Kjetil Våge, UiB Bob Pickart, WHOI Mike Spall, WHOI Kent Moore, UoT H éðinn Valdimarsson , MRI

1. Revised circulation scheme north of the Denmark Strait Kjetil Våge, UiB Bob Pickart, WHOI Mike Spall, WHOI Kent Moore, UoT HéðinnValdimarsson, MRI Steingrimur Jónsson, MRI/UNAK Dan Torres, WHOI Svetlana Erofeeva, OSU Svein Østerhus, UNI BCCR Tor Eldevik, UiB Jan Even Ø. Nilsen, NERSC Blosseville coast, north of the Denmark Strait View from R/V Knorr, October 2008

2. Revised circulation scheme north of the Denmark Strait - background and motivation 1 Sv = 106 m3/s • Nordic seas exchange • Crucial part of climate system • Reasonably well quantified from Hansen et al.(2010) • Greenland-Scotland Ridge • Transformation of warm inflow • into dense overflow waters • north of the ridge from www.whoi.edu

3. Revised circulation scheme north of the Denmark Strait - origin of Denmark Strait Overflow Water • Transformation within the boundary current loop (Mauritzen, 1996)

4. Revised circulation scheme north of the Denmark Strait - origin of Denmark Strait Overflow Water • Transformation within the boundary current loop (Mauritzen, 1996) • Approach along the Iceland slope in the North Icelandic Jet (Jónsson and Valdimarsson, 2004) North Icelandic Jet

5. Revised circulation scheme north of the Denmark Strait - outline Circulation north of the Denmark Strait • The North Icelandic Jet • The East Greenland Current • Revised circulation scheme

6. Revised circulation scheme north of the Denmark Strait - outline Circulation north of the Denmark Strait • The North Icelandic Jet • overflow water masses and pathways • formation of the North Icelandic Jet • overturning circulation schemes • The East Greenland Current • Revised circulation scheme

7. The North Icelandic Jet The East Greenland Current Revised circulation scheme High-resolution surveys off northwest Iceland Hydrographic and direct velocity measurements R/V Knorr • KN194 – October 2008 • vessel-mounted ADCP • R/V Bjarni Sæmundsson • BS010 – August 2009 • lowered ADCP

8. The North Icelandic Jet The East Greenland Current Revised circulation scheme Maximum density above sill depth (650 m) KN194 – October 2008 • Dense water high • on the Iceland slope • Recirculation of dense • EGC waters? KN194 - October 2008

9. The North Icelandic Jet The East Greenland Current Revised circulation scheme Maximum density above sill depth (650 m) KN194 – October 2008 • Dense water high • on the Iceland slope • Hydrography suggests • that this does not • originate from the EGC KN194 - October 2008

10. The North Icelandic Jet The East Greenland Current Revised circulation scheme Mean flow of overflow water Overflow range: σθ > 27.8 and depth < 650 m • Dense water high • on the Iceland slope • Hydrography suggests • that this does not • originate from the EGC • Consistent flow of • overflow water toward • the Denmark Strait • the NIJ KN194 - October 2008

11. The North Icelandic Jet The East Greenland Current Revised circulation scheme Absolute geostrophic velocity KN194 – October 2008 The North Icelandic Jet originates east of the Kolbeinsey Ridge

12. The North Icelandic Jet The East Greenland Current Revised circulation scheme Mean flow of overflow water Overflow range: σθ > 27.8 and depth < 650 m BS010 - August 2009

13. The North Icelandic Jet The East Greenland Current Revised circulation scheme Absolute geostrophic velocity The October 2008 and August 2009 surveys are fully consistent

14. The North Icelandic Jet The East Greenland Current Revised circulation scheme Path of the North Icelandic Jet The NIJ core was typically found above the 650 m isobath – the same depth as the Denmark Strait sill. A coincidence?

15. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet

16. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water upstream of the sill (σθ > 27.8 and depth < 650 m) For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv

17. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water upstream of the sill (σθ > 27.8 and depth < 650 m) For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.1 Sv (densest component)

18. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water upstream of the sill (σθ > 27.8 and depth < 650 m) For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.1 Sv (densest component) Sill

19. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water in the NIJ compared to transport at the sill (σθ > 27.8 and depth < 650 m) For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv For σθ > 27.8 kg/m3: T = 2.9 ± 0.5 Sv

20. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water in the NIJ compared to transport at the sill (σθ > 27.8 and depth < 650 m) For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.1 Sv (densest component) For σθ > 27.8 kg/m3: T = 2.9 ± 0.5 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.2 Sv (densest component)

21. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transport of the North Icelandic Jet Transport of overflow water in the NIJ compared to transport at the sill (σθ > 27.8 and depth < 650 m) The NIJ accounts for about half of the total overflow and nearly all of the densest component For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.1 Sv (densest component) For σθ > 27.8 kg/m3: T = 2.9 ± 0.5 Sv For σθ > 28.03 kg/m3: T = 0.6 ± 0.2 Sv (densest component)

22. The North Icelandic Jet The East Greenland Current Revised circulation scheme Formation of the North Icelandic Jet Idealized model simulations Mean surface temperature and bottom topography Key elements • A sill • An island • Bouancy loss • Cyclonic wind stress curl Key features • Warm inflow • Interior convection MITgcm, idealized configuration, 5 km horizontal resolution

23. The North Icelandic Jet The East Greenland Current Revised circulation scheme Formation of the North Icelandic Jet Model circulation and hydrography C Model processes • NIIC disintegrates • Lateral exchange • Interior convection • Densified water returned • Along-boundary sinking • Feeds the NIJ Water mass transformation in the central Iceland Sea and the NIIC/NIJ current system implicated in the deep limb of the AMOC Mean surface temperature and bottom topography Mean surface temperature and velocity at 650 m Vertical section of meridional velocity Vertical section of temperature

24. The North Icelandic Jet The East Greenland Current Revised circulation scheme Overturning circulation schemes • Transformation within boundary current loop (Mauritzen, 1996)

25. The North Icelandic Jet The East Greenland Current Revised circulation scheme Overturning circulation schemes • Transformation within boundary current loop (Mauritzen, 1996) • Transformation within interior loop (Våge et al., 2011) • Roughly equal contribution from either source

26. Revised circulation scheme north of the Denmark Strait - outline Circulation north of the Denmark Strait • The North Icelandic Jet • The East Greenland Current • Revised circulation scheme

27. The North Icelandic Jet The East Greenland Current Revised circulation scheme Bathymetry The Blosseville Basin

28. The North Icelandic Jet The East Greenland Current Revised circulation scheme Overturning circulation schemes Two currents merging to form the DSOW plume? East Greenland Current (EGC) • Two currents advecting overflow water into the Denmark Strait North Icelandic Jet (NIJ)

29. The North Icelandic Jet The East Greenland Current Revised circulation scheme Observed circulation at the Kögur transect An unknown current in the interior Blosseville Basin EGC ??? NIJ Greenland Iceland Mean absolute geostrophic velocity at Kögur computed from 4 realizations

30. The North Icelandic Jet The East Greenland Current Revised circulation scheme Observed circulation at the Kögur transect The separated East Greenland Current shelf break EGC separated EGC NIJ Greenland Iceland Mean absolute geostrophic velocity at Kögur computed from 4 realizations

31. The North Icelandic Jet The East Greenland Current Revised circulation scheme Overturning circulation schemes Revised circulation scheme East Greenland Current (EGC) North Icelandic Jet (NIJ)

32. Revised circulation scheme north of the Denmark Strait - outline Circulation north of the Denmark Strait • The North Icelandic Jet • The East Greenland Current • presence of separated EGC • a permanent feature of the circulation • hypotheses to explain the separated EGC • gyre scenario • eddy scenario • Revised circulation scheme

33. The North Icelandic Jet The East Greenland Current Revised circulation scheme Mean conditions at the Kögur transect

34. The North Icelandic Jet The East Greenland Current Revised circulation scheme Synoptic realizations of absolute geostrophic velocity Two scenarios for the formation of the separated East Greenland Current Gyre scenario Eddy scenario

35. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transports Freshwater Nearly ¼ of the EGC system FW transport takes place in the interior Reference salinity = 34.8

36. The North Icelandic Jet The East Greenland Current Revised circulation scheme Transports Overflow water Nearly ¼ of the EGC system FW transport takes place in the interior Reference salinity = 34.8 The majority of the OW approaches the strait along the Iceland slope

37. The North Icelandic Jet The East Greenland Current Revised circulation scheme Historical circulation Dynamic height of sea surface relative to 500 db EGC shelf break EGC Iceland Sea Gyre separated EGC

38. The North Icelandic Jet The East Greenland Current Revised circulation scheme Historical hydrography Near-surface layer Potential temperature Salinity Vertically averaged between 50-100 m (potential temperature) and 10-30 m (salinity)

39. The North Icelandic Jet The East Greenland Current Revised circulation scheme Historical hydrography Overflow water layer Potential temperature Salinity Maximum value between 27.9 and 28.0 kg/m3

40. The North Icelandic Jet The East Greenland Current Revised circulation scheme Atmospheric forcing Annual mean sea level pressure and 10 m wind speed/vectors barrier winds Blosseville Basin Icelandic Low ERA-Interim

41. The North Icelandic Jet The East Greenland Current Revised circulation scheme Atmospheric forcing Annual mean wind stress curl and 10 m wind vectors anticyclonic wind stress curl ERA-Interim

42. The North Icelandic Jet The East Greenland Current Revised circulation scheme Atmospheric forcing Annual mean wind stress curl and 10 m wind vectors anticyclonic wind stress curl North American Regional Reanalysis

43. The North Icelandic Jet The East Greenland Current Revised circulation scheme Atmospheric forcing Gyre scenario Annual mean wind stress curl and 10 m wind vectors anticyclonic wind stress curl • anticyclonic wind stress curl • closed bathymetry contours • the separated EGC is part of an anticyclonic gyre North American Regional Reanalysis

44. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations • MITgcm • channel oriented along the east coast of Greenland • southern outflow becomes northern inflow • salinity restored to initial conditions in the northern end (32 at the surface on the shelf, 35 in the deep interior), temperature is constant • 1 km horizontal resolution, 30 vertical levels • forced by steady annual mean wind stress

45. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Mean sea surface salinity over final 2 years • sharp gradient near the shelf break at high latitudes supporting a shelf break jet • offshore diversion of freshwater near y = 500 km

46. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Synoptic section at y = 320 km on day 360 • southward flow near the Greenland shelf break → shelf break EGC • anticyclonic circulation over the deep Iceland slope → a gyre? Meridional velocity Anticyclonic ring Salinity

47. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Sea surface salinity on day 770 • eddies and filaments dominate the Blosseville Basin • freshwater diversion from the shelf break is highly time dependent

48. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Temporal evolution of near-surface layer at y = 320 km

49. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Eddy scenario • southward flow inshore of x = 50 km • offshore salinity front near x = 120 km, coincident with southward flow • the separated EGC arises from eddies that coalesce when encountering the Iceland slope • gyre scenario not supported by the model

50. The North Icelandic Jet The East Greenland Current Revised circulation scheme Numerical simulations Eddy generating mechanism Difference in angle between the mean wind direction and the orientation of the shelf break Not the case at the northern end of the Blosseville Basin → baroclinic instabilities generate eddies • mean winds are generally parallel to the coast • onshore Ekman transport maintains the EGC’s baroclinicity • frontal instabilities are inhibited