1. Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview Western Boundary Circulation (Schott et al. 2004) - shipboard sections at 5°S (8 sections) and 11°S (4 sections) - moored observations at 11°S (2000-2003) Deep Western Boundary Current Eddies Summary Tropical Atlantic Workshop, June 2004 Marcus Dengler Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler Leibniz-Institut für Meereswissenschaften, Kiel, Germany

2. Observational program (1990-2004) 8 sections 4 sections Array 2000 -2004 (gaps!)

3. Mean salinity at 11°S and transport layer distinction Boundary Circulation in the tropical South Atlantic

4. 10 NBUC always at shelf edge DWBC sometimes reversed at slope ( new LADCP post-processing method) Boundary Circulation at 5°S (8 Sections, 1990-2003) Along-shore LADCP/ADCP velocity sections from 5°S

5. NBUC=26.8 Sv 14.3 Sv in SACW layer offshore southward AAIW layer flow (5.2 Sv) net upper 21.5 Sv DWBC= -28.1 Sv upper NADW -14.7Sv m/l NADW -5.3Sv net NADW -20.0 Sv AABW = 1.3 Sv Section total almost closed (10%) Boundary Circulation at 5°S (8 Sections, 1990-2003)

6. NBUC well developed AAIW counter current Large NADW transports and counter currents Boundary Circulation at 11°S (4 Sections, 2000-2003)

7. NBUC = 23.3 Sv 12.1 Sv in SACW layer southward offshore flow at AAIW level net upper 21.2 Sv DWBC = 41.2 Sv 18.2 Sv return offshore net NADW -23.0 Sv AABW = 1.4 Sv Section nearly balanced (<1Sv). T heat = 1.0 PW T fresh = - 0.2 Sv Boundary Circulation at 11°S (4 Sections, 2000-2003)

8. NBUC already fully established at 11°S (i.e. is part of the STC) Box 5-11°S requires westward flow in upper layer (but upper 20m uncertain due to extrapolation) and eastward flow in AAIW layer (300-1200m) EUC layer (main STC carrier): little inflow from east: good site!! Offshore southward flow in AAIW layer! <24.5 24.5-26.8 26.8-32.15 total (Sv) 5°S section boundary 3.7 13.3 8.826.8 Offshore --.2 -5.1 -4.9 net 3.7 13.9 3.020.6 Budget of upper circulation 11°S section boundary 1.5 12.1 9.723.3 Offshore -.4 1.1 -2.6 -1.9 net 1.1 11.0 7.119.2

9. 5 ° S section and AAIW-layer circulation in 1/12 ° Miami model  NBUC retroflection supplies  recirculation Budget of upper circulation Courtesy of Z. Garaffo and E. Chassignet

10. Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview Western Boundary Circulation (Schott et al. 2004) - shipboard sections at 5°S (8 sections) and 11°S (4 sections) - moored observations at 11°S (2000-2003) Deep Western Boundary Current Eddies Summary Tropical Atlantic Workshop, June 2004 Marcus Dengler Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler Leibniz-Institut für Meereswissenschaften, Kiel, Germany

11. Moored observations at 11°S (2000-2003) 100m level (large gaps)

12. 1900m level Moored observations at 11°S (2000-2003) Southward „DWBC“ aliasing during ship section times

13. Box transports (Sv) 1: 12.5 2: 2.8 3. 9.8 4: 2.7 1-4: 27.8 (NBUC) (Ship NBUC was 23.3) 5:-3.0 (+0.7): (AAIW recirculation, -2.6 from ship obs. ) 6: -18.7 (DWBC) (ship mean was –44 Sv) Moored observations at 11°S (2000-2003) Mean section and boxes (1-6) for time series 1 2 3 4 5 6

14. Moored observations at 11°S (2000-2003) NBUC transport variability (box 1-4) current records mapped using EOFs from all time series mode 1+2 explains NBUC variability strong variability on different time scales

15. Moored observations at 11°S (2000-2003) DWBC transport variability (Box 6) strong intraseasonal variability

16. Significant seasonal (semi+annual) cycle of NBUC and DWBC explains lower NBUC transport from ship- board observations Moored observations at 11°S (2000-2003) Annual and semi-annual cycle of transport time series NBUC variability at 11°S may be (partially) caused by SEC bifurcation migration (pers. comm. S. Huettl)

17. Interannual differences and trends 2000-2003 Annual averages: Time NBUC 3/00-3/01 23.6 3/01-3/02 23.7 3/02-3/03 26.4 NBUC Line fit: 1.5 Sv/year Moored observations at 11°S (2000-2003)

18. Correlation of NBUC and DWBC transport with alongshore currents of array Moored observations at 11°S (2000-2003) Variability in the upper ocean is not correlated to deep ocean variability.

19. Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview Western Boundary Circulation (Schott et al. 2004) - shipboard sections at 5°S (8 sections) and 11°S (4 sections) - moored observations at 11°S (2000-2003) Deep Western Boundary Current Eddies Summary Tropical Atlantic Workshop, June 2004 Marcus Dengler Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler Leibniz-Institut für Meereswissenschaften, Kiel, Germany

20. Deep Western Boundary Current Eddies along-shore velocity Structure of DWBC similar to counter current structure

21. Deep Western Boundary Current Eddies Salinity distribution and contours of along-shore velocity Oxygen distribution and contours of along-shore velocity DWBC and offshore counter currents carry same water masses

22. Deep Western Boundary Current Eddies 50-90d band-pass filtered time series EKE maximum largest at 1900m (K3,K4) and 2400m (K5) Maxima away from continental slope

23. Deep Western Boundary Current Eddies 50-90 day band pass filtered velocities at 1900m (2400m) depth Dengler et al., DSR, 2004 Sequence of anti-cyclonic sub-mesoscale eddies (about 6 per year) cross-shore velocity along-shore velocity

24. alongshore component: across-shore component: Eddy fit to mooring data Eddy model based on Gaussian-shaped density distribution as array is one-dimensional - eddy translation LACDP data Eddy fit 2000m

25. Eddy fit to mooring data Eddy model fit: min Data: 14 velocity time series 40h low-pass data 40-80 day ensembles  Explained Variance 46% fit failed for two ensembles

26. Average eddy parameters Eddy translation Along-shore = 3.8 cm/s Cross-shore = 2.1 cm/s 3.9 cm/s westward drift > C R(3. mode) = 2.6 cm/s 1.8 cm/s southward drift   eddy translation not explainable by advection only

27. Average eddy parameters Eddy scale Amplitude (gaussian)= 0.50 m/s Radius (e folding) = 60 km Eddy offshore center = 160 km Height (e folding) = 1100 m Depth of maximum = 2100 m

28. Average eddy parameters Eddy Volume average Volume eddy = (13.9 +/- 5.9) x 10 13 m 3 DWBC- Volume eddy = (10.5 +/- 3.6) x 10 13 m 3 (depth range: 1200-3800m)

29. Box 6 transport from eddy sequence ( - ) and moorings ( -- ) - eddy scales could not be determined Transport: Box 6 (moorings) = -14.2 Sv Eddies Box 6 = -15.2 Sv total eddy DWBC transport (1200- 3800m) = -17.9 Sv Eddy Transport DWBC array sees rectified eddy mean!

30. Deep Western Boundary Current Eddies in Models Eddy kinetic energy and mean kinetic energy at 1900m from FLAME model FLAME Model 1/12° resolution Domain: 18°S-70°N, 100°W-16°E setup includes open boundary at 18°S EKE distribution similar to distribution from 1/6° OPA model. (Trequier et al., JPO, 2003) DWBC-EKE at 11°S only slightly enhanced in 1/12° Miami model.

31. Deep Western Boundary Current Eddies in Models FLAME Model eddy kinetic energy along 10°S Deep EKE maximum similar to observations Snapshot of along-shore and across-shore velocity

32. Deep Western Boundary Current Eddies in Models Velocity vectors and density perturbation Model suggests eddy generation near 8°S. Continental slope turns sharply to the west.

33. Upper layers NBUC - average transport: at 5°S from 8 sections: 26.8 Sv, at 11°S from moorings: 27.8 Sv - 13 Sv as part of STC - SEC bifurcation well south of 11°S - stable NBUC core at 5-11°S - strong annual and semi-annual transport variability - significant interannual differences in NBUC (linear trend: 1.5 Sv/year) - relation to STC variability? - how much near boundary vs. interior? (in Pacific: interior variability partially compensated by wbc) AAIW-Layer - offshore southward recirculation (5 Sv) - how does it connect? - Miami model: NBUC retroflection - - 800m RAFOS show inflow from east Summary

34. Deep circulation DWBC transports at 5-11S - at 5°S (8 sections): net southward flow –20 Sv - at 11°S (4 sections): section net –23 Sv - DWBC transport from eddies: about –18 Sv DWBC Eddies - 5-6 DWBC eddies per year (similar to NBC rings) - carry water masses of the DWBC southward - westward translation agrees with eddy dynamics (southward translation?) - FLAME simulation indicate: - generation at ~7-8°S - eddy size correlates with upstream DWBC transport - eddy existence implies no „classical DWBC“ south of °S Summary