Observing system depiction of circulation in the SE US coastal ocean

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. 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 2000-2007, 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. 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 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 (m) Depth-resolved flow - February • 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. 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. 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 averagedsea level variationsover SAB (’55-’75 period, heatingand atmos. presseffects removed).Can be more than 20 cm variation annually. Postulated due toGulf Stream transportvariations.

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 NNE On-shore CROSS Off-shore ALONG

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