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