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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.

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Ocean circulation in the Nordic Seas

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  1. Ocean circulation in the Nordic Seas Kjell Arne Mork IMR/BCCR NOClim, Geilo 4/9-06

  2. Contents • Results from Argo floats • Combining hydrography and altimetry • Variability in the western branch of the NwAC in the Svinøy secion

  3. 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 • However, few observations of the deep ocean circulation in the Nordic Seas, until some years ago when Argo floats were deployed From Nøst and Isachsen (2003)

  4. 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 days Deployment

  5. June 2002 – April 2005 : first pass : second pass Topographic steering 3000 2000 3000 Bottom depth: contour interval is 500 m Mork and Søiland (2006)

  6. 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)

  7. Monthly means of speed from velocity along and across the isobaths • Quadfasel et al. (2006) ”Mean” velocity ”Residual(eddy)” velocity

  8. Monthly means of speed (bottom depth<3000 m) • ”Mean” velocity ”Residual(eddy)” velocity Quadfasel et al. (2006)

  9. Spring Winter Seasonal anomalous bottom currents (relative to annual mean) Summer Autumn Calculated by combining altimetry and hydrography Mork and Skagseth (2005)

  10. Spring Comparing the changes in the circulation with the integrated wind stress curl over the area (within a closed H-contour) Autumn wind change in speed Jan Mar May Sep Nov Mork and Skagseth (2005)

  11. Mean velocity, April 2003-May/June 2005 Measurement depth: Red vector : 500 m Blue vector: 1000 m Black vector: 2000 m The numbers indicate stability (in %) of the current, from 0 to 100. Stability=average velocity/average speed

  12. Volume transport through the Svinøy section Combine altimetry and hydrography reference velocity (Challenor et al., 1996) Western branch Eastern branch ? Vref Sigma-theta in the Svinøy section Atlantic water Coastal water Arctic water Mork and Skagseth (in prep.)

  13. Method ? Altimeter data (SSH) in the section Mork and Skagseth (in prep.)

  14. Interannual volume transport of Atlantic water in the Svinøy section Seasonal variation removed. One year moving averages.

  15. Time series of the branches

  16. Western branch coupled to wind forcing? • Compare the transport with the averaged wind stress curl over the Norwegian Basin

  17. Western branch coupled to wind forcing? • Compare the transport with the averaged wind stress curl over the Norwegian Basin Wind stress curl moved forward four months Mork and Skagseth (in prep.)

  18. Western branch coupled to wind forcing? Wind stress curl moved forward four months Black line is 5 years moving averages

  19. 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.

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