1. Observing system depiction of circulation in the SE US coastal ocean
H. Seim, University of North Carolina at Chapel Hill
L Leonard, University of North Carolina at Wilmington
M. Fletcher, University of South Carolina
D. Savidge, Skidaway Institute of Oceanography
C. Edwards, University of North Carolina at Chapel Hill

2. Why a circulation climatology?
In general:
Simple characterization of existing data
Important source of validation for models
Motivate archival scheme
For the SE United States coastline:
Confirm existing depictions and develop digital form
Examine adequacy of observing system design
Study the dynamics of the flow field

3. A specific interest – design of Marine Protected Areas – are they connected?
Hare et al., 2007

4. No mean flow presentation
Depiction of Seasonal Cycle by Lee, Yoder and Atkinson (1991),
Based on big DOE-funded deployments in ‘70s and ‘80s
Only variability
Winter/Spring
Summer
Fall
Distinguishes 3 shelf regimes, inner (<20 m), middle (20-40 m) and outer (>40m),
And the Gulf Stream. Cartoon depicts Gulf Stream, outer and mid shelf.
No mean flow presentation

5. Blanton et al. 2004 – digital model climatology, forced by mass field and
climatological winds (COADS) – inner shelf regime hard to distinguish, no
Onslow Bay

6. Observing System measurement locations (for SABSOON, Caro-COOPs,
CORMP, NCCOOS and NDBC)
19 stations occupied between , inner and mid-shelf
Area under study
In this talk

7. What’s new? Bight-wide coverage over 5+ years
Better vertical resolution of currents
Inclusion of nearshore (10m or less)
Not so good:
No observations seaward of 40m isobath
Widely disparate moorings and data management systems

8. Coverage over time in the ‘climatology’ for ADCPs– only months with
50% or greater coverage are included

9. Depth-averaged mean currents and average winds
Cape
Fear
0.005 N/m2
Depth-averaged mean currents
and average winds
Weak mean flow (5 cm/s or less)
inshore of 30 m isobath, divergent
GS-influenced poleward flow seaward
of 40 m isobath
Near-zero flow S off SC
Topographic steering – flow largely
along isobaths
Mean winds are weak and variable
50m
15m

10. MAB depth-averaged mean current – equatorward and relatively uniform
Lentz, JGR, 2008

11. Seasonal depiction – consider:
winds
Limited temperature/salinity time series
Depth-averaged currents
Depth-varying currents

12. 0.03 N/m2
Wintertime
Fairly uniform SE wind stress
Dominated by cold-air outbreaks

13. Wintertime Depth-averaged flow Similar to mean
20 cm/s
Depth-averaged flow
Similar to mean
Reasonable comparison to model

14. Feb bottom
temp
Feb surf
temp
Blanton
climatology
Temp (deg C)

15. Depth-resolved flow - February Depth (m)
Generally little vertical structure
Exception at nearshore stations

16. Summer
Bermuda-high dominated
Northward wind stress

17. Summer SC Depth-averaged flow Whole shelf in motion to NE
Minimum flow off SC – signature of gyre?
Model underestimates inner shelf flow

18. Jul bottom
temp
Jul surf
temp
Blanton
climatology
Temp (deg C)

19. Depth-resolved flow - July SC Depth (m) Significant veering
20 cm/s SC
Depth-resolved flow
- July
Depth (m)
Significant veering
Consistent with upwelling
Should promote nutrient delivery from GS
Exception at shallow stations off SC

20. Fall
Strong southward wind stress
Strength increases seaward

21. Fall SC Depth-averaged flow GA Reduced flow at 40 m isobath
Southward flow on middle, inner shelf
Minima off SC again
Schematic captures flow well
Model misrepresents inner, middle shelf

23. Depth-resolved flow - December Depth (m)
20 cm/s
Depth-resolved flow
- December
Depth (m)
Confused flow, strongly divergent
Veering mostly in opposite sense
Offshore bottom flow – convection?

24. Blaha, JGR ’84 found coherent monthly averaged sea level variations over SAB (’55-’75 period, heating and atmos. press effects removed). Can be more than 20 cm variation annually. Postulated due to Gulf Stream transport variations.

25. Noble/Gelfenbaum – modeled coastal SL impact of GS transport variations.
Low transport
Gulf Stream
Average transport
Coast
Offshore
Fixed “Hinge”
Low transport,
higher CSL
Shelf
Gulf Stream
Average transport
Coast
High transport
Offshore
Fixed “Hinge”
High transport,
lower CSL
Shelf

26. Role of Charleston Bump?
Does turn of GS at the Bump change the surface elevation on the shelf?
Could explain the slowdown/reversal in alongshelf flow off SC

27. Summary
Assembled ADCP observations largely confirm qualitative depiction of Lee et al (1991) – reduced flow off SC consistent with gyre influence but gyre not represented in observations.
Digital climatology of Blanton et al (2004) fails to represent inner shelf and equatorward mid-shelf flows
Strong upwelling circulation in summer is evident
Downwelling circulation present in fall/winter/spring but not shelf-wide
Plan to continue assembly of currents and winds, temperature and salinity measurements

28. MONTHLY MEAN ALONG- AND CROSS-SHORE CURRENT
Climatological along-shore monthly mean wind (scaled 1cm/s:1m/s)
At Station
Off GA
Depth (m above bottom)
Depth (m above bottom)
SSW
ALONG
NNE
On-shore
CROSS
Off-shore

32. Baringer/Larsen

33. 40 cm/s
Jan

34. 40 cm/s
Feb

35. 40 cm/s
Mar

36. 40 cm/s
Apr

37. 40 cm/s
May

38. 40 cm/s
Jun

39. 40 cm/s
Jul

40. 40 cm/s
Aug

41. 40 cm/s
Sep

42. 40 cm/s
Oct

43. 40 cm/s
Nov

44. 40 cm/s
Dec