DOCSLIB.ORG

The Structure and Transport of the Agulhas Return Current

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Structure and Transport of the Agulhas Return Current The Structure and Transport of the Agulhas Return Current Isabelle Jane Ansorge Submitted in fulfillment of the requirements of the Master of Science degree Department of Oceanography University of Cape Town 1996 University of Cape Town The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town I ABSTRACT The Agulhas Current flows along the eastern coast of southern Africa as one of the largest western boundary currents in the world's ocean. On overshooting the southern tip of Africa at approximately 20°E, a retroflection loop is formed that causes the current to double back on itself and to fonn the easterly flowing Agulhas Return Current. No focussed investigation to establish the hydrography of this imponant component of global ocean circulation has to date been carried out. To trace the Agulhas Return Current's passage eastwards into the South Indian Ocean, data collected from six CTD hydrographic cruises; AJAX, ARC, SCARC, Marathon, Discovery 164 and SUZIL, as well as from three XBT temperature cruises,· Marion 83, Gallieni 72 and FIBEX, have been analysed. It is shown that the Agulhas Return Current initially flows zonally from 20°-25°E at a latitude approximately 39°30'-40°30'S before forming a planetary wave over the Agulhas Plateau between 25°-28°E. This wave results in a nonhward shift of the · Current to 37°S. Funher eastward the Current continues it's zonal flow more or less at 40°S, except that over the Mozambique, Madagascar and Southwest Indian Ridges similar waves are evident. Geostrophic speeds (referenced to the bottom) of the Agulhas Return Current show a gradual decrease from an average of 65 cmls in the retroflection region to 17 cmls at 76°E. Volume transpons at the same depth, are similarly reduced from an average of 60 Sv in the retroflection region, to 23 Sv at 76°E. II Temperature-salinity properties show water masses characteristic of the Agulhas Current, > 16 ° C and > 35, 4 psu, to extend as far east as 61 °E. East of this longitude, at approximately 66°E, Subantarctic Swface Water is shown to "cap" the swface waters, as the Agulhas Return Current weakens beyond recognition. Based on these hydrographic results, it is clear that the Agulhas Return Current may be considered to extend across the width of the Southwest Indian Ocean, terminating between 66°E and 76°E. I therefore propose that the name "South Indian Ocean Current", given by Lutjeharms and Van Ballegooyen (1984) and later by Stramma (1992) for the flow just north of the Subtropical Convergence in the Indian Ocean, be retained for the flow east of 70°E only. III ACKNOWLEDGEMENTS I would like to thank Professor J. R. E. Lutjeharms and Mr Henry Valentine for their constructive criticism, guidance and support. A special thank you is extended to Mr Roy van Ballegooyen for his invaluable advice. I thank my fellow students Stefan Kohrs, Craig Matthysen and Andrew Lee-Thorp for their office antics and the many jokes we shared. I also thank Mrs Lesley Staegemann for help and patience with my administrative problems. The financial support of the Foundation for Research and Development and the Department of Environmental Affairs is gratefully acknowledged. CONTENTS IV PAGE Abstract I Acknowledgements ill Contents IV List of Figures V Chapter 1: Chapter 2: Present knowledge of the Agulhas Return Current 4 Chapter 3: Knowledge gaps on the Agulhas Return Current 18 Chapter 4: Data and methodology 22 Chapter 5: Geographic location and general hydrography of the Agulhas Return Current 3 6 Chapter 6: Geostrophic velocities and the volume transported by the Agulhas Return Current 67 Chapter 7: Water masses of the Agulhas Return Current 116 Conclusion 150 References 152 Addenda 162 V LIST OF FIGURES FIGURE PAGE 1.1 A conceptual diagram of the Agulhas Current System 1 2.1 Kerhallet' s 1851 chart of the Agulhas Current System 4 2.2 Petermann's 1865 chart of the Agulhas Current System 5 2.3 Merz' s 1925 chart of the Agulhas Current System 6 2.4 Dietrich's 193 5 chart of the Agulhas Current System 7 2.5 Track showing the path taken by buoy 1210 10 2.6 The l 5°C isotherm and the track of the torpedo-shaped drifter 11 2.7 1976 GOSSTCOMP charts of the Sub-Tropical Convergence 12 2.8 Flow field in the South Indian Ocean 14 4.1 Distribution of CTD and XBT stations 23 4.2 Distribution of CTD stations during AJAX 24 4.3 Distribution of CTD and XBT stations during SCARC 26 4.4 Distribution of CTD stations during ARC and XBT stations during Marion 1983 28 4.5 Distribution of CTD stations during Marathon 29 4.6 Distribution of CTD stations during Discovery 164 30 4.7 Distribution of CTD stations during SUZIL 31 5.1 Location of the AJAX transect 37 5.2a/b Potential temperature and salinity section during AJAX 38 5.3 Location of the three transects during ARC 40 5.4 Dynamic height anomaly of the sea surface during ARC 40 5.5a/b Potential temperature and salinity sections during ARC transect 1 41 5.6a/b Potential temperature and salinity sections during ARC transect 2 42 5.7 Temperature section during XBT cruise Marion 83 43 5.8a/b Potential temperature and salinity sections during ARC transect 3 45 5.9 Dynamic height anomaly during SCARC 46 5.10 Location of the two transects during Marathon 47 VI 5.1 la/bPotential temperature and salinity sections during Marathon transect 1 49 5.12a/bPotential temperature and salinity sections during Marathon transect 2 50 5 .13 Location ofMarion 83 and FIBEX XB T tracks 51 5.14 Temperature section during XBT line Marion 83 52 5.15 Temperature section during XBT lineFJBEX 52 5 .16a/b General location and passage of the Agulhas Return Current from isotherms and isohalines 53 5 .17 Location of the Discovery 164 transect 54 5 .18a/b Potential temperature and salinity sections during Discovery 55 5.19 Location of the Gallieni XBT line 57 5.20 Temperature section between XBT 1-25 57 5.21 Location of the four transects during SUZIL 58 5.22a/b Potential temperature and salinity sections during SUZJL transect 1 60 5.23a/b Potential temperature and salinity sections during SUZIL transect 2 61 5.24a/b Potential temperature and salinity sections during SUZIL transect 3 63 5.25a/b Potential temperature and salinity sections during SUZIL transect 4 64 6.1 Geostrophic velocities during AJAX 71 6.2a-c Location of AJAX(a), geostrophic velocities (b) and volume transport (c) 72 6.3 Geostrophic velocities during ARC transect 1 76 6.4a-c Location of ARC transect 1 (a), geostrophic velocities (b) and volume transport ( c) 77 6.5 Geostrophic velocities during ARC transect 2 78 6.6a-c Location of ARC transect 2 (a), geostrophic velocities (b) and volume transport ( c) 79 6. 7 Geostrophic velocities during ARC transect 3 81 6.8a-c Location of ARC transect 3 (a), geostrophic velocities (b) and volume transport ( c) 82 6. 9 Geostrophic velocities during Marathon transect 1 86 6. lOa-c Location ofMarathon transect 1 (a), geostrophic velocities (b) and volume transport ( c) 87 VII 6.11 Geostrophic. velocities during Marathon transect 2 89 6.12a-c Location of Marathon transect 2 (a), geostrophic velocities (b) and volume transport ( c) 90 6.13 Geostrophic velocities during Discovery 164 92 6.14a-c Location of Discovery (a), geostrophic velocities (b) and volume transport ( c) 93 6.15 Geostrophic velocities during SUZIL transect 1 96 6. l 6a-c Location of SUZIL transect 1 (a), geostrophic velocities (b) and volume transport ( c) 97 6.17 Geostrophic velocities during SUZIL transect 2 99 6.18a-c Location of SUZIL transect 2 (a), geostrophic velocities (b) and volume transport ( c) 100 6.19 Geostrophic velocities during SUZIL transport 3 102 6.20a-c Location of SUZIL transect 3 (a), geostrophic velocities (b) and volume transport ( c) 103 6.21 Geostrophic velocities during SUZIL transect 4 106 6.22a-c Location of SUZIL transect 4 (a), geostrophic velocities (b) and volume transport ( c) 107 6.23 Geostrophic velocities during SUZIL transect 5 109 6.24a-c Location of SUZIL transect 5 (a), geostrophic velocities (b) and volume transport ( c) 110 6.25a/b Calculated surface velocities (a) and volume transport (b )for all stations within the Agulhas Return Current 112 6.26 Volume transports for all stations within the Agulhas Return Current 114 7. la/b TIS (a) and TIO plots for the western Indian Ocean 117 7.2 Distribution of Agulhas Current (inflow) CTD stations 121 7.3a TIS plot of the Agulhas inflow CTD stations 122 7.3b T10 plot of the Agulhas inflow CTD stations 123 7.4 Distribution of AJAX CTD stations 125 7.5a TIS plot of the AJAX CID stations 128 VIII 7. 5b T10 plot of the AJAX CTD stations 129 7. 6 Distribution of Agulhas Return Current (outflow) CTD stations 131 7. 7 a TIS plot comparing Agulhas Current (inflow) CTD stations to Agulhas Return Current (outflow) CTD stations 133 7. 7b TIO plot comparing Agulhas Current (inflow) CTD stations to Agulhas Return Current (outflow) CTD stations 134 7.8 Distribution of Discovery 164 and SUZJL CTD stations 137 7.9a TIS plot comparing Agulhas Return Current (outflow) CTD stations to Discovery CTD stations 17-18 139 7.9b TIO plot comparing Agulhas Return Current (outflow) CTD stations to Discovery CTD stations 17-18 140 7.10a TIS plot comparing Agulhas Return Current (outflow) CTD stations to Discovery frontal stations 141 7.10b TIO plot comparing Agulhas Return Current (outflow) CTD stations to Discovery frontal stations 142 7 .11 a TIS plot comparing Agulhas Return Current (outflow) CTD stations to SUZJL CTD stations 145 7 .11 b T10 plot comparing Agulhas Return Current (outflow) CTD stations to SUZIL CTD stations 146 A.la Potential temperature section of AJAX 162 A.
Load more

Recommended publications
  • Joint Geological Survey/University of Cape Town MARINE GEOSCIENCE UNIT TECHNICAL ^REPORT NO. 13 PROGRESS REPORTS for the YEARS 1
    Joint Geological Survey/University of Cape Town MARINE GEOSCIENCE UNIT TECHNICAL ^REPORT NO. 13 PROGRESS REPORTS FOR THE YEARS 1981-1982 Marine Geoscience Group Department of Geology University of Cape Town December 1982 NGU-Tfc—Kh JOINT GEOLOGICAL SURVEY/UNIVERSITY OF CAPE TOWN MARINE GEOSCIENCE UNIT TECHNICAL REPORT NO. 13 PROGRESS REPORTS FOR THE YEARS 1981-1982 Marine Geoscience Group Department of Geology University of Cape Town December 1982 The Joint Geological Survey/University of Cape Town Marine Geoscience Unit is jointly funded by the two parent organizations to promote marine geoscientific activity in South Africa. The Geological Survey Director, Mr L.N.J. Engelbrecht, and the University Research Committee are thanked for their continued generous financial and technical support for this work. The Unit was established in 1975 by the amalgamation of the Marine Geology Programme (funded by SANCOR until 1972) and the Marine Geophysical Unit. Financial ?nd technical assistance from the South African National Committee for Oceanographic Research, and the National Research Institute for Oceanology (Stellenbosch) are also gratefully acknowledged. It is the policy of the Geological Survey and the University of Cape Town that the data obtained may be presented in the form of theses for higher degrees and that completed projects shall be published without delay in appropriate media. The data and conclusions contained in this report are made available for the information of the international scientific community with tl~e request that they be not published in any manner without written permission. CONTENTS Page INTRODUCTION by R.V.Dingle i PRELIMINARY REPORT ON THE BATHYMETRY OF PART OF 1 THE TRANSKEI BASIN by S.H.
    [Show full text]
  • International Ocean Discovery Program Expedition 361 Preliminary Report South African Climates (Agulhas LGM Density Profile)
    International Ocean Discovery Program Expedition 361 Preliminary Report South African Climates (Agulhas LGM Density Profile) 30 January–31 March 2016 Ian R. Hall, Sidney R. Hemming, Leah J. LeVay, and the Expedition 361 Scientists Publisher’s notes Core samples and the wider set of data from the science program covered in this report are under moratorium and accessible only to Science Party members until 30 September 2017. This publication was prepared by the JOIDES Resolution Science Operator (JRSO) at Texas A&M University (TAMU) as an account of work performed under the International Ocean Discovery Pro- gram (IODP). Funding for IODP is provided by the following international partners: National Science Foundation (NSF), United States Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan European Consortium for Ocean Research Drilling (ECORD) Ministry of Science and Technology (MOST), People’s Republic of China Korea Institute of Geoscience and Mineral Resources (KIGAM) Australia-New Zealand IODP Consortium (ANZIC) Ministry of Earth Sciences (MoES), India Coordination for Improvement of Higher Education Personnel, Brazil (CAPES) Portions of this work may have been published in whole or in part in other International Ocean Discovery Program documents or publications. Disclaimer Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the participating agencies, TAMU, or Texas A&M Research Foundation. Copyright Except where otherwise noted, this work is licensed under a Creative Commons Attribution License. Unrestricted use, distribution, and reproduction is permitted, provided the original author and source are credited. Citation Hall, I.R., Hemming, S.R., LeVay, L.J., and the Expedition 361 Scientists, 2016.
    [Show full text]
  • JOIDES Resolution Expedition 361 (Southern African Climates) Site
    IODP Expedition 361: Southern African Climates Site U1475 Summary Background and Objectives Site U1475 is located on the southwestern flank of Agulhas Plateau (41°25.61′S, 25°15.64′E), ~450 nmi south of Port Elizabeth, South Africa, in a water depth of 2669 mbsl. The Agulhas Plateau, which was formed during the early stages of the opening of the South Atlantic about 90 Ma (Parsiegla et al., 2008), is a major bathymetric high that is variably coated with sediments (Uenzelmann Neben, 2001). The 230,000 km2 plateau, which ascends up to ~2500 m above the adjacent seafloor, is bounded on the north by the 4700 m deep Agulhas Passage and is flanked by the Agulhas Basin in the west and the Transkei Basin in the northeast. The northern part of the plateau is characterized by rugged topography, while the central and southern part of the plateau exhibits a relatively smooth topography (Allen and Tucholke, 1981) and has greater sediment thickness (Uenzelmann Neben, 2001). A strong water mass transport flows across the Agulhas Plateau region (Macdonald, 1993), which involves the water column from the surface down to Upper Circumpolar Deep Water. The hydrography of the upper ocean is dominated by the Agulhas Return Current, which comprises the component of the Agulhas Current that is not leaked to the South Atlantic Ocean but rather flows eastwards from the retroflection (Lutjeharms and Ansorge, 2001). Antarctic Intermediate Water, below the Agulhas Return Current, also follows the same flow path near South Africa as the Agulhas Current showing a similar retroflection (Lutjeharms, 1996).
    [Show full text]
  • Tectonic History, Microtopography and Bottom Water Circulation of the Natal Valley and Mozambique Ridge, Southwest Indian Ocean
    Tectonic history, microtopography and bottom water circulation of the Natal Valley and Mozambique Ridge, southwest Indian Ocean. By: Errol Avern Wiles Submitted in fulfilment of the degree Doctor of Philosophy In the Discipline of Geological Sciences School of Agricultural, Earth and Environmental Sciences College of Agriculture, Engineering and Science University of KwaZulu Natal, Durban South Africa 2014 i Preface The experimental work described in this thesis was carried out in the Discipline of Geological Sciences, University of KwaZulu-Natal, Westville, from July 2010 to November 2014, under the supervision of Prof. Watkeys, Dr. Green and Prof. Jokat. These studies represent original work by the author and have not otherwise been submitted in any form for any degree or diploma to any tertiary institution. Where use has been made of the work of others it is duly acknowledged in the text. Signed:_____________________ Name:________________________ Date:_____________ ii Declaration 1 – Plagiarism I, Errol Avern Wiles, declare that: 1. The research reported in this thesis, except where otherwise indicated, is my original research. 2. This thesis has not been submitted for any degree or examination at any other university. 3. This thesis does not contain other persons’ data, graphs or other information, unless specifically acknowledged as being sourced from other persons. 4. This thesis does not contain any other persons writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted then: a. Their words have been re-written but the general information attributed to them has been referenced b. Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced.
    [Show full text]
  • This Article Appeared in a Journal Published by Elsevier. the Attached
    This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Marine Geology 266 (2009) 65–79 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo Sedimentary deposits on the southern South African continental margin: Slumping versus non-deposition or erosion by oceanic currents? Gabriele Uenzelmann-Neben a,⁎, Katrin Huhn b a Alfred-Wegener-Institut für Polar- und Meeresforschung, Am Alten Hafen 26, 27572 Bremerhaven, Germany b MARUM, Center for Marine Environmental Sciences, University of Bremen, Leobener Str., 28359 Bremen, Germany article info abstract Article history: Seismic profiles extending from the southern South African shelf into the deep sea reveal a strong erosional Received 5 January 2009 activity, which affects large parts of the continental margin. Quaternary to Oligocene units and, in places, the Received in revised form 15 July 2009 complete sedimentary column appear to have been removed. Mass movements were previously considered Accepted 17 July 2009 as the origin of this erosion. However, structures indicating slumping can be identified in only a few places.
    [Show full text]
  • Commission on the Limits of the Continental Shelf
    United Nations Convention on the Law of the Sea ____________________________________________________________ Commission on the Limits of the Continental Shelf SUMMARY OF RECOMMENDATIONS OF THE COMMISSION ON THE LIMITS OF THE CONTINENTAL SHELF IN REGARD TO THE PARTIAL SUBMISSION MADE BY THE REPUBLIC OF SOUTH AFRICA IN RESPECT OF THE AREA OF THE SOUTH AFRICAN MAINLAND 1 ON 5 MAY 2009 Recommendations prepared by the Subcommission established for the consideration of the Partial Submission made by the Republic of South Africa Approved by the Subcommission on 4 August 2016 Approved by the Commission, with amendments, on 17 March 2017 1 The aim of this Summary is to provide information which is not of confidential or proprietary nature in order to facilitate the function of the Secretary-General in accordance with Rule 11.3 of Annex III to the Rules of Procedure of the Commission(CLCS/40/Rev.1). This Summary is based on excerpts of the Recommendations and may refer to material not necessarily included either in the full Recommendations or this Summary. TABLE OF CONTENTS GLOSSARY OF TERMS ...................................................................................................................... IV I. INTRODUCTION ........................................................................................................................... 1 II. CONTENTS OF THE SUBMISSION .............................................................................................. 3 A. The Submission .......................................................................................................................
    [Show full text]
  • References 241
    References 241 References 1. Tchernia, P. (1980). Descriptive Regional Oceanogra- 14. Lutjeharms, J.R.E., W.P.M. de Ruijter and R.G. phy. Pergamon Press, Oxford, 253 pp. Peterson (1992). Interbasin exchange and the Agulhas 2. Lutjeharms, J.R.E. (1972). A Guide to Research Done retroflection; the development of some oceanographic Concerning Ocean Currents and Water Masses in the concepts. Deep-Sea Research, 39(10): 1791–1807. South West Indian Ocean. University of Cape Town, 15. Thomson, C.W. and J. Murray (editors) (1881–1895). Cape Town, 577 pp. Report on the scientific results of the voyage of H.M.S. 3. Lutjeharms, J.R.E. (1980). Fisiese Oseanologie van die Challenger during the years 1873–1876. His Majesty’s Suidwes Indiese Oseaan: ’n bronnelys en oorsig van Stationery Office, London, 50 volumes. 1970 tot 1980. WNNR Verslag, T/SEA 8016, 82 pp. 16. Von Schleinitz, B. (1889–1890). Die Forschungsreise 4. Lutjeharms, J.R.E. (1987). The Physical Oceanology of S.M.S. “Gazelle” in den Jahren 1874 bis 1876 unter the South West Indian Ocean; a bibliography for 1980 Kommando des Kapitän zur Zee Freiherrn von to 1985. WNNR-verslag, T/SEA 8712, 131 pp. Schleinitz. Hydrographische Amt, Reichs-Marine- 5. Lutjeharms, J.R.E. (1992). The Physical Oceanology of Ampts, Berlin, Bände I–V. the South West Indian Ocean; a bibliography from 17. Hydrographic Department (1896). List of oceanic 1986 to 1990. CSIR Research Report, 707, 95 pp. depths and serial temperature observations during the 6. Basto, A. de M. (1945). Diario da viagem de Vasco da years 1895.
    [Show full text]
  • Crustal Structure of the Southern Margin of the African Plate: Results
    Originally published as: Stankiewicz, J., Parsiegla, N., Ryberg, T., Gohl, K, Weckmann, U., Trumbull, R., Weber, M. (2008): Crustal Structure of the Southern Margin of the African Continent: Results from Geophysical Experiments. - Journal of Geophysical Research, 113, B10313 DOI: 10.1029/2008JB005612. 1 Crustal Structure of the Southern Margin of the African 2 Continent: Results from Geophysical Experiments. 3 4 J. Stankiewicz, N. Parsiegla, T. Ryberg, K. Gohl, U. Weckmann, R. Trumbull & M. Weber. 5 6 Abstract 7 A number of geophysical onshore and offshore experiments were carried out along a profile across the 8 southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction 9 seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km 10 at the present coast, before gently thinning out towards the Agulhas-Falkland Fracture Zone, which 11 marks the transition zone between continental and oceanic crust. In the region of the abruptly 12 decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and 13 the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as 14 metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added 15 to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent 16 resolution, and is consistent with the known geological record. A joint interpretation of the velocity 17 model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible 18 to correlate a high velocity anomaly north of the centre of the Beattie Magnetic Anomaly with a highly 19 resistive body.
    [Show full text]
  • The Agulhas Plateau: Structure and Evolution of a Large Igneous Province
    Geophys. J. Int. (2008) 174, 336–350 doi: 10.1111/j.1365-246X.2008.03808.x The Agulhas Plateau: structure and evolution of a Large Igneous Province N. Parsiegla, K. Gohl and G. Uenzelmann-Neben Alfred Wegener Institute for Polar and Marine Research, PO Box 12161, 27515 Bremerhaven, Germany. E-mail: [email protected] Accepted 2008 March 28. Received 2008 March 13; in original form 2008 January 5 SUMMARY Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material.
    [Show full text]
  • The South Atlantic: an Overview of Results from 1983-88 Research
    FEATURE THE SOUTH ATLANTIC: AN OVERVIEW OF RESULTS FROM 1983-88 RESEARCH By Arnold L. Gordon ~ EACH of the major ocean basins is unique, but the Initiative (SAARI) meeting repoll are available from South Atlantic is unrivaled in its number of singular Dr. Garzoli at Lamont-Doherty Geological Observa- features. In its polar region are formed intermediate tory, Palisades, NY, 10964. What follows is an and bottom water masses that cool and lower the abbreviated account of the primary results in the salinity of much of the global ocean. The subtropical SAAR1 report. Names mentioned here without at- region has an eastern boundary that doesn't quite tributive dates refer to the report, in which additional extend to the latitude of maximum westerlies, allow- information and extensive references may be ob- A curious ing thernmcline communication with its counterpart tained: an abbreviated list of references is included at feature of the in the South Indian Ocean. Within the Argentine the end of the article. Basin of the South Atlantic one can sample deep Large Scale Circulation South Atlantic is water from the North Atlantic, deep and intermediate A curious feature of the South Atlantic is the the northward heat water from the Pacific, and bottom water formed in northward heat flux across 30°S, a consequence of the Weddell Sea. These water masses spread and the global thermohaline circulation cell driven by flux across 30°S vigorously mix within tiffs cosmopolitan basin and formation of North Atlantic Deep Water (NADW). throughout much of the South Atlantic. In the classical description, southward-moving Perhaps the most startling attribute of the South NADW is replaced by an upper layer return rio,:,' Atlantic is the significant equalorward heat flux derived from Pacific water, with the introduction of within the subtropical region.
    [Show full text]
  • Meridional Overturning Circulation (MOC)
    The Role of Meridional Overturning Circulation (MOC) on Ancient Climates and implications for Anthropogenic Climate Change ◊ ◊ ◊ Malcolm Cumming AGU Fall Meeting December 11-15, 2017 New Orleans Poster Number: PP23A-1282 Column 1 Sheet 1 MOC: poleward flowing surface currents of warm/salty water (along meridians), which, once chilled, sink (i.e. overturn) and return to lower latitudes as very cold/deep waters. Column 1 Sheet 2 Norwegian Seas Overturning takes place primarily in 4 locations. • Norwegian Seas (2 areas) • Weddell Sea • Ross Sea (Note: overturning sites are fairly shallow which may be a factor in enabling downwelling to occur.) Ross Sea Weddell Sea Overturning in the Norwegian Seas & North Atlantic • Water penetrating into the Nordic Seas & Arctic Ocean flows primarily at the surface. • The volume of cold water flowing out is equals warmer water inflow. • Overturning determines the split between surface and deep outflows. Graphic courtesy of http://www.o-snap.org/ Overturning in the Subpolar North Atlantic Program Column 1 Sheet 3 What Drives Overturning? • Buoyancy loss of surface water is so substantial, it becomes dense enough to sink and displace water at the bottom. • Water density is maximized by a combination of becoming very cold while remaining relatively salty. • The 4 regions where overturning takes place provide needed conditions where salty water can be quickly chilled. Very cold air & shallow bottoms Water must be chilled fast enough to reach its maximum density before it begins to sink from the surface. Deep water formation requires a combination of extreme cold together with inertia against premature sinking. It is likely that shallow bottoms and rough topography are important factors that constrain the outflow of deep water and slow the decent of waters from the surface.
    [Show full text]
  • 449-458 Uenzelmann Et.Al 105-108 Van Dijk
    G. UENZELMANN-NEBEN, M.K. WATKEYS., W. KRETZINGER, M. FRANK AND L. HEUER 449 PALAEOCEANOGRAPHIC INTERPRETATION OF A SEISMIC PROFILE FROM THE SOUTHERN MOZAMBIQUE RIDGE, SOUTHWESTERN INDIAN OCEAN G. UENZELMANN-NEBEN Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany e-mail: [email protected] M.K. WATKEYS AND W. KRETZINGER School of Geological Sciences, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, Republic of South Africa e-mail: [email protected]; [email protected] M. FRANK AND L. HEUER IFM-GEOMAR, Leibniz Institute of Marine Sciences at the University of Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany e-mail: [email protected], [email protected] © 2011 December Geological Society of South Africa ABSTRACT Seismic reflection data from the southern Mozambique Ridge, Southwestern Indian Ocean, show indications for a modification in the oceanic circulation system during the Neogene. Major reorganisations in the Indian Ocean circulation system accompanying the closure of the Indonesian Gateway led to the onset of current controlled sedimentation in the vicinity of the Mozambique Ridge at ~14 Ma. The modifications in water mass properties and path are documented in changes in reflection characteristics in the Mozambique Ridge area. Correlating these with identified changes of the Nd isotope evolution in deep water masses the general present day large scale circulation in the southern Indian Ocean is suggested to have prevailed for the last 9 Ma. References should not be included in abstracts. Introduction manganese nodules and crusts from Heuer (2009) are The exchange of water masses between the Indian and used to provide a record of oceanic circulation and South Atlantic Oceans has a crucial influence on the weathering inputs using radiogenic isotopes of Nd, Pb, global thermohaline circulation and thus also the general and Hf over the past 20 Ma.
    [Show full text]