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Wind driven seasonal variations Wind driven seasonal variations in the transport of NAW through in the transport of NAW through
the Faroe-Shetland Channelthe Faroe-Shetland Channel
also acknowledge:
• European Commission (MOEN – ASOF EU-E, Framework V)
Toby SherwinScottish Association for Marine Science, Oban
Acknowledgements: Bill Turrell, Sarah Hughes, Bee Berx (Marine Scotland), Bogi Hansen (FFL), Svein Østerhus (UiB)
based on:
Sherwin, T.J., Hughes, S.L., Turrell, W.R., Hansen, B., Østerhus, S., 2008. Wind-driven monthly variations in transport and the flow field in the Faroe–Shetland Channel, Polar Research 27 (1), 7-22.
Sherwin, T.J., Williams, M.O., Turrell, W.R., Hughes, S.L., Miller, P.I., 2006. A description and analysis of mesoscale variability in the Faroe-Shetland Channel. JGR 111, ISI:000235995000002 (C3).
Bathymetry of the Faroe – Shetland Channel
8 7 6 5 4 3 2 1 0
Longitude (w )
60
60
61
61
62
62
63La
titud
e
AB
CDE
Faroe
ShetlandsFIM1
15
NSDW +
NAWMNAW
THE PROBLEM OF MESOSCALE VARIABILITY
Velocities and transport in the NAW at 4 ADCPs
From different methodologies:
Hughes et al., (2006) mean NAW transport = 4.0 Sv (trend ~ -0.1 Sv per annum)
Sherwin et al,. (2008) mean NAW transport = 3.5 Sv±0.1 Sv (2.1 Sv barotropic, 1.4 Sv baroclinic)
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
-50
-25
0
25
50 Trend is:0.86cm/s/year
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
-50
-25
0
25
50 Trend is:0.21cm/s/year
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-50
-25
0
25
50
75 Trend is:-0.50cm/s/year
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
-25
0
25
50
75 Trend is:-1.06cm/s/year
B
C
D
E
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
0
5
10
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-100
0
100
200
300
400
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-100
0
100
200
300
Sv
cm/s
Hughes et al., (2006)
9 8 7 6 5 4 3 2 1 0
Longitude (W )
59
60
61
62La
titud
e
B
C D
E
0.4 m /s
-0.4 m /s A ltim eter cross track velocities
0423 h 19 May 1999
10 9 8 7 6 5 4 3 2 1 0
Longitude (W )
59.0
59.5
60.0
60.5
61.0
61.5
62.0
62.5
63.0
Latit
ude
23233
359397
209
Surface drifter and ADCP EKE (cm2 /s2)
SEASONAL VARIATION IN THE MEAN FLOW
10 9 8 7 6 5 4 3 2 1 0
Longitude (W )
59.0
59.5
60.0
60.5
61.0
61.5
62.0
62.5
63.0
Latit
ude
0.5 m /s
3.8 Sv
2.0 Sv 5.3 Sv
Mean surface drifter velocities (6000 days of data)
Monthly mean velocities in the upper 500 m
8 7 6 5 4 3 2 1 0
Longitude (w )
60
60
61
61
62
62
63
Latit
ude
AB
CDE
Faroe
ShetlandsFIM1
15
Correlation between monthly mean transport in the upper 500 and SW wind stress from NCAR/NCEP
2.3 Sv
4.1 Sv
BAROCLINIC / BAROTROPIC BREAKDOWN
Examples of strong and weak geostrophic transport (from CTD sections)
0 50 100 150 200
D istance/ km
11001000
900800700600500400300200100
0
Dep
th /
m
ShetlandShelf
Faroe Shelfc)
0 50 100 150 200
D istance/ km
11001000
900800700600500400300200100
0
Dep
th /
m
ShetlandShelf
Faroe Shelfb)
0 50 100 150 200
D istance / km
-50
0
50
100
Tra
nspo
rt /
m²/
s
a )
12 September 1999, Qgeo = 1.7 Sv
NAW
15 June 1997, Qgeo = 0.33 Sv
0 50 100 150 200
D istance/ km
11001000
900800700600500400300200100
0
Dep
th /
m
Faroe Shelfc)
ShetlandShelf
0 50 100 150 200
D istance/ km
11001000
900800700600500400300200100
0
Dep
th /
m
Faroe Shelfb)
ShetlandShelf
0 50 100 150 200
D istance / km
-50
0
50
100T
rans
port
/ (m
²/s)
a )
MNAW
Mean and mode 1 transports across the FSC
Mean
Mode 1
ADCP
Geostrophic
HOW THOR IS ADDRESSING BOTH PROBLEMS
(mesoscale variability and the shelf edge)
In THOR we add 2 extra ADCPs
Trawl proof frame for shallow mooring
SUMMARY• Contrary to earlier shorter term analysis there does appear to be a seasonal variation in the transport of NAW of ~ 1.8 Sv (± 50%)
• Identified some difficulties that limit the accuracy of absolute mean transport
• Large mesoscale variability makes it difficult to determine means and trends with this configuration
• After 15 years of observation it is difficult to argue for a relocation of the moorings