Poster # 739
On the Nature of the Atlantic Water Recirculation in Fram Strait
Wilken-Jon von Appen
1, Ursula Schauer
1, Agnieszka Beszczynska-Möller
2, and Eberhard Fahrbach
11 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany, Wilken-Jon.von.Appen@awi.de
2 Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
I NTRODUCTION
The Atlantic Water (AW) inflow is the major oceanic heat source of the Arctic Ocean and keeps the eastern Fram Strait ice-free year-round.
20°W 15°W 10°W 5°W 0° 5°E 10°E 15°E 20°E 75°N
76°N 77°N 78°N 79°N 80°N 81°N 82°N 83°N
Maximum extent
Minimum extent
Mean sea ice concentration [%]
0 10 20 30 40 50 60 70 80 90 100
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Mean sea-ice extent: AMSR-E and AMSR- 2 2002-2013 mean sea-ice concentration.
The maximum [minimum] sea-ice extent is defined as the line where the sea-ice con- centration is 60% or less [or more] in 95% of the realizations.
Some the AW turns westward and then flows southward as part of the cyclonic gyre of the Nordic Seas. What is the nature of this AW recirculation and its variability?
AWI/NPI M OORING A RRAY
31
Fig. 3.1: Map with the position of CTD stations and moorings during ARK-XXVII/1
Fig. 3.2: The moored array in Fram Strait redeployed in 2012 during ARK-XXVII/1 for the deployment period 2012-2014 (dashed box indicated moorings operated by AWI)Mooring array during ACOBAR: Most
comprehensive variant of the mooring array maintained by the Alfred Wegener Institute and the Norwegian Polar Institute.
The array as shown has been maintained from 1997 to 2014 with the exception of moorings F15, F16, and F17 which were only added in 2002.
Starting in 2014, the array will be continued in a re- duced form: Moorings F7, F8, F15, F16 are not re- deployed. This is due to the fact that the northward fluxes in the recirculation area are insufficiently re- solved by the array.
Additionally, glider activities in Fram Strait are not continued. Navigation of the gliders under sea-ice continues to be challenging and it is therefore im- possible to substitute under ice mooring measure- ments to monitor the inflow/outflow.
B ATHYMETRY
The barotropic component of the flow is strongly influenced (“steered”) by the complex bathymetry ranging from 1200 m (Hovgaard Fracture Zone) to 5600 m (Molloy Deep) away from the shelves.
10°W8°W 6°W 4°W 2°W 0° 2°E 4°E 6°E 8°E 10°E 77°N
20ʼ 40ʼ 78°N 20ʼ 40ʼ 79°N 20ʼ 40ʼ 80°N 20ʼ 40ʼ 81°N
Elevation [m]
−5000
−4500
−4000
−3500
−3000
−2500
−2000
−1500
−1000
−500 0
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−1165m
−5573m
G EOSTROPHIC F LOW
0
500
1000
1500
2000
2500
Depth [m]
Mean zonal section
5°W 0° 5°E
10°W 10°E
Mean meridional section
79°N
78°N 80°N
potential Density [kg/m3]
27.4 27.5 27.6 27.7 27.8 27.9 28 28.1 28.2
Mean CTD sections: Mean potential den- sity from 16 (1997–2012) CTD sections in east-west direction (left) and from 4 (1997, 1998, 1999, 2001) CTD sections in north- south direction (right).
The baroclinic flow is proportional to the isopyc- nal tilt. The AW Recirculation and the EGC both have strong baroclinic flow while the WSC is mostly barotropic and has a weaker baroclinic signal.
M EAN C URRENTS
10°W 8°W 6°W 4°W 2°W 0° 2°E 4°E 6°E 8°E 10°E 78°N
10’
20’
30’
40’
50’
79°N 10’
20’
30’
0.5 m/s
WSC
AW Recirculation
EGC
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Mean currents in ≈50–100 m: The means are shown in red and the standard deviation ellipses around the means in green.
The mean velocity is larger than the variability only in the West Spitsbergen Current (WSC). The west- ward flow in the central strait is as strong as East Greenland Current (EGC).
S EASONAL C YCLE OF AW R ECIRCULATION
Distance [km], Longitude
F17 F14 F13F12F11F10 F9 F16F15 F8 F7 F6 F5 F4 321
−150 −100 −50 0 50 100 150
2°W 2°E
4°W 4°E
6°W 6°E
8°W 0°E 8°E
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean northward velocity [m/s]
−0.25
−0.2
−0.15
−0.1
−0.05 0
0.05 0.1 0.15 0.2 0.25
Distance [km], Longitude
F17 F14 F13F12F11F10 F9 F16F15 F8 F7 F6 F5 F4 321
−150 −100 −50 0 50 100 150
2°W 2°E
4°W 4°E
6°W 6°E
8°W 0°E 8°E
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean westward velocity [m/s]
−0.25
−0.2
−0.15
−0.1
−0.05 0
0.05 0.1 0.15 0.2 0.25
Mean velocities in ≈50–100 m: 1997–2012 monthly averages of the upper ocean velocity. The meridional velocity is shown on the left and the zonal velocity on the right; note that westward velocity is positive.
Distance [km], Longitude
F17 F14 F13F12F11F10 F9 F16F15 F8 F7 F6 F5 F4 321
−150 −100 −50 0 50 100 150
2°W 2°E
4°W 4°E
6°W 6°E
8°W 0°E 8°E
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Eddy kinetic energy [m2 /s2 ]
0
0.005 0.01 0.015 0.02 0.025 0.03
0.035 Left: Eddy kinetic energy in ≈50–
100 m: 1997–2012 monthly averages of the upper ocean eddy kinetic energy:
1 2
(u − u¯ )2 + (v − v¯ )2
where ¯u and ¯v are the three month lowpass filtered velocities indica- tive of the seasonal and interannual variability.
The flow in the Fram Strait is weaker in the sum- mer months. The westward velocity maximum of the AW recirculation in the central strait coincides with the strongest eddy activity in the offshore branch of the West Spitsbergen Current. This suggests that the recirculation is related to eddy generation in the eastern Fram Strait.
EKE D ISTRIBUTION WITH D EPTH
0
500
1000
1500
2000
2500
Distance [km], Longitude
Depth [m]
F17 F14 F13F12F11F10 F9 F16F15 F8 F7 F6 F5 F4 321
−200 −150 −100 −50 0 50 100 150 200
2°W 2°E
4°W 4°E
6°W 6°E
8°W 8°E
10°W 0°E 10°E
Eddy kinetic energy [m2 /s2 ]
0
0.005 0.01 0.015 0.02 0.025
Section of eddy kinetic energy: The 1997–2012 mean EKE is shown at the hor- izontal and vertical locations of the moored current meters.
Most of the variability/instabilities is happening in the upper ocean suggesting that the eddy activity in the AW recirculation is mostly baroclinic.
E XAMPLE OF E DDY A CTIVITY
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 0
50 100 150 200 250
Depth [m] Eastward velocity [m/s]−0.5
−0.4
−0.3
−0.2
−0.1 0 0.1 0.2 0.3 0.4 0.5
Northward velocity [m/s]
−0.5
−0.4
−0.3
−0.2
−0.1 0 0.1 0.2 0.3 0.4 0.5
F6
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 0
50 100 150 200 250
Depth [m]
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Mooring record at F6 in 2011-2012: The meridional and zonal (eastward positive) ve- locities are shown in color and the black lines show the vertical mooring motion which is proportional to the full water column mean speed.
There is significantly less eddy activity in the AW recirculation in September, October, and Novem- ber 2011 than in the rest of the year consistent with the 1997–2012 mean. Events lasting several days to weeks are otherwise prevalent.
