rela 
tively high minimum bottom O2 concentrations being above 
7.3 mg O2 L _ 1 in the entire model domain. The areas directly 
north and south of the Doggerbank also reveal lower bottom 
O2 concentrations in both years. 
The stratification periods derived from the simulation are 
in good agreement with the different stratification regimes 
described by Pingree et al. (1978) and van Leeuwen et al. 
(2015). The latter applied a density-based stratification crite 
rion on model results to subdivide the North Sea into areas of 
different stratification characteristics, and showed that most 
areas of the seasonally stratified central and northern North 
Sea reveal stratification periods of 170 to 230 days. 
The increased potential for low O2 conditions north and 
south of the Doggerbank corresponds well to observed bot 
tom O2 time series in these regions (Greenwood et ah, 2010). 
Queste et al. (2013) also observed lower bottom O2 concen-