Arnold L. Gordon Retroflections and Bifurcations Johann Lutjeharms Memorial Lecture

Transcription

1 The Agulhas System and its Role in Changing Ocean Circulation, Climate & Marine Ecosystems Spier Hotel, Stellenbosch, Western Cape, South Africa 8 12 October 2012 Brazil/Malvinas Agulhas Retroflection Arnold L. Gordon Retroflections and Bifurcations Johann Lutjeharms Memorial Lecture Brazil Current Agulhas Agulhas Retroflection

2 The Agulhas System and its Role in Changing Ocean Circulation, Climate & Marine Ecosystems Spier Hotel, Stellenbosch, Western Cape, South Africa 8 12 October 2012 Brazil/Malvinas Agulhas Retroflection Arnold L. Gordon Retroflections and Bifurcations Johann Lutjeharms Memorial Lecture Brazil Current Retroflection Agulhas Agulhas Retroflection

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4 Station #5 of the ARC cruise: a trapped blob [eddy] of Indian Ocean water within a anticyclonic mesoscale feature in the SE Atlantic. The homogeneous layer from 75 to 280 meters was clearly winter modification of Indian Ocean water. The region between the coast and the eddy was mainly Indian Ocean water, stirred into the Cape Basin winter mixed layer 16 C Cape Town Eddy core Salty eddy Red Sea water Cape Town Eddy 16 C Stad Agulhas Retroflection Retroflection on Eddy 19 C Stad 19 C Stad

5 SAC Benguela C 0/1500 db Agulhas C. The primary attention has been directed at the eddy field, but the T/S, CFC-11 and geostrophic calculations reveal that the Cape Basin is tagged with Indian Ocean thermocline and intermediate water, Between the eddy corridor and the African coast there is frequent streamers [filaments] of Indian Ocean water. Benguela is ~50% Agulhas Agulhas C, low CFC [on isotherms] Benguela C, more like Agulhas CFC South Atlantic Current, High CFC Gordon, A.L., R.F. Weiss, W. M. Smethie, Jr., and M. J. Warner (1992) Thermocline and intermediate water communication between the South Atlantic and Indian Oceans. J. Geophys. Res., 97(C5): R. Rodrigues, M.Wimbush, D. R. Watts, L. M. Rothstein and M. Ollitrault South Atlantic mass transports obtained from subsurface float and hydrographic data. Journal of Marine Research, 68, , 2010

6 Bigger picture south equatorial current Subtropical gyre eddy d corridor eddy dy core leakage *Brazil eddies? South Atl current supergyre * Smythe-Wright et al (1996) CFC-113 shows Brazil Current Ring crossing the South Atlantic to the Agulhas Retroflection Region. JGR 101(C1): Byrne et al

7 Agulhas Leakage makes the Benguela Current warmer/saltier than it would be if the S. Atlantic gyre were closed by the cooler, fresher South Atlantic Current. Additionally: Export of North Atlantic water vapor across Central America and South Atlantic subtropical derived water vapor into the Indian Ocean within the South Atlantic Current. These factors conspire to make for a salty Atlantic, conditioning it for deep ocean overturning, the AMOC The Benguela Blend Sv Eddy/Ring Agulhas leakage e Eddies and streamers streamer Agulhas Agulhas Retroflection Sv Agulhas Return C. Maximum westerlies South Atlantic C. Δ Δ Sv = 10 6 m 3 /sec Latitudal shifts in Maximum westerlies can alter the leakage and by inference the AMOC

8 Gordon 1986 ITF warm water route Cold water route ARC cruise led me to a global pattern of interocean thermocline exchange: the return of the NADW Broecker [Monnier], 1987 A plethora of schematics from cartoons to complex wiring diagrams Richardson, P. L. 2008, On the history of meridional overturning circulation schematic diagrams. Progress in Oceanography 76: Schmitz, 1996

9 From the 1983 ARC cruise to the 2012 AGU Chapman Conference, a lot of progress, a lot more questions: relation of Agulhas transport to Agulhas leakage, relation to ocean and global scale ocean and climate systems?

10 Johann, December and the 2006 seminal book Johann R.E. Lutjeharms has produced a truly scholarly work, drawing from a multitude of research papers, many from his own pen, and presenting to us with clarity the complex oceanography of the Agulhas Current system, including its rich history of exploration, from the early days to the current research endeavors. Johann indeed has met his objectives, as stated in the preface: to bring together and synthesize what has been learnt a user-friendly key to unlocking the wealth of information scattered throughout the primary literature.

11 salt added downstream upstream 2003, Nature

12 SSS SSS 33S 38S 10E 12E SSS 33S salty bridge between the S. Atlantic and Indian Oceans, blocks closure of subtropical gyre by the low SSS South Atlantic Current 38S 10E 12E

13 salty hill Aviso 10E Aquarius 33S 33S 38S 10E 12E Note- Aquarius data in evaluation mode Elevated sea level with high SSS are suggestive of Agulhas eddies. Satellite altimeter and sea surface salinity may be effective tool in tracking Agulhas leakage. 12E 38S salty hill Offset? Aquarius data processing?

14 Kuroshio N. Pacific Bifurcation Grand Banks N. Atlantic Bifurcation The Great Whirl NEC Bifurcation Mindanao Retroflection Loop Current North Brazil Current S. Atlantic Bifurcation Agulhas retroflection Madagascar Current East Australian Current S. Pacific Bifurcation Brazil-Malvinas Confluence Agulhas retroflection October 2006 Retroflections (Circles) and Bifurcations (Squares) are a primary mechanism for interocean and regional gyre/gyre exchange

15 South Atlantic Bifurcation surface expression at the Brazilian Bump, as shown by the surface drifters? The thermocline bifurcation near 10 S; poleward tilt of bifurcations are common. What sets the bifurcation latitude? How vulnerable is it shifting? For AMOC issues, the SA Bifurcation is as important as the Agulhas leakage [so is the ITF] Ocean upper layer thermocline Mesoscale rich

16 SST, oscar currents

17 Mindanao leakage Nemo could have easily ended up in the Indian Ocean Rather than running a model

18 The studies of the Agulhas system, so dramtically advanced, synthesized, schematized by Johann R.E. Lutjeharms, fostering collaborations across 3 continents, have far reaching importance, not just to the AMOC and NADW formation, present and paleo-climate systems, but also to inter-gyre and inter-ocean exchange of mass, heat and fresh water, throughout the world ocean- Retroflections (and Bifurcations) link the major circulation cells into a global network. The Johann s Agulhas Retroflection is the prototype. Lutjeharms, 2006