F. Große et al.: Looking beyond stratification 
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Biogeosciences, 13, 2511-2535, 2016 
Appendix A: Evaluation of the stratification and MLD 
criterion 
Figure A1 shows the Hovmoller diagram of simulated T at 
station North Dogger (see Fig. 2, region 2) for the year 2007, 
including the MLD (dashed magenta line) derived from the 
simulated T field according to Eqs. (1) and (2). Regarding 
the onset of stratification in late March, it is shown that the 
near-surface (0-25 m) T starts to increase relatively to T in 
the deeper layers. r CI it = 0.5 K is reached on 26 April, mark 
ing the beginning of the stratified period according to Eq. (1). 
The maximum vertical T gradient at the onset occurs in 25 m 
depth. From that moment stratification according to Eq. (1) 
persists until 31 October, which may represent a slight over 
estimation as the maximum T gradient is found in 60 m depth 
already, indicating deep mixing. 
During the first two months of the stratified period 
(April/May) the MLD shows stronger fluctuations in terms 
of its actual depth. This results from the relatively weak near 
surface stratification, and thus, the stronger effect of mainly 
wind-induced mixing reaching depths of up to 35 m. These 
events are indicated by the episodic increase and decrease of 
surface T. From early June to end of July, the MLD is less 
variable in depth due to the persistent surface heating and less 
strong wind events. In late June a short-term decrease in sur 
face T indicating enhanced mixing occurs which also results 
in a deepening of the MLD. From August until the end of the 
stratified period the MLD shows a deepening trend which is 
caused by the decreasing surface heating and increasing wind 
activity. 
Figure Al. Hovmoller diagram of simulated T and MLD accord 
ing to Eqs. (1) and (2) at Cefas station North Dogger (see Fig. 2, 
region 2) in 2007. Depth levels represent the centre depth of model 
layers. 
The main assumption behind the rather small critical T 
difference of 0.05 K is, that even if the surface mixed layer is 
interrupted due to mixing, this does not necessarily result in 
a complete overturning of the water column. Thus, even a mi 
nor difference in T indicates a bottom layer unaffected from 
vertical mixing. Despite this significant difference to com 
mon MLD criteria (e.g Kara et al., 2000, therein Table 1), 
the criterion applied in this study represents the stratification 
conditions quite well. However, it should be noted that the 
end of the stratified period may be slightly overestimated. In 
addition, in regions with a less pronounced onset of stratifica 
tion, i.e., a less distinct increase in surface T, the determined 
timing of the onset may be slightly too early. The use of the 
maximum T gradient to determine the MLD under stratified 
conditions yields reasonable results, and is closely related to 
real conditions as the thermocline is defined as the layer with 
the maximum T gradient.