Rebecca Hummels 1 , Peter Brandt 1 , Marcus Dengler 1 , Jürgen Fischer 1

1. Western boundarycirculation in thetropical South Atlanticanditsrelationto Tropical AtlanticVariability Rebecca Hummels1, Peter Brandt1, Marcus Dengler1, Jürgen Fischer1 1GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany Workshop Brazil-Germany, Fortaleza, Brazil, 27.05.2014

2. Tropical AtlanticVariability • Twomodesofclimatevariability • Meridional gradientmode • Zonal mode • interannual SST anomaliesassociatedwiththesepatternsarerelatedtorainfallanomaliesovertheadjacentcontinents Kushnir et al.2006

3. Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC) Kuhlbrodt et al. 2007

4. Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC) 2) Shallow Subtropical-tropical Overturning Circulation 3) Wind driven circulation Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC) 2) Shallow Subtropical-tropical Overturning Circulation Subduction (blue) Upwelling (green) Equatorward transport (circles and numbers in [Sv]) Poleward Ekman transport (red arrows) Schott et al. 2004

5. Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC) 2) Shallow Subtropical-tropical Overturning Circulation 3) Wind driven circulation  Interaction betweenthehemispheresisfocused on the western boundary

6. Observationsat 5°S and 11°S between1990-2004: • 9 researchcruises: repeatedlyoccupiedthe 5°S and 11°S section • Mooring arrayat 11°S Schott et al. 2005

7. Observationsat 5°S and 11°S between 2000-2004: Meanstate cm/s cm/s Average transportsat 11°S: NBUC 21.7 +/-5.3 Sv NADW 24.6 +/-5 Sv Average transportsat 5°S: NBUC 22.1 +/- 5.3 Sv [=1 x106 m3s-1] NADW 20.3 +/-10.1 Sv

8. Observationsat 5°S and 11°S between 2000-2004: Variability • NBUC: • Average transportissimilartoshipsections (23.3 Sv) • Seasonalvariability : amplitudeofannualand semi-annualharmonics < 2 Sv • Interannualvariability: forthe 4 yearsisestimatedto +/- 1.2 Sv • NADW: • Average transportsimilartoshipsections • Extremely large variability Schott et al. 2005

9. Observationsat 5°S and 11°S between 2000-2004: Break upof DWBC in toeddiesataround 8°S Dengler et al. 2004

10. Other studies afterobservationalperiod: Biastoch et al. 2008 extremelyclosecorrespondencebetween AMOC strengthat 6°S (red) and NBUC transport (black) in a modelstudy

11. Other studies afterobservationalperiod: Biastoch et al. 2009 Salinityanomalieswithinthe NBUC arerelatedtothevariabilityoftheAgulhasleakageandmighthaveimplicationsforfurtherevolutionof MOC

12. Other studies afterobservationalperiod: Zhang et al. 2011 Strong interannual NBUC transportvariabilitywhenconstructingthegeostrophictransporttimeseriesbased on historicalhydrographicobservations in the NBUC region

13. Other studies afterobservationalperiod: Biastoch et al. 2008 Biastoch et al. 2009 ? Zhang et al. 2011 2000 2010 2005 2015

14. New observations: a) velocities 5°S

15. New observations: a) velocities 11°S

16. New observations: b) watermasscharacteristics 5°S

17. New observations: b) watermasscharacteristics 5°S

18. New observations: b) watermasscharacteristics 5°S and 11°S °C Biastoch et al., 2009 Average differences (5°S and 11°S) ΔS1 (27.7<γn<24.5) = 0.136 / decade ΔS2 (28.135<γn<27.7) = -0.005 /decade Average differences (5°S and 11°S) ΔO1 (27.7<γn<24.5) = 3μmol/kg/decade ΔO2 (28.135<γn<27.7) = 1.36μmol/kg/decade

19. New observations: c) mooringarray Mooring arraydeployed in July 2013 Recoveredandredeployed in April/May 2014

20. cm/s New observations: mooringarray K1 K2 K3 K4  furtherprocessingnecessarybeforeprovidingthecontinuationofthetransporttimeseries

21. Further aims • estimatethenorthwardtransportofcentraland intermediate waterwithinthe NBUC aspartofthe AMOC and STC

22. Further aims • estimatethenorthwardtransportofcentraland intermediate waterwithinthe NBUC aspartofthe AMOC and STC • monitorthetransportvariabilityofthe NBUC on intraseasonaltointerannualtimescales

23. Further aims • estimatethenorthwardtransportofcentraland intermediate waterwithinthe NBUC aspartofthe AMOC and STC • monitorthetransportvariabilityofthe NBUC on intraseasonaltointerannualtimescales • analysetheconnectionbetweentransportvariations in the western boundarycurrentsystemofthetropical South Atlantic (warm andcoldwaterroute) andthevariabilityofthe subpolar North AtlanticwithrespecttothesignalpropagationwithintheAMOC

24. Further aims • estimatethenorthwardtransportofcentraland intermediate waterwithinthe NBUC aspartofthe AMOC and STC • monitorthetransportvariabilityofthe NBUC on intraseasonaltointerannualtimescales • analysetheconnectionbetweentransportvariations in the western boundarycurrentsystemofthetropical South Atlantic (warm andcoldwaterroute) andthevariabilityofthe subpolar North AtlanticwithrespecttothesignalpropagationwithintheAMOC • analysethepropagationofwatermassanomalies in the AMOC, whichcan e.g. becausedbythevariability in theAgulhasleakage

25. Further aims • estimatethenorthwardtransportofcentraland intermediate waterwithinthe NBUC aspartofthe AMOC and STC • monitorthetransportvariabilityofthe NBUC on intraseasonaltointerannualtimescales • analysetheconnectionbetweentransportvariations in the western boundarycurrentsystemofthetropical South Atlantic (warm andcoldwaterroute) andthevariabilityofthe subpolar North AtlanticwithrespecttothesignalpropagationwithintheAMOC • analysethepropagationofwatermassanomalies in the AMOC, whichcan e.g. becausedbythevariability in theAgulhasleakage • analysetheconnectionbetween NBUC variabilityat 11° S and EUC variabilityat 23°W on theequatoranditsrelevanceforclimatevariability

27. m

28. Newobservations: b) T/S characteristics 5°S