1. WHOI, 20-Dec-2006 Cold Event in the South Atlantic Bight during Summer 2003: Observations and Model Simulations Alfredo Aretxabaleta Applied Ocean Physics & Engineering WHOI J. Nelson, J. Blanton (SkIO) B. Blanton, H. Seim, F. Werner, J. Bane (UNC-CH) E. Chassignet (FSU) R. Weisberg (USF)

2. WHOI, 20-Dec-2006 Baroclinic process: associated with density structure (density and pressure surfaces intersect) Barotropic response is “simpler” e.g.: tides, direct wind forcing Difficult both to measure and to model Examples: fronts, plumes, internal waves Implications: mixing, circulation, biological processes Baroclinic processes

3. WHOI, 20-Dec-2006 Introduction Observations Model simulations Implications Conclusions Outline

4. WHOI, 20-Dec-2006 South Atlantic Bight (SAB) Region Bane et al., 2001

5. WHOI, 20-Dec-2006 Summer 2003 beach report http://www.washingtonpost.com/ac2/wp-dyn/A25865-2003Aug6? Shivering in the Surf Atlantic's Sudden Temperature Dive A Midsummer Mystery for Scientists By John F. Kelly Washington Post, Thursday, August 7, 2003

6. WHOI, 20-Dec-2006 Anomalous hydrographic and atmospheric conditions A. Aretxabaleta, J. Nelson, J. Blanton, H. Seim, F. Werner, J. Bane, R. Weisberg Published JGR, 2006

7. WHOI, 20-Dec-2006 2003 observations Cross-shelf hydrographic transects Wind observations Bottom temperature NOS coastal water level

8. WHOI, 20-Dec-2006 Surface Temperature Anomaly NDBC (NOAA)

9. WHOI, 20-Dec-2006 R2 Bottom Temperature Year 2003 Temperature ( o C) 6 o C temperature anomaly

10. WHOI, 20-Dec-2006 SST anomaly July 2003 MODIS, NASA Earth Observatory

11. WHOI, 20-Dec-2006 SAB processes http://www.skio.peachnet.edu/ Inner-shelf: coast-20m isobath Mid-shelf: 20-40m isobaths Outer-shelf: 40-75m isobath

12. WHOI, 20-Dec-2006 Wind temporal variability 2003 Upwelling favorable wind stronger than in previous years Apr May Jun Jul Aug Sep Oct

13. WHOI, 20-Dec-2006 Wind Persistence Direction (degrees) July 2003 July previous years (1988-2002) 2003 wind persistently in upwelling favorable direction (red line)

14. WHOI, 20-Dec-2006 River Discharge Higher than average river discharge during the study period (USGS)

15. WHOI, 20-Dec-2006 Cruise tracks R/V Savannah Most of the cruise observations in inner and mid-shelf

16. WHOI, 20-Dec-2006 Temperature transects Cold water intrusion increases with time

17. WHOI, 20-Dec-2006 Salinity transects Strong salinity gradients during spring Very homogeneous conditions during the summer

18. WHOI, 20-Dec-2006 Density transects Level of stratification increases as season progresses

19. WHOI, 20-Dec-2006 May Salinity Large surface pool of fresher water Strong salinity stratification 07-May-2003 observed salinity May climatological salinity

20. WHOI, 20-Dec-2006 June Temperature By June Transformed into thermal stratification June climatological temperature 25-Jun-2003 observed temperature

21. WHOI, 20-Dec-2006 August Temperature August climatological temperature 27-Aug-2003 observed temperature By August Strong stratification with water 8°C colder!!

22. WHOI, 20-Dec-2006 T-S characteristics

23. WHOI, 20-Dec-2006 Salinity Temperature ( o C) T-S characteristics MAY Surface water greatly influenced by river discharge

24. WHOI, 20-Dec-2006 Salinity Temperature ( o C) T-S characteristics JUNE “Deep”  related to Gulf Stream water “Surface”  still influenced by fresh water

25. WHOI, 20-Dec-2006 Salinity Temperature ( o C) T-S characteristics AUGUST Closely linked to Gulf Stream water

26. WHOI, 20-Dec-2006 Origin of upwelled water where α is the thermal expansion, β is the saline contraction, T GS is the Gulf Stream temperature and T j is the observed temperature

27. WHOI, 20-Dec-2006 ~10-15 cm lower than climatology During Jul + Aug Virginia Key Cape Hatteras Water Level anomaly (2003 – climatology)

28. WHOI, 20-Dec-2006 Virginia Key Cape Hatteras Water Level anomaly (2003 – climatology)

29. WHOI, 20-Dec-2006 Enhanced Ekman Transport Relationship between Ekman velocity and stratification (Niiler and Krauss, 1977; Ralph and Niiler, 1999) Therefore, in shallow areas (not fully developed Ekman layer) the total Ekman transport is constraint to a shallower layer, resulting in increased Ekman velocities.

30. WHOI, 20-Dec-2006 Anomalous upwelling favorable winds Increased river discharge produced stronger stratification Salinity and thermal stratification enhanced upwelling process The cold water upwelled onto the shelf and the Gulf Stream water had similar characteristics No single mechanism explains the cold anomaly Conclusions from observations

31. WHOI, 20-Dec-2006 Model Simulations and Implications A. Aretxabaleta, B. Blanton, H. Seim, F. Werner, J. Nelson, E. Chassignet Submitted JGR

32. WHOI, 20-Dec-2006 GOALS Relative importance of different forcings Anomalous wind River discharge and increased stratification Gulf Stream intrusion Comparison with observations Estimate volume of anomalous water on the shelf Possible biological implications Summer 2003 cold event model study

33. WHOI, 20-Dec-2006 Model QUODDY Nonlinear prognostic, finite element model (Lynch & Werner, 1991) Uses higher order advection scheme (Kliem, 2004)

34. WHOI, 20-Dec-2006 Model Procedure HYCOM/ CLIM Post-processing Comparison with obs. River discharge USGS EDAS

35. WHOI, 20-Dec-2006 Model forcings Generated by Brian Blanton Initial and open-ocean boundary conditions: HYCOM 1/16 North Atlantic analysis climatological temperature and salinity ( Blanton et al., 2003 ) Tidal boundary conditions: ADCIRC tidal database ( Blanton et al., 2004 ) Atmospheric forcing: atmospheric model (EDAS)

36. WHOI, 20-Dec-2006 HYCOM mass fields Generated by Brian Blanton

37. WHOI, 20-Dec-2006 UNC-SAB model nested regional-scale QUODDY (right). Within the 1/12 deg operational HYCOM-GODAE model (left). SAB Nested Regional Model

38. WHOI, 20-Dec-2006 Heat flux Atmospheric forcing: EDAS model

39. WHOI, 20-Dec-2006 HYCOM as IC and BC estimator HYCOM (NRL) QUODDY Resolution~1/12 deg O(10 km)50 m to 5 km Bathymetry ETOPO5 + local improvements NGDC Coastal Relief Model Coastal Wall~10-m isobath 0-m contour line in NGDC CRM RiversNONE in SAB7 TidesNONETides (Blanton et al, 2004.)

40. WHOI, 20-Dec-2006 Surface Salinity Inclusion of River Discharge

41. WHOI, 20-Dec-2006 Summer 2003 hindcast model results Temperature ( o C) 2003 Surface Temperature @ Gray’s Reef Gray’s Reef Year 2003 RMS difference 1.81 o C

42. WHOI, 20-Dec-2006 2003 Bottom Temperature @ R2 R2 Temperature ( o C) Summer 2003 hindcast model results RMS diff. 0.90 o C

43. WHOI, 20-Dec-2006 2003 Bottom Temperature @ CAP3 CAP3 Temperature ( o C) Summer 2003 hindcast model results

44. WHOI, 20-Dec-2006 Model results Heat flux errors affect the surface temperature field and the total stratification

45. WHOI, 20-Dec-2006 Model results Inclusion of river discharge provides approx. to salinity field

46. WHOI, 20-Dec-2006 Model results (river effect) Effect of river discharge (river -no river simulations) ~0.5 o C in subsurface

47. WHOI, 20-Dec-2006 Model results Effect of atmospheric forcing (2003 simulation minus simulation with 2002 wind) 1-2 o C in subsurface

48. WHOI, 20-Dec-2006 Skill assessment Model skill definition Hetland & Signell (2005) where d i are available observations, m i are model results and c i are climatological values

49. WHOI, 20-Dec-2006 Bottom Temperature Animation Temperature ( o C) Bottom Temperature < 23 o C

50. WHOI, 20-Dec-2006 Remote upwelling dynamics

51. WHOI, 20-Dec-2006 Across isobath flow Influx to outer-shelf along shelf- break in central and southern areas Influx to mid- shelf in southern areas and near-bottom in central areas

52. WHOI, 20-Dec-2006 Vertically-averaged temperature anomaly Influx of anomalously cold water (2 o C colder than normal) along shelf- break in central and southern areas

53. WHOI, 20-Dec-2006 Volume of anomalous water Volume (km 3 ) Percentage of shelf volume 16-May-2003735.3 01-Jun-200360243.8 16-Jun-200351237.2 01-Jul-200368750.0 16-Jul-200378857.3 01-Aug-200384661.6 16-Aug-200365547.8 01-Sep-200347734.9 16-Sep-2003695.0

54. WHOI, 20-Dec-2006 Implications Bottom nitrate estimate from temperature Large pool of extra nitrate in mid-shelf

55. WHOI, 20-Dec-2006 Model provides realistic description of event Anomalous wind stress conditions Main driver (inner- and mid-shelf) High river discharge Effect on stratification Controlled inner-shelf structure Strong stratification Enhanced upwelling process Gulf Stream Source of cold water Anomalous cold water >50% shelf Biological implications (to be determined) Event Conclusions

56. WHOI, 20-Dec-2006 Improve atmospheric forcing Improve initial conditions More elaborate nesting between basin-scale (HYCOM) and shelf-scale Evaluation of nitrate fluxes Next steps

57. WHOI, 20-Dec-2006 Acknowledgments Funding SEACOOS (ONR) SABLAM (NOPP) Institutions UNC-CH Skidaway Institute of Oceanography Dartmouth College People Dennis McGillicuddy, Keston Smith, J.L. Pelegri, Rick Luettich

58. WHOI, 20-Dec-2006 Inner-shelf coast-20m isobath Tides, Winds, Rivers Mid-shelf 20m-50m isobaths Winds, Tides, Q-Flux Outer-shelf 50m-Shelfbreak W.B.C. Influences Currently, neither basin-scale nor regional-scale models can include all relevant scales/physics for shelf prediction We augment basin-scale HYCOM TS initializations and estimates of TS and sea level along a limited-area domain open boundary, with locally enhanced resolution, physics, and forcing, through nesting procedures.

59. WHOI, 20-Dec-2006 Summer 2003 hindcast model results Gray’s Reef RMS difference 1.81 o C

60. WHOI, 20-Dec-2006 R2 Summer 2003 hindcast model results RMS diff. 0.90 o C

61. WHOI, 20-Dec-2006 2003 Bottom Temperature @ CAP3 CAP3 Temperature ( o C) Summer 2003 hindcast model results