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 ( ) Drift at m depths. Total: 35 floats IMR: 9 floats Univ. Hamburg: 26 floats About 2000 observations (June 2006) IMR: +2 floats (2006) 10 days Deployment

June 2002 – April 2005 Topographic steering : first pass : second pass 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 Quadfasel et al. (2006) ”Residual(eddy)” velocity

Seasonal anomalous bottom currents (relative to annual mean) Winter Spring SummerAutumn Calculated by combining altimetry 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 MarMaySepNov 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 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 water Arctic water Coastal water Eastern branch Western branch ? Mork and Skagseth (in prep.)

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

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

Time series of the branches

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 Wind stress curl moved forward four months Mork and Skagseth (in prep.)

Western branch coupled to wind forcing? 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: 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.