1. www.sams.ac.uk Effect of the Subpolar Gyre on the eastern North Atlantic Clare Johnson (SAMS)*, Mark Inall (SAMS), Toby Sherwin (SAMS) * cljo@sams.ac.uk NACLIM Annual Science Meeting 2013

2. www.sams.ac.uk Introduction - Subpolar Gyre (SPG) stronger gyre weaker gyre Satellite altimeter data from 1992 + (Häkinnen and Rhines, 2009) Model data back to 1960 (Hätun et al., 2005)

3. www.sams.ac.uk Methods NAW: North Atlantic Water ENAW: Eastern North Atlantic Water WNAW: Western North Atlantic Water mod-WNAW: modified Western North Atlantic Water salinity potemp. nitrate phosphate Time-series of upper water (200-700 dbar) properties from 1996 to 2011 from Ellett Line dataset.

4. www.sams.ac.uk Results As Subpolar Gyre weakens: salinity (r -0.86) potemp. (r -0.85) nitrate (r +0.87) phosphate (r +0.81) Threshold reached mid-2000s (SPG index -5 cm)

5. www.sams.ac.uk Results Mixing between: (1) mod-WNAW from Subpolar Gyre. (2) NAW and ENAW from boundary Subpolar and Subtropical Gyres. Increased southern water masses when weak Subpolar Gyre. WNAW not an important upper water mass. weakening Subpolar Gyre

6. www.sams.ac.uk Results Strong gyre: ~ 50 % mod-WNAW from Subpolar Gyre. ~ 50 % NAW and ENAW from southern source. Weak gyre: < 25 % mod-WNAW from Subpolar Gyre. > 75 % NAW and ENAW from boundary Subpolar and Subtropical Gyres. r -0.88 Mean value from S-θ and S-PO 4 Value from S-θ only

7. www.sams.ac.uk Results Since the mid-2000s, the Subpolar Gyre has been weaker than anything observed since 1992, or modelled since 1960-1970. We suggest the maximum proportion of southern water masses (and minimum proportion subpolar water masses) reached. Time-series 5-300 m Bay of Biscay (ICES Report Ocean Climate 2009)

8. www.sams.ac.uk Conclusions Between 1996 and mid-2000s, upper water properties in the Rockall Trough primarily controlled by the extent of the Subpolar Gyre (1975 to mid-2000s for θ and S). As the gyre weakens, the upper waters become: warmer, saltier and reduced in nitrate and phosphate. Linked to increased advection of southern-origin water masses (and decreased influence of subpolar-origin water masses) as the Subpolar Gyre contracts. Since the mid-2000s the gyre has been particularly weak. Upper water properties no longer directly related to changing ratio of southern to subpolar water masses. Instead suggest related to changes in properties of southern water masses. Johnson, C. et al., 2013, Declining nutrient concentrations in the northeast Atlantic as a result of a weakening Subpolar Gyre, DSR I, 82, 95-107.

9. www.sams.ac.uk The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299 NACLIM www.naclim.euwww.naclim.eu