Ocean circulation in the Nordic Seas

Kjell Arne Mork

IMR/BCCR

NOClim, Geilo 4/9-06

Contents

• Results from Argo floats

• Combining hydrography and altimetry

• Variability in the western branch of the NwAC in the Svinøy secion

The bottom/deep circulation in the Nordic Seas

Models of the deep circulation:

•The time-mean circulation is along f/H contours (Nøst and Isachsen, 2003; Eldevik et al., 2005)

•Wind forced fluctuations in circulation along f/H contours (e.g. Legutke, 1991; Isachsen et al., 2003; Eldevik et al., 2005)

Red vectors are observations

From Nøst and Isachsen (2003)

However, few observations of the deep ocean circulation in the Nordic Seas, until some years ago when Argo floats were deployed

Argo floats locations (2002-2006)

Drift at 1000-2000 m depths.

Total: 35 floats

IMR: 9 floats

Univ. Hamburg: 26 floats

About 2000 observations (June 2006)

IMR: +2 floats (2006)

10 daysDeployment

Argo-id: 6900217, start: 20-Jun-2002, end: 05-Apr-2005

8oW 4oW 0o 4oE 62oN

64oN

66oN

68oN

70oN

Number of days: 1020Total distance: 3338 kmMean speed: 3.8 cm/sMax speed: 9.2 cm/s

Green: last position

Blue: first position

Argo-id: 6900217

8oW 4oW 64oN

66oN

Red last pass

Blue first pass

June 2002 – April 2005

Topographic steering

: first pass : second pass

3000

2000

3000

Bottom depth: contour interval is 500 m

Mork and Søiland (2006)

Deep currents from Argo floats

The floats drift at: 1000, 1500 or 2000 m depth.

Red vectors: speed larger than 10 cm/s

Quadfasel et al. (2006)

Monthly means of speed from velocity along and across the isobaths

Quadfasel et al. (2006)

”Mean” velocity ”Residual(eddy)” velocity

Monthly means of speed (bottom depth<3000 m)

”Mean” velocity

Feb May Aug Nov0

1

2

3

4

5

cm/s

Monthly means of speed from current across bottom slope (H<3000)

Lofoten BasinNorwegian BasinGreenland SeaIceland Sea

Feb May Aug Nov0

1

2

3

4

5

cm/s

Monthly means of speed from current along bottom slope (H<3000)

Lofoten BasinNorwegian BasinGreenland SeaIceland Sea

Quadfasel et al. (2006)

”Residual(eddy)” velocity

Seasonal anomalous bottom currents

(relative to annual mean)

Winter Spring

Summer AutumnCalculated bycombiningaltimetry and hydrography

Mork and Skagseth (2005)

Comparing the changes in the circulation with the integrated wind stress curl over the area (within a closed H-contour)

Spring

Autumn

wind

change in speed

Jan Mar May Sep Nov

Mork and Skagseth (2005)

Mean velocity, April 2003-May/June 2005

Measurement depth:

Red vector : 500 m Blue vector: 1000 m Black vector: 2000 m

171830

35

2111

3126

12

79

51

(84)

3 cm/s

20 oW

10oW

0o 10

oE 20

oE

30o E

70oN

75oN

80oN

The numbers indicate stability (in %) of the current, from 0 to 100.

Stability=average velocity/average speed

Combine altimetry and hydrography reference velocity (Challenor et al., 1996)

Sigma-theta in the Svinøy section

Vref

Volume transport through the Svinøy section

Atlantic waterArctic water Coastal water

Eastern branch

Western branch

?

Mork and Skagseth (in prep.)

Method

0( , ) , ,refv z t v z t f density altimetry

Altimeter data (SSH) in the section

?

Mork and Skagseth (in prep.)

Interannual volume transport of Atlantic water in the Svinøy section

Kilometer

Yea

r

50 100 150 200 250 300 350

1994

1996

1998

2000

2002

2004

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

2000 m 200 m1000 m

Western branch Easternbranch

Sv km-1

Seasonal variation removed. One year moving averages.

Time series of the branches

1992 1994 1996 1998 2000 2002 2004-3

-2

-1

0

1

2

3

Sv

1992 1994 1996 1998 2000 2002 2004-3

-2

-1

0

1

2

3

Year

Sv

Western branch

Eastern branch

Western branch coupled to wind forcing?

Compare the transport with the averaged wind stress curl over the Norwegian Basin

Western branch coupled to wind forcing?

Compare the transport with the averaged wind stress curl over the Norwegian Basin

1994 1996 1998 2000 2002 2004

-1.5

-1

-0.5

0

0.5

1

1.5

Year

Sv

/ 10

-7 N

m

Volume transportWind stress curl

Wind stress curl moved forward four months

Mork and Skagseth (in prep.)

Western branch coupled to wind forcing?

1950 1960 1970 1980 1990 2000

-1.5

-1

-0.5

0

0.5

1

1.5

Year

Sv

/ 10

-7 N

m

Volume transportWind stress curl

Wind stress curl moved forward four months

Black line is 5 years moving averages

Conclusion

Deep currents are cyclonic and show strong topographic steering

Annual speed of current along the isobaths: 1.5-4 cm/s with seasonal variation: 0-2 cm/s.

Highest residual(eddy) current in the Lofoten Basin

Transport estimates coupled to the wind stress curl over the Norwegian Basin.