Box 1: Dynamics of Agulhas leakage
As a western boundary current, the Agulhas Current is primarily driven by positive wind stress curl over the subtropical Indian Ocean. However, beyond the tip of Africa, there is no western boundary to steer the current southwards, towards the latitude where the westerlies are a maximum and the wind stress curl is zero (Fig. 1). Hence, without inertia the full Agulhas Current would turn west into the Atlantic and feed the Southern Hemisphere supergyre8. Conversely, for large inertia, or with the latitude of zero wind stress curl much closer to the continent, the full Agulhas Current would loop back into the Indian Ocean. In this case, there is no leakage, owing to ‘inertial choking’. Lower inertia would lead to partial leakage. Hence, the main controls on retroflection and leakage are the latitude of maximum westerlies and the southward inertia of the Agulhas Current at separation, which are both largely determined by the strength and position of the wind field over the Indian Ocean. Instabilities, interaction with bathymetry and other mesoscale, nonlinear dynamics are also important, generating Agulhas rings that propagate westwards, conveying most of the leakage into the Atlantic2, 95, 96.
When interpreting glacial–interglacial cycles in the Agulhas system implied from palaeoproxy data, or present-day changes implied from model simulations, the combination of these controls should be taken into account. For instance, if the wind pattern is fixed but its strength reduced, then a weaker Agulhas Current would result in more leakage, owing to the smaller inertial overshoot97, 98. Similarly, for increased wind strength a stronger Agulhas Current combines with less leakage. But if the wind pattern shifts northwards and weakens (for example in relation to glacial climate99) then the Agulhas Current would be weaker at separation, and the latitude of zero wind stress curl would simultaneously be closer to Africa. The weaker Agulhas Current may then still have enough inertia to reach the closer wind curl change, so that a weaker Agulhas Current combines with less leakage17, 91. In present-day climate change, global wind data point to a southward expansion of the wind pattern (a southward shift of the westerlies) but no robust trend in wind curl over the Indian Ocean. In such a situation, the latitude of zero wind stress curl or maximum westerlies is farther south and leakage increases, but the response of the Agulhas Current and its change in inertia is unclear9, 10.
To simulate Agulhas leakage correctly, it is essential that inertial mechanisms and ring formation—that is, nonlinear dynamics—are resolved. However, most Intergovernmental Panel on Climate Change (IPCC)-type simulations have coarse resolution (large eddy viscosity). Agulhas retroflection appears, but as a result of ‘viscous choking’ instead of inertial choking. As a result, the amplitudes of Agulhas transport and leakage and their sensitivity to change may be completely wrong100. An effective resolution of at least 0.1° is necessary to resolve Agulhas leakage and could be achieved by a range of regional and global numerical grids15, 70.
Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Florida 33149, USA
- Lisa M. Beal
Institute for Marine and Atmospheric Research, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
- Wilhelmus P. M. De Ruijter
Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany
- Arne Biastoch
Institució Catalana de Recerca i Estudis Avançats (ICREA), Institut de Ciència i Tecnologia Ambientals (ICTA), Departament de Física, Universitat Autònoma de Barcelona, Bellaterra E-08193, Spain
- Rainer Zahn
NOAA Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, Washington 98115, USA.
- Meghan Cronin
South African Environmental Observation Network, Private Bag X2, Roggebaai 8012, South Africa.
- Juliet Hermes
Department of Oceanography, University of Cape Town, 7700 Rondebosch, South Africa.
- Johann Lutjeharms
National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK.
- Graham Quartly &
- Paolo Cipollini
Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
- Tomoki Tozuka
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
- Sheekela Baker-Yeboah
South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa.
- Thomas Bornman
Institute for Marine and Atmospheric Research, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands.
- Henk Dijkstra
School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, UK.
- Ian Hall
Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany.
- Wonsun Park
Faculty of Earth and Life Sciences, Section Marine Biogeology, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
- Frank Peeters
Laboratoire de Physique des Oceans (UMR 6523 CNRS, IFREMER, IRD, UBO), LMI ICEMASA, Department of Oceanography, University of Cape Town, Cape Town, 7700 Rondebosch, South Africa.
- Pierrick Penven
NIOZ Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, The Netherlands.
- Herman Ridderinkhof &
- Jens Zinke
SCOR/WCRP/IAPSO Working Group 136
- Meghan Cronin,
- Juliet Hermes,
- Johann Lutjeharms,
- Graham Quartly,
- Tomoki Tozuka,
- Sheekela Baker-Yeboah,
- Thomas Bornman,
- Paolo Cipollini,
- Henk Dijkstra,
- Ian Hall,
- Wonsun Park,
- Frank Peeters,
- Pierrick Penven,
- Herman Ridderinkhof &
- Jens Zinke
The ideas presented here were developed jointly by L.M.B., W.P.M.D.R., A.B., R.Z. and the SCOR/WCRP/IAPSO Working Group 136 on the Climatic Importance of the Greater Agulhas System. Working Group 136 is sponsored by the Scientific Committee for Oceanic Research (SCOR), the World Climate Research Program (WCRP) and the International Association for the Physical Sciences of the Ocean (IAPSO).
Competing financial interests
The authors declare no competing financial interests.
Lisa M. Beal
Wilhelmus P. M. De Ruijter