Ocean circulation in the Nordic Seas
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Transcript of Ocean circulation in the Nordic Seas
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.