F. Große et al.: Looking beyond stratification 
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www.biogeosciences.net/13/2511/2016/ 
Biogeosciences, 13, 2511-2535, 2016 
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Figure 3. Annual time series of observed and simulated bottom 
Cb concentrations at Cefas station North Dogger in (a) 2007 and 
(b) 2008, and at MARNET station Ems in (c) 2010 and (d) 2011. 
Same legend for all panels. Grey shaded areas indicate the stratifi 
cation periods derived from simulated T according to Eq. (1). 
also affect the difference between simulated and observed 0 2 
concentrations. 
At MARNET station Ems the observed bottom 0 2 concen 
trations show significantly larger intra-seasonal fluctuations 
than at North Dogger. This applies to both years 2010 and 
2011, and mainly results from the shallower station depth, 
i.e., sampling depth (sensor in 30 m). As at North Dogger, 
differences may also relate to different depths of the time se 
ries (32.5 m for observation) and the vertical resolution with 
only six layers. 
In 2010, the onset of 0 2 decline in the observations is in 
good agreement with that in the simulations. Stratification 
lasts shorter and is less persistent than at North Dogger. As at 
North Dogger, intra-seasonal fluctuations in the 0 2 evolution 
are not fully reproduced. The model tends to overestimate 
bottom 0 2 in 2010, revealing a maximum difference of about 
1 mg0 2 L -1 . 
In 2011, persistent stratified periods derived from the 
simulation do not exceed 2 months at station Ems. Con 
sequently, the temporal evolution of bottom 0 2 represents 
mainly the temporal evolution of the 0 2 saturation concen 
trations. Again large fluctuations of up to ±2 mg0 2 L -1 can 
be seen in the observations which are not fully reproduced 
by the model. Besides these short-term changes, the differ 
ence between simulated and observed bottom concentrations 
is less than 0.8 mg0 2 L -1 with higher summer values in the 
simulation. 
The validation of bottom 0 2 at the stations North Dog 
ger and Ems shows that the HAMSOM-ECOHAM model 
is capable of reproducing the main features of the bottom 
0 2 dynamics at these two stations. The minor differences 
in the concentrations (< ±0.4mgO 2 L _1 ) at the beginning 
and end of the year, representing mainly the saturation con 
centrations, show that the general physical setting provided 
by the model is reasonable. The slightly slower 0 2 reduction 
in the simulation may indicate an underestimation of the bi 
ological consumption, e.g., due to benthic remineralisation. 
Intra-seasonal fluctuations at both stations are not fully re 
produced, due to the limited spatial resolution of the model 
grid. Additionally, the tides may have an effect at station Ems 
on the short-term. However, they are not resolved due to the 
daily time step of the simulated current fields. 
The generally good agreement between simulation and ob 
servation is also shown by the Taylor diagram (Taylor, 2001, 
see Fig. 5, markers “a” for North Dogger and “b” for Ems), 
which presents the correlation coefficients (COR), standard 
deviations (SDs) and centred root-mean-square differences 
(RMSD) of the simulation relative to the observations. SDs 
and RMSDs are normalised by the SD of the correspond 
ing observations. For analysis, the data of each data set was 
merged into a continuous series of data. For both stations, 
COR is high with values of about 0.95 and the normalised 
RMSD is less than 0.38. The RMSD values are mainly due 
to the larger range and higher (seasonal and intra-seasonal) 
variability in the observed bottom 0 2 , which is also indi 
cated by the normalised SDs of about 0.73 and 0.82 for Cefas 
North Dogger and MARNET Ems, respectively. 
3.1.2 Spatial distribution of late summer bottom 0 2 
Figure 4 shows the spatial distribution of the average simu 
lated and observed 0 2 concentrations in the model bottom 
layer for the years 2001 (a), 2005 (b) and 2008 (c), and the 
SD related to the averages in 2005 (d). The averaging period 
for the simulations corresponds to the complete observation 
period for each year, listed in the bottom right comer of each 
panel. 
In 2001, the observations show the lowest concentrations 
of all years with minimum values of 5.9mg0 2 L _1 in the 
area 54-57° N, 4.5-7° E. This minimum is similarly present 
in the model yielding 6.96mg0 2 L _1 . Maximum observed 
concentrations were found off the southern tip of Norway 
(9.3mg0 2 L _1 ) and in the deepest parts of the Norwegian 
Trench (8.7 to 8.8 mg0 2 L -1 ). The very high observed value 
at the northwesternmost sampling site represents an outlier