Towards a climatology of the SE US coastal ocean

1. Towards a climatology of 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, Florida State University

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. 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

4. Blanton et al. 2004 – digital model climatology, forced by mass field and climatological winds (COADS) – inner shelf regime hard to distinguish, limited northern extent

5. 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

6. 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 - Disparate moorings and data management systems

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

8. Seasonal depiction – consider: • Winds • Limited temperature/salinity time series • Depth-averaged currents • Depth-varying currents

9. 0.03 N/m2 Wintertime Fairly uniform SE wind stress Dominated by cold-air outbreaks

10. Wintertime 20 cm/s 40m Depth-averaged flow 400m • Similar to mean • Reasonable comparison to model 20m Mean position of GS

11. Feb bottom temp Feb surf temp Blanton bottom temp clim. Blanton climatology

12. Feb surf salinity Feb bot salinity Blanton surface salinity clim Blanton climatology

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

14. Summer Bermuda-high dominated Northward wind stress

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

16. Jul bottom temp Jul surf temp Blanton bot temp clim Blanton climatology

17. July surf salinity July bot salinity Blanton climatology Blanton Surf salinity

18. 20 cm/s SC Depth-resolved flow - July Depth (m) • Significant vertical shear/veering • Consistent with upwelling • Should promote nutrient delivery from GS

19. Fall Strong southward wind stress Strength increases seaward

20. 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

21. Oct surf temp Oct bot temp Blanton Bot Temp climatology Blanton climatology

22. Oct surf salinity Oct bot salinity Blanton climatology Blanton Surf Salinity clim

23. 20 cm/s Depth-resolved flow - October Depth (m) Flow strongest on inner shelf Weak offshore bottom flow

24. 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

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

27. Some notion of dynamics: Wind stress weak – but curl? Alongshore pressure gradient important but possibly non-constant Cross-shelf baroclinic gradient - working on it.

28. 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

30. Summary • Assembled ADCP observations largely confirm qualitative depiction of Lee et al (1991) • 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 • Reduced mean flow off SC consistent with gyre influence but gyre not represented in observations. Other form of GS influence?

31. 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

35. Baringer/Larsen

36. Blaha, JGR ’84 found coherent monthly averagedsea level variationsalong shelf (’55-’75 period, heatingand atmos. presseffects removed).Can be more than 20 cm variation annually. Postulated due toGulf Stream transportvariations.

37. 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