NOVEMBER 2020 
LIN ET AL. 
3747 
10 
9 
RR, (Vertical relative PV / Planetary stretching PV) 
1.5 4 05 A 05 1 1.5 
8 
6 
5 
A 
61 
ae 
-2 
Y 
1 
-1 Ü ® 
Total PV (X 1019 m! s1) 
FIG. 12. Scatterplot of the shear Rossby number R, vs total the potential vorticity IT, using 
‚he 22 Lätrabjarg sections with velocity. The data points are colored by the value of the 
Rossby number R, except for cases when R, > —1 and II > 0, which are shaded gray. The 
»lack line with open circles is the average value of R, when it is less than —1, for each PV bin 
‘bin size of 0.4 X 10719 m71 5871. The standard errors are included. 
-2 
To determine where in the water column the conditions for 
symmetric instability occur, we tabulated the occurrences of 
R,< —1 for all of the sections (Fig. 13). This reveals that the 
instability occurs mainly in the trough, with a few instances 
near the surface on the Iceland shelf and near the bottom in 
che vicinity of the Greenland shelf break. However, the ma- 
jority of cases are clustered into two areas: a deeper region 
near the western side of the trough and a shallower region 
closer to the eastern side of the trough. To shed light on the 
underlying reasons for this pattern, we constructed a com- 
’osite hydrographic section for all of the occurrences in the 
deeper region, then did the same for the shallower region. 
These are shown in Fig. 14. For the shallower occurrences 
ihere is a large amount of cold overflow water filling the 
:rough, while for the deeper occurrences there is only a thin 
ayer of this water banked on the western side of the trough. 
These two states correspond nearly identically with the hy 
drographic patterns of boluses and pulses, respectively (von 
Appen et al. 2017). Note that in both cases the instability 
(akes place in the steep frontal zone, where the horizontal 
relative PV is strongly negative. This result suggests that 
strong vertical mixing occurs at the top of the overflow layer, 
regardless of whether there is a large or small amount of 
dense water present. The implication is that, even though 
strait-wide hydraulic control may not be achieved until 
downstream of Denmark Strait, the mixing/entrainment 
process that modifies the overflow water begins at the sill 
(also see North et al. 2018). 
6. Summary and discussion 
In this study we have used 22 occupations of the Lätrabjarg 
line from 1993 to 2018, together with reanalysis wind fields and 
satellite absolute dynamic topography data, to investigate 
{he kinematic structure and dynamics of the Denmark Strait 
Jverflow Water. While the Lätrabjarg section has been 
accupied over 100 times through the years, the unique as- 
zect of the subset considered here is that it includes vessel- 
nounted/lowered ADCP velocity data that were used to 
construct vertical sections of absolute geostrophic velocity. 
The mean velocity section reveals the presence of the 
shelfbreak EGC in the vicinity of the Greenland shelf edge, the 
merged NIJ-separated EGC banked against the western side 
‚00 
200 
E 300 
Ss 
3 400 
A 
500 
son 
700 
„120 -80 40 
0 40 80 120 
Distance (km) 
FIG. 13. Occurrence of symmetric instability for the 122 hydrographic 
Lätrabjarg sections. colored by number of realizations. 
3rancnht to van hy RBUNDFSAMT FEIIP 
SEFESCHIET ALIPT 
| Inanuthenticataer |! Dawnlaarleadll 01/12/72 AB- 
‘m 
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