1. Water mass transformation in the Iceland Sea Irminger Sea, R/V Knorr, October 2008 Kjetil Våge Kent Moore Steingrímur Jónsson Héðinn Valdimarsson

2. Water mass transformation in the Iceland Sea - the Denmark Strait Overflow Water Denmark Strait  Largest overflow plume  Source of densest water to the lower limb of the AMOC Iceland Sea  Wintertime convection  First definitive scenario for the source of DSOW (Swift et al., 1980)

3. Water mass transformation in the Iceland Sea - overturning circulation schemes  Formed in the Iceland Sea (Swift et al., 1980)  Transformation within boundary current loop (Mauritzen, 1996)

4. Water mass transformation in the Iceland Sea - the North Icelandic Jet – another source of overflow water? from Jónsson and Valdimarsson (2004)

5. Water mass transformation in the Iceland Sea - overturning circulation schemes  Formed in the Iceland Sea (Swift et al., 1980)  Transformation within boundary current loop (Mauritzen, 1996)  Transformation within interior loop (Våge et al., 2011)

6. Water mass transformation in the Iceland Sea - climatological winter total turbulent heat flux from Moore et al. (2012) Winter (DJFM) climatological mean total turbulent heat flux from ERA-Interim

7. from Jónsson (2007)  Cyclonic circulation in the central Iceland Sea  Typical wintertime mixed layer depths about 150-200 m  Surface densities exceeding 27.8 kg\m 3 common in winter Surface circulation Water mass transformation in the Iceland Sea - circulation in the Iceland Sea

8. Water mass transformation in the Iceland Sea - historical hydrographic measurements in the Iceland Sea Collection of historical hydrographic measurements (1980 - present) Determination of mixed- layer depth and properties  visual inspection of all profiles  automated detection routines employed  manually determined when those failed → robust data set

9. Water mass transformation in the Iceland Sea - February-April mixed-layer depths Map of mixed-layer depths

10. Water mass transformation in the Iceland Sea - February-April mixed-layer depths Map of mixed-layer depths Contours of dynamic height

11. Water mass transformation in the Iceland Sea - February-April mixed-layer densities Map of mixed-layer potential densities

12. Water mass transformation in the Iceland Sea - convection in the north-central Iceland Sea Profiles located within the north-central Iceland Sea

13. Water mass transformation in the Iceland Sea - the annual cycle Mixed-layer depths

14. Water mass transformation in the Iceland Sea - the annual cycle Mixed-layer depths Mixed-layer potential densities

15. Water mass transformation in the Iceland Sea - convective activity in the north-central Iceland Sea Potential density in the central Iceland Sea (time vs. depth)

16. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet σ θ > 28.03 kg/m 3 Potential density in the central Iceland Sea (time vs. depth) Transport of σ θ > 28.03 kg/m 3 in the NIJ: 0.6 ± 0.1 Sv

17. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet Mixed-layer depths Mixed-layer potential densities

18. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet Mixed layers denser than σ θ = 28.03 kg/m 3 5 profiles from 2013 Important caveats  sparse data set  huge spatial and temporal variability

19. Temporal evolution of potential vorticity along Argo float trajectory Water mass transformation in the Iceland Sea - convective activity as recorded by Argo float winter 2008

20. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet Profiles at the outer end of the Langanes section Langanes repeat hydrographic section Langanes 6

21. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet Depth of the 28.03 kg/m 3 isopycnal at Langanes 6

22. Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet Difference: ~60 m Depth of the 28.03 kg/m 3 isopycnal at Langanes 6 → Reduced production of dense water? → Different circulation regime?

23. Water mass transformation in the Iceland Sea - atmospheric forcing Decrease in the total turbulent heat flux, discontinuity around 1995

24. Water mass transformation in the Iceland Sea - atmospheric forcing Decrease in the total turbulent heat flux, discontinuity around 1995 Decrease in the wind stress curl, discontinuity around 1995

25. Water mass transformation in the Iceland Sea - change in wintertime atmospheric circulation 1980-1995 1996-2013 Difference between the periods 1980-1995 and 1996-2013  Increased pressure  Reduced northerly winds  Anti-cyclonic circulation anomaly

26. Water mass transformation in the Iceland Sea - change in wintertime atmospheric circulation Difference between the periods 1980-1995 and 1996-2013  Increased pressure  Reduced northerly winds  Anti-cyclonic circulation anomaly 1980-1995 1996-2013Difference between the periods

27. Water mass transformation in the Iceland Sea - frequency of high heat flux events Frequency of high heat flux events  Decreasing occurrence of heat flux events exceeding the 90 th percentile value  Consistent with a weakening of the northerly winds

28. Water mass transformation in the Iceland Sea - composite means of high heat flux events Nature of high heat flux events  Retreat of sea ice  Northward shift of the highest fluxes  Narrowing of marginal ice zone  Reduced number of events (75 during first period, 65 during last) 1980-1989 2004-2013

29. Water mass transformation in the Iceland Sea - ramifications of reduced forcing November profiles from the Iceland Sea – initial conditions from Moore et al. (2014)

30. Water mass transformation in the Iceland Sea - ramifications of reduced forcing 1D mixed-layer model in the Iceland Sea from Moore et al. (2014) Ramifications of reduced forcing  Gradual reduction in depth and density of convection  If this continues, it may weaken the overturning loop that feeds the NIJ and reduce the supply of the densest water to the AMOC

31. 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 Water mass transformation in the Iceland Sea

32. - NAO and ILD indices North Atlantic Oscillation (NAO) index Icelandic Lofoten Dipole (ILD) index

33. Water mass transformation in the Iceland Sea - ramifications of reduced forcing from Moore et al. (2014) Model-data comparisons suggest that the 1D mixed-layer model is reasonable

34. Water mass transformation in the Iceland Sea - Summertime stratification Difference in potential density between 10 and 250 m

35. Water mass transformation in the Iceland Sea - June-August mixed-layer densities Map of mixed-layer potential densities

36. Polar inflow Arctic domain Surface salinity, from Swift and Aagaard (1981)  Local modification leads to formation of Arctic Intermediate Water  Contributes to overflows east and west of Iceland Atlantic inflow Water mass transformation in the Iceland Sea - the Arctic domain

37. 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.eu