48 
JOURNAL OF PHYSICAL OCEANOGRAPHY 
(a) Potential temperature 
for the shallow region of high occurrence 
VOLUME 50 
state and the noncyclonic state. The former is characterized 
’y a strong southward flow of the merged NIJ-separated EGC 
adjacent to a strong northward flow of the NIIC. This structure 
was present in 15 of the 22 occupations. In this state the NIIC 
ıs located farther to the west and occupies part of the trough. 
The remaining 7 sections corresponded to weaker southward 
ınd northward flows, with the NIIC shifted eastward and the 
antire trough associated with the merged NIJ-separated EGC. 
Jsing the reanalysis wind data, it was demonstrated that the 
zyclonic state corresponds to negative wind stress curl north of 
:he strait in the Blosseville Basin and strong northeasterly 
winds within the strait. The former is conducive for an en- 
aanced merged flow as demonstrated previously (Väge et al. 
2013). Using the satellite surface geostrophic velocity data, we 
;howed that the NIIC becomes stronger and shifts closer to the 
ihelf break under northeasterly winds, although the physical 
nechanism for this remains unresolved. 
The hydraulic criticality of the flow was assessed using a 
zomposite Froude number that can account for two moving 
‚ayers—the overflow layer and the lighter water above. This 
vevealed that roughly two thirds of the cyclonic realizations 
ıad regions of supercritical flow in the trough, and this con- 
lition was present in the mean for the strongest flow in the 
nerged NIJ-separated EGC. This suggests that hydraulic 
zontrol could be occurring intermittently during the cyclonic 
state. However, the presence of such a confined region of 
‚arge Froude number does not necessarily imply that strait- 
wide hydraulic control is occurring (Pratt and Helfrich 2005). 
A potential vorticity (PV) analysis of the 22 occupations 
.ndicated that the flow through Denmark Strait is subject to 
3ymmetric instability. This occurs when the total PV is nega- 
:ive, which tends to happen when the horizontal relative PV 
Jecomes strongly negative. We determined that the shear 
Rossby number (R;,) is a good proxy for determining when 
symmetric instability is active. In particular, when R-, is less 
:han —1, the total PV is typically negative. This proxy, which 
does not rely on the flow speed but only the density structure, 
was then applied to the full set of 122 Lätrabjarg occupations. 
This revealed that symmetric instability tends to occur at the 
:op of the overflow layer, regardless of whether there is a 
'arge or small amount of dense water in the strait. Symmetric 
‚nstability is a fast-growing instability that generally reaches 
inite amplitude in a matter of hours, leading to intense ver- 
cal mixing. This implies that, even though hydraulic criti- 
zality may not be achieved until downstream of the strait, the 
nixing/entrainment process that modifies the overflow water 
Jegins at the sill. 
Previous work has implied that the dominant mesoscale 
variability in Denmark Strait is due to baroclinic instability of 
:he hydrographic front that separates the overflow water 
[rom the subtropical-origin water in the NIC (Spall et al. 
2019). The resulting meanders of the front propagate equa- 
:orward through the strait and are associated with the well- 
<nown boluses and pulses of overflow water (Mastropole et al. 
2017; von Appen et al. 2017). In particular, meander crests are 
associated with boluses, which correspond to a thick layer of 
averflow water, whereas meander troughs coincide with pulses, 
which are characterized by a thin layer of overflow water. 
0: 
100 -P 
200 
E. 300 
s 
5 400 
_ 
500 
800 
(75 
5 
25 
1.25 
15 
700 
120 
„80 
40 
H} 
40 
80 
120 
(b) Potential temperature 
or +ha daaepn region of high occurrence 
D 
00 
200 - 
5, 300 + 
5 400 - 
A 
500 - 
500 
175 
5 
75 
1.25 
41.5 
700 
-80 -40 0 40 80 
Distance (km) 
FIG. 14. Composite average sections of potential temperature 
(°C, colors) overlain by potential density (kg m, contours) cor- 
:esponding to the (a) shallow and (b) deep regions of high occur- 
:ence of symmetric instability in Fig. 13. The highlighted isopycnal 
of 27.8 kg m? is the upper boundary of the overflow water. 
120 
of the deep trough, and the northward-flowing NIIC near the 
Iceland shelf break. 
The mean transport of the overflow water (denser than 
27.8kg m) is 3.54 + 0.29 Sv, which includes an extrapolated 
estimate of the unresolved component on the Greenland shelf 
(0.54 Sv). This is close to previously published estimates of the 
mean overflow transport (Harden et al. 2016; Jochumsen et al. 
2017). We partitioned the transport in terms of water masses 
and current components. For the former we used a hydro- 
graphic end-member analysis to distinguish Atlantic-origin 
Overflow Water (AtOW) from Arctic-origin Overflow Water 
(ArOW). Assuming that the unresolved overflow transport on 
:he Greenland shelf is AtOW, this gives 1.72 + 0.15Sv for 
ArOW and 1.49 + 0.10 Sv for ALOW, indicating that the mean 
:ransports of the two types of overflow water are comparable in 
Denmark Strait. For the currents, we distinguished the shelf- 
reak EGC and the merged NIJ-separated EGC using a geo- 
graphical boundary, and assumed that the unresolved overflow 
water on the Greenland shelf emanated from the shelfbreak 
EGC upstream of the strait. This gives 1.39 + 0.14 Sv for the 
shelfbreak EGC and 2.15 + 0.15 Sv for the merged flow, which 
is in line with similar partitioning done by Harden et al. (2016) 
upstream in the Blosseville Basin. Notably, both currents 
:ransport both types of overflow water, implying a significant 
degree of exchange between the branches as they converge in 
Denmark Strait. 
With regard to temporal variability, there were two domi- 
nant configurations of the flow which we refer to as the cyclonic 
3rancht to van hy RBUNDFSAMT EUR SFPFESCHIFTAHR" 
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