NOVEMBER 2020 
Wind vector 
1 
x. 
u. 
m ; 
E {Pa 
5 6 ! 
-12 
-12-6 0 6 12 
ums 
LIN ET AL. 
Along-strait surface velocity 
(a) 
I. 
A 
’b) 
J.1 
u“ 
— 
di 
0) 
A 
°C) 
= a 
_ _ _ _Merged_ _ _ ms U 
NiJ/separated EGC A" 
a 
0.1 
rn 
ln 
x 
7.1 
120 -100 -80 -60 1-40 
-20 0 20 40 60 80 2100 120 
Distance (km) 
/ a— 
00 E 
00 SE 
© 
500 6 
37243 
FIG. 8. (right) Composites of along-strait surface geostrophic velocity (m s !) corre- 
sponding to (left) different wind conditions in Denmark Strait. The shading represents the 
standard error. (a) Average of all instances where the southwesterly wind is greater than the 
nean plus one standard deviation. (b) Instances where the wind is close to the mean. 
<c) Instances where the northeasterly wind in the strait is greater than the mean plus one 
standard deviation. (bottom) The bathymetry 
velocity corresponding to the associated wind conditions 
n the strait (averaged in a 1° X 1° box around the trough). 
In particular, we composited all of the satellite crossings for 
strong northeasterly wind (greater than the mean plus one 
standard deviation), strong southwesterly wind (same crite- 
rion), and for all remaining cases. The results are shown in 
Fig. 8. Each of these composites contains on the order of a 
thousand crossings, and the small standard errors indicate the 
‚obustness of the results. The composites demonstrate that 
when the winds are strongly out of the northeast the NIIC is 
’oth stronger and located more seaward, plus the merged 
NIJ-separated EGC is enhanced as well. This is consistent with 
‘he fact that the cyclonic state (Fig. 6a) corresponds to strong 
northeasterly winds (Fig. 7a). 
We note that in Fig. 8c that the signatures of the NIIC 
and merged NIJ-separated EGC are much broader than in 
:he cyclonic velocity composite, plus the NIIC is located 
on the Iceland shelf and the merged flow is located near 
:he Greenland shelf break, versus being situated close to the 
:rough. This is likely due in part to the resolution of the 
altimeter (12 km), which is not well suited for resolving ei- 
:her flow, plus the compositing process. However, it is also 
partly due to the fact that not all instances with strong 
northeasterly wind correspond to a strong NIIC displaced 
[O0 the west—although this is clearly the case in the mean 
‘Fig. 8c). To assess this, we composited the surface geostrophic 
velocity for all of the instances when the NIIC was at the 
edge of the Iceland shelf, regardless of wind conditions. 
This revealed a significantly narrower, stronger NIIC along 
with an enhanced southward-flowing merged NIT-separated 
3GC. Importantly, the mean wind for these instances was 
strongly out of the northeast. This, together with Fig. 8c, 
ndicates that the cyclonic state in Denmark Strait is 
zlearly associated with enhanced northeasterly winds through 
<he strait. 
It remains to be determined what the physical mechanism is 
sehind this change in the NIIC. Upwelling-favorable north- 
sasterly winds should drive southward flow on the Iceland shelf 
due to Ekman setup, i.e., the opposite of an enhanced NIIC, 
yut the altimeter data are too inaccurate near the coast to 
zonfirm this. The strong cyclonic flow offshore, in the vicinity of 
‚he trough, is associated with a depression of the sea surface 
ı1eight. Such a signature would arise if there was an increase in 
wind speed near the center of the strait, due to divergence of 
‚he offshore Ekman transport. Unfortunately, the spatial 
resolution of ERAS (—-30 km) is insufficient to resolve such a 
zhange. It should be noted that a two-dimensional view may 
aot be appropriate here because the presence of warm, rel- 
atively light water along the south coast of Iceland and cold, 
dense water along the north slope, will result in an anticy- 
clonic propagation of a high sea surface height signal around 
‘he west coast of Iceland (Spall et al. 2017). This would act to 
maintain a high sea surface height over the Iceland shelf to 
he east of the trough, even in the presence of upwelling- 
[avorable winds. Thus, the enhanced southward flow of the 
nerged NIJ-separated EGC due to the negative wind stress 
curl, lowering the sea surface height in the trough, would be 
zoncomitate with a stronger northward-flowing NIIC. These 
ıdeas warrant further investigation, but are beyond the scope 
of the present study. 
Zroncht to van hy RUNDFSAMT FÜR SFPFFESCHIFTAHPR" 
| Inaunthenticatern |! Dawnlaadend 0171272 AR- 
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