1. Measuring the eastern boundary inflow to the Labrador Sea
Robert S. Pickart (WHOI)
Outline
1. Insights on the overturning circulation of the Labrador Sea
2. Challenges associated with measuring the eastern boundary current
3. Strawman array design and remaining questions
Greenland coast viewed from R/V Knorr , Oct 2008
(Photo by Ben Harden)

2. Labrador Sea general circulation

3. Wintertime storm tracks (from 45 years of ECMWF re-analysis data)
Mean storm track
Våge et al. (2009)

4. Heat loss from wintertime cold-air outbreaks
Ice
Color = heat loss in Watts/m2
(from NCEP reanalysis)

5. Heat loss from wintertime cold-air outbreaks
Ice
Color = heat loss in Watts/m2
(from NCEP reanalysis)
Contours = Absolute geostrophic
pressure (Lavender et al. (2000)

6. Surface eddy speed Color = surface eddy speed (cm/s)
Contours = Absolute geostrophic
pressure (Lavender et al. (2000)
After Lilly et al. (2003)

7. Surface eddy speed and regions of deep convection
Color = surface eddy
speed (cm/s)
Contours = Absolute geostrophic
pressure (Lavender et al. (2000)

8. Surface eddy speed and regions of deep convection
Color = surface eddy
speed (cm/s)
Contours = Absolute geostrophic
pressure (Lavender et al. (2000)

9. Mixed-layer depth in winter 1997
Color = mixed layer depth (m)
Contours = Absolute geostrophic
pressure (Lavender et al. (2000)

10. WOCE AR7W line Inverse calculation
Time period of high NAO in
the early to mid 1990s

11. Float data Mean circulation at 700m 1995–1999 AR7W
Lavendar et al. (2000)

12. Referencing the geostrophic velocity section
Mean velocity at 700m from PALACE

13. Referencing the geostrophic velocity section
Mean velocity at 700m from PALACE
Mean temperature/density section X

14. Referencing the geostrophic velocity section
Mean velocity at 700m from PALACE
Mean temperature/density section X

15. Referencing the geostrophic velocity section
Mean velocity at 700m from PALACE
Total transport in: 35.4 Sv
Total transport out: 35.5 Sv
Mean temperature/density section X

16. Absolute geostrophic velocity
AR7W

17. Absolute geostrophic velocity
Throughput
Overflow water transport = 12.4 Sv
Recirculation transport = 2.5 Sv
Total Boundary Current transport = 28.5 Sv

18. Absolute geostrophic velocity
Near zero
velocity!
Throughput
Overflow water transport = 12.4 Sv
Recirculation transport = 2.5 Sv
Total Boundary Current transport = 28.5 Sv

19. Depth space: Overturning and horizontal components of the flow
Deviation
Decompose the full velocity:
Horizontal
gyre
Baroclinic
gyre
Then decompose the deviation velocity:

20. Transport components
Overturning Cell
Horizontal cell

21. Transport components
95% of sinking happens
in boundary current

22. Density space: Overturning transport

23. Density space: Overturning transport
> 95% of the transport imbalance
occurs in the boundary current

24. What are the important points for OSNAP?
Seaward of the 700m isobath there is mass balance across
the section (i.e. excluding the shallow shelf-edge flows).
The horizontal cell is significantly larger than the overturning cell.
Nearly all of the overturning, in depth space and density space,
occurs along the rim of the basin. Flows in the middle of the
Basin are weak.
There is substantial mesoscale variability, so low-passing will be necessary.

25. Ongoing Timeseries in the Labrador Sea
OSNAP-east moorings

26. Ongoing Timeseries in the Labrador Sea
OSNAP-east moorings

27. Considerations for OSNAP eastern boundary array

28. Considerations for OSNAP eastern boundary array
1997 hydrographic
Sections

29. Two eastern boundary crossings in Feb 1997
North
Salinity (color) overlain
by density (contours)
26 Feb
South

30. Considerations for OSNAP eastern boundary array

31. Considerations for OSNAP eastern boundary array

32. Considerations for OSNAP eastern boundary array

33. Considerations for OSNAP eastern boundary array
2001 hydrographic
Section

34. Eastern boundary crossing in Aug 2001
Salinity (color) overlain by
density (contours)
Absolute geostrophic velocity (color)
overlain by density (contours)

35. Eastern boundary crossing in Aug 2001
Salinity (color) overlain by
density (contours)
Absolute geostrophic velocity (color)
overlain by density (contours)

36. Eastern boundary crossing in Aug 2001

37. Eastern boundary crossing in Aug 2001

38. Considerations for OSNAP eastern boundary array

39. Eastern boundary crossing in Aug 2001

40. Eastern boundary crossing in Aug 2001

41. Eastern boundary crossing in Aug 2001

42. Eastern boundary crossing in Aug 2001
Use of current meters and Microcats is safer
and allows for 2-year deployments

43. Strawman OSNAP-West Boundary Arrays

44. Remaining issues: shelf, near-surface, recirculation

45. Remaining issues: shelf, near-surface, recirculation

46. Considerations for OSNAP eastern boundary array

47. Positive aspects of eastern array location
Bottom topography is gentle enough to effectively resolve
the boundary current.
Array will coincide with an altimeter line (providing additional surface velocity information).
By moving south, the array is a true input boundary condition for
the Labrador Sea (to compare to the western export array).
The offshore recirculation is geographically confined, which means it can be effectively sampled with floats.

48. Negative aspects of eastern array location
It is not the AR7W line! (which provides a wonderful context).
Logistically more difficult to service the array.
Closer to OSNAP Cape Farewell array (less contrast).